DESIGN MANUAL FOR ARTICULATING - HCFCD Associates preparing this "Design Manual for Articulating...
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Transcript of DESIGN MANUAL FOR ARTICULATING - HCFCD Associates preparing this "Design Manual for Articulating...
P.O. Box 270460Fort Collins, Colorado 80527
(970) 223-5556, FAX (970) 223-5578
Ayres Project No. 32-0366.00HAR9-TX.DOC
September 2001
DESIGN MANUAL FOR ARTICULATINGCONCRETE BLOCK SYSTEMS
Prepared for: Harris County Flood Control District
ACKNOWLEDGEMENT
The Harris County Flood Control District (the "District") acknowledges with appreciationAyres Associates preparing this "Design Manual for Articulating Concrete Block (ACB)Systems" (the "Manual").
DISCLAIMER
THIS MANUAL WAS PREPARED AND IS BEING DISTRIBUTED MERELY AS AN OVERVIEW WITHREGARD TO SOME ISSUES THAT COULD RELATE TO DESIGNING ARTICULATING BLOCKSYSTEMS. THE DISTRICT AND AYRES ASSOCIATES DO NOT REPRESENT, WARRANT, ORGUARANTEE THAT THE MANUAL IS FREE OF ERRORS OR OMISSIONS. THIS MANUAL ISBEING RECEIVED AND USED AT THE USER�S OWN RISK �AS IS,� �AS IT STANDS,� AND �WITHALL FAULTS.� THE DISTRICT AND AYRES ASSOCIATES EXPRESSLY DISCLAIM ALL LIABILITYFOR ANY CLAIMS OR DAMAGE, DIRECT OR CONSEQUENTIAL, FROM MATERIAL CONTAINEDWITHIN THE MANUAL, AND FURTHER EXPRESSLY DISCLAIM ALL WARRANTIES, EXPRESS ORIMPLIED, WHICH RELATE OR IN ANY WAY COULD RELATE TO THE MANUAL. THE USERSHOULD OBTAIN INDEPENDENT, COMPETENT PROFESSIONAL ADVICE TO DETERMINEWHETHER THE MATERIAL CONTAINED WITHIN THE MANUAL IS CURRENT, APPLICABLE, ANDCORRECT AT THE TIME OF ITS USAGE.
NEITHER THE DISTRICT NOR AYRES ASSOCIATES SHALL BE RESPONSIBLE FOR ANYERRORS OR OMISSIONS INCLUDED WITHIN OR IN ANY WAY ASSOCIATED WITH THEMANUAL, AND THEY SHALL BE HELD HARMLESS FOR SAME.
LIMITED REPRODUCTION OR MODIFICATION
It is expressly understood and agreed by accepting and using the manual, that neither themanual nor any part thereof shall be copied, otherwise reproduced, or modified without theprior express written permission of the District�s Director. However, the District�s Directorgrants permission to copy and distribute the Manual for noncommercial and non-profit use,so long as the content remains unaltered and credit is given to the District and AyresAssociates in connection with such use. The user also understands and agrees that theabove disclaimer and this limitation on reproduction or modification shall accompany eachreproduction or use of the materials contained herein.
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TABLE OF CONTENTS
C.1 INTRODUCTION .......................................................................................................C.1
C.1.1 Background.............................................................................................................C.1C.1.2 Special Conditions Unique to Harris County............................................................C.3
C.2 OPEN CHANNEL HYDRAULICS FOR ACB DESIGN ...............................................C.4
C.3 GEOMORPHIC CONSIDERATIONS FOR ACB DESIGN ..........................................C.7
C.4 DESIGNING ACB SYSTEMS FOR HYDRAULIC STABILITY ...................................C.8
C.4.1 Performance Testing of ACB Systems ....................................................................C.9C.4.2 Extrapolation of Test Data .....................................................................................C.10C.4.3 Factor of Safety Design Equations ........................................................................C.12C.4.4 Pre-Selecting a Design Factor of Safety................................................................C.13C.4.5 Extent of Revetment Coverage .............................................................................C.17C.4.6 Cabled Versus Non-Cabled ACB Systems ............................................................C.18C.4.7 Drainage Layers....................................................................................................C.18C.4.8 ACB Design Procedure and Example....................................................................C.18
C.5 GEOTEXTILE AND GRANULAR FILTER DESIGN..................................................C.26
C.5.1 Filter Functions......................................................................................................C.27C.5.2 Base Soil Properties..............................................................................................C.30C.5.3 Geotextile Filter Properties ....................................................................................C.31C.5.4 Granular Filter Properties ......................................................................................C.32C.5.5 Filter Design Procedure and Example ...................................................................C.32
C.6 INSTALLATION GUIDELINES ...............................................................................C.41
C.6.1 Subgrade Preparation ...........................................................................................C.41C.6.2 Placement of Geotextile ........................................................................................C.42C.6.3 Placement of ACB System ....................................................................................C.42C.6.4 Finishing................................................................................................................C.47C.6.5 Inspection..............................................................................................................C.48
C.7 Worksheets, Proprietary Block Properties, and Typical Detail Drawings...................C.48
C.8 Annotated Bibliography ............................................................................................C.56
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LIST OF FIGURES
Figure C.1.1. Examples of proprietary ACB systems shown in plan view..........................C.1
Figure C.1.2. Geologic formations in Harris County ..........................................................C.3
Figure C.2.1. (A) Plan view of a river meander bend with region of increased shear stress indicated (B) Cross section A-A� illustrating superelevation at outer bank of the bend ............................................................................C.5
Figure C.2.2. Two-dimensional model results with velocity vectors at a waterway constricted by bridge approach embankments ............................................C.5
Figure C.2.3. Horseshoe vortex flow pattern observed at bridge piers ..............................C.6
Figure C.2.4. Schematic of a block protruding above ACB matrix resulting in added drag and lift forces overturning the block ..........................................C.7
Figure C.4.1. Moment Balance on an ACB at Incipient Failure .......................................C.10
Figure C.4.2. Typical laboratory flume configuration and photographs of the full-scale testing facility..............................................................................C.11
Figure C.4.3. Three-dimensional view of a block on a channel side slope with factor of safety variables defined...............................................................C.14
Figure C.4.4. Figure of a block showing moment arms �1, �2, �3, and �4 ............................................. C.14
Figure C.4.5. Factor of Safety Decision Support for ACB Systems...................................................... C.16
Figure C.4.6. Definition sketch of example problem setting and ACB installation................... C.20
Figure C.4.7. Velocity Distribution at River Mile 23.4 from HEC-RAS Model............................... C.21
Figure C.5.1. Channel cross sections showing filter and bedding orientation ..................C.26
Figure C.5.2. Examples of soil and filter subgrades ........................................................C.27
Figure C.5.3. Time series of channel and groundwater level changes resulting from a storm event ....................................................................................C.29
Figure C.5.4. Geotextile selection based on soil retention...............................................C.34
Figure C.6.1. Granular filter detail showing granular filter encapsulation.........................C.43
Figure C.6.2. ACB mats being placed with a crane and spreader bar .............................C.44
Figure C.6.3. Close-up of spreader bar and ACB mat.....................................................C.44
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LIST OF FIGURES (continued)
Figure C.6.4. Conceptual detail of minimum radius-of-curvature for bed and bank protection ..................................................................................................C.45
Figure C.6.5. Bed and bank protection with minimum radius-of-curvature at grade changes and top-of-slope termination points ...................................C.45
Figure C.6.6. Conceptual detail of minimum radius-of-curvature for bank protection ..................................................................................................C.46
Figure C.6.7. Conceptual detail of toe termination for spillways or embankment overtopping flow........................................................................................C.46
Figure C.6.8. Embankment dam overtopping protection with radius-of-curvature at top-of-slope termination.........................................................................C.47
LIST OF TABLES
Table C.4.1. Design Equations for ACB Systems ...........................................................C.15
Table C.4.2. HEC-RAS Model Output for River Mile 23.4 ...............................................C.19
Table C.5.1. Porosity for Alluvial Soils ............................................................................C.31
Table C.5.2. General Soil Sample Information and Classification ...................................C.32
Table C.5.3. Results from Sieve and Hydrometer Analysis .............................................C.33
Table C.5.4. Pit Run Sand Gradation for Granular Filter .................................................C.35
C.1
C.1 INTRODUCTION
C.1.1 Background
This Design Manual provides guidelines and procedures for the design and installation ofarticulating concrete block revetment systems. Articulating concrete block (ACB) systems areused to provide erosion protection to underlying soil from the hydraulic forces of movingwater. An ACB system is comprised of a matrix of individual concrete blocks placed togetherto form an erosion-resistant revetment with specific hydraulic performance characteristics.The term "articulating" implies the ability of the matrix to conform to minor changes in thesubgrade while remaining interconnected with geometric interlock and/or additional systemcomponents such as cables. Several varieties of ACB systems are available: interlockingand non-interlocking block shapes, cable-tied and non-cable-tied matrices, and open cell andclosed cell varieties. Figure C.1.1 shows a few ACB varieties in plan view.
Figure C.1.1. Examples of proprietary ACB systems shown in plan view. Note that this is not all inclusive of what is available in the industry and no endorsement or recommendation is intended.
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The ACB system includes a filter component that allows infiltration and exfiltration to occurwhile retaining the soil subgrade. The filter layer requires a geotextile and may include agranular transition layer. In some cases a highly permeable drainage layer, either granularor synthetic, may be included in the system design for sub-block pressure relief, particularlyin turbulent flows.
Articulating concrete blocks can be used in a broad range of erosion control applications withgood success. Since ACB systems have a very high armoring potential, application is notlimited to subcritical flow or locations of low turbulence. ACB systems have been used withexcellent success at installations generating high velocities such as culvert outlets, spillways,and grade control structures. In many laboratory studies, ACB systems did not fail atvelocities exceeding 20 fps, where failure was defined as any loss of contact between theblock and the subgrade. In many geographic regions, ACB systems offer a less expensiveand more aesthetic alternative to more traditional treatments such as riprap, structuralconcrete, and soil cement.
ACB systems are well suited to channel lining applications, in particular for lateral streamstability. The articulating characteristic allows the systems to be placed effectively at bendsand regions of vertical change, such as sloping grade control structures. Many ACB systemsare manufactured with voids or open cells to accommodate vegetation.
The systems should not be placed on slopes that are geotechnically unstable; ACB systemsare intended for erosion control, not slope stabilization. Geotechnical engineering and slopestabilization references should be sought for problems in this area.
Ayres Associates, under contract to the Harris County Flood Control District (HCFCD),developed this ACB Design Manual. This manual is intended to provide a standardizedbasis for the analysis, design, and installation of ACB systems for erosion controlapplications in open channels. Sections provided in this manual include:
C.2 Open Channel Hydraulics for ACB Design � provides background of open channelhydraulics related to ACB design that supplements the Design Criteria Manual;
C.3 Geomorphic Considerations for ACB Design - provides insight and references forstability assessment of a site prior to ACB application;
C.4 Designing ACB Systems for Hydraulic Stability � provides discussion of laboratorytesting, design criteria and equations, special topics, and an ACB design procedure;
C.5 Geotextile and Granular Filter Design - provides information regarding thesignificance of the filter layer, base soil and filter properties, and a filter designprocedure;
C.6 Installation Guidelines � provides recommended/required ACB installation proceduresfor effective erosion control performance;
C.7 Worksheets, Proprietary Block Properties, and Typical Detail Drawings � providesdesign worksheets to assist with calculations, tables of manufacturer supplied blockproperties for proprietary systems, and HCFCD detail sheet(s);
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C.8 Annotated Bibliography � provides a list of cited references and additional referencesfor subjects beyond the scope of this manual, with a short narrative describingrelevant aspects of each document.
C.1.2 Special Conditions Unique to Harris County
There are four major surficial geologic units which outcrop in Harris County. From youngestto oldest, these units are
Deweyville FormationBeaumont FormationLissie FormationWillis Formation
Figure C.1.2 illustrates the distribution of these geologic formations in Harris County. Soilsassociated with the Lissie Formation are highly dispersive and tend to be susceptible topiping failures along stream banks. Essentially, these soils suffer from both internal andexternal erosion.
Because of the known problematic nature of Lissie soils, articulating concrete blockrevetment systems shall not be used in these areas unless site-specific geotechnicalengineering controls are incorporated into the project design. Such designs must receivespecial approval by the Director on a case-by-case basis.
Figure C.1.2. Geologic formations in Harris County.
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C.2 OPEN CHANNEL HYDRAULICS FOR ACB DESIGN
Effective design of ACB revetment systems depends upon proper characterization of thehydraulic conditions expected during the design event. The vast majority of revetmentfailures, whether riprap or manufactured systems, have occurred where the designer did notadequately quantify the hydraulics of flow.
