Austroads AP-T196-11 Design Guide for Structure

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AP-T196-11

AUSTROADS TECHNICAL REPORT

Guidelines for Design, Construction,Monitoring and Rehabilitation of Buried

Corrugated Metal Structures


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Guidelines for Design, Construction, Monitoring andRehabil itation of Buried Corrugated Metal Structures


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Guidelines for Design, Constr uction, Monitoring and Rehabilit ation of Buried CorrugatedMetal Struc tures

Published December 2011

Austroads Ltd 2011

This work is copyright. Apart from any use as permitted under the Copyright Act 1968 ,no part may be reproduced by any process without the prior written permission of Austroads.

Guidelines for Design, Constr uction, Monitoring and Rehabilit ation of Bur ied CorrugatedMetal Struc tures

ISBN 978-1-921991-10-3

Austroads Project No. TS1603

Austroads Publication No. APT196-11

Project Manager

Dr Ross PritchardQueensland Department of Transport and Main Roads

Prepared by

Dr Neal Lake ARRB Group

Published by Austroads LtdLevel 9, Robell House287 Elizabeth Street

Sydney NSW 2000 Australia

Phone: +61 2 9264 7088Fax: +61 2 9264 1657

Email: [email protected] www.austroads.com.au

Austroads believes this publication to be correct at the time of printing and does not acceptresponsibility for any consequences arising from the use of information herein. Readers should

rely on their own skill and judgement to apply information to particular issues.
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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Guidelines for Design, Construction, Monitoring andRehabil itation of Buried Corrugated Metal Structures

Sydney 2011


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About Aust roads

Austroads purpose is to:

promote improved Australian and New Zealand transport outcomes provide expert technical input to national policy development on road and road transport

issues promote improved practice and capability by road agencies. promote consistency in road and road agency operations.

Austroads membership comprises the six state and two territory road transport and trafficauthorities, the Commonwealth Department of Infrastructure and Transport, the Australian LocalGovernment Association, and NZ Transport Agency. Austroads is governed by a Board consistingof the chief executive officer (or an alternative senior executive officer) of each of its elevenmember organisations:

Roads and Maritime Services New South Wales

Roads Corporation Victoria

Department of Transport and Main Roads Queensland

Main Roads Western Australia

Department of Planning, Transport and Infrastructure South Australia

Department of Infrastructure, Energy and Resources Tasmania

Department of Lands and Planning Northern Territory

Department of Territory and Municipal Services Australian Capital Territory

Commonwealth Department of Infrastructure and Transport

Australian Local Government Association

New Zealand Transport Agency.

The success of Austroads is derived from the collaboration of member organisations and others inthe road industry. It aims to be the Australasian leader in providing high quality information, adviceand fostering research in the road transport sector.


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CONTENTS

1 INTRODUCTION ................................................................................................................... 1

1.1 Background ........................................................................................................................... 1 1.2 Aims ............. ......... ......... ......... ......... ........ .......... ......... ......... ......... ......... ......... ......... ........ ..... 1 1.3 Scope .................................................................................................................................... 2 1.4 Outline ................................................................................................................................... 2

2 DESIGN OF BCMS................................................................................................................ 3

2.1 Behaviour of BCMS ............................................................................................................... 3 2.2 Failure Mechanisms ............................................................................................................... 5

2.2.1 Corrosion and Abrasion ............................................................................................ 6 2.2.2 Strength-related Failures .......................................................................................... 8 2.2.3 Construction Failures ............................................................................................... 8

2.3 Overall Design Methodology .................................................................................................. 9 2.3.1 Overall Design Process ............................................................................................ 9

2.4 Preliminary Assessment ...................................................................................................... 11 2.4.1 Consideration of BCMS as Appropriate Culvert Type ......... ......... ......... ......... ......... 11 2.4.2 Structure Classification (Importance level) Intended Use (Design Working

Life) ........................................................................................................................ 12 2.4.3 BCMS Configuration and Application ..................................................................... 12 2.4.4 BCMS Fabrication and Material Types ................................................................... 13 2.4.5 Site Investigation .................................................................................................... 15

2.5 Structural Analysis Approaches ........................................................................................... 17 2.5.1 Design Loads ......................................................................................................... 17 2.5.2 Ring Compression Method ..................................................................................... 23 2.5.3 Limit State Method ................................................................................................. 26 2.5.4 FE Analysis Method ............................................................................................... 28 2.5.5 Design Method Selection ....................................................................................... 29

2.6 Design for Durability ............................................................................................................. 31 2.6.1 Material Selection ......... ......... ......... ......... ......... ......... ......... ......... ......... ......... ......... 32 2.6.2 Corrosion Allowance Methods for Durability Design ........ ......... ......... ......... ......... ... 34 2.6.3 Site Investigations/Tests ........................................................................................ 38

2.7 Detailing ............................................................................................................................... 40 2.7.1 Footings ................................................................................................................. 40 2.7.2 Longitudinal Stiffeners ............................................................................................ 42 2.7.3 End Treatments ...................................................................................................... 42 2.7.4 Invert Lining ........................................................................................................... 45 2.7.5 Spacing .................................................................................................................. 45 2.7.6 Cover ..................................................................................................................... 46 2.7.7 Location and Alignment Considerations ................................................................. 46

3 CONSTRUCTION GUIDELINES ......................................................................................... 49

3.1 Material Handling ................................................................................................................. 49 3.1.1 Material Delivery .................................................................................................... 49 3.1.2 Handling Damage .................................................................................................. 50

3.2 Site Preparation ................................................................................................................... 51 3.2.1 Installation Type ..................................................................................................... 51 3.2.2 Grade ..................................................................................................................... 52


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3.2.3 Camber .................................................................................................................. 52 3.2.4 Foundation Requirements ...................................................................................... 53 3.2.5 Bedding .................................................................................................................. 55

3.3 Pipe Assembly ..................................................................................................................... 55 3.3.1 Assembly Instructions ................. ......... ......... ......... ......... ......... ......... ......... ......... ... 55 3.3.2 Shape Tolerances .................................................................................................. 58

3.4 Backfilling Specifications ...................................................................................................... 59 3.4.1 Material Selection ......... ......... ......... ......... ......... ........ ......... .......... ......... ........ ......... . 59 3.4.2 Compaction Process and Equipment ..................................................................... 60

3.5 Construction Loads .............................................................................................................. 62

4 STRUCTURAL MANAGEMENT AND INSPECTION OF BCMS ......................................... 63

4.1 Structure Management Planning .......................................................................................... 63 4.2 Workplace Health and Safety ........ ......... ......... ......... ......... ......... ......... ......... ......... ......... ...... 64 4.3 Level 2 Structural Inspections: Defect Identification ............................................................. 64 4.4 Level 2 Structural Inspections: Condition States .................................................................. 67

4.5 Level 3 Structural Inspections: Information Collection ......... ......... ......... ......... ......... ......... .... 68 4.5.1 Type of BCMS ........................................................................................................ 68 4.5.2 Size and Shape ...................................................................................................... 69 4.5.3 Corrugations Pitch and Depth ............................................................................. 70 4.5.4 Height of Fill Material.............................................................................................. 70 4.5.5 Material Thickness ................................................................................................. 70 4.5.6 Maximum Outside Diameter ................................................................................... 70 4.5.7 Voids Present in Fill ............................................................................................... 70 4.5.8 Estimated Maximum Sag in Pipe due to Settlement ........ ......... ......... ......... ......... ... 71 4.5.9 Waterway Description ............................................................................................ 71 4.5.10 Environmental Conditions ...................................................................................... 71 4.5.11 Water/Soil Samples ......... ......... ......... ......... ......... ......... ......... ......... ......... ......... ...... 71

