28989377 Design of Reinforecment in Piles by J P Tyson

~ All exacutlve aGilitY 01 • THE DEPARTMENT 4f• OF TRANSPORT

Design of reinforcement in piles

byJPTyson (Trafalgar House Technology Limited)

TRL Report 144

I I

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TRANSPORT RESEARCH LABORATORY An Executive Agency of the Department of Transport

TRL REPORT 144

DESIGN OF REINFORCEMENT IN PILES

by J P Tyson (Trafalgar House Technology Limited)

This report describes work commissioned by the Bridges Engineering Division of the Highways Agency under E553C/BG, Reinforcement in Piles (Desk Study)

Crown Copyright 1995. The contents of this report are the responsibility of the authors and the ChiefExecutive oflRL. They do not necessarily represent the views or policies of the Department of Transport.

Transport Research Laboratory Old Wokingham Road Crowthome, Berkshire, RG45 6AU

1995 ISSN 0968-4107

Highways Agency St Christopher House

Southwark Street, London SE1 OTE

CONTENTS

EXECUTIVE SUMMARY

ABSTRACT

1.0 INTRODUCTION

1.1 Scope of Study 1.2 Research Strategy

2.0 DATA COLLATION

2.1 General 2.2 Data Sources

3.0 CODE REQUIREMENTS

3.1 Current UK Codes 3.2 Historic/Superceded Standards/Codes 3.3 Non UK Standards/Codes

4.0 DESIGN OF REINFORCEMENT IN Pll..ES

5.0 DISCUSSION

5.1 Changing Design & Construction Practice 5.2 Pile Reinforcement Design

5. 2.1 Concrete Strength & Stiffness 5. 2. 2 Steel Reinforcement Strength 5. 2. 3 Design for Bending 5.2.4 Design for Shear 5.2.5 Design for Buckling 5.2.6 Early Thermal Cracking 5.2.7 Corrosion and Durability 5.2.8 Nominal Reinforcement 5.2.9 Curtailment of Reinforcement

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PAGE

1

2

2

2 3

3

3 4

4

4 10 12

13

16

16 19 19 20 20 21 22 22 23 25 26

CONTENTS (cont'd)

PAGE

6.0

5.3 Design Relating to Free Standing Lengths of Piles

5.4 Design Relating to Piled Retaining Walls 5.4.1 Concrete Strength and Stiffness 50402 Steel Reinforcement Strength 504.3 Design for Bending 50 4 0 4 Design of Shear 5.405 Design for Buckling 5 .40 6 Thermal Cracking 5.407 Corrosion and Durability 5.408 Nominal Reinforcement 5.4.9 Curtailment of Steel

SUMMARY AND RECOMMENDATIONS

6.1

6.2 6.3

Summary 6 01.1 Fully Embedded Piles 6.1.2 Free Standing Lengths of Piles &

Pile Retaining Walls Recommendations Areas for Further Study

7.0 ACKNOWLEDGEMENTS

8.0 REFERENCES

FIGURES

APPENDICES

Appendix 1 - Data Sources for Research Study Appendix 2 - Buckling Resistance of Fully Embedded Piles Appendix 3 - Example Method of Calculating Spacing of Links to Prevent

Local Buckling of Embedded Pile

26

27 27 27 27 28 28 28 28 29 29

30

30 30

31 31 34

Appendix 4 - Calculation of Depth to Pile Fixity of Free Standing Length of Pile

ii

EXECUTIVE SUMMARY

Feedback obtained from two construction sites in the UK has suggested that current design practices for pile reinforcement may be overconservative.

This report investigates the development of the design of reinforcement in piles and assesses the applicability of current design codes to pile design. It also gives recommendations for amendments to the Standard BD 32/88 (DMRB 2.1) for piled foundations and suggestions for clarifying existing British Standard requirements. Areas for further research are highlighted.

It is shown that developments in the understanding of concrete, steel and structural design, together with the development of geotechnics have led to an increase in the use of vertical piles to resist lateral loads. This in tum has resulted in a parallel requirement for increased steel quantities to resist lateral forces. Computer design techniques have also developed rapidly, allowing the effects of temporary loadings and deflections to be incorporated into the design. Higher design loads, and hence increased reinforcement, are invariably the result.

It is shown that, although conventional structural analyses can be applied to pile reinforcement design, consideration must be given to factors unique to the piling situation. In particular, the supporting effect of the surrounding ground and protection provided against corrosion are significant factors in determining reinforcement requirements.

For fully embedded piles nominal requirements for links, minimum numbers of bars and crack control steel can be ignored.

Crack control steel need only be applied to the control of early thermal cracking and then only if this is required to ensure the serviceability of the pile. Some evidence suggests that crack control steel may not be effective in reducing long term corrosion of steel. Dense concrete, resistant to carbonation, should be used with external sleeving or steel coatings provided in extreme corrosion environments to achieve a durable pile.

Free standing lengths of piles and the upper portions of pile retaining walls should, however, be designed as columns in air but only down to a point of fixity below ground level. A method for determining the point of fixity is suggested.