The hydraulic variables of greatest interest in ACB system design are shear stress andvelocity. The design procedure presented in this ACB Design Manual is based on anapproach that considers the hydraulic forces imposed on a single block at incipient failure ofthe system. In formulating the equations for practical use, a ratio of design shear stress to"critical" shear stress is used. Cross section averaged shear stress can be calculated fordesign using the following simple equation:
f0 RSγ=τ (Eqn. C.2.1)
where:
τ0 = Cross section averaged shear stress (lbs/ft2)γ = Unit weight of water (62.4 lb/ft3)R = Hydraulic radius (ft)Sf = Local friction slope (ft/ft)
Manufacturers of ACB systems should provide performance data from full-scale testsperformed in accordance with the Federal Highway Administration (Clopper and Chen 1988),"Minimizing Embankment Damage During Overtopping Flow." The performance results arethen provided to the designer in the form of critical shear stress (also known as maximumallowable shear stress). A background discussion of laboratory flume testing of ACBsystems is provided later in this ACB Design Manual.
For some applications, cross section averaged shear stress is not suitable for design. Suchcases include bends, confluences, constrictions, and flow obstructions. An example of howshear stress can vary in a complex flow field is illustrated in the river meander bend of FigureC.2.1. The superelevation of the water surface against the outside bank of the bendproduces a locally steep downstream water surface slope and, as a result, a region ofincreased shear stress. A similar phenomenon can occur at bridge crossings whereapproach embankments encroach on a floodplain. A locally steep water surface isdeveloped near the bridge abutment between the water backed up behind the embankmentand the water moving through the bridge opening at a much higher velocity.
For complex hydraulic systems, more sophisticated modeling is generally an appropriatesolution. For example, a 2-dimensional model would be the appropriate method fordetermining shear stress through a bridge where the approach embankment(s) constrict awide floodplain. A 2-dimensional model showing velocity vectors through a constrictedwaterway is shown in Figure C.2.2. More sophisticated modeling tools are discussed in theannotated bibliography provided at the end of this ACB Design Manual, along with availabilityand ordering information.
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Figure C.2.1. (A) Plan view of a river meander bend with region of increased shear stress indicated (B) Cross section A-A� illustrating superelevation at outer bank of the bend.
Figure C.2.2. Two-dimensional model results with velocity vectors at a waterway constricted by bridge approach embankments.
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If a simplified modeling approach, such as the Manning equation or the HEC-2 model, isused to model a complex hydraulic system then conservatism should be incorporated intothe design shear stress and factor of safety (discussed later in this ACB Design Manual). Inthe case of flow around a bend, velocity can range between 0.9 and 1.7 times the crosssection averaged velocity (Lagasse et al. 2001b). Shear stress is proportional to the squareof velocity; therefore, the range of multipliers that is suggested for shear stress is 0.8 to 2.9.Some example shear stress multipliers are provided as follows:
• 0.8 for a location near the bank of a straight reach• 1.4 for a location in the main current of flow of a meander bend• 2.9 for a location in the main current of flow of an extreme bend
To date, there is limited information available for quantifying how velocity and shear stressincrease locally at obstructions to a flow field, such as bridge piers or pipelines. Flow aroundlocal obstructions is very turbulent and generally results in some vortex flow pattern, bothcontributing to very erosive flows. A schematic of the horseshoe shaped vortex oftenobserved at flow around bridge piers is provided in Figure C.2.3. The rearranged Isbashriprap equation for piers from Lagasse et al. (2001b) uses a velocity multiplier of 1.5 forround piers and 1.7 for rectangular piers. These values correspond to shear stressmultipliers of 2.3 and 2.9 for round and square piers, respectively. It is suggested that thesevalues be used along with an increased design factor of safety for bridge piers.
Figure C.2.3. Horseshoe vortex flow pattern observed at bridge piers.
Flow velocity becomes a significant hydraulic variable when considering the potential forincreased drag on individual blocks that can result from blocks protruding above thesurrounding ACB matrix due to local subgrade irregularities or imprecise placement. Theproblem is presented in the schematic of Figure C.2.4. The added drag on the block is afunction of the velocity squared according to the following relationship:
2DD Vb)Z(C2/1F ρ∆⋅=′ (Eqn. C.2.2)
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where:
FD′ = Drag force due to block protrusion (lbs)CD = Drag coefficient (CD ≈ 1.0)∆Z = Height of protrusion (ft)b = Block width perpendicular to flow (ft)ρ = Density of water (1.94 slugs / ft3)V = Velocity (ft/s)
The added lift force (FL′) due to the block protruding above the ACB matrix is conservativelyassumed equal to the drag force. With the added drag force imposed on the blockproportional to velocity squared, proper subgrade preparations and installation quality controlare very important, especially in regions of high velocity, such as supercritical reaches andovertopping spillways. In the design procedure that follows, allowable height of blockprotrusion is specified by the designer and should be used by inspectors as a criterion foracceptance or rejection of the installation.
Figure C.2.4. Schematic of a block protruding above ACB matrix resulting in added drag and lift forces overturning the block.
C.3 GEOMORPHIC CONSIDERATIONS FOR ACB DESIGN
Ascertaining whether or not a stream is stable requires a functional definition of stability. Inthe context of ACB design, stability implies that the geomorphic state of the stream, with theACB system in place, is such that adverse conditions to the revetment do not develop overtime.
"Stream Stability at Highway Structures" (Lagasse et al. 2001a) provides a stabilitycharacterization system that classifies several stream properties as being unstable or stable.The system is qualitative in nature, but provides a quick method for ascertaining stability of astream using very little data: annual hydrograph characteristics, soil properties, aerialphotography, and land topography. Thirteen stream properties are used in the method,
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which can be categorized into temporal flow characteristics, channel boundarycharacteristics, topographic relief, plan geometry, and cross section geometry.
Many natural streams migrate laterally without impacting the stream as a system (i.e., effectsof migration do not propagate upstream and downstream). However, lateral migrationbecomes a concern when the security of nearby infrastructure from erosion is jeopardized.In such cases, ACB systems can be used as a countermeasure or as a component of acountermeasure to arrest lateral migration. The designer is referred to "Bridge Scour andStream Instability Countermeasures" (Lagasse et al. 2001b) for lateral instabilitycountermeasure options.
In many applications, an ACB system is used for embankment lining while a "soft" channelbed is maintained for environmental, habitat, or economic reasons. The vertical stability ofthe project site, in terms of aggradation or degradation, should be quantified to determine thesufficient toe-down depth for the revetment. Long-term bed elevation changes are usuallythe result of change(s) to the watershed system, such as: urbanization, deforestation,channelization, meander cutoff, and changes to downstream base level control elevation.Since vertical instability is typically indicative of system-wide response, local use ofarticulating concrete blocks should not be used as the sole countermeasure to arrestdegradation.
Prediction of long-term bed elevation changes is a multi-disciplinary problem that must besolved using a system analysis approach. Analysis of the problem requires the considerationof all influences to the system: runoff from the watershed (hydrology), sediment delivery tothe channel reach (sedimentology), sediment transport capacity of the reach (hydraulics),and the response of the channel to these factors (geomorphology). Lagasse et al. (2001a)offers a three level system approach to fully characterize stream stability:
Level 1: Application of simple geomorphic concepts and other qualitative analyses.Level 2: Application of basic hydrologic, hydraulic, and sediment transport engineering
concepts.Level 3: Application of mathematical or physical modeling studies.
Not all three levels of analysis must be completed, it is suggested that each level of analysisbe carried out until adequate characterization of stream stability is achieved. Given adequatecharacterization of stream stability, the designer can then utilize Lagasse et al. (2001b) forcountermeasure design if needed.
C.4 DESIGNING ACB SYSTEMS FOR HYDRAULIC STABILITY
This section defines a procedure for designing ACB systems based on hydraulic stabilityconcepts. A linkage between performance testing in laboratory flumes and real-world fieldapplications is described and a method that uses results from full-scale performance tests ispresented.
In the design of ACB systems, a factor-of-safety is calculated for the proposed product andthen assessed against a pre-selected target value. This section presents equations forcalculating a factor-of-safety for a specific ACB system, a rational approach to pre-selectinga target factor-of-safety, and a design procedure that compares the calculated and pre-selected values. Special topics related to ACB design are also addressed in this section.
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C.4.1 Performance Testing of ACB Systems
Starting in 1983, the Federal Highway Administration (FHWA) led a group of federalagencies in a multi-year research program to evaluate the performance of different erosioncontrol systems for embankment overtopping flow. Clopper and Chen (1988) summarizesresults from the investigation. The erosion control systems in the 1988 report included threeproprietary articulating concrete blocks. Test results indicated that ACB systems showedpromise as an erosion control countermeasure under severe hydraulic loading; however, theperformance of tested systems varied significantly. The scope of the 1988 study does notprovide a thorough understanding of the failure mechanisms associated with ACB systemsand does not provide reasons for the broad range in system performance. Clopper (1989)provides a follow up report that more thoroughly addresses these issues.
Concurrent with FHWA testing, researchers in Great Britain were evaluating the performanceof similar erosion control systems. Both the FHWA and British researchers agreed that asuitable definition of "failure" for ACB systems is the local loss of intimate contact betweenthe ACB and the subgrade that it protects. Clopper (1989) outlines four causativemechanisms that will result in this definition of failure:
1. Loss of embankment soil beneath the system by gradual erosion along the slope beneaththe system or washout through the system at joints and open cells;
2. Deformation of the underlying embankment through liquefaction and shallow slip of theembankment soil caused by the ingress of water beneath the system;
3. Loss of a block or group of blocks (uncabled systems) which directly exposes thesubgrade to the flow;
4. Local uplift of a block or group of blocks due to hydraulic loading.
It should be noted that these are not the only mechanisms for failure, but those observedduring FHWA laboratory testing.
Given the nature of ACB system installation in typical applications on slopes of 2H:1V orless, the probability of failure due to slipping or sliding of the system matrix along the plane ofthe subgrade is remote. The loss of intimate contact is most often the result of overturning ofa block or group of blocks, in which incipient failure occurs when the overturning momentsequal the restraining moments about the downstream contact point of an individual block.The hydraulic stability of a block is thus a function of its restraining moments (block weightand inter-block restraint) versus the applied overturning moments from hydrodynamic dragand lift. Inter-block restraint is the force from block-to-block contact that resists overturning.The process of incipient failure is illustrated in the moment balance of Figure C.4.1.
Summing moments acting on the block at incipient failure produces an equation defininghydraulic stability. The following equation, which conservatively ignores inter-block restraintis proposed, with the restraining moments on the left side of the equation and the overturningmoments on the right side:
)FF()FF(WW LL4DD31S12S2 ′++′++= ���� (Eqn. C.4.1)
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Figure C.4.1. Moment Balance on an ACB at Incipient Failure.
The above figure illustrates that the ability of any ACB system to provide a stable erosionresistant boundary under a given set of hydraulic conditions is a function of its weight, inter-block restraint, geometry, and quality of installation. In addition, the ability of a system toprovide a degree of flexibility through block-to-block articulation is an important factor inmaintaining intimate contact between the system and the subgrade that it protects. Sincethese characteristics can vary greatly between ACB systems, laboratory flume testing of asystem is necessary to quantify the performance of a particular system. Using test results,the manufacturer can provide performance criteria in the form of "critical" shear stress to thedesigner of the ACB system. The term critical implies the condition at the brink of failure of asingle block.
A schematic of a typical laboratory flume is shown in Figure C.4.2, along with photographs ofactual testing facilities. Flume configurations vary greatly depending on the laboratorysetting provided by the testing Contractor, but are most commonly used for full scale testingof the blocks. Reference can be made to the annotated bibliography provided at the end ofthis ACB Design Manual for further documentation on laboratory flume testing of ACBsystems.
C.4.2 Extrapolation of Test Data
Often laboratory flume testing of blocks is conducted using a steep bed slope. In order touse the design procedure that follows, the critical shear stress for a horizontal surface mustbe known. An equation for extrapolation of test results from one bed slope to results foranother bed slope has been developed. The equation is based on a moment balanceapproach that assumes inter-block restraint to be the same for the tested and untestedconfigurations. The following equation is suggested for extrapolation of test results from onebed slope to another for the same ACB system:
C.11
Revetment
A. Schematic of a typical laboratory flume for ACB performance testing
B. Photograph of full-scale flume test (courtesy of Colorado State University)
C. Photograph of subgrade inspection after a series of full-scale tests (courtesy of Colorado State University)
Figure C.4.2. Typical laboratory flume configuration and photographs of full-scale testing facility.