4.5.12 Other Defects and Cause (Construction/In-service) ............................................... 71 4.5.13 Sketches ................................................................................................................ 71

4.6 Risk Assessment Method and Treatment Action .................................................................. 71 4.6.1 Situation 1 .............................................................................................................. 73 4.6.2 Situation 2 .............................................................................................................. 74 4.6.3 Situation 3 .............................................................................................................. 75 4.6.4 Situation 4 .............................................................................................................. 76 4.6.5 Situation 5 .............................................................................................................. 77 4.6.6 Situation 6 .............................................................................................................. 78 4.6.7 Situation 7 .............................................................................................................. 79

5 MAINTENANCE AND REPAIR PROCEDURES ................................................................. 81

5.1 Emergency Propping ........................................................................................................... 81 5.2 Repair Methods ................................................................................................................... 81

5.2.1 Repair and Maintenance Methods .......................................................................... 82 5.2.2 Concrete Lining of Invert ........................................................................................ 82 5.2.3 Painting the Invert .................................................................................................. 84 5.2.4 Joint Repairs .......................................................................................................... 85 5.2.5 Replacement of the Culvert .................................................................................... 85 5.2.6 Shotcrete Lining ..................................................................................................... 85 5.2.7 Slip Lining .............................................................................................................. 86 5.2.8 Pipe Jacking Around the Existing Culvert ............................................................... 89 5.2.9 Filling the Culvert ................................................................................................... 89


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6 CONCLUSIONS .................................................................................................................. 90

6.1 Future Directions ................................................................................................................. 90

REFERENCES ............................................................................................................................. 92

APPENDIX A BCMS MANUFACTURERS AND COMPANIES PROVIDINGREHABILITATION SERVICES ................................................................ 95

APPENDIX B REVIEW OF STATE ROAD AUTHORITY EXPERIENCE ......... ......... .... 100


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Figure 3.2: Trench installation ......... ......... ......... ......... ......... ......... ........ .......... ......... ......... ........ 51 Figure 3.3: Embankment installation ........ ......... ......... ......... ......... ......... ......... ......... ......... ........ 52 Figure 3.4: Camber under a high fill ......... ......... ......... ......... ......... ......... ......... ......... ......... ........ 53 Figure 3.5: Bedding on soft, rock and firm foundations ......... .......... ........ ......... .......... ......... ...... 54 Figure 3.6: Component sub-assembly method for multi-plate structure ........ ......... ......... ......... . 57 Figure 3.7: Backfilling with plum-bob monitoring ......... ......... ......... ......... ......... ......... ......... ....... 61 Figure 4.1: Cracks in metal plate probably caused by excessive side pressures

during backfill ......................................................................................................... 65 Figure 4.2: New culvert damaged at joint during backfill, probably due to

construction overload ............................................................................................. 66 Figure 4.3: Multi-plate culvert ........ ......... ......... ......... ......... ......... ......... ......... ......... ......... ......... . 69 Figure 4.4: Helically wound culvert ........ ......... ......... ......... ......... ......... ......... ......... ......... ......... .. 69 Figure 4.5: Corrugation profile for steel pipes ........ ......... ......... ......... ......... ......... ......... ......... .... 70 Figure 4.6: Flowchart for risk assessment and treatment ......... ......... ......... ......... ......... ......... ... 72 Figure 4.7: Standing water in BCMS ......... ......... ......... ......... ......... ........ ......... .......... ......... ....... 74 Figure 4.8: BCMS has significant invert corrosion and will need a reinforced

concrete invert in the next 2 years .......................................................................... 75 Figure 4.9: Heavy corrosion in invert with small perforations to metal structure ......... ......... ...... 76 Figure 4.10: Heavy corrosion considerable loss of metal thickness ......... ......... ......... ......... ..... 76 Figure 4.11: BCMS invert corroded away (loss of granular bedding material in invert) ......... ...... 78 Figure 4.12: BCMS ring movement ......... ......... ......... ......... ......... ......... ......... ......... ......... .......... . 79 Figure 4.13: BCMS soil arch failure ......... ......... ......... ......... ......... ......... ......... ......... ......... .......... . 80 Figure 5.1: Example of emergency propping ......... ......... ......... ......... ......... ......... ......... ......... .... 81 Figure 5.2: A thin concrete invert lining which has separated from the culvert and

washed away in flood ............................................................................................. 83 Figure 5.3: HDPE lining being installed ......... ......... ......... ......... ......... ......... ......... ......... ......... ... 87 Figure 5.4: Relining process ......... ......... ......... ......... ......... ......... ......... ......... ......... ......... ......... .. 87 Figure 5.5: Estimating largest liner diameter ......... ......... ......... ......... ......... ......... ......... ......... .... 88

Figure 5.6: Typical pipe jacking set-up ......... ......... ......... ......... ......... ......... ......... ......... ......... .... 89


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SUMMARY

Buried corrugated metal structures (BCMS) have been used in Australia as an attractive solution tounder road drainage requirements due to the low cost and fast construction times achievable.Several incidents of significant failures of BCMS, however, have been reported in current practice.In most observed failures, corrosion has been a critical issue which resulted in subsequentmaintenance, rehabilitation and replacement. In addition, thinner sections have been introduced tothe Australian market and included in Australian Standards, potentially increasing future problemswith premature corrosion and deterioration.

It is critical that road authorities, consultants and contractors use these structures in appropriatelocations and use appropriate design procedures, detailing and construction techniques. Inaddition, it is vital that appropriate structural management plans be developed during the designand planning phase of a project to ensure cost effective and safe management of these higher riskstructures. These plans need to include regular inspections and maintenance processes with

appropriate feedback loops to enhance the management and future design of BCMS.These guidelines provide essential information regarding BCMS from the design process,installation, in-service monitoring, through to maintenance and repair procedures. The content ofthe guidelines include the following key topics:

A discussion on the available methods for structural designing of BCMS. In addition,durability design considerations are also included in determining the metal and coatingthickness in order to achieve the desired service life.

Methods of installation and construction required to satisfy the design performance. Itincludes construction procedures such as handling of the BCMS, the necessary sitepreparation, assembly instructions, backfilling specification and consideration of construction

loading. Guidelines for structural management and inspection of BCMS. The guidelines include two

major aspects, being defect identification condition rating system and suitable structuralmanagement plans.

A discussion of a number of repair methods for damaged BCMS.

A list of BCMS manufacturers in Australia as well as companies which provide repair andrehabilitation services for pre-existing BCMS is provided in the appendix. A summary of theexperiences of state road authorities when dealing with design, construction and maintenance ofBCMS is also included as part of the appendix.


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1 INTRODUCTION

1.1 Background

Buried corrugated metal structures (BCMS) offer an attractive solution to under road drainagerequirements due to the low cost and fast construction times achievable. BCMS are particularlysuited to deep culvert installations where more traditional material types start to become lessappropriate. BCMS work well in these situations due to the flexible nature of the culvert allowingsoil structure interaction to develop and thus significantly improving the structural resistance of aculvert.

Due to the low cost advantages offered by BCMS, their use has become widespread; however,there have been several incidents of significant failures in Australia. In most observed failures,corrosion has been a critical issue which resulted in subsequent maintenance, rehabilitation andreplacement. In addition, thinner sections have been introduced to the Australian market andincluded in Australian Standards, potentially increasing future problems with premature corrosion

and deterioration.