1

DESIGN OF REINFORCEMENT IN PILES

ABSTRACT

The quantity of reinforcement installed in concrete piles appears to have increased significantly over the years. Recent case histories have suggested that overly conservative designs may be generated when current design standards are applied. This report considers the historical development of the design of reinforcement in piles and reviews the reasons behind the increase in pile reinforcement quantities. The report also researches the concepts underlying current design requirements and their applicability to pile design.

Advice is given on the implementation of existing British Standards and recommendations given for amendments to the Standard for piled foundations BD 32/88 (DMRB 2.1).

1.0 INTRODUCTION

1.1 Scope of Study

In July 1994, the Transport Research Laboratory (TRL) commissioned Trafalgar House Technology to undertake a desk study into the design of reinforcement in piles. The specific requirements were to identify reasons for the increase in pile reinforcement in recent years and to establish whether the present high levels of reinforcement are justified.

The catalyst for this work is feedback from two completed projects. The first was an unpublished study, commissioned by the DOT, into the design of the Holmesdale and Bell Common Tunnel retaining walls. This reviewed various methods of deriving the lateral forces applied to the walls and considered the implications for quantities of reinforcement. For the diaphragm walls of Holmesdale tunnel, one of the findings was that the application of crack control criteria significantly increased the steel reinforcement requirements.

The second project was work being undertaken for the Medway Crossing. Here, a number of piles were exposed adjacent to a marine environment and, despite the relatively light reinforcement, all appeared to be in good condition.

This study researches the current and historic methods of the design of the reinforcement in piles necessary to resist the calculated design forces. It covers fully embedded piles, piles exposed along part of their length and those acting as retaining walls.

The study deals principally with reinforcement provided to resist the forces applied to the pile and to provide for a durable pile. The derivation of such forces, however, is not included within this study. Pre-cast concrete piles are excluded as the reinforcement for these is generally controlled by the handling and insertion forces and not the in-service forces.

2

1. 2 Research Strategy

To consider as many aspects of pile reinforcement design as possible in the time available, a research strategy was devised which incorporated a review of current and superseded design codes, published literature and consultation with external organisations.

A flow chart indicating the design strategy is presented in Fig. 1.

2.0 DATA COLLATION

2.1 General

Reference material from a variety of sources including UK and non-UK Standards, published literature, private correspondence and internal company case histories has been gathered and collated.

This material has been analysed and the key issues influencing pile reinforcement design identified. These are listed below and discussed in detail in Section 5.2 of this report.

o Changing design and construction practice o Concrete strength and E values o Steel reinforcement strength and E values o Design for bending o Design for shear o Design for buckling o Thermal cracking o Corrosion and durability o Nominal reinforcement o Curtailment of steel at depth

The literature search was supplemented by a consultation process instigated to gather the experience of a cross section of external organisations. Three consultants and three contractors were chosen to ensure a broad cross section of experience.

External consultees were as follows:-

Consultants

Mott MacDonald Group Ove Arup and Partners Rendel Palmer and Tritton Ltd

3

Contractors

Bachy Limited Cementation Piling & Foundations Ltd Keller Foundations

The consultation was in two stages. Firstly the companies were approached for their willingness to participate and sent an initial questionnaire canvassing their views. Their responses were collated and analysed and subsequently re-circulated to the respondents for further comment. Results of the consultation are considered in Section 4.

2.2 Data Sources

Design codes and published literature were generally obtained using standard library database searches. Some unpublished data was obtained from TRL and external consul tees.

A complete list of data sources used is given in Appendix 1.

3.0 CODEREQUIREMENTS

3.1 Current UK Codes

At present, specific references in UK codes to the design of reinforcement in piles is limited to two documents, BS 8004:1986 "Foundations" and BD 32/88 "Piled Foundations" (DMRB 2.1). This latter document is mandatory on DOT jobs only. A summary of the requirements of these codes is given below:

i) BS 8004: 1986 "Foundations"

(a) Vertical piles which are axially loaded need not be designed as structural columns unless part of the pile extends above ground level (Cl7.3.3.3). For this latter case, only the upper portion of the pile need be considered as a column down to a point of fixity. Para 2 Cl 7.3.3.3 states:-

"where part of the finished pile projects above ground, that length should be designed as a column in accordance with BS 8110, CP114 orBS 449. The effective length to be taken in the calculation is dependent on the lateral loading if any and on the degree_of fixity provided by the ground, by the structure which the pile supports and by any bracing. The depth below the ground surface to the point of contraflexure varies with the type of soil. In firm ground it may be taken as about lm below the ground surface; in weak ground, such as soft clay or silt, it may be as much as one half of the depth of -penetration into the stratum but not necessarily more than 3m. The degree of fixity, the position and inclination of the pile top and the restraint supplied by any bracing should be estimated as in normal structural calculations".

4

(b) All forces acting on the pile are to be determined and the pile reinforced accordingly (Cl7.3.3.4, Cl 7.3.3.6, and Cl7.4.4.3.2). Some or all of the pile length may be unreinforced (Cl 7.4.4.3.2). Pre-cast concrete piles are to be designed to BS 8110 (or CP 116).