C.12
���
����
�
θ−θθ−θ
⋅τ=τ θθT1T2
U1U2TCUC sincos
sincos��
�� Eqn. (C.4.2)
where:
τCθU = Critical shear stress for untested bed slope (lbs/ft2)τCθT = Critical shear stress for tested bed slope (lbs/ft2)θU = Untested bed slope (degrees)θT = Tested bed slope (degrees)�x = Moment arms (ft)
Note that the moment arms used in this equation should apply to the orientation of the blockduring testing and are not necessarily the same as those suggested later in this document fordesign.
Similar to extrapolation based on bed slope, an equation for extrapolating test results from atested block to an untested block of similar characteristics has been developed. Theequation should only be used to extrapolate results from a block within a similar family (i.e.,having a similar footprint area and interlock mechanism) but with different thickness andweight. This equation is also based on a moment balance approach that neglects inter-blockrestraint. The following equation is suggested for extrapolation of test results from one blockto another within the same family:
���
����
�
++
⋅⋅τ=τU4U3
T4T3
T2ST
U2SUCTCU W
W��
��
�
� Eqn. (C.4.3)
where:
τCU = Critical shear stress for untested block (lbs/ft2)τCT = Critical shear stress for tested block (lbs/ft2)WSU and WST = Submerged weight of untested and tested blocks (lbs)�XU and �XT = Moment arms of untested and tested blocks (ft)
Note that the moment arms used in this equation should apply to the orientation of the blockduring testing and are not necessarily the same as those suggested later in the document fordesign.
C.4.3 Factor of Safety Design Equations
The following design equations quantify a "Factor of Safety" for application of an ACB systembased on an approach that considers the hydraulic forces imposed on a single block. Theprocedure was originally developed by Stevens et al. (1971) for riprap design and has beenmodified by Julien (1995) to account for the case of riprap placed on a steep longitudinalslope and a steep lateral slope (e.g., a revetment system protecting the bank of anovertopping spillway). Julien�s equations are the most general formulation to date and canbe applied to any hydraulic system where the water surface slope is approximately equal tothe bed slope (i.e., gradually varied flow). Julien�s equations have been modified slightly forthis procedure to consider the known geometric dimensions of concrete blocks and the"critical" shear stress determined from performance testing. Clopper (1991) first presented
C.13
the process of adapting the factor of safety equations to ACB systems in "ProtectingEmbankment Dams with Concrete Blocks," Hydro Review. The major adjustment to theequations is to use the known block geometry for the moment arms rather than anappropriate angle of repose for riprap.
Changes have also been incorporated into the design procedure to account for the additionalforces imposed on a block that protrudes above the surrounding ACB matrix due to localsubgrade irregularities or imprecise placement. Since a slight disruption of intimate contactbetween a block and the subgrade constitutes failure, the equations do not account for therestraining forces due to cables. The potential restraining force imposed on the block matrixby cables is intentionally limited so that block-to-block articulation is maintained. Similarly,the additional stabilizing forces offered by vegetation and/or mechanical-anchoring devicesare ignored in the procedure because such effects are difficult to quantify and are assumedto be of limited value for the sake of design conservatism.
The safety factor (SF) for a single block in the ACB system is defined as the ratio ofrestraining moments to the overturning moments. Rearranging Equation C.4.1 from pageC.9 and adding terms to account for a block placed on a three-dimensional surface results inthe following equation for SF:
L4D3L4D32
S1
S2
FcosFFcosFcosa1W
aWSF′+δ′++δ+β−
=θ
θ
�����
� (Eqn. C.4.4)
The forces, dimensions, and angles in the equation for SF are presented in Figure C.4.3.Dividing Equation C.4.4 by �1WS and substituting terms yields the final form of the factor ofsafety equations as presented in Table C.4.1. The equations can be used in any consistentset of units; however, variables are indicated here in English units.
The submerged block weight, WS, is the weight of the block after subtracting out the force ofbuoyancy. The moment arms �1, �2, �3, and �4 are determined from the block dimensionsshown in Figure C.4.4. In the general case, the pivot point of overturning will be at the frontcorner of the block; therefore, the horizontal distance from the center of the block to thecorner should be used for both �2 and �4. Since the resultant of weight is through the blockcenter of gravity, one half the block height should be used for �1. The drag force acts both onthe top surface of the block (shear drag) and on the body of the block (form drag).Considering both elements of drag, eight-tenths the height of the block is considered a goodestimate of �3.
Extensive research has been conducted to determine the critical shear stress for virtually allsizes of granular soil particles and riprap, but there are limited test data available forproprietary ACB products. Therefore, critical shear stress for a horizontal surface, τC, shouldcome from performance testing of the ACB system being considered. Determination ofdesign shear stress, τdes, is discussed in Section C.2.
C.4.4 Pre-Selecting a Target Factor of Safety
The target factor of safety should be a function of the complexity of the hydraulic system, theconsequence of failure, and the uncertainty in the hydrologic and hydraulic analyses. FigureC.4.5 is a flow chart that offers a simple decision support system that is based on theseelements of ACB design.
C.14
Figure C.4.3. Three-dimensional view of a block on a channel side slope with factor of safety variables defined.
Figure C.4.4. Figure of a block showing moment arms �1, �2, �3, and �4.
C.15
Table C.4.1. Design Equations for ACB Systems.
S1
L4D3121
2
12
w)FcosF()/(cosa1
a)/(SF
�
����
��
′+δ′+η+β−
=
θ
θ C.4.5
radians2or90 π°=θ+β+δC.4.6
034
0341 1/
)(sin/��
����
�
+β+θ+θ+
=�
��C.4.7
�����
�
�
�����
�
�
θ+θ+η
−+
θ+θ=β
θ )sin()/(
a1)1/(
)cos(arctan
0120
2
34
0
����
C.4.8
���
����
�
θθ
=���
����
�
θθ
⋅θθ
=θ1
o
0
1
1
0
tantanarctan
coscos
sinsinarctan
C.4.9
02
12 sincosa θ−θ=θ
C.4.10
2desDL Vb)Z(5.0FF ρ∆⋅=′=′
C.4.11
C
des0 τ
τ=η
C.4.12
���
����
� −⋅=C
CS S
1SWW C.4.13
aθ = Projection of WS into subgrade beneath blockb = Block width (ft)FD
′ & FL′ = additional drag
and lift forces (lbs)�x = Block moment arms (ft)SC = Specific gravity of concrete (assume 2.1)SF = Calculated factor of safetyVdes = Design velocity (ft/s)W = Weight of block (lbs)WS = Submerged weight of block (lbs)∆Z = Height of block protrusion above ACB matrix (ft)β = Angle of block projection from downward direction, once in motionδ = Angle between drag force and block motionη0 = Stability number for a horizontal surfaceη1 = Stability number for a sloped surfaceθ = Angle between side slope projection of WS and the verticalθ0 = Channel bed slope (degrees or radians)θ1 = Channel side slope (degrees or radians)Note - the equations cannot besolved for θ1 = 0 (i.e., division by0); therefore, a negligible sideslope must be entered for thecase of θ1 = 0.ρ = Mass density of water (1.94 slugs/ft3)τC = Critical shear stress for block on a horizontal surface (lbs/ft2)τdes = Design shear stress (lbs/ft2)
C.16
Figure C.4.5. Selecting a target factor of safety for ACB systems.
C.17
In the flow chart, a base factor of safety is selected that considers the complexity of thehydraulic system. If the flow is relatively uniform and predictable, then a lower value for SFBcan be assumed. Alternatively, if the flow is complex and highly erratic, then a larger valueshould be used to account for the possibility of high transient velocities and shear stresses.
Two multipliers are then selected in the flow chart. The first, Xc, is used to consider thesocial and economic losses associated with failure. The range for each of four categories(low, medium, high, and extreme) is provided in the flow chart. The selection of Xc should bebased on the assumed cost of failure relative to the cost of the ACB system. The secondmultiplier, XM, is used to consider the degree of uncertainty in the hydrologic and hydraulicmodeling used for the design. There are three approaches to determining the hydrologic andhydraulic variables of a system for design: deterministic, empirical or stochastic, andestimation. Deterministic techniques are physical or numerical models where empirical andstochastic techniques are based on statistical analysis and regression equations. Estimationis the use of educated guess, often based on observations of previous events.
Suggested values of SFB, XC, and XM are provided in the flow chart for common applications.The balance between the cost of conservatism and more sophisticated engineering analysismust always be considered. If the flow chart yields a factor of safety that is too conservativethen it may be necessary to redo the modeling or consider a more sophisticated approach toselecting a target factor of safety, SFT. In all cases, prudent engineering judgment should beused.
C.4.5 Extent of Revetment Coverage
Longitudinal Extent: The revetment should be continuous for a distance which extendsupstream and downstream of the region which experiences hydraulic forces severe enoughto cause dislodging and/or transport of bed or bank material. The minimum distancesrecommended are an upstream distance of 1.0 channel width and a downstream distance of1.5 channel widths. The channel reach which experiences severe hydraulic forces is usuallyidentified by site inspection, examination of aerial photography, hydraulic modeling, or acombination of these methods.
Many site-specific factors have an influence on the actual length of channel that should beprotected. Channel controls (such as bridge abutments) may produce local areas ofrelatively high velocity and shear stress due to channel constriction, but may also createareas of ineffective flow further upstream and downstream in "shadow zone" areas of slackwater. In straight reaches, field reconnaissance may reveal erosion scars on the channelbanks that will assist in determining the protection length required. In meandering reaches,since the natural progression of bank erosion is in the downstream direction, the present limitof erosion may not necessarily define the ultimate downstream limit. Lagasse et al. (2001a)provide guidance for the assessment of lateral migration. The design engineer isencouraged to review this reference for proper implementation.
Vertical Extent. The vertical extent of the revetment should provide ample freeboard abovethe design water surface. A minimum freeboard of 1 to 2 ft should be used for unconstrictedreaches and 2 to 3 ft for constricted reaches. If the flow is supercritical, the freeboard shouldbe based on height above the energy grade line rather than the water surface. Therevetment system should either cover the entire channel bottom or, in the case of unlinedchannel beds, extend below the bed far enough so that the revetment is not underminedfrom local scour or degradation. Techniques for estimating local scour are provided byRichardson et al. (2001c) and long-term degradation is discussed in more detail in SectionC.3.
C.18
C.4.6 Cabled Versus Non-Cabled ACB Systems
Some manufacturers of ACB systems provide the option of cables for block-to-blockconnection. The use of cabled systems is recognized as advantageous when installation canbe done via a pre-assembled mat of blocks held together by cable using heavy equipment.Such installation procedures can result in significant cost savings but are limited byequipment access to the site. Under the precepts of the definition of failure and the factor ofsafety design procedure, cables are not considered to increase the hydraulic stability of theACB system and no explicit terms are incorporated into the procedure for block-to-blockconnections. However, the use of cables can be considered beneficial in preventing localareas of instability from expanding and progressively "unzipping" large areas of the ACBmatrix.
C.4.7 Drainage Layers
A drainage layer may be used in conjunction with an ACB system. A drainage layer liesbetween the blocks and the geotextile and/or granular filter. This layer allows "free" flow ofwater beneath the block system while still holding the filter material to the subsoil surfaceunder the force of the block weight. This free flow of water can relieve sub-block pressureand has appeared to significantly increase the hydraulic stability of ACB systems based onfull-scale performance testing conducted since the mid 1990s.
Drainage layers can be comprised of coarse, uniformly sized granular material, or can besynthetic mats that are specifically manufactured to permit flow within the plane of the mat.Granular drainage layers are typically comprised of 1- to 2-inch crushed rock in a layer 4inches or more in thickness. The uniformity of the rock provides significant void space forflow of water. Synthetic drainage mats typically range in thickness from 0.25 to 0.75 inchesand are manufactured using stiff nylon fibers or high density polyethylene (HDPE) material.The stiffness of the fibers supports the weight of the blocks with a low fiber density, thusproviding large hydraulic conductivity within the plane of the mat.
Many full-scale laboratory performance tests have been conducted with a drainage layer inplace. When evaluating a block system, for which performance testing was conducted with adrainage layer, a drainage layer must also be used in the design. This recommendation isbased on the apparent increase in the hydraulic stability of systems that have incorporated adrainage layer in the performance testing.
Vertical components of velocity in highly turbulent flow can create conditions wheredetrimental quantities of flow may penetrate beneath the block system in local areas. Forthis reason, the designer may wish to incorporate a drainage layer with any ACB systemdesign in areas where very turbulent flows are expected.