It is critical that road authorities, consultants and contractors use these structures in appropriatelocations and use appropriate design procedures, detailing and construction techniques. Inaddition, it is vital that appropriate structural management plans be developed during the designand planning phase of a project to ensure cost effective and safe management of these higher riskstructures. These plans need to include regular inspections and maintenance processes withappropriate feedback loops to enhance the management and future design of BCMS.

Another key issue related to BCMS is a general lack of expertise and technicalresources/reference material. What are needed are guidelines that will provide engineers whohave little experience in the application of BCMS, with a comprehensive document that addresses

most of the critical issues related to BCMS.

1.2 AimsThe aims of this project are to:

1 Review existing Australian and international literature on buried corrugated metal pipeculverts.

2 Collect and report state road authority experiences with the design, construction inspection,maintenance, repair and failures of buried corrugated metal pipe culverts.

3 Develop Austroads guidelines addressing critical issues in the use of BCMS to ensureappropriate performance of BCMS over the specified design life.

4 Identify specific research and development investigations that will deliver the data relevant tounderstanding the performance of these structures in the Australian environment.


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1.3 ScopeThe guidelines will consider the following key elements relating to BCMS:

structural design

construction

structural management and inspection

rehabilitation.

Hydraulic performance of BCMS is outside the project scope.

1.4 OutlineThese guidelines provide essential information regarding BCMS from the design process,installation, in-service monitoring, through to maintenance and repair procedures.

Section 2 presents the method for designing BCMS which include structural and durabilityconsiderations. The structural design covers two design methods, the ring compression and thelimit state design methods, which are described in the detail in draft AS/NZS 2041.1 (2010). Ageneral description of the Finite Element Method (FEM) is also provided. Durability design includesthe calculation of metal and coating thickness in order to achieve the desired service life.

Section 3 outlines the method of installation and construction required to satisfy the designperformance. It includes construction procedures such as handling of the BCMS, the necessarysite preparation, assembly instructions, backfilling specification and consideration of constructionloading.

Section 4 provides guidelines for structural management and inspection of BCMS. This sectionconsiders two major aspects. Firstly it covers a defect identification condition rating system and thenecessary information needed to be collected during the inspection in order to assess suitabletreatment/repair methods. This aspect of the section is aimed primarily at dealing with structuresthat have not been managed well in the past and have progressed to various serious levels ofdeterioration. Secondly it addresses aspects of developing suitable structural management plansto adequately manage structures throughout their life ensuring that serious deterioration levels arenot reached.

Section 5 includes a discussion of a number of repair methods and outlines the advantages anddisadvantages of each method.

Section 6 concludes the guidelines and highlights areas which still require further work once thedraft AS/NZS 2041.1 (2010) is completed.

Appendix A provides the user of these guidelines with the list of BCMS manufacturers in Australiaas well as companies which provide repair and rehabilitation services for pre-existing BCMS. Theexperiences of state road authorities when dealing with design, construction and maintenance ofBCMS are summarised in Appendix B.


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2 DESIGN OF BCMS

2.1 Behaviour of BCMS

BCMS are flexible members that rely on soil-structure interaction to function. This makes theresponse of BCMS complex. Both the soil and metal structure play a vital part in the structuraldesign and performance of BCMS and proper installation plays a key role in ensuring that thestructure performs as per the assumptions of the design.

The flexible metal culvert can be considered a composite structure made up of the steel culvertand the surrounding soil. Both the barrel and the soil are vital elements in the structuralperformance of the culvert. As a load is applied to the culvert it attempts to deflect as illustrated inFigure 2.1 and Figure 2.2. In the case of a round pipe, the vertical diameter decreases and thehorizontal diameter increases. When good embankment material is well compacted around theculvert, the increase in horizontal diameter of the culvert is resisted by the lateral soil pressure.With a round pipe the result is a relatively uniform radial pressure around the pipe that creates a

compressive thrust in the pipe walls (Connecticut DOT 2000).

Source: Connecticut DOT (2000).

Figure 2.1: Behaviour of buried corrugated metal pipes


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Source: TMR (2010).

Figure 2.2: Ring compression theory whereby overburden and live load stresses are evenly distributed to thesurrounding soil

A number of factors may affect the soil-structure interaction for BCMS including structuralparameters (profile, size and stiffness), construction method (trench, embankment or tunnel), thetype and placement of the backfill material, and external loading.

Typically soil-structure interaction is ensured by the following:

Passive pressure reaction above the crown needs to be developed for stability by adequatedepth of overburden. Minimum cover depths must be adhered to.

Soil compaction during installation must be adequate. AS/NZS 2041 requires that a value of90% compaction be obtained in order to use the ring compression method.

The structural resistance mechanisms are formed during the incremental backfilling process andrely on soil-structure interaction.

The backfilling of a culvert normally includes three stages (Pritchard 2008):

Stage 1 placement of the culvert on a prepared base. A small amount of surcharge isplaced on the culvert crown prior to placing backfill to limit the vertical deformation of theculvert.

Stage 2 progressive placement of the backfill layers, each typically 200 mm thick, from theinvert until the mid-plane horizontal axis is reached.

Stage 3 progressive placement of the backfill layers above the mid-plane horizontal axis

until completion.

For large diameter BCMS having spans of up to more than 15 m, the quality and properties of thebackfill are important for the proper performance of the structure (Sandford 2000).

Pritchard (2008) points out that for helical steel culverts, interaction during backfilling involves highlateral earth pressure on the culvert due to compaction of the backfill. The peak bending effectsoccur during incremental backfilling instead of when the maximum cover is reached, and thesebending effects are not increased due to legal live loads. Thus the incremental nature of backfillingis a critical design consideration and should be a fundamental part of the design process.


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Figure 2.3 shows the possible peak bending locations during construction. The figure also showsthe possible effects of rolling which highlights the need for symmetric installation of the fill layers.

Source: CSPI (2007).

Figure 2.3: Possible deformed shape due to backfill sequence

Due to the fact that BCMS are flexible buried structures, they rely on the soil-structure interactionfor their strength. Proper compaction all around the culvert is a vital factor in the constructionstage. Lack of proper compaction of the foundation can make the culvert deflect up and down asloads pass over it. The culvert can bulge sideways due to live load if the compaction of the soilwithin the culverts height is not sufficient. It can also be crushed due to over-concentrated liveloads, if the soil on the top of the culvert is not well compacted (ARTC 2006).

In addition, BCMS special features, such as stiffeners and relieving slabs, have effects onsoil-structure interaction. Longitudinal stiffeners on long spans can improve compaction and liveload distribution. Transverse stiffeners on the top part of the BCMS can resist peakingdeformations from compaction and live loads acting on the finished structure. A reliving slab helpsreinforce the soil above the crown and distribute live load on a wider area (Sandford 2000).

Factors affecting the structural resistance mechanisms include minimum cover and minimumspacing for multiple installations. Detailed discussion on the minimum spacing and cover ispresented in Section 2.7.5 and Section 2.7.6, respectively.

2.2 Failure MechanismsFailures of BCMS may result from serviceability and/or strength-related problems. General types ofculvert problems include (Connecticut DOT 2000):

Serviceability-related problems:

scour and erosion of streambed and embankments

inadequate flow capacity

corrosion and abrasion of culvert metal

sedimentation and blockage by debris

separation and/or drop off of sections of modular culverts

inadequate length.