(c) Where tensile forces are to be resisted by the pile, adequate reinforcement is required to resist the entire tension stresses. The reinforcement should be provided for the full length of the pile or where tensile forces are small, to a depth at which the tensile forces have been fully transmitted to the ground (Cl 7.3.3.7 and Cl 7.4.5.3.2).

(d) Minimum spacing of links is given as 150mm (Cl 7.4.4.4.2).

(e) For raking piles, loads may be considered as axial with an applied bending force at the top (Cl 7.3.3.5).

(f) Durability and protection of reinforcement against corrosion is provided by dense impermeable concrete free from defects (Cl10.4.7). Nominal cover for various exposure conditions should be as BS 8110.

'ii) Standard BD 32/88: 1988 "Piled Foundations" (DMRB 2.1).

(a) This standard applies to both the design of driven and bored piles (Cl 2.1) and is mandatory on all DOT projects.

(b) Pile caps are to be designed to BS 8004 but Cl 3 .1 states that the structural design of all concrete elements of the pile is to be to BS 5400 Pt 4.

BS 8110 states that embedded piles need not be designed as columns and piles carrying axial load only need not be reinforced. Some rules are provided regarding calculation of axial forces which may be accommodated without reinforcement but no guidance is given on calculation of shear capacity. Where reinforcement is required, BS 8110 (and CP116 and CP114) is mentioned for design but is not specifically invoked, except for pre-cast piles. No guidance is given for curtailment of longitudinal steel.

BS 5400, the design code for bridges, is widely accepted as being a more stringent design standard than the general civil engineering concrete code BS 8110. Additional forces are imposed on a bridge structure such as impact and braking forces and abutment earth pressures. Also the often exposed and relatively long and flexible nature of bridges leads to high wind and thermal expansion forces. The difficulties in determining the magnitude of these forces and their effect on the structure has required a more conservative design approach which is reflected in the bridge code.

The requirements of BS 8004, BS 5400 Pt 4 and BS 8110 are summarised in Tables 3.1.1 and 3.1.2.

For piles used as earth retaining structures, (ie contiguous bored pile walls) the exposed portion of the pile may be designed either to BS 8002, code of practice for retaining walls or, if applicable, BD 30/87 (DMRB 2.1) for backfilled retaining walls. However, a new Standard, BD42/94 (DMRB 2.1) has just been released which deals

5

Table 3.1.1 Summary of Code Requirements

Code/Standard Cover to Longitudinal Reinforcement Transverse Reinforcement Crack Control (Date) Reinforcement Min Min. Max Min Max Min Max Crack Width

No. Dia. Spacing %- Spacing Diameter

BS 8110 75mm 4 Tension: 12 times ~ times Only checked if Pt 1 (Concrete cast (Rectangular) 0.8%- smallest largest main N < 0. 2fcu.A: (1985) against ground) 6 12mm - (mild main bar bar size or Then max. width

(Circular) (steel) size 6mm = 0. 3mm 0.45%-

(high yield)

0'1

BS 5400 45mm 4 1%- or 12 times )( times 0.2Smm j

Pt 1 (Buried C30 (Rectangular) 0.15 Nry smallest main largest main (Buried concrete) j

(1990) Concrete) 6 12mm 300mm bar or 0.74 bar size 0 .lmm if (Circular) times effective groundwater

depth pH< 4.5 I

BS 8004 As BS8110 ---- As BS 8110 ----------- ------As BS8110--------------- Not mentioned but add 40mm for concrete cast against ground -------· -~--------- --------

Table 3.1.1 cont/d Summary of Code Requirements

-.l

Code/Standard (Date)

Permissible Stresses Maximum Axial Load Concrete Steel Without Reinforcement With Reinforcement

BS 8110

Pt 1 (1985)

BS 5400

Pt 4 (1990)

BS 8004

(1986)

Compression 0. 67 feu/Ym

Shear

Ve + 0 . 6 . N. V. h Ac.M

ComPression

0. 67. fcu/Ym

Shear

0. 5. feu (Triangular Stress Distribution)

0. 3.8. feu (Uniform Stress Distrubtion)

As BS 8110

Ac - area of concrete A'sl - area of compression A.2 - area of reinf. in

other face A..e - area of vertical

reinf. b - width of section

fy/ym

(Tension & 0. 4 feu. Ac

and compression)

Compression

fy/ (Ym+fy/2000) 0 . 4 feu. b. de

Tension

fy/ym

As BS 8110 As BS 8110

de - depth of concrete in compression fw - characteristic concrete cube strength fd - stress in reinf. in other face fy - characteristic strength of reinforcement h - overall depth of section

0 . 4 . f cu . Ac + 0 . 7 5 . A.c . f y

0 . 4 . feu. b. de + fye. A 1 sl + fsl. A.2

As BS 8110

M - applied design moment N - applied design axial load V - applied design shear ve - design concrete shear stress Ym - partial safety factor for strengh of

material

Table 3.1.2 Summary of Current UK Design Codes "

