Studies on Geosynthetics Reinforced MaterialsNovember 2013: Technical Presentation – Session I

Civil Engineering Contractors, Consultants, Architects & Planners, Interior Designers

Kensetsu Kaihatsu Ltd

Republic of KenyaMinistry of Transport &Infrastructure,

Department of Infrastructure, Materials Testing & Research Division,

Prepared and Presented by Sirmoi Wekesa

Introduction to Geosynthetics: Type and Applications

Session I

1. Preamble

2. Different types of geosynthetics

3. Main functions of geosynthetics

4. Major applications of geosynthetics

5. Major benefits of geosynthetics

Presentation guidelines

A planar product manufactured from polymeric material used within geomaterials to enhance geotechnical engineering/geo-structural properties through reinforcement and/or improvement.

Geosynthetics is a generic term for all synthetic materials used in geotechnical engineering applications including geotextiles, geogrids, geomembranes, geocells, geocomposites, geonets etc.

What is a Geosynthetic

material?

Preamble

TYPES OF GEOSYNTHETICS

A geosynthetic formed by a regular network of tensile elements and apertures, typically used for reinforcement purposes

1. GEOGRIDS

1. GEOGRIDS

Type 1:

Categorized by the method/mode of manufacturing:

Triaxial Geogrids Biaxial Geogrids

Punched and Extruded Geogrids

Welded Geogrids

GEOGRIDS

Type 2:

Categorized by the orientation of ribs Uniaxial Geogrids

Triaxial Geogrids Biaxial Geogrids

Quaxial Geogrids

A geotextile/geofabric is a permeable textile used with foundation, soil, rock, earth, or any other geotechnical engineering-related materials as an integral part of a human-made project, structure, or system.

2. GEOTEXTILES

2. GEOTEXTILESType:

Woven Geotextiles

Non-woven Geotextiles

Uniaxial Geogrids

Geonets are made of stacked, criss-crossing polymer strands that provide in-plane drainage.

Nearly all geonets are made of polyethylene.

Two layers of strands are called “bi-planar”.

Three layers are called “tri-planar”.

3. GEONETS

3. GEONETSType:

Biplanar

Triplanar

Biplanar Geonets

Triplanar Geonets

These are products manufactured by combining the superior features of various types of geosynthetics.

The objective is to produce materials which are multi-functional and are faster to install than the individual components.

Interface friction becomes an issue when geosynthetics are placed on slopes and bonded materials address this potential problem.

4. GEOCOMPOSITES

4. GEOCOMPOSITES

Geocomposites

Geomembranes are relatively impermeable sheets of plastic.

5. GEOMEMBRANES

5. GEOMEMBRANES

Geosynthetic clay liners (GCLs) include a thin layer of finely-ground bentonite clay. When wetted, the clay swells and becomes a very effective hydraulic barrier.

GCLs are manufactured by sandwiching the bentonite within or layering it on geotextiles and/or geomembranes, bonding the layers with needling, stitching and/or chemical adhesives.

6. GEOSYNTHETICS

CLAY LINERS [GCLs]

6. GEOSYNTHETICS CLAY LINERS

TYPES OF GEOSYNTHETICS

Geocellular confinement systems (GCS) are 3-dimensional honeycomb-like structures filled with soil, rock or concrete.

The GCS structure, often called a Geocell, is made of strips of polymer sheet or geotextile connected at staggered points so that, when the strips are pulled apart, a large honey-comb mat is formed.

The GCS provides both a physical containment of a depth of soil and a transfer of load through

6. GEOCELLULAR CONFINEMENT

SYSTEMS

6. GEOCELLULAR CONFINEMENT

SYSTEMS

Geomat is a three-dimensional erosion control mat consisting of a UV-stabilized labyrinth-like extruded polymer core mounted on a warp knitted mesh

The Geomats act in three major mechanisms: Surface reinforcement and confinement of

the soil;

Protection against rain drops

Reinforcement of the slope and at the same time allowing vegetation [grass] growth

7. GEOMATS

7. GEOMATS

Biodegradable Geomats

Non-biodegradable Geomats

Another significant product which has been “adopted” as a geosynthetic is plastic pipe.

There is a wide variety of civil engineering applications for these products, including: highway and railway edge drains,

interceptor drains, and

leachate removal systems.