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Strength-related problems:

cracking of culvert due to force-induced effects such as compression, seam failure andglobal buckling

undermining and loss of structural support

loss of the invert of culverts due to corrosion and abrasion causing failure of ringcompression resistance

over-deflection and shape deformation.

The main sources of failure of BCMS are discussed below.

2.2.1 Corrosion and Abrasion

Working permanently in wet areas, BCMS are subjected to corrosion and abrasion due toenvironmental effects. Corrosion occurs in several locations such as on the surface being incontact with the soil, on the inside face at the invert where flowing water is present, or on thesurface exposed to the air. It is due to aggressive agents in the air, water or the fill material such assalts, metals or other corrosive chemicals. Figure 2.4 shows an example of corrosion failure of aBCMS.

Figure 2.4: Heavy corrosion of a BCMS considerable loss of metal thickness

Abrasion, on the other hand, occurs mainly at the invert of the structure when the flowing watercontains a bed load of sand or gravel. Figure 2.5 shows an example.


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Figure 2.5: BCMS invert corroded away due to loss of granular bedding material in invert

Corrosion is the major cause of structural failure of buried metal structures. Determination of thecorrosion level for buried metal structures is different to those above the ground. In theatmosphere, metal corrosivity can be predicted based on relative humidity, pollution level andtemperature. In-ground corrosion is more difficult to predict since it is dependent on local variables,such as soil chemistry and water content/quality of the soil.

The corrosivity of a site can be determined through the following tests of the water and soil:

1 pH condition is the indicator of whether water or soil is acidic (pH less than 7) or alkali (pHmore than 7). Most of the coating material used in BCMS is expected to perform well in andaround a neutral pH (pH = 7). Determination of soil pH should be in accordance with AS1289.4.3.1. In addition, California test method 643 details the pH and resistivity test methods

for both water and soil (California DOT 2007).2 Resistivity is an indicator of the inability of water or soil to carry an electrical current and is a

function of the concentration of salt ions dissolved in the water. The higher the concentrationof salt ions that exist in the water, the easier it is to conduct electrical current (less resistivity),which increases the soils potential for corrosion. Resistivity should be determined inaccordance with AS 1289.4.4.1 (1997).

3 In addition, the draft AS/NZS 2041.1 (2010) suggests that the measurement of concentrateof chloride and sulphate ions in the fill material should also be considered if the results of thetests approach the limits given in Table 2.1. The acceptable levels of chloride and sulphateions are less than or equal to 200 ppm and 1000 ppm by weight respectively.

The results of the tests should be used in conjunction with Table 2.1 to determine the site specificpotential for corrosion.


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Table 2.1: Corrosive level determination

Corrosive level pH Resistivity(ohmcm)

Normal condition 5.88 > 2000

Mildly corrosive 5.05.8 15002000

Corrosive < 5 < 1500

Source: Draft AS/NZS 2041.1 (2010).

2.2.2 Strengt h-related Failures

The main strength-related failures include:

Ring compression failure: this failure may occur if the allowable compressive wall stress isexceeded due to the compression force resulting from the design load combinations. In canalso occur in combination with invert corrosion. Once the invert has corroded, the integrity ofthe ring is lost and with it the primary structural resistance mechanism.

Bending failure: occurs due to excessive combined effects of compression force and bendingmoment, which result in plastic hinge formation.

Connection failure: may occur in longitudinal bolted joints.

These failures can be avoided by proper design checks during the design process.

In earthquake-prone areas particular attention must be given to the potential of ground shaking toundermine the soil support for the culvert and thus resulting in a strength-related failure. Criticalelements include:

settlement induced by shaking

pore pressure build-up in the bedding soils

strain softening of the embedment material

potential for liquefaction of the fill

permanent deformation of the surrounding material.

All of these factors need careful consideration during the design phase.

2.2.3 Const ruc tion Failures

Pritchard (2008) reports the results of various tests to failure of helical steel culverts during

installation. The failure of the culvert may be due to: High lateral earth pressure acting on the culvert during incremental backfilling that leads to

the formation of plastic hinges particularly at the crown.

In shallow cover applications, the passage of a vehicle that imposes a larger load effect onthe culvert than the compaction pressure during backfilling (Figure 2.6) .


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Source: Pritchard (2008).

Figure 2.6: Collapse of a steel culvert during backfilling

BCMS material includes corrugated, galvanised steel and corrugated aluminium. The selection ofstructural materials and fill materials should take into consideration the effects of corrosion andabrasion in order to meet the design working life requirement. The following situations should betaken into consideration:

permanent water

marine or salt spray locations

aggressive soils such as clay soils, saline and sulphate soils

highly acidic or alkaline environments.

For fill materials, the pH value and resistivity value R b are key factors. Refer to Section 2.6.1 fordetails.

2.3 Overall Design Methodology2.3.1 Overall Design Process

The design of BCMS takes into account durability, structural failure, bearing failure of thesurrounding soil and handling stresses during construction. The flow chart in Figure 2.7 defines thedesign process of BCMS as presented in the draft AS/NZS 2041.1.


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Source: Based on draft AS/NZS 2041.1 (2010).

Figure 2.7: BCMS design action s

START

Structure Classification

Design Consideration

Determine Minimum Cover

Minimum spacing for multiple structures

Site Investigation/Test

Durability Design Analytical Design (Structural)

Determine Design Loads Determine EnvironmentalCharacteristics

Determine MaterialSuitability

Determine Service Life

Determine Minimum Thickness

Forces for Footing of Arch Structure

Longitudinal Stiffness

End Treatment

Invert Protection

DetermineDesignMethod

RingCompression

Method

LimitState

Method

Finite Element AnalysisMethod


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2.4 Preliminary AssessmentThe purpose of preliminary assessment is to establish the suitability of BCMS for the particularsituation. The aspects that need to be considered at this stage, before proceeding with theanalytical design process, include:

the design working life

the appropriateness of BCMS for the situation

alignment issues (Section 2.7.7)

geotechnical investigation of in situ soil, upstream, downstream and at the culvert site

the initial shape and configuration of BCMS

the most suitable BCMS material.

2.4.1 Consideration of BCMS as Appropriate Culvert Type

BCMS are typically selected for use due to the low capital cost of installation. Particularly insituations where there is a lot of fill, other structure types typically become uneconomical due to thelarge section increases required to resist the fill and surcharge loads. BCMS can be cost-effectivein situation of fill 12 m or more depending on the shape and size and properties of the culvert.

The lower capital cost of installing BCMS must be carefully weighed up against the whole-of-lifecost. In the past, poor durability performance has resulted in the need to repair or replace manyBCMS. Typically design lives have not been achieved particularly in more aggressiveenvironments. Along with achievable design life and whole-of-life cost implications, specialconsideration also needs to be given to the implications of replacement or repair. There have beeninstances within Australia where BCMS have required replacement on major highways with manythousands of vehicle movements per day. Logistically, closing the road or staging replacement can

be very difficult and costly.

It is suggested that BCMS be avoided in the following instances:

major highways and freeways

roads of emergency or economic significance to which there are no other suitable/timeeffective alternative roads

in areas expected to suffer significant flood inundation with overtopping of the roadway

where cover over the culvert is less than the minimum cover requirements stipulated in therelevant BCMS codes

circular pipe culverts are not recommended where water is likely to pond in the culvert due tothe surrounding geometry.