Global Reinforcement BD 32/88 Design Requirements BS8004 BS8110 (BS5400 Pt 4)

o Axial Compression No guidance Detailed Detailed Refer to design design rules BS 8110 rules for for

structural structural design design (See (See Table Table 3.1.1) 3.1.1)

o Axial Tension General " " guidance given on length of pile to be reinforced

o Flexure No guidance " " Refer to BS 8110

o Shear No guidance " " Refer to BS 8110

o Buckling Check for No checks No checks Buckling if Cu < 20kN/m2

required required

Rules for:

oMinimum No minimum " " Reinforcement reinforcement

required

0 Links Minimum spacing 150mm " "

o Calculation of Refer to " " Rebar BS8110. Only

upper part of pile above ground level to be designed as a column

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Table 3.1.2 cont/d Summary of Current UK Design Codes

BD 32/88 Global Reinforcement BS8004 BS8110 (BS5400 Pt 4) Design Requirements

o Cover As BS8110 75mm 45mmmax plus 40mm +40mm in

(Concrete accordance with cast against BS8004 cl 2.4.5 ground)

o Crack Control Not Only O.lmm to mentioned required 0.25mm

for depending N <0.2fcu on exposure

conditions

o Durability Some Detailed Detailed guidance reqmts requirements given for for concrete

concrete design and design nominal cover and to nominal reinforcement cover to reinforcement

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specifically with the design of embedded retaining walls and bridge abutments. There is a separate Standard BD32/88 (DMRB 2.1) which covers piled foundations in general.

In all these codes, structural design of the piles is referred back to structural codes (BS 5400 Pt 4 orBS 8110).

3.2 Historic/Superceded Standards/Codes

Historically, there are very few codes relating specifically to design of piles. The only documents dealing with this subject are British Standards documents CP2:1951 "Earth Retaining Structures", CP4:1954 "Foundations" and CP101:1972" Foundations and Substructures for Non-Industrial Buildings of Not More than Four Storeys". However, as far as the design of steel reinforcement in the piles is concerned, there were no codes which specifically dealt with it, and therefore the general design standards for reinforced concrete were used instead. These included CP114: 1948 "Reinforced Concrete for Buildings", CP110:1972 "The Structural use on Concrete" and BEl/73:1973 "Reinforced Concrete for Highway Structures". A summary of the various requirements of these codes for pile design is presented in Table 3. 2 .1.

In the absence of specific pile design standards, many aspects of deriving the forces acting on piles and therefore the required reinforcement was based on key reference documents, such as:-

o Caquot and Kerisel (1948) "Tables for the calculation of passive pressure, active pressure and bearing capacity of foundations".

o Terzaghi (1955) "Evaluation of coefficients of subgrade reaction".

o Rowe (1957) "Sheet Pile Walls in Clay" .

o British Steel Piling Handbook (1963)

o Broms (1964) "The Lateral Resistance of Piles in Cohesive Soils"

o Tomlinson (1977) "Pile Design and Construction Practice"

o Hambly (1979) "Bridge Foundations and Substructures"

o Randolph (1981) "The response of flexible piles to lateral loading".

o Burland, Potts and Walsh (1981) "The overall stability of free and propped embedded cantilever retaining walls".

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Table 3.2.1 Sutmnary of Superceded Codes Requirements

Code/Standard (Date)

CP114 (1948)

~ IBE1/73 (1973)

CP110 (1972)

Minimum Cover to Reinforcement (mm)

38

40

40

* Mild Steel Grade is also allowed.

Minimum Concrete 2

Strength (Nmm )

25.0

22.5

20.0

2 Concrete (N/mm ) *High Yield Steel (N/mm

2)

Direct Bending Shear

Direct Bending Shear

Direct Bending Shear

PERMISSIBLE STRESSES

5.50 Tension 10.70 Compression

0.70

5.70 Tension 7.50 Compression 0. 72

ULTIMATE LIMIT STATE

8.90 8.90 0.60

Tension Compression

190 143

230 175

400 333

Notes

Limit state design not introduced

Limit state design not introduced

Limit state design introduced

All of the above, except for the Piling Handbook, relate to methods of determining the forces acting on the piles and not to the provision of reinforcement to resist these loads.

3 .3 Non UK Standards/Codes

A review of codes from Europe and America was undertaken to look at current design requirements outside the UK.

Appendix 1 contains a list of a selection of relevant standards dealing with reinforced concrete and/or piling. This is not, however, intended to be exhaustive.