C.4.8 ACB Design Procedure and Example
The following example illustrates an ACB design procedure that uses the factor of safetyequations presented in Table C.4.1. The procedure is presented in a series of steps that canbe followed by the designer in order to select the appropriate ACB system based on a pre-selected target factor of safety. The major criterion for product selection is if the computedfactor of safety for the ACB system meets or exceeds the pre-selected target value.
C.19
Problem Statement:
A hydraulic structure is to be constructed at the downstream end of a reach on MeanderingRiver, Texas. The river has a history of channel instability, both vertically and laterally. Aquantitative assessment of channel stability has been conducted using the multi-levelanalysis from "Stream Stability at Highway Structures" (Lagasse et al. 2001a). Usingguidelines from "Bridge Scour and Stream Instability Countermeasures" (Lagasse et al.2001b), a drop structure has been designed at the indicated reach to control bed elevationchanges. However, there is concern that lateral channel migration will threaten the integrityof the structure. An ACB system is proposed to arrest lateral migration. Figure C.4.6presents a definition sketch for this example problem.
The design discharge for the revetment is the 100-year event, which is 6,444 cfs. The bedslope of the reach upstream of the proposed drop structure is 0.01. The bed material is clay,the bank material is silty clay with sand.
The design procedure assumes that appropriate assessment of hydraulic and geomorphicconditions has been made prior to the design process. The HEC-RAS package has beenused to model the design hydraulics for the reach upstream of the proposed drop structureon Meandering River. Table C.4.2 presents pertinent results from the hydraulic model at thecross section that is exposed to the most severe hydraulic conditions.
Table C.4.2. HEC-RAS Model Output for River Mile 23.4.Channel Discharge (cfs) 6,444Cross Section Averaged Velocity (ft/s) 8.1Hydraulic Radius (ft) 4.3Energy Grade Line Slope (ft/ft) 0.007
A horizontal velocity distribution was calculated at River Mile 23.4 using HEC-RAS. FigureC.4.7 presents a reduced form of the velocity distribution with 9 velocity subsections; HEC-RAS originally calculated a distribution of 20 velocity subsections. The distribution indicatesthat the maximum velocity expected at the bend is 11.0 ft/s, which will be used as the designvalue in the factor of safety calculations. The cross section averaged shear stress can becalculated with Equation C.2.1 as τ0 = γRSf = 62.4(4.3)(0.007) = 1.9 lb/ft2.
Section C.2 provides guidance for increasing cross section averaged shear stress atmeander bends. For this example, the velocity distribution in Figure C.4.7 can be usedinstead, knowing that shear stress is proportional to the square of velocity. The maximumshear stress for design can be estimated as follows:
222
avg
des0max ft/lbs5.3
1.80.119.1
VV
=��
���
�⋅=��
�
�
��
�
�⋅τ=τ (Eqn. C.4.14)
For this example, the estimated maximum shear stress is used as the design value (τdes =τmax).
C.20
Figure C.4.6. Definition sketch of example problem setting and ACB installation (not to scale).
C.21
Figure C.4.7. Velocity Distribution at River Mile 23.4 from HEC-RAS Model.
The suggested design procedure follows:
Step 1. Select a target factor of safety
Use Worksheet 1 from Section C.7 to assist in the selection of a target factor of safety. Forthis example, a completed worksheet is included, and a target factor of safety of 2.4 isselected.
Step 2. Select potential ACB products for design
Contact ACB manufacturers and/or review ACB catalogs and select several systems that areappropriate for the given application based on a preliminary assessment of the hydraulicconditions. At the same time obtain the block properties necessary for design. Theseproperties generally include the moment arms in Figure C.4.4, the submerged weight of theblock, and the critical shear stress for the block on a horizontal surface.
For this Example, two products from ACB Systems, Inc. are selected based on guidancefrom the manufacturer. ACB Systems, Inc. suggests that the 105-S or 105-L blocks wouldbe appropriate for velocities in the range of 10 to 15 ft/s. The block properties provided bythe manufacturer are shown on the worksheet. Section C.7 provides tables of blockproperties supplied by manufacturers of commonly used ACB systems.
Step 3. Calculate the factor of safety for each product
Use Worksheet 2 from Section C.7 to assist in the factor of safety calculations using theequations from Table C.4.1. For this example the calculations are presented for the 105-Sblock and a completed worksheet with both blocks is included.
C.22
a) Assuming a specific gravity of 2.1 for the concrete, calculate the submerged unit weight:
���
����
� −⋅=C
CS S
1SWW (see Eqn. C.4.13)
lbs6.341.2
11.20.66WS =��
���
� −⋅=
b) Calculate the stability number on a horizontal surface:
C
des0 τ
τ=η (see Eqn. C.4.12)
14.00.25
5.30 ==η
c) Calculate the additional lift and drag forces from block protrusion out of the ACB matrix:
2desDL Vb)Z(5.0FF ρ∆⋅=′=′ (see Eqn. C.4.11)
lbs87.5)0.11)(94.1)(25.1)(04.0(5.0FF 2DL ==′=′
d) Calculate aθ:
02
12 sincosa θ−θ=θ (see Eqn. C.4.10)
8943.0)57.0(sin)57.26(cosa 22 =−=θ
e) Calculate θ:
���
����
�
θθ
=���
����
�
θθ
⋅θθ
=θ1
o
0
1
1
0
tantanarctan
coscos
sinsinarctan (see Eqn. C.4.9)
reesdeg14.1)57.0cos()57.26cos(
)57.26sin()57.0sin(arctan =��
�
����
�⋅=θ
f) Calculate β:
C.23
�����
�
�
�����
�
�
θ+θ+η
−+
θ+θ=β
θ )sin()/(
a1)1/(
)cos(arctan
0120
2
34
0
����
(see Eqn. C.4.8)
reesdeg48.19)14.157.0sin(
)21.0/88.0(14.08943.01)133.0/88.0(
)14.157.0cos(arctan2
=
�����
�
�
�����
�
�
++−+
+=β
g) Calculate the stability number on a sloped surface:
034
0341 1/
(sin/��
����
�
+β+θ+θ+
=�
�� (see Eqn. C.4.7)
12.014.0133.0/88.0
)48.1914.157.0sin(33.0/88.01 =
���
���
++++=η
h) Calculate δ:
radians2or90 π°=θ+β+δ (see Eqn. C.4.6)
reesdeg38.69)14.148.19(90 =+−=δ
i) Calculate the actual factor of safety for the 105-S block under these hydraulic conditions:
S1
L4D3121
2
12
w)FcosF()/(cosa1
a)/(SF
�
����
��
′+δ′+η+β−
=
θ
θ (see Eqn. C.4.5)
2.2
)6.34(21.0))87.5(88.0)38.69(cos)87.5(33.0()21.0/88.0(12.0)48.19cos(8943.01
8943.0)21.0/88.0(SF2
=+++−
=
Steps a) through i) are then repeated for the 105-L block.
Step 4. Assess the suitability of each product and select a final ACB System
Compare the calculated factors of safety for the considered blocks with the design factor ofsafety and select the product that best meets the design needs. For this example the 105-Lproduct is the only choice because the alternative did not satisfy the target factor of safety.Once a product has been selected, fill in the block specifications at the bottom of theworksheet.
C.24
Worksheet 1 - Target Factor of Safety SelectionProject Information Site Description: Meandering River
Company: ACB Consultants, Inc. Modeling Approaches
Designer: John Doe Date: 1/10/01 Design Event: 100-Year
Project Name/Number: 10-466-007 Hydrologic Model: USGS Regression Equations
Client: Harris County, TX Hydraulic Model: HEC-RAS
Determine Variables for Target Factor of Safety Equation from Figure C.4.5Rank complexity and turbulence of hydraulicflow field (low, medium or high): Medium
1.4SFB =
Comments related to SFB:
The river is very sinuous and high velocitiescan be expected at the outside of bends.
Rank consequence of failure (low, medium,high, or extreme): medium
1.4XC =
Comments related to XC:
None
What type of hydrologic/hydraulic modelswere used? Deterministic
1.2XM =
Comments related to XM:
Hydrology from recent restudy and SWMMmodel; cross-sections for HEC-RAS modelhave not been updated since last study.
Calculate the Target Factor of Safety
SFT = SFBXCXM = 1 2.4
C.25
Worksheet 2 - ACB Design and SelectionProject Information Site Information
Company: ACB Consultants, Inc. Description: Meandering River
Designer: John Doe Date: 1/10/01 Bed Slope (ft/ft): 0.01 (degrees): 0.57
Project Name/Number: 10-466-077 Side Slope (H/V): 2.0 (degrees): 26.57
Client: Harris County, TX Hydraulic Design Data
Target Factor of Safety: 2.4 Discharge (cfs): 6444 Description: 100-Yr
Additional Comments Velocity (ft/s): 11.0 Flow Depth (ft): 4.3
Hydraulic design data comes from XS 23.4 Friction Slope (ft/ft): 0.007000
in the HEC-RAS model Design Shear Stress (lbs/ft2): 3.5Factor of Safety Calculations from Table C.4.1
ACB Systems for SF CalculationVariables in SFCalculation 105-S 105-L
W (lbs) 66.0 72.9WS (lbs) 34.6 38.2b (ft) 1.25 1.25ℓ1 (ft) 0.21 0.21ℓ2 & ℓ4 (ft) 0.88 0.98ℓ3 (ft) 0.33 0.33τC (lbs/ft2) 25.0 30.0η0 0.14 0.12∆Z (ft) 0.04 0.04F�L & F�D (lbs) 5.87 5.87aθ 0.8943 0.8943θ (degrees) 1.14 1.14β (degrees) 19.48 17.34η1 0.12 0.10δ (degrees) 69,38 71.52SF 2.2 2.5
Manufacturer/Selected ACB System
ACB Systems, Inc./Class 105-L Block Width (in.): 15
Critical Shear Stress (lb/ft2): 30.0 Block Length (in.): 18
Calculated Factor of Safety: 2.5 Block Height (in.): 5
Block Weight (lbs): 72.9 Acceptable Protrusion (in.): 0.5
C.26
Step 5. Design horizontal and vertical extent of the ACB system
Following guidelines from Section C.4.5, the ACB system should terminate against the dropstructure and extend 2200 ft upstream, which is more than one channel width beyond theobserved limits of channel erosion. The drop structure is expected to arrest verticaldegradation, therefore, bed erosion is not expected to undermine the revetment. A toe downinto the bed of 2 ft is specified so that lateral movement of the lowest point in the channel willnot undermine the revetment. The specified freeboard for this application is 1.5 ft above thewater surface profile computed in the HEC-RAS model. The maximum side slope for anyACB system should be 2H:1V.
Step 6. Design the filtration component of the ACB system
The procedure outlined in Section C.5 should be followed for filtration design. A workedexample problem is provided in Section C.5.5 to illustrate the procedure. If performancetesting of the selected ACB system was conducted with a drainage layer in place, then adrainage layer of the same type is required for the design.
C.5 GEOTEXTILE AND GRANULAR FILTER DESIGN
The importance of the filter component of an ACB system should not be underestimated. Iflaboratory testing of an ACB system was conducted with a filter in place then the design,which uses the ACB system, should include a filter. Geotextiles and granular layers performthe filtration function. Some situations call for a composite filter consisting of both a granularlayer and a geotextile. The specific characteristics of the existing base soil determinewhether a granular filter is required.
The filter is installed between the ACB and the base soil (Figure C.5.1). The primary role of afilter component is to retain the soil particles while allowing the flow of water through theinterface between the ACB system and the underlying soil. A granular filter also provides asmooth and free-draining surface to rocky or otherwise irregular subgrades, therebymaximizing intimate contact. The need for granular material is fully addressed in theinstallation section. Careful design, selection, and installation of the appropriate filtermaterial all play an important role in the overall performance of ACB systems.
Figure C.5.1. Channel cross sections showing filter and bedding orientation.
C.27
C.5.1 Filter Functions
The primary function of filter components is to prevent fine particles from washing away whileallowing water flow through the filter material. These two contrary objectives must beconsidered to achieve an effective functional balance between retention and permeability.
Filters assist in maintaining intimate contact between the revetment and the subgrade byproviding stability at the interface. Depending upon the internal stability of the soil, severalprocesses can occur over time along the interface of the base soil and filter material. Thefilter pore size and the base soil stability dictate these processes. As an example, considerthe process of "piping." Piping is basically the washing away of very fine particles, resultingin greater void space in the soil structure. Piping is more likely to occur in non-cohesive/unstable soils that are in contact with a filter material that has large openings. Thelarge openings do not retain the smaller particles and therefore these particles are removedby flowing water and only the larger particles remain. This process increases the potential forsoil erosion by weakening the soil structure. Correct filter design reduces the effects ofpiping by limiting the loss of fines. Figure C.5.2 illustrates a stable versus unstable soil andseveral common filtering processes that can occur (modified from Koerner 1997). The largearrow indicates flow direction.