BCMS structures do not have sufficient internal strength to resist ground deformation, as such; theintegrity of the structure is totally dependent on the backfill and in situ materials remaining intact.The use of BCMS should be carefully considered in the following instances:

where there is a high potential for liquefaction especially where the site will experienceearthquake or other shaking effects

where hydraulic conditions are such that the headwater depth is sufficiently high tocompromise the integrity of the backfill or in situ material; end treatments are critical here


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where hydraulic conditions are such that there is a high potential for debris load or barrelblockage.

The total reliance on soil structure interaction for stability also highlights the need for uniformsupport conditions along the length of the structure and the importance of the in situ foundingmaterial. If the foundation is rock then sufficient bedding material of a typical pliable backfill soilneeds to be placed under the culvert to ensure that the soil and the structure interact. Sitting theculvert on a non-yielding foundation is not acceptable because the necessary soil-structureinteraction may not develop.

2.4.2 Structure Classifi cation (Import ance level) Intended Use (Design Worki ng Life)

The intended use of the structure greatly influences the shape, configuration and the overalldimensions of BCMS. For example, if a structure is intended for a traffic tunnel, clearanceconsiderations will dictate the vertical geometry of the structure. If it is to be used as a culvertwhere continuous water flow is to be expected, an arch shaped structure is preferable to eliminatethe chance of invert corrosion often found in closed circular BCMS. Alternatively, concrete lining ofthe invert could be considered.

The draft AS/NZS 2041.1 (2010) recommends classifying the importance of BCMS according tothe consequences of loss of human life as well as economic and environmental consequences inthe event of a failure.

Structures are classified into five importance levels as follows:

Level 1 for minor structures such as farm structures

Level 2 for structures not categorised in other levels

Level 3 and 4 structures for BCMS supporting major railways or roadways with high traffic

volume Level 5 is reserved for special-case structures.

These classifications are used to determine the structure design life. If it has not already beendetermined by the owner the following recommendations for design life can be adopted:

100 years is recommended for BCMS supporting major roads or railways or where accessfor replacement is difficult or where the existing fill is high or there is a lack of an alternativeroute (typically Level 3 and 4 structures).

50 years for minor and local roads or secondary railway lines a 50 year design life iscommonly adopted (typically Level 2 structures).

An appropriate but shorter design life can be adopted for less important roads such astemporary, private or forestry roads (typically Level 1 structures).

2.4.3 BCMS Configuration and Application

There are a number of cross-sectional shapes of BCMS which have been commonly used,manufactured and acknowledged in the design codes in Australia. These shapes and thedescription of their typical applications are listed in Table 2.2 and are intended to give designers astarting point in selecting an appropriate BCMS for their project.


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Table 2.2: BCMS configuration and appli cation

Structure shape Pitch(mm)Depth(mm)

Range of standard structure spans(mm) Typical application

Steel Aluminium

Pipe68

200

13

55300 to 1950 and

1500 to 8550300 to 1950

1600 to 4800

Culverts, sewers and sub-drains, but is also appropriate

for tunnels and bridges

Pipe-arch orunderpass

68

200

13

55

450 to 1800

1925 to 6578450 to 1800 and

1925 to 5521

Bridges and underpasses withlimited overhead clearance,

culverts and sewers

Horseshoearch

Ellipticalarch

200

200

55

552400 to 85002334 to 8486

2400 to 7100 and2334 to 7720

Ideal for projects that includelarge end areas or large spans.It is also used for highway grade

separations

Semi circle-arch

68

200

13

55

300 to 1950

2000 to 8500

300 to 1950

2000 to 8000Stream enclosures, culverts and

storm sewers

Part-arch68 and

20013 and 55 4000 to 8500 4000 to 8500

Stream enclosures, culverts andstorm sewers

Verticalellipse

200 55 1363 to 8055 1600 to 4752Service tunnel, single lane

vehicular and railwayunderpasses

Horizontalellipse

200 55 1507 to 8750 1600 to 4362Multi-lane vehicular

underpasses


The use of closed circular pipes is not recommended where there is potential for continuousflowing water, water ponding or high abrasion on the site. The arch shaped structures are moresuitable in these conditions. Some closed circular pipes, however, can still be considered if theappropriate materials and detailing are used such as invert lining.

2.4.4 BCMS Fabrication and Material TypesThis section describes base material used in buried corrugated metal structures including coatingoptions and their suitability.

Type of fabrication

There are two types of BCMS in the market based on how these structures are fabricated:

helically formed

bolted plate.


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Helically formed structures are fabricated by spiralling a corrugated strip into a helical form so thatthe opposite sides of the strip come together, interlocking them by seams. A typical lock seamedinterlock section is shown in Figure 2.8.

AS 1761 (1985) specifies fabrication requirements for helical lock-seam corrugated pipes includinga specification for connection between the pipe sections.

Source: AS 1761 (1985).

Figure 2.8: Typical lock-seam cross-section of helically formed structures

A multi-plate structure is constructed by bolting sheets of metal together, following a certainarrangement to form a pre-determined shape. The plates arrangement is staggered to form seamsin either the longitudinal or circumferential direction as illustrated respectively in Figure 2.9 (a) and(b).

The details of fabrication are specified in AS/NZS 2041 (1998) which is soon to be replaced by AS/NZS 2041.4 and AS/NZS 2041.5.

(b) Sheets staggered in circumferentialdirection

(a) Sheets staggered in longitudinaldirection


Figure 2.9: Typical configu ration of bolted plate structures

The types of metals used for BCMS are either steel sheets or aluminium alloy sheets. The steelsheets are typically galvanised, aluminised (Type 2), polymer coated or have other protectivecoatings such as bitumen or concrete/grout to prolong resistance to corrosion and abrasion.


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Base metal

Steel is the most common base material used in BCMS. It is a highly corrosive metal and henceneeds protective coating to ensure its durability. The use of steel within close proximity to the seais not recommended, unless it is coated with a suitable material.

Aluminium alloy sheets can be used as an alternative in the marine environment. Aluminium offerssignificantly better corrosion resistance than steel but has low resistance to abrasion. Thealuminium sheets are naturally light in weight compared to galvanised structures and consequentlyare advantageous in shipping and handling. Having a lower strength, however, results ininstallation difficulties such as susceptibility to damage during backfill operation, surface puncturingfrom rocks or granular fill material and shape deformation from pressure during installation.

Coatings

There are three types of protective coating discussed in these guidelines which are currentlyrecognised by the draft AS/NZS 2041.1 (2010):

Galvanised coating: the standard coating of steel base metal is galvanised coating Z600.Galvanised coating is applied to steel sheet to increase its corrosive resistance. The methodof application is by hot-dipping the metal into zinc. Zinc corrodes much slower than steel inthe natural environment. Its corrosive resistance comes from oxide films that develop on itssurface. This oxide is a product of corrosion in all metals, not just zinc. It is more stable thanpure zinc and its build-up helps to reduce the rate of corrosion. Zinc reacts with both acids(pH less than 7) and alkalis (pH more than 7), which means that its ideal workingenvironment is when the pH is neutral (pH of 7).

Aluminised (Type 2) coating: may be used when the environment is more corrosive. Similarto galvanised coatings, the corrosive resistance is gained by the formulation of an oxide filmon its surface. This oxide is stable in neutral and many acid solutions but is attacked by

alkalis. Polymer coating: for an even harsher condition with a high corrosion level and moderate

level of abrasion, polymer coating can be considered. Polymer coating is applied before thecorrugation process, by milling. The corrugation process, especially at the tight bends and atthe lock seams, has been found to weaken the bond between the steel and the coating. Thishas resulted in separation of the coating under extreme conditions (Meacham et al. 1982). Acomparative laboratory study of coatings on corrugated metal culverts indicated that apre-coated polymer performs better than a bituminous coating under different abrasivebed-loads (Curtice & Funnell 1971).