The following non-UK codes are discussed below in relation to pile design: DIN 4014, ACI 336.3R-72, ACI 318:1992, ACI 318.1 9992, ACI Committee Report 543, ENV 1992.

i) DIN 4014 "Bored Piles"

Reinforcement in piles is designed to structural Code DIN 1045. Piles over 0.5m diameter need not be reinforced unless required for structural reasons. Piles less than 0. 5 m diameter may be unreinforced if there is no structural requirement and load dispersing features such as grating plates and pile bents are provided. Tension piles must have reinforcement for their full length.

ii) ACI 336.3R-72 "Pier Foundations"

This document deals specifically with bored concrete piles over 0. 76m in diameter. Design of plain concrete piers (piles) are to ACI 318.1 and reinforced concrete to ACI 318, both structural codes. Where the soil SPT N value exceeds 2, sufficient lateral support is provided by the soil to prevent buckling of the pile.

iii) ACI 318:1992 "Building Code Requirements for Reinforced Concrete"

Design of piles is specifically excluded and reference made to ACI Committee report 543 "Recommendations for Design, Manufacture and Installation of Concrete Piles".

iv) ACI 318.1:1992 "Building Code Requirements for Structural Plain Concrete"

Unreinforced concrete piles continuously supported by soil are dealt with in ACI 318.1 provided compression occurs across the entire cross section under all loading conditions. The tensile strength of the concrete is allowed in design providing structural failure is not induced by uncontrolled cracking.

A minimum concrete strength of 2500 psi (17 .5 Nlmm2

) is specified for unreinforced concrete. Shear in the concrete for any section shape is calculated using a simple formula. When calculating stresses, the cross section of the concrete is reduced by 2" (50mm) for concrete cast against soil.

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v) The ACI Committee Report 543 (1973) "Recommendations for Design, Manufacture and Installation of Concrete Piles"

This is the most comprehensive of the codes dealing with concrete pile design and recommendations are made on all of the following:-

Lateral support of ground, lateral capacities of piles, uplift, tension and shear stresses, allowable design stresses, allowable design loads, unsupported piles, direct tension, corrosion and reinforcement.

The requirements of DIN 4014, ACI 336.3R-72, ACI 318.1 (1992) and ACI 543R-74 are summarised in Table 3.3.1

4.0 DESIGN OF REINFORCEMENT IN PILES

As a result of the consultation exercise described in Section 2.2 above, it is apparent that there is a distinct difference in pile design practice between piles designed for, or influenced by the requirements in DMRB using BS 5400 and those designed to other requirements.

For a conventional design situation (non DMRB), a consulting engineer generally produces a design which is then put out to tender to various Contractors. Using BS 8004, the engineer is not required to design the pile as a column in free air and instead must, to a large extent, select elements of the existing structural codes which are appropriate to complete the design. Alternatively a specialist piling contractor may be requested to take responsibility for the pile design. In either case, there is significant scope for variations to occur in the quantities of the fmal pile reinforcement depending upon, for instance, whether nominal links or crack control are included in the design. As illustrated in Section 5 this can lead to disputes between designers if an external design office is used for checking the design.

Since the issue of BD 32/88 (DMRB 2.1), it is mandatory in highway projects for the structural design of the concrete elements of piles to be designed in accordance with BS 5400 Pt 4. Current design practice therefore requires piles to be treated as columns in air for the purposes of reinforcement design.

It is now normal practice in large civil engineering schemes to incorporate the full requirements of the structural-codes when determining the reinforcement requirements for piles. The latest draft of Eurocode 7 (ENV1997-1) perpetuates this approach.

The use of the current structural codes for pile design requires that provision must be made for nominal reinforcement links, minimum numbers of bars, maximum bar spacings and minimum bar diameters. Checks for allowable crack widths are also required. Many of these may be inapplicable to pile reinforcement.

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Table 3.3.1 Summary of Current Non-UK Design Codes

Global Reinforcement DIN 4014 ACI ACI ACI Design Requirements 336.3R-72 318.1 543R-74

(Revised (1992) 1985) Revised

o Axial Compression No guidance Refer to Detailed Detailed specifically structural guidance guidance for rebar code

o Axial Tension II II II

o Flexure Methods of II II

determining lateral loads but no guidance on rebar

o Shear Not mentioned II Not mentioned

o Buckling Only Only considered considered ifCu ifN > 2 < 15 kN/m2 (N=SPT N

value)

Rules for:

o Minimum General Refer to No No Reinforcement comments struct. reqmts reqmts

only code

o Links Minimum II " To be diameter & provided maximum where spacing loads given indicate

a reqmt

14

Table 3.3.1 cont/d Summary of Current Non-UK Design

Global Reinforcement DIN 4014 ACI ACI ACI Design Requirements 336.3R-72 318.1 543R-74

(Revised 1995) 1985)

o Calculation of Limited II II Refer to Rebar comment structural

Refer DIN code. 1045

o Cover 70mm II II 40-75mm

o Crack Control Not II II Not mentioned mentioned

o Durability Concrete II II Little with high guidance. chemical Aggressive resistance environments required require if ground protective is coatings, aggressive sleeving or

cathodic protection

15

During construction, reinforcement cages are assembled in accordance with the detail drawings. For bored piles or driven cast-in-place piles, the reinforcement is inserted into the preformed hole and concrete pumped around it to form the pile. Alternatively, for cfa piles, the reinforcement is pushed or vibrated through previously placed concrete.

In either case, the insertion of large quantities of reinforcement can result in difficulties in ensuring a satisfactory construction of the pile. For concrete pumped around the reinforcement, it may be difficult to provide proper compaction around the steel in heavily reinforced piles which may result in defects in the pile. Where reinforcement is inserted through concrete, it may not be possible to achieve sufficient penetration of the steel. Excessive vibration of the cage to aid penetration may damage the cage and cause segregation of the concrete.