Figure C.5.2. Examples of soil and filter subgrades.
C.28
Figure C.5.2. Examples of soil and filter subgrades (continued).
As illustrated in the above Figure, matching the correct filter opening to the characteristics ofthe base soil is critical for obtaining the desired retention of the filter component.
Filters should be permeable enough to allow unimpeded flow of water through the filtermaterial. This is necessary for two reasons: regulation of the filtration process along thebase soil and filter interface, as illustrated above, and reduction of hydrostatic pressure buildup from local groundwater fluctuations in the vicinity of the channel bed and banks (e.g.,seasonal water level changes and storm events) that can weaken the channel soil structure.The permeability of the filter should never be less than the layer below it (whether base soilor another filter layer).
Figure C.5.3 illustrates a process that can result in an increase of hydrostatic pressurebeneath the filter. The figure is a time series view of channel cross sections showingchanging water levels and seepage resulting from a storm event. A properly designed filterwill help alleviate problems associated with fluctuating water levels.
C.29
Figure C.5.3. Time series of channel and groundwater level changes resulting from a storm event.
C.30
C.5.2 Base Soil Properties
Base soil is defined here as the existing material of the channel bed and banks. Thefollowing properties represent a minimum level of information that should be obtained for usein the design process:
General Soil Classification. Soils are classified based on laboratory determinations ofparticle size characteristics and the physical effects of varying water content on soilconsistency. Typically, soils are described as coarse-grained if more than 50 percent byweight of the particles are larger than a #200 sieve (0.075 mm mesh), and fine-grained ifmore than 50 percent by weight is smaller than this size. Sands and gravels are examples ofcoarse-grained soils, while silts and clays are examples of fine-grained soils.
The fine-grained fraction of a soil is further described by changes in its consistency causedby varying water content and by the percentage of organic matter present. Soil classificationmethodology is described in ASTM D 2487 "Standard Practice for Classification of Soils forEngineering Purposes (Unified Soil Classification System)."
Particle Size Distribution. The single most important soil property for design purposes is therange of particle sizes in the soil. Particle size is a convenient and relatively simple way toassess soil properties. Also, particle size tends to be an indication of other properties suchas permeability. Characterizing soil particle size involves determining the relative proportionsof sand, silt, and clay in the soil. This characterization is usually done using either atechnique called sieve analysis for coarse-grained soils or sedimentation (hydrometer)analysis for fine-grained soils. ASTM D 422 "Standard Test Method for Particle-Size Analysisof Soils" outlines the specific procedure.
Plasticity. Plasticity is defined as the property of a material that allows it to be deformedrapidly, without rupture, without elastic rebound, and without volume change. A measure ofplasticity is the Plasticity Index (PI), which should be determined for soils with a largepercentage of fines or clay particles. The results associated with plasticity testing arereferred to as the Atterberg Limits. ASTM D 4318 "Standard Test Methods for Liquid Limit,Plastic Limit, and Plasticity Index of Soils" defines the testing procedure.
Permeability. Permeability is a measure of the ability of soil to transmit water. Permeability isrelated to particle size distribution, dominated by the finest 20 percent, and can bedetermined using an equation that has been developed for this purpose or through laboratoryanalysis. ASTM provides two standard test methods for determining permeability. They areASTM D 2434 "Standard Test Method for Permeability of Granular Soils (Constant Head)" orASTM D 5084 "Standard Test Method for Measurement of Hydraulic Conductivity ofSaturated Porous Materials Using a Flexible Wall Perimeter." Soil permeability is used aspart of the design process to help select an appropriate filter material.
For granular soils, the permeability may be estimated by the Fair-Hatch Equation in lieu ofperforming laboratory testing. The Fair-Hatch Equation relates permeability to soil porosityand the particle size distribution. Porosity is defined as the ratio of void space to the totalvolume of the soil. The pores in the soil are the means of conducting water; therefore,permeability of soil is influenced by the soil porosity. The Fair-Hatch Equation is:
C.31
( )�����
�
�
�����
�
�
���
����
�φ−φ×=
�=
2N
1n n
n
2
36
dP49
11
10958.1K (Eqn. C.5.1)
where:
K = Soil permeability (cm/s)φ = Dimensionless soil porosity determined from Equation C.5.2 or
Table C.5.1, both shown belowP = Percentage of material in the distribution between adjacent
particle sizesd = Geometric mean of adjacent particle sizes in the distribution (mm)N = Number of intervals between adjacent particle sizes
If the particle size distribution does not include a particle size at 0 percent, this value shouldbe estimated by extrapolation and included in the calculation. This is important because thepresence of small particles representing the fine end of the particle size distributionsignificantly influences permeability.
Commonly observed values of porosity for alluvial soils are presented in Table C.5.1. If thesoil has been compacted in place rather than naturally deposited, the following equation thatrelates porosity to compaction and dry unit weight of the soil is recommended:
���
����
� γ⋅−=φ 3
d
ft/lb4.165100C1 (Eqn. C.5.2)
where:
φ = Soil porosity (dimensionless)C = Soil compaction in percent of Standard Proctor Density (90 to 100)γd = Dry unit weight of the soil at 100 percent of Standard Proctor
Density (lbs/ft3)
Table C.5.1. Porosity for Alluvial Soils (after Terzaghi et al. (1996)).Uniform sand, loose 0.46Uniform sand, dense 0.34Mixed sand, loose 0.40Mixed sand, dense 0.30
C.5.3 Geotextile Filter Properties
The following geotextile properties are available from manufacturers. The ASTM standardtest is cited where applicable.
Permittivity. This is a measure (used to compare geotextiles of various thicknesses) of amaterial�s cross-plane permeability that when multiplied by the geotextile thickness gives atraditional permeability value. (ASTM D 4491)
C.32
Apparent Opening Size (AOS). Also known as Equivalent Opening Size, this measure isgenerally reported as O95. O95 represents the aperture size such that 95 percent of theopenings are smaller. In similar fashion to a soil gradation curve, a geotextile holedistribution curve can be derived. (ASTM D 4751)
Percent Open Area (POA). POA is a comparison of the total open area to the total geotextilearea. This measure is applicable to woven geotextiles only. POA is used to estimate thepotential for long term clogging.
Thickness. As mentioned above, thickness is used to calculate traditional permeability.
C.5.4 Granular Filter Properties
Generally speaking, most required granular filter properties can be obtained from the particlesize distribution curve for the material. Granular filters serve as a transitional layer betweena predominantly fine-grained base soil and a geotextile.
Particle size distribution. The gradation curve of the granular filter material should beapproximately parallel to that of the base soil. Parallel gradations will minimize particlesegregation.
Permeability. See above explanation of permeability in Section C.5.2. Often thepermeability for a granular filter material is estimated by the Fair-Hatch equation ordetermined by laboratory analysis. The permeability of a granular layer is used to select ageotextile when designing a composite filter. The permeability of the geotextile should be atleast 10 times the permeability of the soil.
Thickness. Practical issues of placement suggest that a typical minimum thickness of 6inches be specified. For placement under water, thickness should be increased by 50percent.
C.5.5 Filter Design Procedure and Example
The following example illustrates a six-step design procedure for the filter component of anACB system. The major criteria for geotextile and granular filter design are permeability andretention, which need to be compatible with the base soil.
Problem Statement:
A filter needs to be designed for the ACB system that was designed in Section C.4.7 forMeandering River, Texas. See Section C.4.8 for an overall description of the site and theneed for the ACB System. Tables C.5.2 and C.5.3 provide the needed soil properties fromgeotechnical laboratory testing for this example problem.
Table C.5.2. General Soil Sample Information and Classification.Sample ID No. 3 (in Channel)Test Date 6/18/00
Soil description Silty Clay with SandUSCS Classification CL-ML
Moisture Content 9.90%Liquid Limit (LL) 26%Plastic Limit (PL) 19%
Plasticity Index (PI) 7%Permeability 7.5 x 10-7 cm/s
C.33
Table C.5.3. Results From Sieve and Hydrometer Analysis.Sieve Size Particle Size (mm) Percent Finer
3/4 inch 19.05 100.01/2 inch 12.70 100.03/8 inch 9.52 100.0No. 4 4.75 100.0No. 10 2.00 100.0No. 20 0.85 99.8No. 40 0.425 99.6No. 80 0.180 99.6No. 100 0.150 99.0No. 200 0.075 71.9
0.005 mm 0.005 24.2
The suggested design procedure follows:
Step 1. Obtain base soil information
Section C.5.2 can be consulted for a definition of common soil properties. Typically, thenecessary base soil information is a grain size distribution curve, permeability, and thePlasticity Index (PI is required only if the base soil is more than 20 percent clay). Worksheet3 from Section C.7 can be used to plot the grain size distribution on a gradation curve. Forthis example, the information is provided in the problem statement and a gradation curve isincluded.
Step 2. Determine the geotextile retention criterion
Use the decision tree in Figure C.5.4, modified after Luettich (1991), to determine the geotextileretention criterion. The figure has been modified to include the Terzaghi rules when a granulartransition layer is necessary. A granular transition layer is required when the base soil isgreater than 20 percent clay or there is predominantly fine-grained soil. If a granular transitionlayer is required, the geotextile selection should be based on granular layer properties.
Note: If the required AOS is smaller than that of available geotextiles, then a granulartransition layer is required, even if the base soil is not clay. However, this requirement iswaived if the base soil exhibits the following conditions:
K < 10-7 cm/sPI > 40
Under these soil conditions there is sufficient cohesion to prevent soil loss through thegeotextile. A woven monofilament geotextile with an AOS ≤ #70 sieve (approximately 0.2mm) should be used with soils meeting these conditions.
Document the percentages of gravel, fines, and clay that were observed in the sample.Gravel is characterized by particle sizes greater than 4.75 mm, fines are defined as theparticles that passed the No. 200 sieve, and clay is characterized by particle sizes less than0.005 mm per ASTM D422. Also, document the plasticity index (PI) if the percentage of clayis greater than 20 percent. Worksheet 4 from Section C.7 can be used for documenting thegeotextile selection process. For this example, the sample contains no gravel, 71.9 percentfines, and 24.2 percent clay.
C.34
Figure C.5.4. Geotextile selection based on soil retention.
C.35
From Figure C.5.4, determine if a granular transition layer will be necessary. If a granularfilter is used, the remaining steps in the geotextile selection should be based on the granularfilter properties. Go to step 2b to design the granular filter before continuing on withgeotextile selection.
From Figure C.5.4, no wave attack is expected at Meandering River, therefore the UniformityCoefficient will be used for the final step in determining the retention criteria. The UniformityCoefficient, CU, is defined as follows:
10
60U d
dC = (Eqn. C.5.3)
where:
dx = Particle size of which X percent is smaller
For this example, the computation of CU and selection of the geotextile retention criterion forO95 is shown on Worksheet 4, page C.36.
Step 2b. Determine the granular filter retention and permeability criteria if needed
Use the Terzaghi rule to specify d15 of the granular transition layer (filter) such that:
d15 FILTER < 5d85 BASE
Use the Terzaghi rule to specify d15 of the granular transition layer (filter) such that:
d15 FILTER > 4 d15 BASE
The gradation curve of the granular transition layer should be approximately parallel to that ofthe base soil. At this point the granular transition layer design, when required, is complete.For practical considerations related to constructability and inspection, the granular filterthickness should not be less than 6 inches. For placement under water, thickness should beincreased by 50 percent.
For this example, a granular filter is required and should be 9 inches thick because therevetment will be continuously under water. Pit run sand is selected for the filter based onthe d15 criteria. The particle size gradation is provided in Table C.5.4 and is plotted onWorksheet 3. Notice that the gradation of the pit run sand is approximately parallel to that ofthe base soil. Calculations for d15 of the granular filter are presented on Worksheet 4.
Table C.5.4. Pit Run Sand Gradation for Granular Filter.Sieve Size Particle Size (mm) Percent Finer
3/8 in. 9.52 100No. 4 4.75 98.7No. 8 2.36 95.5
No. 16 1.18 89.3No. 30 0.600 71.8No. 50 0.300 26.0No. 100 0.150 5.0No. 200 0.075 4.1
C.36
Step 3. Determine the geotextile permeability criterion
The permeability criterion is specified as a function of the base soil permeability as follows:
Kg ≥ 10Ks (Eqn. C.5.4)
where:
Kg = Permeability of the geotextile (cm/s)Ks = Permeability of the base soil or granular filter (cm/s)
To obtain the permeability of a geotextile in cm/s, multiply the thickness of the geotextile incm by its permittivity in s-1. Typically, the designer will need to contact the geotextilemanufacturer to obtain values of permittivity.