There are other types of coatings provided by various manufacturers such as bituminous coating,epoxy coating, aluminium-zinc alloy (Galvalume) and aluminised Type 1 coating, but theireffectiveness is yet to be recognised by the Australian Standard.

2.4.5 Site Investi gation

The performance of BCMS is highly dependent on the site-specific environmental parameters. It istherefore essential for a detailed site investigation to be conducted in order to ensure compatibilityof the structure and the surrounding environment.

The site inspection can be classified as follows:

General site investigation including observations of local site factors that contributes to thedetermination of appropriate BCMS type, configuration and material.


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Specific tests which include the testing of water and soil content for corrosive substances.This information will be used to determine the aggressiveness level of the site whendesigning for durability criteria. This is covered in Section 2.6.3.

Geotechnical investigation of structural design-related factors.

General site investigation and specific tests

The following aspects should be considered during the site investigation:

Access to the site plays a role in determining the configuration of the BCMS and themethod of construction, i.e. whether or not on-site assembly should be considered.

Assessment of the required waterway area, formation width and embankment slope whichcontrol the dimension, number and layout of the structure. Consideration should be given toover-sizing the culvert to allow for the possible future loss of cross-sectional area due toinvert lining and/or other rehabilitation techniques.

Proximity to the sea is related to the appropriate material selection of BCMS. As previouslydiscussed, aluminium pipes may be used in maritime environments where galvanised pipesshould not be used.

Soil condition investigation should include the assessment of the native soil as backfillmaterial. The assessment can be done by conducting soil pH, resistivity testing. This isdetailed in Section 2.6.3. Scour potential should also be assessed.

Watercourse depending on the intended use of BCMS, the site investigation should includean observation of the existing drainage and water flow, including upstream land use toconfirm the risk of high debris load and/or chemically contaminated water. Flow velocityshould be determined based on typical hydraulic engineering principles. A general guide tothe corrosion potential of the area may also be made by observation of old corrugated steelstructures in the same location on the same watercourse.

Geotechnical investigations

A geotechnical investigation is a critical element in the preliminary assessment of suitability ofBCMS for a given situation. BCMS structures do not have sufficient internal strength to resistground deformation; therefore, the integrity of the structure is totally dependent on the backfill andin situ materials remaining intact. Investigation of engineering properties of the soil should includeelements such as:

in situ density and other relevant material properties

settlement potential

groundwater, slope stability and ability to excavate rock/in situ material

vertical bearing capacity

other factors that contribute to the soil structure interaction such as vertical and lateral soilstiffness.


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2.5 Structural Analysis Approaches2.5.1 Design Loads

In general, design loads used for BCMS can be classified into permanent loads and imposed

loads. Permanent loads comprise of the loads due to self weight of soil and other permanentmaterials directly above the structure. Imposed loads may include construction, highway, railway,aircraft, stockpile, mine vehicle and abnormal loads.

Earthquake loadings may also need to be considered if applicable.

Permanent loads

The load due to the self weight of the soil is calculated using the density of the soil. For normal fillmaterial a unit weight of fill of 22 kN/m 3 is used; however, the unit weight of soil may varysignificantly depending on the soil origin, i.e. volcanic or oxide origins. The load factors forpermanent loads on buried structures are specified in AS 5100.2 (2004).

The load effects resulting from permanent loads are taken as the calculated pressure at the crownof the BCMS as follows in Equation 1.

h pG = 1where

pG = pressure at the crown resulting from dead load, in kPa

= backfill density, in kN/m 3

h = height of fill from surface to neutral axis of corrugated section, in m,

(refer to Figure 2.10) .

Figure 2.10: Height of fil l for calculation of dead load pressure


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The soil weight experienced by a culvert can increase or decrease based on the characteristicsand compaction of the soil above the culvert. This is known as soil arching. Pritchard (2008)described arching as follows. Positive arching occurs when the weight of the soil acting on theculvert is less than the soil immediately above the culvert. Negative arching occurs when theweight of the soil acting on the culvert is in excess of the soil above the culvert.

Typically, positive arching should not be considered when calculating soil loads because conditionscan change over time resulting in possible non-conservative assumptions. AS/NZS 2041 (1998)and the current draft AS/NZS 2041.1 (2010) does not make provisions for positive soil arching.

A soil arching factor is used in the draft AS/NZS 2041.1 (2010) standard for the limit statesapproach to structural analysis (Section 2.5.3) . This factor depends on the effective vertical andhorizontal dimensions as well as the height of cover of the structure. The soil envelope surroundingthe structure is assumed to always exhibit negative arching i.e. soil settlement increases the loadon the structure, using the limit states method.

Construction loadsConstruction loads are often the largest load effect the BCMS may be subjected to. Such loadsoccur when the cover depth is not fully constructed or the roads or other structures above are notcompleted. Besides normal road vehicles that may pass over, loads may include large constructionequipment such as scrapers, dumpers or similar equipment. Loading parameters associated withthis equipment such as axle load, footprint, axle spacing, load factor and dynamic load allowanceare largely dependent on the specific construction site. These parameters should therefore bedetermined on a site-specific basis.

Where the actual construction equipment to be used is not known at the time of design, a typicalconstruction vehicle load as shown in Figure 2.11 can be used.


Figure 2.11: Typical heavy construc tion vehicle load


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Construction loads are calculated as a uniform pressure at the level of the structure crown, inwhich the wheel load is distributed through fill over the structure, from the imprint of the rectangularwheel contact area at the road surface to a rectangular distribution area at the level of the structurecrown. The length of the sides of this distribution rectangle is determined as follows (AS 5100.2):

For a cover depth of up to 200 mm sides of distribution rectangle = sides of wheel contactrectangle + 0.5 h, where h is the depth of fill cover in mm.

For cover depth greater than 200 mm sides of distribution rectangle = sides of wheelcontact rectangle + 100 mm + 1.2 x ( h 200).

Where distribution areas from several wheels overlap, the total load may be considered to beevenly distributed on the surface over the total area of distribution. The un-factored imposed loadpressure (

Q p ) is calculated using Equation 2.

)(

)1(

lt

Q

ll

DLAP p

+=

2

where

Q p = unfactored imposed pressure, in kPa

P = unfactored wheel, axle or track load applied over the footprint, in kN. Formultiple axle vehicles, P is the total load of all axles being considered

lt and ll = sizes of the distribution rectangle at structure crown level, in m

DLA = dynamic load allowance.

Highway loads

At the time of publication the following load cases are required by AS 5100.2 to determine the mostadverse effects on a culvert:

single wheel load W80

axle load A160

S1600 load

M1600 tri-axle group

M1600 load.

Heavy load platform loads HLP320 or HLP400 may be included as required by the relevant roadauthority.

In all the above load cases, the corresponding load factors, accompanying lane factors anddynamic load allowances are taken as per AS 5100.2 provisions.

The highway load is calculated as a uniform pressure at the level of the structure crown. Dynamicload allowances are considered at the crown level. It is also important to note that the dynamicload allowance diminishes with depth. AS 5100.2 (2004) dictates that the dynamic load allowance


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diminishes linearly to 0.1 at 2 m depth. The distribution of highway load through fill is calculated asper AS 5100.2 (2004) and is illustrated in Figure 2.12.