8. GEOPIPES

8. GEOPIPES

Geofoam is manufactured into large blocks which are stacked to form a lightweight, thermally insulating mass buried within a soil or pavement structure.

Typical applications of geofoams include: within soil embankments built over

soft, weak soils; under roads, airfield pavements and

railway track systems subject to excessive freeze-thaw conditions; and

beneath on-grade storage tanks containing cold liquids.

9. GEOFOAMS

9. GEOFOAM

Main Functions Of Geosynthetics

1. Reinforcement

2. Filtration

3. Separation

4. Drainage

5. Erosion Control

6. Barrier/Protection

Main Functions:

1. REINFORCEMENT: REDUCTION OF STRESS INTENSITY (CONCENTRATION) THROUGH WIDER DISTRIBUTION

The stresses over the subgrade are higher in

unreinforced flexible pavements than in

geosynthetic-reinforced pavement due to

stress distribution factor

1Relative Load Magnitudes at Subgrade Layer Level for:

(a) Unreinforced Flexible Pavement; and,

(b) Geosynthetics-Reinforced (Improved) Flexible Pavement.

INTEGRAL MECHANISMS THAT CONTRIBUTE TO PERFORMANCE

Lateral restraint of the base and subgrade through friction and interlock between the aggregate, soil and the geosynthetic .

Increase in the system bearing capacity by forcing the potential bearing capacity failure surface to develop along alternate, higher shear strength surfaces.

Membrane support of the wheel loads.

Geosynthetics provide reinforcement through

three possible mechanisms.

INTEGRAL MECHANISMS THAT CONTRIBUTE TO PERFORMANCE

Reinforcement Mechanisms Induced by Geosynthetics: (a) Lateral Restraint (b)

Increased Bearing Capacity; and, (c) Membrane Tension Support

Aperture Stability Aperture Size Junction Integrity Radial stiffness

Geosynthetics Characteristics

Influencing Reinforcing

Functions

2. SEPARATION: Preventing intermixing of soil types

or soil/aggregate to maintain the integrity of each material yet still allow the free passage of liquids/gases. Commonly used in between sub-base/subgrade and around drainage materials.

Contamination of the base course layers leads to a reduction of strength, stiffness and drainage characteristics, promoting distress and early failure of roadway.

SEPARATION MECHANISMS

3. FILTRATION: Restraining soil particles subject

to hydraulic forces whilst allowing the passage of liquids/gases. This function is often partnered with separation.

4. DRAINAGE: Allowing fluids and gases to flow

both through the plane of the material. Commonly used as components in geocomposites used for surface water runoff or for gas collection under membranes.

Piping Resistance: Apparent Opening Size - AOS (as related to soil retention),

Permeability: Flow capacity, and clogging potential.

Strength and Durability: Grab, Puncture strengths

Geosynthetics Characteristics

Influencing Filter, Separation

and Drainage Functions

5. BARRIER/PROTECTION:

Isolating one material form another. The most frequent use of this function is in landfills where impermeable linings prevent contamination of surrounding soils

Preventing or limiting localized damage to an adjacent material, usually a geomembrane used to line a lagoon or a landfill. Thick geotextiles prevent puncture or excessive strain in the membrane.

5. EROSION CONTROL:

Protecting and reinforcing slopes and drainage channels from erosive agents whilst allowing the establishment of vegetation cover.

Major Applications of Geosynthetics

1. GEOSYNTHETICS IN ROADS AND PAVEMENTS:

Subgrade Separation and Stabilization;

Base Reinforcement;

Overlay Stress Absorption and

Overlay Reinforcement

SUBGRADE SEPARATION

Separation refers to the ability of a Geosynthetics to provide and maintain physical separation between the base course aggregate and the underlying fine grained subgrade.

It does prevent mixing of the two dissimilar materials, where mixing is caused by mechanical action generally induced by construction and operation traffic.

The ingress of fines by as little as 10% by weight results in the reduction of strength by more than 80%.