Generally speaking, if the permeability of the base soil or granular filter has been determinedfrom laboratory testing, that value should be used. If testing was not conducted, then theFair-Hatch Equation should be used. For this example, the calculation of permeability of thegranular filter using the Fair-Hatch Equation is shown below. A dry unit weight of 115 lbs/ft3and 95 percent compaction are assumed for the selected pit run sand filter material.
Calculate the porosity:
4.165100C1 dγ⋅−=φ (see Eqn. C.5.2)
339.04.1650.115
100951 =⋅−=φ
Calculate the permeability for the pit run sand. For the gradation in Table C.5.4 there will beeight particle size intervals, the seven shown in the table plus one to extrapolate down to 0percent (particle size 0.008 mm scaled from Worksheet 3).
( )�����
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10958.1K (see Eqn. C.5.1)
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8
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n
mm476.408)008.0)(075.0(
010.4)075.0)(150.0(
1.40.5)150.0)(300.0(
0.50.26)300.0)(600.0(
0.268.71)600.0)(18.1(
8.713.89)18.1)(36.2(
3.895.95)36.2)(75.4(
5.957.98)75.4)(52.9(
7.98100dP
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C.37
The permeability for the granular filter and the calculated criterion for the geotextile areindicated on Worksheet 4.
Step 4. Select potential geotextiles for design
Using results obtained in Steps 2 and 3, select several geotextile candidates. A valuablereference is the annual "Geotechnical Fabrics Report - Specifier�s Guide" (published by theIndustrial Fabrics Association International).
For this example, three products from three different manufacturers are selected ascandidates for design. The selected systems are 121F, 113-004, and XW45. All threeproducts satisfy the retention and permeability criteria.
Step 5. Screen potential geotextiles using the following considerations
1. Geotextile strength relating to installation. This refers to the ability of the geotextile towithstand installation, the weight of the block system, and additional compaction.Minimum strength requirements are as follows:
• Grab Strength � 90 lbs. (ASTM D 4632)• Elongation � 15% (ASTM D 4632)• Puncture Strength � 40 lbs. (ASTM D 4833)• Trapezoidal Tear � 30 lbs. (ASTM D 4533)
2. Durability and the ability to withstand long term degradation. This is particularly aconcern for geotextiles exposed to ultraviolet light during installation. Follow manufacturerrecommendations for protection against ultraviolet light degradation. For additionalguidelines regarding the selection of durability test methods refer to ASTM D 5819,"Standard Guide for Selecting Test Methods for Experimental Evaluation of GeosyntheticDurability."
3. Other criteria for selection. HCFCD requires the use of a woven monofilament geotextilein conjunction with ACB systems. Select a system that has a percent open area greaterthan or equal to 4 percent (POA ≥ 4%). Typically, the geotextile with the largest AOS thatsatisfies the retention criteria, and all other minimum standards, should be used.
For this example, the strength values are indicated in the table on Worksheet 4. Two of thethree products satisfy all of the strength requirements.
Step 6. Make a final geotextile selection by assessing cost and availability
The XW45 system from Geotextile Fabrics, Inc. is selected because of availability and cost.
Note: During construction, but before the geotextile is placed, collect soil samples foranalysis to ensure that the geotextile selected in the design process is still appropriate, perHCFCD Standard Specification 02379 "Geotextiles for Erosion Control Systems." SeeSection C.6 for required testing frequency and laboratory tests.
C.38
Worksheet 3 – Grain Size Distribution Curve
C.39
Worksheet 4 – Geotextile Selection and Granular Filter Design
Project Information Site Description: Meandering River
Company: ACB Consultants, Inc. Soil Information
Designer: John Doe Date: 1/10/01 Description: Reddish Brown Clayey Silt
Project Name/Number: 10-466-007PercentGravel: 0 Fines: 71.9 Clay: 24.2
Client: Harris County, TX Plasticity Index: 7
From Figure C.5.4 – Geotextile Criterion Based on
Base Soil Properties □ Granular Filter Properties ■
For Granular Filter Only
d85 BASE (mm): 0.1 d15 FILTER < 5d85 BASE < 0.5 mm
d15 BASE (mm): 0.0026 d15 FILTER ≥ 4 d15 BASE ≥ 0.0104 mmDescription ofSelected Material: Pit Run Sand d15: 0.21 mm
Geotextile Retention Criterion from Figure C.5.4d60 0.48
Base Soil or Granular Filter Particle Sizes CU = d10= 0.18 = 2.7
d10 (mm): 0.18 currents are
d50 (mm): 0.41 mild ■ severe □
d60 (mm): 0.48 geotextile retention criteria for O95
d90 (mm): 1.2 0.41 mm < O95 < 1.2 mm
Geotextile Permeability Criterion
Soil permeability determined from
Fair-Hatch Equation ■ laboratory testing of soil □
Other □ Explain
Ks (cm/s): 0.02 geotextile permeability criterion: Kg ≥ 10Ks ≥ 0.20 cm/s
C.40
Worksheet 4 Continued – Geotextile Selection and Granular Filter Design
Geotextile Strength Screening TableSelected Woven Geotextile Products
121F 113-004 XW45GeotextileStrengthProperties Value Satisfactory? Value Satisfactory? Value Satisfactory?Grab Strength(lbs) ≥ 90 81 No 200 Yes 310 YesElongation(%) ≥15 50 Yes 15 Yes 30 YesPunctureStrength (lbs)≥ 40
43 Yes 120 Yes 155 Yes
TrapezoidalTear (lbs) ≥ 30 34 Yes 65 Yes 55 YesNote: use additional tables if more than three products are being evaluated
Manufacturer/Selected Geotextile Percent Open Area: 4
Geotextile Fabrics, Inc./XW45 O95 (mm): 0.60
Does the geotextile have a woven structure? Yes Kg (cm/s): 0.40
C.41
C.6 INSTALLATION GUIDELINES
The proper installation of an ACB revetment system is essential to achieve suitable hydraulicperformance and maintain stability against the erosive force of flowing water during thedesign hydrologic event. These guidelines are intended to maximize the conformity betweenthe design intent and the actual field-finished conditions of the project. Quality workmanshipis important to the ultimate performance of the system. The following sections address thesubgrade preparation, geotextile placement, block system placement, backfilling andfinishing, and inspection. These guidelines apply to the installation of ACB revetmentsystems, whether hand-placed or placed as a mattress.
These guidelines do not purport to address the safety issues associated with installation ofACB revetment systems, including use of hazardous materials, mechanical equipment, andoperations. It is the responsibility of the Contractor to establish and adopt appropriate safetyand health practices. Also, the Contractor shall comply with prevalent regulatory codes, suchas OSHA (Occupational Health and Safety Administration) regulations, while using theseguidelines.
At the completion of rough grading, soil samples representative of subgrade conditions shallbe obtained at a frequency of one sample per 50,000 blocks, or additional fraction thereof,and tested for the following properties:
1. Grain size distribution (ASTM D 422)2. Atterberg Limits (ASTM D 4318)3. Standard Proctor Density (ASTM D 698)
Results of laboratory tests shall be submitted to Engineer to ensure conformance with designparameters prior to placement of the geotextile and ACB revetment system. When agranular filter is used, it shall be tested for grain size distribution at the same frequency asthe subgrade soil testing.
C.6.1 Subgrade Preparation
Stable and compacted subgrade soil should be prepared to the lines, grades, and crosssections shown on the contract drawings per HCFCD Standard Specification 02315"Excavating and Backfilling." Termination trenches and transitions between slopes andembankment crests, benches, berms, and toes should be compacted, shaped and uniformlygraded to facilitate the development of intimate contact between the ACB revetment systemand the underlying grade. Secure the revetment in a manner that prevents soil migrationwhen the ACB matrix is terminated at a structure, such as a concrete slab or wall.
Subgrade soil should be approved by the Engineer to confirm that the actual subgrade soilconditions meet the required material and compaction standards. Soils not meeting therequired standards should be removed and replaced with acceptable material.
Care should be exercised so as not to excavate below the grades shown on the contractdrawings, unless directed by the Engineer to remove unsatisfactory materials and anyexcessive excavation should be filled with approved backfill material per HCFCD StandardSpecification 02314 "Fill Material" and compacted. Where it is impractical, in the opinion ofthe Engineer, to dewater the area to be filled, over-excavations should be backfilled withcrushed rock or stone conforming to the grading and quality requirements of well-gradedcoarse aggregate in ASTM C33 "Standard Specification for Concrete Aggregates."
C.42
When placing in the dry, areas to receive the ACB system should be graded to a smoothsurface to ensure that intimate contact is achieved between the subgrade surface and thegeotextile, and between the geotextile and the bottom surface of the ACB revetment system.Unsatisfactory soils, soils too wet to achieve desired compaction, and soils containing roots,sod, brush, or other organic materials, should be removed, replaced with approved material,and compacted. The subgrade should be uniformly compacted per HCFCD StandardSpecification 02315 "Excavating and Backfilling." Should the subgrade surface for anyreason become rough, eroded, corrugated, uneven, textured, or traffic marked prior to ACBinstallation, such unsatisfactory portion should be scarified, reworked, recompacted, orreplaced as directed by the Engineer.
Excavation of the subgrade, above the water line, should not be more than 2 inches belowthe grade indicated on the contract drawings. Excavation of the subgrade below the waterline should not be more than 4 inches below the grade indicated on the contract drawings.Where such areas are below the allowable grades, they should be brought to grade byplacing approved material and compacting in lifts not exceeding 8 inches in thickness.Where such areas are above the allowable grades they should be brought to grade byremoving material, or reworking existing material, and compacting as directed by theEngineer. The subgrade should be raked, screeded, or rolled by hand or machine to achievea smooth compacted surface that is free of loose material, clods, rocks, roots, or othermaterials that would prevent satisfactory contact between the geotextile and the subgrade.Immediately prior to placing the geotextile and ACB system, the prepared subgrade shouldbe inspected and approved by the Engineer.
C.6.2 Placement of Geotextile
The geotextile should be placed directly on the prepared area, in intimate contact with thesubgrade, and free of folds or wrinkles. The geotextile shall be placed in such a manner thatplacement of the overlying materials will not excessively stretch or tear the geotextile. Aftergeotextile placement, the work area should not be disturbed so as to result is a loss ofintimate contact between the articulating concrete block and the geotextile, or between thegeotextile and the subgrade. The geotextile should not be left exposed longer than themanufacturer�s recommendation to minimize damage due to ultraviolet radiation.
The geotextile should be placed so that the upstream strips of fabric overlap downstreamstrips, and so that upslope strips overlap downslope strips. Overlaps should be in thedirection of flow wherever possible. The joints should be overlapped at least 3 ft for below-water installations and at least 2 feet for dry installations. When a granular filter is used, thegeotextile should be placed so as to encapsulate the granular filter as shown in Figure C.6.1.The distance between encapsulation points should not exceed 20 feet. The geotextileshould extend to the edge of the revetment within the top, toe, and side termination points ofthe revetment. If necessary to expedite construction and to maintain the recommendedoverlaps, anchoring pins or 11 gauge, 6- by 1-inch U-staples may be used; however, weights(e.g., sand filled bags) are preferred to prevent creating holes in the geotextile.
C.6.3 Placement of ACB System
The articulating concrete block system should be placed on the geotextile in such a manneras to produce a smooth plane surface in intimate contact with the geotextile. For blockswithin the mat and blocks that are hand set, the joint spacing between adjacent blocks is tobe maintained so that binding of blocks does not occur and block-to-block interlock isachieved. In curvature and grade change areas, alignment of the individual block and theorientation of the neighboring adjacent block is to provide for intimate block to fabric contactand block-to-block interlock. Care shall be taken during block installation so as to avoid
C.43
damage to the geotextile or subgrade during the installation process. Preferably, when ageotextile is used, the ACB system placement should begin at the upstream end andproceed downstream. If an ACB system is to be installed from downstream up, a Contractoroption is to place a temporary toe on the front edge of the ACB system to protect againstundermining when flows are anticipated. On sloped sections, where practical, placementshall begin at the toe of the slope and proceed up the slope. Block placement shall not bringblock-to-block interconnections into tension. Individual blocks within the plane of the finishedsystem shall not exceed the protrusion tolerance beyond that used in the stability design ofthe system. Typical protrusion tolerance specifications range from 0.5 to 1.0 inches. SeeHCFCD Standard Specification 02374 "Articulating Concrete Block for Erosion Control."
Figure C.6.1. Granular filter detail showing granular filter encapsulation.
Do not use the ACB revetment system as a road for heavy construction traffic unlessdesigned as a flexible pavement that can handle the expected wheel loads. Light traffic,such as single axle trucks and mowing equipment, may operate on installed ACB systems.