Figure 2.12: Distribution of vehicle loads through fill

Figure 2.13 shows that for shallow applications (cover depth 1.45 m) the wheel load W80controls, while at cover depths of greater than 1.45 m, the M1600 load is the controlling load case.When HLP load is considered, it controls when the cover depth is greater than 1.45 m.


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1

10

100

1000

0.1 1 10

Depth o f Fill above Crown of Culvert, H (m)

L i v e

L o a

d P r e s s u r e ,

p Q

( 1 +

) ( k P a

)

HLP400M1600 x two lanes

M1600 A160W80

1.45

Source: TMR nd b.

Figure 2.13: Live load pressure vs. depth of fill fo r MS1600 and HLP loadings

Railway loads

The current railway design load is 300LA as specified in AS 5100.2. The multiple track factor,dynamic load allowance and distribution of railway loads are calculated as also specified in

AS 5100.2. The railway load is calculated as a uniform pressure at the level of the structure crown.

Aircraft loads

The required imposed loads, load distributions and dynamic load allowance for the calculation ofpressure due to aircraft should be obtained from the relevant regulatory authority. The method ofdesign should be in accordance with the relevant regulatory authoritys specification.

Stockpile loads

Vertical loads at the base of stockpiles are considered permanent loads and are calculated usingthe specified average density of the stockpiled material. A stockpile influence factor ( ks), whichaccounts for stockpile geometry and internal stockpile arching should be included in calculating thevertical loads. A value of 1.0 is taken for ks unless a value has been determined for the situationbeing considered.

The pressure at the crown level due to stockpile load is ksp s = ks s h s , where s is the unit weight ofstockpile material, in kN/m 3 per cubic metre, and h s is the stockpile height above the crown at thesection being considered in metres.


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Mine vehicles and abnormal loads

If vehicles other than those included in construction load or highway load categories are used suchas mine haul vehicles or heavy earthmoving plant, the loads are calculated using the expectedvehicle loads with appropriate load factor and dynamic load allowance.

Earthquake

A design of BCMS should also consider earthquake loadings. The draft AS/NZS 2041.1 (2010)suggests that earthquake design be considered where any of the following occurs:

The structure falls into Importance Level 4 (AS/NZS 1170.4-2007).

The structure is in Importance Level 3 with a design working life of 50 years or more and theearthquake hazard factor ( Z) is greater than 0.09.

The structure is in Importance Level 2 with design working life of 50 years or more and theearthquake hazard factor ( Z) is greater than 0.12.

For any case other than the above cases where a special requirement is more stringent.Structures falling into Importance Level 1 and structures with a diameter dh of less than or equal to3000 mm need not be designed for earthquake loadings.

Earthquakes generate actions in vertical and horizontal directions. The vertical component isusually calculated as 50% of the maximum horizontal component. Only permanent or long-termloadings are considered for design action because of the low occurrence of earthquakes and thelow probability of a vehicle being on a structure when an earthquake strikes.

For design of BCMS, only vertical earthquake forces are considered because the horizontalearthquake forces are restrained by the stiffness of the surrounding backfill.

Equation 3 is used to calculate earthquake design action (draft AS/NZS 2041.1-2010).

)5.0(5.0 h pu ZC k G E = 3

where

E u = vertical ultimate earthquake action

G = permanent action as specified for ring compression or limit state designmethod

kp = probability factor as given in AS 1170.4 (2007) appropriate to the annualprobability of exceedance given in AS/NZS 1170.0 (2002) or AS 5100.2(2004)

Z = earthquake hazard factor as given in AS/NZS 1170.4 (2007)

Ch(0.5) = spectral shape factor for the site sub-soil class appropriate to the site,using T (natural period of structure) = 0.5 s as given in AS 1170.4 (2007).


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2.5.2 Ring Compression Method

The ring compression method assumes hoop compression in the culvert without any bending andis the traditional method used for the design of BCMS based on the ring compression theory. It is apermissible stress design method based on the thrust generated in the structure side wall. The

structural requirement (wall thickness) is determined by comparing the calculated ring compressionforces (design thrust) to the allowable compressive stresses.

This method is based on the assumption that the ring has negligible bending strength, which needsto be ensured by satisfying the minimum cover requirement.

Using the ring compression method, the main steps for design of a BCMS include:

1 Determine material properties from the product standard.

2 Determine the design pressure at the crown, including permanent, imposed and earthquakeloads.

3 For pipe-arch shape structures, check the maximum allowable haunch pressures.4 Calculate the ring compression force ( F r ).

5 Check seam strength (ultimate shear strength for multi-plate structures).

6 Calculate allowable compressive wall stress ( f a).

7 Ensure adequate wall strength in compression by determining the structural wall thicknessbased on the required wall area and the product standard.

8 Determine the stiffness required for handling and installation.

9 Determine the minimum wall thickness.

The draft AS/NZS 2041.1 standard provides comprehensive information on the ring compressionmethod and is recommended for this approach.

The minimum wall thickness of the structure will be the greater of:

the wall thickness based on seam strength and wall compression strength requirements forpermanent and imposed loads plus any durability allowance required

the wall thickness based on seam strength and wall compression strength requirements forpermanent and imposed construction loads excluding durability allowance

the wall thickness required for handling and installation.


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Figure 2.14 illustrates the process of the ring compression method when designing BCMS.


Figure 2.14: Ring compression design method flow chart


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2.5.3 Limit State Method

This method is a simplified method based on limit state principles and the ring compressionmethod with a more detailed consideration of soil-structure modelling. The design approachcompares the calculated stresses on the wall in compression as a result of factored loads to wall

resistance capacity.

For ultimate limit states, three conditions need to be checked as follows:

compression failure (buckling failure)

connection failure

combined bending and compression during construction and in-service including handlingduring construction.

For serviceability limit states, deformation during construction will also need to be checked.

This method includes the following steps:

1 Determine material properties from the product standard.

2 Determine the design actions and their combination, including permanent, imposed andearthquake loads.

3 Check the wall strength in compression (buckling failure) for the finished structure.

4 Check the seam strength (connection failure) for the finished structure.

5 Check the plastic hinge failure (combined bending and axial compression) for the finishedstructure and during construction.

6 Check haunch pressure for pipe-arch structures.

7 Determine the minimum structural base metal wall thickness.

The draft AS/NZS 2041.1 (2010) standard provides comprehensive information on the limit statesmethod and is recommended for this approach.

The minimum wall thickness of the structure will be the greater of:

the wall thickness based on buckling failure, connection failure, and plastic hinge failurerequirements for permanent and imposed loads plus any durability allowance required

the wall thickness based on buckling failure, connection failure, and plastic hinge failurerequirements for permanent and imposed construction loads excluding durability allowance.


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Figure 2.16 illustrates the action steps in designing BCMS with the limit state method.

Source: Based on Draft AS/NZS 2041.1 (2010).

Figure 2.16: Limit state design method flow chart

The followings points should be noted:

For the compression failure limit states, the wall compression is assumed to be constantaround the structure.

The load combination of permanent and imposed action during construction is used only for

combined bending and compression checks. Checks during the construction stage replace the flexibility factor check in the ring

compression method. Maximum bending moment and axial force in the wall are likely tooccur during construction where additional axial force caused by construction equipmentunder a shallow cover is to be expected.

The bending moment occurring in the wall M is the combination of bending moments due tothe weight of backfill up to crown level, backfill from the crown to surface level ( h c) and thelive load applied. It depends on unit weight of the backfill, the axle load of live load, the heightof cover, effective span of the structure and a number of empirical parameters based on finiteelement analysis.