It is therefore necessary for the designer to specify the minimum reinforcement to satisfy the structural requirements of the pile. If this is not done, erstwhile economical piling techniques may be excluded through the specification of excessive reinforcement.

5.0 DISCUSSION

5.1 Changing Design and Construction Practice

Over many years structural reinforced concrete design and concrete foundation design has undergone a continuing development. Advances have been made in the understanding of the behaviour of materials, mechanisms of failure, magnitude of forces applied to structures and methods of ultimate and serviceability limit state design. This has resulted in a greater knowledge of the nature and interaction of materials and forces. Developments in structural and foundation analyses have continued along essentially parallel but often separate paths. Superstructures including in many cases pile caps and basement constructions, have been designed in accordance with the relevant structural codes whilst foundation design for the same structure followed a separate design code.

As these developments took place, designers were provided with the tools to analyse, with greater confidence, the forces, deflections and reactions generated by and applied to the structure. Over the same period, the science of geotechnics developed significantly, making it possible to determine, at least theoretically, the response of the ground surrounding the foundations. Landmark publications giving methods of deriving the lateral resistance of piles (Broms, 1964) and coefficients of subgrade reaction (Terzaghi 1955) were significant in moving pile design forward. Techniques therefore became available for estimating the bending moments and shear forces which a laterally loaded pile must resist.

This led to a significant change in the way piles were used. Lateral forces on bridge decks, for instance, were historically resisted by raking piles, passive pressure on the pile caps or abutment keys. As the theoretical understanding and analytical tools advanced it became more popular to resist lateral forces on vertical piles in flexure, albeit with increased steel reinforcement. Publication of the BSC Steel Designers Handbook (1963) greatly aided the engineer's task in assigning steel reinforcement quantities for piled retaining structures. Over the same period, the introduction of more rapid concrete piling techniques, (such as driven cast-in-place and cfa piling), reduced

. 16

the cost of concrete piling and increased the range of available pile diameters, thus further encouraging the use of vertical piles to resist lateral loads.

The advances in techniques and reduced costs also led to an increased use of concrete piles in retaining wall applications. Where, prior to the early 1970's, the majority of cantilever retaining walls would be designed using steel sheet piling, it is now common for concrete piles to be a viable alternative. The larger cross-sections and therefore greater stiffnesses achievable with concrete has also meant that larger and deeper excavations in difficult ground can be completed using concrete retaining walls with minimum prop requirements. Such uses obviously generate higher pile shaft forces which must be resisted by increased quantities of steel reinforcement.

It has therefore become necessary for designers to develop procedures for designing pile reinforcement to resist the induced pile forces. As previously stated, foundation design codes offered only limited guidance and the designer was forced to tum to structural codes. The applicability of the structural codes to the design of fully embedded piles has, however, been a subject for debate amongst designers. For example, it is questionnable whether a fully embedded pile should be provided with hoops or links in accordance with the column codes. This issue was the subject of an adjudication on the QEII bridge, a DOT project. The pile designer argued that the surrounding soils provided sufficient restraint to prevent bar buckling, and detailed widely spaced hoops for cage rigidity only. The checker called for hoops in accordance with BS 5400 (Part 4, Clause 5.8.4.3). The level of curtailment of the main axial reinforcement was also a subject of adjudication.

The adjudication concluded that BS 5400 does not address itself to pile design and BS 8004 only addressed piles as a structural member when in free air. It was also considered that "the subject of the design and specification of steel reinforcement cages for bored cast-in-place piles is not addressed adequately in current Codes of Practice". Furthermore that it is therefore necessary to rely on traditional practice for pile design and the experience of specialist piling contractors in relation to their particular types of piles. Steel reinforcement cages could be required in the upper portions of pile shafts to resist flexural stresses from lateral loads or eccentricities of loads, but the remainder of pile shafts, subjected only to axial compressive stresses, may be unreinforced or provided with nominal cages designed primarily to resist handling and insertion forces.

During the adjudication period BD 32/88 (DMRB 2.1) was issued and calls for full compliance with BS 5400 Pt 4, including:

o nominal links throughout the length of the main reinforcement o minimum vertical steel bar numbers, and maximum and minimum bar spacings o crack width checks

The code design requirements are thus clearer but perhaps unduly conservative.

In addition to the above, other factors affecting the design of pile reinforcement include a greater understanding of the effects of lateral earth pressures, increased use of design software and the routine use of pile integrity testing techniques.

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Prior to the early 1980's, the full effect of ground movements on piles caused by loading of adjacent ground was not well understood. Increased use of piled abutments particularly on soft ground led to a number of cases being recorded where unacceptable movements were occurring at pile cap level. Advice Note BA 25/88 (DMRB 2.1) was promulgated to address this issue.