If assembled and placed as large mattresses, the articulating mats can be attached to aspreader bar to aid in the lifting and placing of the mats in their proper position with a crane.Figure C.6.2 contains a photo of a crane placing bank protection with a spreader bar whileFigure C.6.3 is a close-up of an ACB mat and spreader bar. The mats should be placed sideby side and/or end to end so the mats abut each other. Seams between individual matsshould be laterally connected so that adjacent mats are physically joined together.
Mat seams or openings between mats greater than the typical separation distance betweenblocks in the matrix should be filled with grout. Whether placed by hand or in largemattresses, distinct grade changes should be accommodated with a well-rounded transition(i.e., minimum radius determined by individual system characteristics). Figure C.6.4 is aconceptual detail showing a minimum radius for a top and toe-of-slope transition for bed andbank protection. The trapezoidal channel in Figure C.6.5 shows a properly finished ACBrevetment system with minimum radius-of-curvature. A top-of-slope transition and a typicaltoe detail for bank protection is shown in Figure C.6.6. Figure C.6.7 is a conceptual detail forspillways or embankment overtopping flow and Figure C.6.8 is a photo of an ACB systemthat has been installed to protect an embankment during overtopping flow.
C.44
Figure C.6.2. ACB mats being placed with a crane and spreader bar.
Figure C.6.3. Close-up of spreader bar and ACB mat.
C.45
Figure C.6.4. Conceptual detail of minimum radius-of-curvature for bed and bank protection.
Figure C.6.5. Bed and bank protection with minimum radius-of-curvature at grade changes and top-of-slope termination points.
C.46
Figure C.6.6. Conceptual detail of minimum radius-of-curvature for bank protection.
Figure C.6.7. Conceptual detail of toe termination for spillways or embankment overtopping flow.
C.47
Figure C.6.8. Embankment dam overtopping protection with radius-of-curvature at top-of-slope termination.
If a discontinuous revetment surface exists in the direction of flow, a grout seam at the gradechange location shall be provided to produce a continuous, flush finished surface. Groutseams should not be wider than one-half the maximum dimension of a single block.
Termination trenches should be backfilled with approved fill material and compacted flushwith the top of the blocks. The integrity of a soil trench backfill must be maintained so as toensure a surface that is flush with the top surface of the articulating blocks for its entireservice life. Top, toe, and side termination trenches should be backfilled with suitable fillmaterial and compacted immediately after the block system has been placed.
Anchors or other penetrations through the geotextile should be grouted or otherwise repairedin a permanent fashion to prevent migration of subsoil through the penetration point.
C.6.4 Finishing
The open area of the articulating concrete block system is typically either backfilled withsuitable soil for revegetation or with 3/8- to 3/4-inch diameter uniform crushed stone or amixture thereof. Crushed stone can enhance the interlock restraint, but can make the ACBrevetment system less flexible. Backfilling with soil or granular fill within the cells of thesystem should be completed, as soon as possible, after the revetment has been installed.When topsoil (HCFCD Specifications 02911 - Topsoil and 02921 - Turf Establishment) isused as a fill material above the normal waterline, overfilling by 1 - 2 inches may be desirableto allow for consolidation.
C.48
C.6.5 Inspection
The subgrade preparation, geotextile placement and ACB revetment system installation, andoverall finished condition including termination points should be inspected and approved bythe Engineer.
Each step of installation - subgrade preparation, geotextile and granular filter placement,ACB revetment placement, and the overall finished condition, including termination points,shall be inspected and approved by the Engineer.
C.7. WORKSHEETS, PROPRIETARY BLOCK PROPERTIES, AND TYPICAL DETAIL DRAWINGS
Subsequent pages in this section provide design worksheets, tables of manufacturersupplied block properties for proprietary systems, and HCFCD detail sheet(s). Theworksheets are provided as a design aid given the complexity of some of the calculationsnecessary for ACB system design. They are intended to guide the designer through theappropriate series of calculations and decisions and to expedite the review process on thepart of HCFCD. The tables of block properties are provided for efficiency of the designprocess; however, they do not include all available proprietary systems and no endorsementor recommendation is intended. It is the responsibility of the designer to contactmanufacturers and investigate alternative products that may not be included in this manual.The HCFCD detail sheet(s) provide typical details that can be adapted by the designer forspecific applications.
C.49
Worksheet 1 - Target Factor of Safety SelectionProject Information Site Description:
Company: Modeling Approaches
Designer: Date: Design Event:
Project Name/Number: Hydrologic Model:
Client: Hydraulic Model:
Determine Variables for Target Factor of Safety Equation from Figure C.4.5Rank complexity and turbulence of hydraulicflow field (low, medium or high):
SFB =
Comments related to SFB:
Rank consequence of failure (low, medium,high, or extreme):
XC =
Comments related to XC:
What type of hydrologic/hydraulic modelswere used?
XM =
Comments related to XM:
Calculate the Target Factor of Safety
SFT = SFBXCXM = 1
C.50
Worksheet 2 - ACB Design and SelectionProject Information Site Information
Company: Description:
Designer: Date: Bed Slope (ft/ft): (degrees):
Project Name/Number: Side Slope (H/V): (degrees):
Client: Hydraulic Design Data
Target Factor of Safety: Discharge (cfs): Description:
Additional Comments Velocity (ft/s): Flow Depth (ft):
Friction Slope (ft/ft):
Design Shear Stress (lbs/ft2):Factor of Safety Calculations from Table C.4.1
ACB Systems for SF CalculationVariables in SFCalculation
W (lbs)WS (lbs)b (ft)ℓ1 (ft)ℓ2 & ℓ4 (ft)ℓ3 (ft)τC (lbs/ft2)η0
∆Z (ft)F�L & F�D (lbs)aθ
θ (degrees)β (degrees)η1
δ (degrees)SFManufacturer/Selected ACB System
Block Width (in.):
Critical Shear Stress (lb/ft2): Block Length (in.):
Calculated Factor of Safety: Block Height (in.):
Block Weight (lbs): Acceptable Protrusion (in.):
C.51
Worksheet 3 – Grain Size Distribution Curve
C.52
Worksheet 4 – Geotextile Selection and Granular Filter Design
Project Information Site Description:
Company: Soil Information
Designer: Date: Description:
Project Name/Number:PercentGravel: Fines: Clay:
Client: Plasticity Index:
From Figure C.5.4 – Geotextile Criterion Based on
Base Soil Properties □ Granular Filter Properties □
For Granular Filter Only
d85 BASE (mm): d15 FILTER < 5d85 BASE < mm
d15 BASE (mm): d15 FILTER ≥ 4 d15 BASE ≥ mmDescription ofSelected Material: d15: mm
Geotextile Retention Criterion from Figure C.5.4d60
Base Soil or Granular Filter Particle Sizes CU = d10= =
d10 (mm): currents are
d50 (mm): mild □ severe □
d60 (mm): geotextile retention criteria for O95
d90 (mm):
Geotextile Permeability Criterion
Soil permeability determined from
Fair-Hatch Equation □ laboratory testing of soil □
Other □ Explain Ks (cm/s): geotextile permeability criterion: Kg ≥ 10Ks ≥ cm/s
C.53
Worksheet 4 Continued – Geotextile Selection and Granular Filter Design
Geotextile Strength Screening TableSelected Woven Geotextile ProductsGeotextile
StrengthProperties Value Satisfactory? Value Satisfactory? Value Satisfactory?Grab Strength(lbs) ≥ 90Elongation(%) ≥ 15PunctureStrength (lbs)≥ 40TrapezoidalTear (lbs) ≥ 30Note: use additional tables if more than three products are being evaluated
Manufacturer/Selected Geotextile Percent Open Area:
O95 (mm):
Does the geotextile have a woven structure? Kg (cm/s):
C.54
Manufacturer:
Contact Person: Date:
Phone: Fax:
Address:
Other Information:
Block Group Block Size/Designation
Physical PropertiesBlock Dimensions Moment Arm 1 (in.) Moment Arm 2 (in.) Moment Arm 3 (in.) Moment Arm 4 (in.)PercentOpen AreaBlock Weight inWater (lbs)BlockManning�s nPerformance PropertiesCritical Shear Stress(lbs/ft2) at HorizontalPeak Tested Velocity(ft/s)Tested Bed Slope(%)Tested/ExtrapolatedData? (T/E)Tested with DrainageLayer? (Y/N)
)HeightBlock(8.03ArmMoment)LengthBlock()WidthBlock(5.04ArmMoment2ArmMoment
)HeightBlock(5.01ArmMomentConcreteGravitySpecific
1ConcreteGravitySpecificWeightBlockWaterinWeightBlock
22
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C.55
11 x 17 Detail Sheet
C.56
C.8 ANNOTATED BIBLIOGRAPHYH
ydra
ulic
s
ACB
Des
ign
Filtr
atio
n
� American Society for Testing and Materials (ASTM). Standardspecifications, standard test methods, standard classifications, andstandard guides.
The following are the ASTM standards that are referenced in this manual andothers that are related to ACB design, filter design, and system installation:
• C 33 Standard Specification for Concrete Aggregates• D 422 Standard Test Method for Particle-Size Analysis of Soils• D 698 Standard Test Methods for Laboratory Compaction Characteristics
of Soil Using Standard Effort (12,400 ft-lbf/ft3 (600 kN-m/m3))• D 2434 Standard Test Method for Permeability of Granular Soils (Constant
Head)• D 2487 Standard Classification of Soils for Engineering Purposes (Unified
Soil Classification System)• D 4221 Standard Test Method for Dispersive Characteristics of Clay Soil
by Double Hydrometer• D 4318 Standard Test Methods for Liquid Limit, Plastic Limit, and
Plasticity Index of Soils• D 4491 Standard Test Methods for Water Permeability of Geotextiles by
Permittivity• D 4533 (1996) Standard Test Method for Trapezoid Tearing Strength of
Geotextiles• D 4632 (1996) Standard Test Method for Grab Breaking Load and
Elongation of Geotextiles• D 4751 Standard Test Method for Determining Apparent Opening Size of
a Geotextile• D 4833 Standard Test Method for Index Puncture Resistance of
Geotextiles, Geomembranes, and Related Products• D 5084 Standard Test Methods for Measurement of Hydraulic
Conductivity of Saturated Porous Materials Using a Flexible WallPermeameter
• D 5101 Standard Test Method for Measuring the Soil-Geotextile SystemClogging Potential by the Gradient Ratio
• D 5819 Standard Guide for Selecting Test Methods for ExperimentalEvaluation of Geosynthetic Durability
ASTM standards summary can be previewed and the standard orderedthrough the ASTM web site:
http://www.astm.org/
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n� Chow, V.T. (1959). Open-Channel Hydraulics. McGraw-Hill, New York.
This famous textbook has been the definitive reference in open channelhydraulics since the time it was published. It was reissued in 1988 inresponse to a high demand after discontinued printing. The textbook is agood reference for any design professional working in open channelhydraulics, including erosion control and ACB design.
Open-Channel Hydraulics covers a wide range of topics related to ACBdesign, including evaluation of shear stress, uniform flow, hydraulic backwaterprofiles, flow over spillways, hydraulic jumps, and flow in meanderingchannels. The reasons for its popularity probably include the broad range oftopics and the fact that the textbook provides a useful balance between theoryand application.
� Clopper, P.E. (1989). Hydraulic stability of articulated concrete blockrevetment systems during overtopping flow. Technical Report FHWARD-89 199. Federal Highway Administration, Washington, D.C.
This document summarizes findings from full-scale laboratory testing of fiveproprietary ACB systems. The research was conducted as a follow up studyfrom FHWA (1988) testing to provide a more comprehensive understanding ofACB performance. The goal of the testing was to define hydraulic processescausing ACB failure and isolate the hydraulic conditions at failure for eachsystem. A secondary goal of the testing was to develop preliminary designguidelines for protection of embankments against erosion. The researchconducted for this document has become the industry standard practice andset the starting point for continued research and development by ACBmanufacturers.
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n� Clopper, P.E. (1991). Protecting embankment dams with concrete block
systems. Hydro Review, April.
The article represents the first time the factor of safety equations, as appliedto ACB system, were published in a peer-reviewed journal. This documentprovides a good background and history of concrete block testing programs.Particular emphasis has been placed on research conducted by Simons, Li &Associates, Inc. that evaluated a number of methods for protectingembankments from erosion caused by overtopping flow. From this research amethod was developed for assessing the stability of block systems under fieldhydraulic conditions. A thorough explanation introduces the resulting methodreferred to as the "factor-of-safety" procedure. This procedure has beenadapted from previous research on the stability of riprap. Application of thefactor-of-safety method is illustrated through a design example.