Characteristics of Pavement Structure Subjected to Black Cotton Soil Intrusion

After Repeated Dynamic Loading and Cyclic Seasonal Effects

ANALYSIS OF IMPACT OF INFERIOR MATERIAL INTRUSION INTO UPPER PAVEMENT LAYERS

0

20

40

60

80

100

120

140

160

180

0 10 20 30 40 50 60So

aked

CBR

Plasticity Index, PI (%)

'1:1 '2:1 '3:1 '4:1 '5:1 '1:0

Reduction in CBR Practically Linear

Rate of Reduction and Reduction Characteristics Dependent on Batching Ratio and Quality of Bearing Material

Lower Bound Limits are Distinctly Dependent on Batching Ratio

CBR Reduction ~ PI Threshold @PI =40%

Relation with Structural Thickness

Tendency to Residural (Threshold)

Optimum Batching Ratio

Impact of Black Cotton Soil Intrusion

Impact of Varying Geomaterial Intrusion

SUBGRADE STABILIZATION

Stabilization of weak subgrades entails the confinement and mechanical interlocking of aggregates within the apertures of the geosynthetics to increase the bearing capacity.

The three main important functions of reinforcement: Lateral restraint is the lateral interaction between

the aggregate and the geosynthetic. The presence of the geosynthetic creates pressure in the aggregate that improves the strength and stiffness of the road structure.

Membrane action is the ability of a geosynthetic material to reduce and spread stress arising from the weak subgrade. Additionally, when a geogrid is involved, a third function can be described:

enhanced load distribution within the aggregate.

SUBGRADE STABILIZATION

BASE REINFORCEMENT

Base Reinforcement is achieved through lateral restrain [confinement].

With the addition of an appropriate geosynthetic, the Soil-Geosynthetic-Aggregate (SGA) system gains stiffness. The stiffened SGA system is better able to provide the following structural benefits: Preventing lateral spreading of the base

Increasing confinement and thus stiffness of the base

Improving vertical stress distribution on the subgrade

Reducing shear stress in the subgrade

OVERLAY STRESS ABSORPTION

A geosynthetic interlayer can be placed over the distressed pavement or within the overlay to create an overlay system. The geosynthetic interlayer can contribute to the life of the overlay via stress absorption, strain relief and provision of tensile strength.

A stress relieving interlayer retards the development of reflective cracks by absorbing the stresses that arise from the damaged pavement. It also waterproofs the pavement so that when cracking does occur, water ingress cannot worsen the situation.

OVERLAY STRESS ABSORPTION

OVERLAY REINFORCEMENT

Reinforcement occurs when an interlayer is able to contribute significant tensile strength to the pavement system. The reinforcement attempts to prevent the cracked old pavement from moving under traffic loads and thermal stress by holding the cracks together.

The benefits of geosynthetic interlayers include: Reduction of overlay thickness

Delaying the appearance of reflective cracks

Lengthening the useful life of the overlay

(MODEL TESTING) – ASPHALT CONCRETE CRACK PROPAGATION CHARACTERISTICS

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-4

-3

-2

-1

0

1

2

3

4

0.00E+00 2.00E+05 4.00E+05 6.00E+05 8.00E+05 1.00E+06 1.20E+06

Refe

renc

e lin

es fo

r ob

serv

ation

s of

cra

ck

prop

agati

on

Number of the cycles

Propagation of the primary crack in non-reinforced sampleInterlayer bonding level of the binder and wearing course (the level of the reinforcement)Propagation of the primary crack in the geocomposite reinforced samplePropagation of the secondary crack in the geocomposite reinforced samplePropagation of the primary crack in the geogrid reinforced sample

Average test temperature T± 2σ:non-reinforced sample T=13,2 ± 0,4°Creinforced sample T=13,4 ± 0,7°C

OVERLAY REINFORCEMENT

2. GEOSYNTHETICS IN SUBSURFACE DRAINAGE:

Subgrade Dewatering;

Road Base Drainage, and

Structure Drainage

SUBGRADE DEWATERING:

A high groundwater table can, and often does, interfere with the stability of subgrade soils. For instance, some clay soils can swell or shrink as their water content increases or decreases, respectively.

Geosynthetic materials have become commonplace in subsurface drainage applications. Commonly, geotextiles are being used in lieu of select grades of sand because they are less expensive, provide more consistent properties, and are much easier to install.

ROAD BASE DRAINAGE :

The introduction of geotextiles into drainage applications has enhanced the economical application of blanket and trench drains under and adjacent to the pavement structure, respectively.