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The factored compression force F and bending moment M are calculated using appropriateloads and load combinations in construction and service stages.

It is important for designers to specify or communicate to the contractor the maximumallowable construction axle loads that can be used above the structure.

For the construction stage, when the ratio ofh

c

d

h < 0.2, no axial force is considered in the

wall ( F = 0 ).

2.5.4 FE Analysis Method

This method uses a computer program to simulate a real-life structure by a mathematical model bydiscretising the structure into a number of small elements and connecting them by mathematicalrelations. Results of a FE analysis can be in the form of stresses, strains, moments anddeformations.

The response of a BCMS is complex due to the soil-structure interaction. As a result, the numericalmodelling should be undertaken by a special-purpose FE (or finite difference) computer packagedeveloped to perform deformation and stress analyses for geotechnical applications. A suitable FEpackage must have capacity to consider the following:

soil models, including popular soil models such as Morh-Coulomb, Drucker-Prager,Cam-Clay and Duncan-Chang models. Each model has an applicable range, which issuitable to a specific soil type with associated parameters. Mlynarski et al. (2008) indicatedthat the Duncan and Duncan/Selig soil models are very representative of the non-linear soilbehaviour in most culvert installations

in situ stresses in the soil due to excavation during construction

staged construction, to include the incremental backfilling process during construction. Thisincludes the capability to simulate the physical process of placing and compacting soil layers,one lift at a time, below, alongside and above the culvert as the installation is constructed

the interface between structure elements and soil elements. the ability to simulate thefrictional sliding, separation and re-bonding of two bodies originally in contact. Typically theseelements are used between the culvert and soil and between trench soil and in situ soil.

An example of a suitable software package is CANDE (Culvert Analysis and Design). It is apublic-domain 2D finite element program and is widely used among the state departments oftransport in the US (Mlynarski et al. 2008). The report prepared for the NCHRP summarises theprocess involved in using CANDE along with tutorial examples and links to other relevantdocuments (Mlynarski et al. 2008). The solution output provides an evaluation of the BCMS interms of safety (as specified by the user) for all potential modes of failure associated with thestructural material and shape of the BCMS. The program conducts a number of design iterationcycles to determine the required thrust area, moment of inertia and section modulus. Sectionproperties are included in the system, which allows the program to search through the corrugationtables to produce design solutions in terms of corrugation size and thickness.

Another example is FLAC3D a 3D model mesh package which also comes with built-in soilmodels. The software was used in developing a simplified design equation for live load distributionon buried structures in the US (Petersen et al. 2010).

Other FE packages that can be used for modelling soil-structure interaction include ANSYS,CosmosM, NLSSIP, PLAXIS, SPIDA, and STRAND7.


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FE modelling should always be carried out under the guidance and supervision of an experiencedengineer with extensive finite element modelling experience particularly in non-linear/stagedanalysis. The draft AS/NZS 2041.1 has a comprehensive and informative appendix on numericalmodelling with a number of useful and informative references on the challenges and aspects thatneed to be considered in FE analysis.

There is always a risk in using FE modelling without reality checks to ensure the magnitude of theresults is of the correct order. Simple hand calculations should always be used to confirm that theresults of FE modelling are realistic.

For BCMS, the following ultimate limit states must be considered in the analysis:

combined thrust and bending

seam strength

global buckling. Although buckling does not normally govern in metal box culverts, for highcover (> 3 m) or large span structures (> 11 m), buckling should be checked. Where the legis longer than 1.2 m, buckling of the straight lower section of the box culvert should bechecked

serviceability deflection checks.

Suitable load factors, combination and failure criteria can be found in the draft AS/NZS 2041.1standard.

2.5.5 Design Method Selection

For a specific BCMS, the selection of a suitable design method plays an important role in ensuringthat the installation procedure, structural features and behaviour are correctly simulated. Eachmethod has advantages and limitations.

Ring compression

The ring compression method is the most simplified design method for BCMS. The followingparameters are the basis for selecting this method for design of BCMS:

This method is only applied to structures that are symmetrical about the vertical axis(AS/NZS 2041-1998).

This method is only valid if the metal structure has a minimum cover of correctly installed filland adequate side support (that requires 90% compaction) so that arching of the surroundingmaterial can occur. The purpose is to reduce the bending in the metal wall so thatcompression governs the design of the finished structures.

Failure of a metal structure designed by the ring compression method is assumed to occuron the horizontal axis.

The assumed modes of failure include crushing or yielding, ring buckling, and the transitionzone between crushing and buckling.

Structures with rib stiffening are not recommended for the ring compression method sincethe rib stiffening will add significant bending stiffness to the structural wall and as a result,bending stresses will occur.

Pritchard (2008) points out that the ring compression method does not consider the incrementalbackfilling process and the resulting bending effects on the culvert during installation. Thus it doesnot represent the physical behaviour of a culvert subject to installation loadings. Consequently, it


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may lead to either a conservative design or a lack of check against failure of the culvert duringinstallation.

Limit state method

This method covers the BCMS that satisfies the following requirements: at any point in the structure wall, the radius of curvature is not less than 2 r t, where r t is the

top radius (centre line of corrugation) of the structure

ratio of the radii of mating pates at a longitudinal connection is not greater than 8, except forpipe-arch structures that comply with the haunch pressure requirements

maximum difference in structural base metal wall thickness of lapping plates at a longitudinalconnection is 2 mm for a thinner plate of thickness less than 3.1 mm.

This method can be used if transverse stiffeners, such as steel rib and encased concrete ribs, arepresent. The section properties for steel stiffeners can be calculated as cumulative, while for

concrete stiffeners, as composite.Being developed from the ring compression method, the limit state method is limited by a numberof factors such as:

simplified soil-structure modelling/behaviour

a set of specific failure modes

construction loading sequence, which is governed by a minimum cover.

These assumptions may be critical for BCMS requiring a high degree of design accuracy such aslarge span BCMS under shallow fill.

For metal box structures, limit state procedures have been developed; however, these proceduresare only valid for structures with the geometrical limits given in Table 2.3.

Table 2.3: Structures geometrical limits for the limit analysis method

Material Span range (m) Structural rise (m) Crown radius (m) Haunch radius (m)

Steel or aluminium 2.6 to 7.8 0.75 to 3.2 7.6 0.75

Aluminium 7.8 to 11.0 0.75 to 3.2 7.6 0.75

Steel 7.8 to 15.0 1.96 to 3.17 8.0 to 8.82 1.02 to 1.14

FEA method

This is a rigorous limit state method using FE modelling, which can be applied to almost anyBCMS. However, current practice shows that this method normally is applied to structures withspecial features or complex geometries that are beyond the scope of application of other simplifiedmethods such as the ring compression or limit state method. For instance, this method is suitablefor metal box structures beyond the limits given in the simplified method or when railway, aircraft orheavy off-road vehicles are involved. Bolted plate structures greater than 3.0 m in span or anybolted plate structures with transverse stiffeners can also use the FEA method.

The design of BCMS with the FEA method is not limited to any shape, size and material and maybe analysed to withstand dead weight, incremental soil-layer loading, temporary construction loadsand surface loads due to traffic.


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Suggested design methods

Table 2.4 presents the scope of application of different design methods for BCMS.

Table 2.4: Selection of d esign methods

Structure types Span d h (mm) Design method

Helically formed sinusoidal pipes 3000 Limit state method or ring compression method

> 3000 Limit state method

Helically formed ribbed pipes 3000 Limit state method or ring compression method

Austroads AP-T196-11 Design Guide for Structure

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