� Clopper, P.E. and Y. Chen (1988). Minimizing embankment damageduring overtopping flow. Technical Report FHWA RD-88 181. FederalHighway Administration, Washington, D.C.
This document provides a discussion and background literature review on themechanics of overtopping flow (steep slope, high velocity conditions) andsummarizes findings from full scale tests of bare soil, gabion, geosynthetics,soil cement, asphalt, cellular confinement systems, and ACB system on a 6foot high earthen test embankment. The FHWA and the Bureau ofReclamation sponsored the research to provide pilot testing of each of thesystems so that their performance and feasibility could be evaluated for fieldapplications. Since the research was so broad in scope, the document doesnot provide substantial information related to ACB performance; however, itdoes define some of the observed failure mechanisms. A broad range ofperformance was observed between the three proprietary ACB systems thatwere tested, indicating the need for further research and development of thetechnology.
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n� � � Escarameia, M. (1998). River and Channel Revetments. A Design
Manual. Thomas Telford Ltd., Heron Quay, London.
Escarameia begins with background information including geotechnicalfactors affecting bank stability and the modes of bank failure most common inriver engineering. A concise section on geotechnical stability addresses soilcharacteristics and applicable geotechnical parameters.
The design manual identifies common revetment types and design equationsfor each, including ACB systems. Escarameia separates the discussion ofblock revetments into two sections: interlocking blocks and cabled blocks, andprovides design equations for both. Parameters for these two designequations differ. Additional parameters included with the cabled block designequation include porosity of the revetment, water depth, and a slope factor.These two design equations determine required thickness only. Theequations are empirical in form and very simple to apply. However, given thelarge variability in block performance observed in laboratory testing, theequations may not be suitable for all ACB systems. The manual does provideuseful information related to the suitability of each block type to variousapplications.
As an indication of the relative importance of bedding component designwithin the overall framework of revetment design, an individual Chapterentitled, "Use of granular filters and geotextiles," is included in this manual.The provided flowchart for filter design can be used to compare/contrast withthe design steps recommended in this design manual. The explanation ofgeotextile types is brief yet very informative and useful. Also, provided areseveral examples of situations where drainage layers are not advisable. Insummary, Escarameia provides a compact presentation on filter design that isto the point and easy to follow.
� � � Hewlett, H.W.M., L.A. Boorman, and M.E. Bramley (1987). Design ofgrassed waterways. Construction Industry Research and InformationAssociation, London.
This manual addresses a number of issues relating to grassed waterwaydesign. Methods to reinforce grassed waterways are outlined and basicchannel design is reviewed. Within the erosion resistance section, arecommendation is made to use a 2-dimensional woven fabric when thechannel design process specifies a geotextile underlayer. A lower limit on thegeotextile opening size of O90 > 0.5 mm is recommended here as well.Several field and laboratory experiences with erosion resistancereinforcement systems are reviewed within the ACB Design Manual.
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n� Industrial Fabrics Association International (2000). "2001 Specifiers
Guide." In Geotechnical Fabrics Report, v. 18, No. 9, December.
This guide is a special edition of the trade journal "Geotechnical FabricsReport." It is updated annually, and provides tables of values for variousphysical properties of geotextiles. Tables are organized by manufacturer andproduct name (or alphanumeric acronym), and include most geotextilestypically specified for use in conjunction with articulating concrete blockrevetment systems.
http://www.ifai.com
� � Julien, P.Y. (1995). Erosion and Sedimentation. Cambridge UniversityPress, Cambridge, UK.
This sediment transport textbook is referenced because it provides the mostgeneral form of the factor of safety equations (i.e., steep slope in bothlongitudinal and lateral directions). The equations presented in the text areformulated for riprap design, and therefore can not be used to replace thisdesign manual as an ACB design reference. The text is an importantreference for subjects related to hydraulics and sediment transport. Inparticular, Julien�s book gives an excellent presentation of turbulent velocityprofiles and incipient motion analysis, both subjects pertinent to erosioncontrol applications.
� Koerner, Robert M. (1998). Designing with Geosynthetics, 4th edition.Prentice Hall, Upper Saddle River, New Jersey.
Koerner presents a thorough coverage of geosynthetic design. IndividualChapters are devoted to designing with, geotextiles, geogrids,geomembranes, geosynthetic clay liners, geopipes, and geocomposites.Specifically for purposes of design with geotextiles, Koerner details thefunctions and mechanisms of geotextiles as well as their properties andrelated test methods. A section addressing geotextile design for filtrationproves somewhat useful although the applicable example problem is of ageotextile below riprap used as a coastal inlet protection. The description andanalysis of geotextile property test methods is the strength of this reference.The soil retention function of geotextiles presented here parallels the methodchosen for the HCFCD design manual.
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n� Lagasse, P.F., J.D. Schall, and E.V. Richardson (2001a). Stream stability
at highway structures, 3rd edition. Hydraulic Engineering Circular No.20. Federal Highway Administration, Washington, D.C.
This FHWA publication is most often referred to as HEC-20. The documentprovides background and methodology for stream reconnaissance andrestoration projects. HEC-20 uses a multi-disciplinary approach includingmethods from geomorphology, sedimentology, hydrology, and hydraulics.
An excellent feature of HEC-20 is that it is written for a broad range ofaudiences; it provides sufficient background for general planning, technicalanalysis, and design. It presents quantitative procedures for assessing localscour at piers, local scour at abutments, contraction scour, and long termdegradation scour. The document suggests a three level approach to streamanalysis/restoration projects that is systematic and general enough to apply tomost projects. To date, HEC-20 is the most comprehensive and applieddocument related to stream reconnaissance and restoration projects.
� � Lagasse, P.F., L.W. Zevenbergen, J.D. Schall, and P.E. Clopper (2001b).Bridge scour and stream instability countermeasures, experience,selection, and design guidance, 2nd edition. Hydraulic EngineeringCircular No. 23. Federal Highway Administration, Washington, D.C.
This FHWA publication is most often referred to as HEC-23. The documentprovides guidance for scour countermeasure designs at bridge crossings.HEC-23 was developed in response to the recognized need for FHWAsupport to design professionals related to scour countermeasures. Includedis a countermeasure matrix that provides tabular information related to scourtype and river environment suitability. The matrix also provides states whereeach countermeasure has been used successfully.
HEC-23 provides specific design guidance for ACB systems that is similar tothat presented in this document. However, this design manual is much morecomprehensive and the design procedure presented here uses a moregeneral set of equations.
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n� Luettich, Scott M. (1991). Geotextile filter design manual. Nicolon Mirafi
Group.
This design manual was prepared for the Nicolon Corporation by Luettich andreviewed by Dr. Robert C. Bachus and Dr. Jean-Pierre Giroud of GeoSyntecConsultants. The document closely follows a similarly titled article, "GeotextileFilter Design Guide", authored by the three individuals just mentioned, thatappeared in the "Journal of Geotextiles and Geomembranes" in 1992. Themanual covers many of the same topics and procedures as those presentedin the HCFCD Criteria Manual bedding section. A good overview of thefiltration processes associated with bedding components is presented. A stepby step design procedure shows in detail the process for selecting anappropriate geotextile. Many topics relating to geotextile application designare briefly introduced and references for further information are provided. Anumber of design examples are included addressing a broad range ofapplications.
� Richardson, E.V. and S.R. Davis (2001). Evaluating scour at bridges, 4th
edition. Hydraulic Engineering Circular No. 18. Federal HighwayAdministration, Washington, D.C.
This FHWA publication is most often referred to as HEC-18. The documentprovides guidelines for estimating scour at riverine and tidal bridges underhydraulic loading.
HEC-18 presents two major classifications for scour: live-bed and clear water(indicating if sediment is being transported into the subject reach). Scour isalso classified into three sub-types: contraction scour, pier scour, anddegradation. In terms of ACB design, the scour of greatest interest iscontraction scour and degradation. These variables need to be estimatedwhen considering toe-down depth of the ACB revetment, as discussed inSections C.3 and C.4.5.
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n� Stevens, M.A. and D.B. Simons (1971). Stability analysis for coarse
granular material on slopes. In River Mechanics, Shen, H.W. (ed.), FortCollins, Colorado.
This document provides background information and development of thefactor of safety design procedure. Stevens is the original developer/inventorof the design procedure for stability analysis using the moment balanceapproach. The document provides thorough insight into the development ofthe factor of safety equations; however, it is of limited use for ACB designpurposes because the original equations are not tailored to analysis of blocksof known geometric dimensions.
� Terzaghi, K., G. Mesri, and R.B. Peck (1996). Soil Mechanics InEngineering Practice, 3rd edition. John Wiley & Sons, New York.
This widely used and renowned textbook covers in great detail many of thesoil mechanic topics critical to the field of civil engineering. Of particularimportance to filter design are Articles One through Nine devoted to the indexproperties of soils and several articles included within Chapter 3 (permeabilityof soils, and particle migration and erosion). The Terzaghi rules used todetermine the appropriate grain size of granular filter material are presentedand briefly explained. These are the same rules used to design granularbedding components as presented within the HCFCD Design Manual. Thistext is particularly useful at providing definitions and explanations of thebackground soil information required by the bedding component designprocedure.
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n� United States Army Corps of Engineers (1997). RMA-2V Version 4.35.
USACE Waterways Experiment Station, Vicksburg, Mississippi.
RMA-2V is a sophisticated 2-dimensional model for free surface flowapplications. The model is written in the FORTRAN programming languageand is executed through DOS. RMA-2V solves the 2-dimensional version ofthe momentum and continuity equations at each node in a finite elementmesh to calculate depth and velocity. The program is limited to sub-criticalflow and longitudinal bed slopes less than 10 percent. Time dependent windfields can also be added to the model as a boundary condition. RMA-2V wasoriginally developed by Resource Management Associates but is currentlymaintained by Waterways Experiment Station. The program itself does notprovide editing utilities for the input file or a post processor for viewing modelresults. The most efficient way to develop the model and interpret results isusing Surface-Water Modeling System (SMS) developed and supported bythe Environmental Modeling Research Laboratory (EMRL) at Brigham YoungUniversity. SMS is a pre- and post-processor that can be used to develop thefinite element mesh geometry and boundary condition/run control file and viewthe model solution using several graphical tools. The RMA-2V source code,program, and manuals can be downloaded from the Environmental ModelingSystems Incorporated (EMS-I) web site:
http://www.ems-i.com/
The SMS program is not free but can be purchased at the above web addressor alternatively using the following contact information:
EMS-I719 North 1890 West, Suite 38BProvo, Utah 84601Phone: (801)373-5200Fax: (801)375-6410
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n� United States Army Corps of Engineers (1998). HEC-RAS Version 2.2.
USACE Hydrologic Engineering Center, Davis, CA.
This is a widely used software package for 1-dimensional hydraulic modelingof open channel flow. Version 2.2 provides only steady flow modeling;however, Version 3.0 is currently being tested and offers unsteady flowcapabilities. For most applications the model offers similar function to theHEC-2 program that is currently recommended for use by HCFCD, exceptthat it is executed using a graphical user interface rather than DOS. A featurethat makes HEC-RAS suitable for ACB design is that it will provide ahorizontal velocity distribution at a cross section for a specified number ofintervals that is based on the conveyance of each interval. However, thisprocedure can not replace 2-dimensional modeling for complex systemsbecause the velocity distribution is not based on principals of momentum.The upcoming unsteady flow feature of HEC-RAS will make it suitable for tidalapplications, where time dependent tide elevations can be used as aboundary condition. The HEC-RAS program and manuals are available freeof charge from the HEC web site:
http://www.wrc-hec.usace.army.mil/
� Wilson-Fahmy, R.F., G.R. Koerner, and R.M. Koerner (1996). Geotextilefilter critique. In Recent Developments in Geotextile Filters andPrefabricated Drainage Geocomposites, ASTM STP 1281, Shobha K. andL. David Suits, Eds., American Society for Testing and Materials.
This paper compares data collected from exhumed highway drainage fieldsites with existing geotextile design criteria for permeability, soil retention, andlong term performance (clogging). The purpose here is to verify currentdesign practices with actual in-field performance. The exhumed sites wereeach given a letter grade based on a visual assessment of their performance.This rating is followed by a review of the three primary requirements ofgeotextile filter design. Each of the specific design criteria widely in use(permeability, soil retention, and long term performance) are then presented intabular form. These design criteria are assessed through comparison withactual in-field performance. The authors conclude by recommending a set ofdesign criteria based upon the results of the comparison. This paper presentsa very good summary of the current design methods in practice and providessome useful insight into observed behavior of geotextiles under actual fieldconditions.
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