The excellent filtration and separation characteristics associated with filtration geotextiles permits the use of a single layer of open-graded base or trench aggregate enveloped in a geotextile.

STRUCTURE DRAINAGE:

It has become customary to place a vertical blanket of “pervious” sand or gravel behind retaining walls for protection against hydrostatic pressures.

One of the best ways to assure effective aggregate drainage is to sandwich an aggregate layer within layers of filtration geotextiles. The inclusion of a perforated drain pipe that collects and discharges seepage will increase the drain’s efficiency. Back fill is placed directly against the drain.

GEOSYNTHETICS IN SUBSURFACE DRAINAGE

3. GEOSYNTHETICS IN EROSION AND SEDIMENT CONTROL:

Slope Protection;

Channel Protection, and

Coastal Protection

GEOSYNTHETICS IN EROSION AND SEDIMENT CONTROL:

4. GEOSYNTHETICS IN REINFORCED SOIL SYSTEMS:

Embankments over Soft Foundations;

Reinforced Steepened Slopes; and

Mechanically Stabilized Earth Walls

EMBANKMENTS OVER SOFT FOUNDATIONS :

The primary problem with these soft soils results from their low shear strength and excessive consolidation settlements requiring special construction practices and leading to high construction costs.

Several methods of treatment are available to reduce the problems associated with soft foundations. These methods include: Removal and replacement of soft soil.

Displacement of compressible material by end-loading.

Staged construction - placing fill at controlled rates to allow for consolidation and strength gains.

Installation of drains to facilitate consolidation.

Pre-loading the site to reduce settlements of the structure and provide higher strength.

Deposit improvement using admixtures (e.g. soil, cement, lime) or injections

Reinforcement of the soil matrix using a structural element.

EMBANKMENTS OVER SOFT FOUNDATIONS :

soil reinforcement has emerged as an efficient, economical and effective solution to the problem of constructing embankments over soft soils.

REINFORCED STEEPENED SLOPES [RSS]:

For many years, retaining structures were almost exclusively made of reinforced concrete and were designed as gravity or cantilever walls which are essentially rigid structures and cannot accommodate significant differential settlements unless founded on deep foundations.

The economic advantages of constructing a safe, steeper RSS than would normally be possible are the resulting material and rights-of-way savings. For example, in repair of landslides it is possible to reuse the slide debris rather than to import higher quality backfill.

REINFORCED STEEPENED SLOPES [RSS]:

MECHANICALLY STABILIZED WALLS [MSE];

5. GEOSYNTHETICS IN REINFORCED SOIL SYSTEMS:

Structure waterproofing;

Water Supply Preservation; and

Environmental Protection,

STRUCTURE WATERPROOFING

WATER SUPPLY PRESERVATION

ENVIRONMENTAL PROTECTION

Benefits Based On Study Findings

Summary of Benefits categorized into Structural and Value Engineering

STRUCTURAL BENEFITS

Enhanced geotechnical engineering properties including bearing capacity, structural capacity, shear strength and deformation resistance [achievement of higher resilient/elastic modulus (stiffness)].

Increased ranges of permissible resilient/linear elastic and lateral strains.

Improvement of the subgrade strength and deformation resistance through stress mobilization and expanded distribution, as well as further tension cut-off.

By spreading and distributing the imparted stresses over a wider area of the foundation, geosynthetics may be improving the foundation/subgrade in a mode that is analogous to stage loading consolidation.

Enhanced structural performance resulting from increased resistance to deformation.

Prevention of the migration of inferior material into the upper pavement layers. This results in the significant enhancement of structural performance and elongation of the life-span of the pavement structure.

Structural benefits analyzed and realized on

the basis of theoretical considerations and

experimental data determined in this Study

include:

VALUE ENGINEERING BENEFITS

Construction cost-time savings through the reduction of required pavement material quantities, whilst maintaining enhanced structural performance.

Elongated pavement structural life – span particularly as a result of incorporating the filtration/separation geotextile.

Reduction in maintenance requirements as a result of enhanced structural performance.

Environmental conservation mainly due to reduction in material quantities and erosion control.

Appropriate application of geosynthetics can

realize the following benefits.