The recent increased availability of complex and comprehensive structural and geotechnical design software has also had a significant effect on structural design. Forces due to flexure and stiffness and the effects of expansion joints etc, can now be easily calculated and then added to the forces acting on the foundations. If, however, the interaction between soil and structure is not similarly investigated, the pile design will be over conservative. In the past, soil/structure interaction effects would have been ignored due to the complexity of the analyses or lack of design tools. Lower calculated design forces may have been compensated for by the use of overall safety factors prior to CP 110: 1972. Existing partial safety factors are based on CIRIA Report No. 63 (1977), into safety and serviceability factors in structural codes and were introduced to achieve roughly similar effects to the previous overall factors but allow greater flexibility in design. There may now, however, be a case for reviewing the partial load and material safety factors used in current design to allow for the increased sophistication of the design process.

For example, designers have recently become more aware, through the use of computer programs, of the sensitivity of lateral forces to the design model chosen and parameters, particularly geotechnical input into the model. Sensitivity analyses are therefore frequently run and designers may then perhaps use the more conservative analyses in their final design.

The routine use of non destructive integrity testing of piles has lead to the discovery that many piles suffer significant cracking after installation. This has lead to concerns over durability and a desire to limit crack widths. Checks for crack widths as required in BS 5400 can have significant effects on reinforcement quantities. Examples for embedded retaining walls are found in cases such as the A406 North Circular contiguous piled walls and the Holmesdale Tunnel diaphragm walls. Although both these involve reinforcement design, where checking for crack widths may be applicable (see Section 5.4), steel requirements are often applied over the full length of the wall. This was certainly the case for the A406 piles where provision of crack control steel was the governing design critera. Crack width checks are also often specified for fully embedded piles.

At Holmesdale, the ground comprised up to 6m of Terrace Gravels overlying another 6m of the basement beds of the London Clay with Woolwich and Reading Beds below. The tunnel structure was formed by reinforced concrete diaphragm walls. The roof served to prop the walls and form a cut and cover tunnel. A study was made into the effects of designing the diaphragm retaining walls using CIRIA report 104, BS 5400 Pt 4 and the then draft Standard BD 42 (DMRB 2.1). Under certain conditions, it was found that serviceability limit state crack control became the critical design criteria and that, depending on the particular analysis method used, a reduction of up to 28% steel could be made if cracking were discounted.

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BS 8110 has a slightly less onerous crack width requirement than BS 5400 and is therefore less severe in its requirements for additional steel. Further discussion on the relevance of structural codes including crack control requirements for buried reinforced concrete may be found in Section 5.2 of this report.

Relatively recently, developments affecting the wider field of civil engineering have also affected the design of piles. An increase in professional liability, larger and more frequent claims for negligence and the introduction of widespread internal mandatory checking procedures under BS 5750, or similar quality assurance systems have all affected engineers' attitudes to design. The increased threat of litigation has meant that companies, both consultants and contractors, are less willing to amend design code requirements to fit their needs and since there is no comprehensive code dealing with pile design, it is often easiest to invoke a structural code as a basis for design to speed both internal QA and external checking.

5. 2 Pile Reinforcement Design

The design of pile reinforcement has been discussed in Section 5 .1 in terms of existing and historic design practice. The following discussion, however, deals with the design of pile reinforcement from a consideration of the fundamentals on which design is based. The key issues identified in Section 2.1 are discussed and their relevance to pile design highlighted.

These key issues are listed below:

o Concrete strength and stiffness o Steel reinforcement strength o Design for bending o Design for shear o Design for buckling o Thermal cracking o Corrosion and durability o Nominal reinforcement o Curtailment of steel at depth

Where applicable, existing code requirements are reviewed and amendments suggested. These amendments are then summarised and recommendations made in Section 6.

5. 2.1 Concrete Strength & Stiffness

The concrete used to construct a pile obviously has a profound effect on pile capacity and the forces attracted to it. Concrete design for foundations is a subject in itself and is the subject of an ongoing TRL study. For pile reinforcement, however, its effect can essentially be reduced to two elements, strength and stiffness.

Concrete strengths and stiffnesses are closely interrelated and will vary with type of aggregate, aggregate cement ratio and age of concrete. They will also vary with the load conditions, whether short term, long term or dynamic. For a given concrete mix and load case, an increased concrete strength will result in an increase in stiffness.

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Over the years, minimum concrete strength for use in foundations has increased (see Fig. 2). This has been driven by a desire for denser, more durable concrete and has also had the effect of increasing the load capacity of piles. As a result the pile stiffness has increased resulting in an increase in the relative differences between soil and pile stiffnesses. This in tum has increased the magnitude of shear forces and bending moments which can act on a pile and these tend to act over a longer length of the pile.

For axially loaded piles, concrete strength is the governing requirement and an increase in strength directly reduces reinforcement requirements. Stiffness only plays a role where small groups of piles require design for moments resulting from nominal design eccentricities. For laterally loaded piles and retaining walls, however, stiffness is also important in the design for providing lateral resistance. Retaining wall design in particular is largely governed by requirements for stiffness rather than axial load carrying capacity.

Increased concrete strength, although allowing greater axial and lateral loads to be carried, also has drawbacks since it produces a greater tendency for thermal cracking to occur. The greater the concrete strength, the higher the curing temperature due to increased cement content. This leads to higher thermal strains and larger concrete shrinkage. The concrete shrinkage is resisted by the surrounding soil thus generating tensile forces within the pile. If these forces exceed the tensile strength of the concrete, a horizontal crack will develop at some depth which in severe cases may affect the structural integrity of the pile. A more detailed discussion of this is given in Section 5.2.6.

5 .2 .2 Steel reinforcement strength

Permissible stresses quoted in design codes for steel rebar have increased with time (Fig. 3). There has also been an increase in the availability and relative reduction in cost of high yield steel. This has meant that fewer and smaller diameter bars are used to accommodate larger bending moments and shear forces allowing for a reduction in overall steel quantities for axially loaded piles and greater resistance to lateral forces for laterally loaded piles.

5 .2. 3 Design for bending

Lateral forces when applied to a pile set up bending moments within the pile. Eccentricities of loading also apply moments to the pile. Where these moments exceed the design bending resistance of the pile, reinforcement is required to strengthen the concrete section.

Design charts for calculating the required area of steel for a given rectangular concrete section and given bending moment are provided in structural design code BS 8110 Part 3. BS 8110 does not provide design charts for circular sections and design for these is often based on an equivalent rectangular area. BD 44/90 (DMRB 3.4.4) allows circular columns to be assessed using the design charts for circular sections given in CPllO. Dedicated computer design software is also available for design of circular sections.

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For design of reinforcement to resist bending, therefore, the major issue is not the method of calculating steel quantities but the method of determining the magnitude and distribution of the bending moments to be resisted.

A fully embedded pile, by definition is laterally supported by the surrounding ground. When the pile is loaded laterally it will deflect until sufficient resistance is provided by the ground and the pile to resist the load. The bending moment in the pile, will depend on the strength and stiffness of the ground and the stiffness of the pile. A stiff soil will be able to mobilise greater resistance against the pile resulting in less deflection of the pile for a given load. Bending moments will therefore be large. Soft soil will have the reverse effect with a trade off between decreased bending moment and increased deflection.

Any determination of bending moment on a pile must therefore take account of soil/structure interaction effects. This will normally result in the use of computer software to model the pile and surrounding ground.

The effect of lateral deflections of the pile can be assessed using dedicated design software using either non linear elastic spring model, p-y curves or finite elements to model the soil response. Soil parameters and factors of safety must be carefully chosen taking account of the type of analysis to be used. BA 25/88 (DMRB 2.1) provides a recommended method of determining additional forces on piles from soil movements related to loading of adjacent ground. CIRIA Technical Note 109 provides advice on assessing the forces on a laterally loaded pile.

Nominal reinforcement requirements such as minimum numbers of longitudinal bars as required in BS8110 and BS 5400 Pt 4 may not be applicable to buried piles again due to the supporting effects of the surrounding ground.

5.2 .4 Design for Shear

Until recently, design for shear in circular sections was one of the least defined aspects of column and therefore pile design. Both BS 8110 and BS 5400 Pt 4 require columns to be treated as beams for the purposes of shear. This requires that an equivalent rectangular area be derived from the circular section and the shear capacity determined accordingly. BD 44/90 (DMRB 3.4.4) and BA 44/90 (DMRB 3.4.4) now specify a design method for circular sections based on the ACI code published in 1983 and confirmed by Clarke and Birjandi (1993). Design using these codes is now, therefore, relatively straightforward.

BS 5400 requires an increase of 15% in design load when calculating shear in columns plus provision of an increase of 0.4 N/mm

2 in shear capacity above the calculated value.

The basis of these requirements appears to be an attempt to:

i) reduce the possibility of sudden brittle collapse by the provision of a larger safety factor and

ii) to account for a reduction in the contribution to shear resistance of the concrete under repeated loading by the provision of extra capacity.

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BS 8110 has neither of these requirements.

For piles not subject to repeated loading, such as fully embedded piles and many retaining walls, Clarke suggests that the additional 15% load factor in BS 5400 appears overly conservative. Clarke also suggests that for distributed loading, as applies along the length of a pile, shear capacity of a given section is approximately twice that for a concentrated load and suggested that the addition of 0.4 N/mm

2 extra capacity therefore

seems unnecessary. Finally, Clarke also suggests that the full shear resistance of the concrete can be taken into account when calculating the necessary reinforcement but recommends further research to confirm this view. Clarke Is findings confirm a general impression that BS 5400 is overly conservative in its requirements for design for shear particularly for foundation work.

The case often made for allowing for significant conservatism in shear design is the brittle mode and possible catastrophic consequences of such a failure. Unlike the case of beams and columns in air, buried foundations have the support of the ground to modify their failure mode. Additionally, considerable redundancy is often incorporated into pile group designs such that the failure of a single pile is not catastrophic to the whole structure. As an example many elastic computer. analysis models of pile groups generate large design forces in the comer piles of a group. The resultant large steel requirements are therefore, for simplicity, often provided for all piles in the group.

5.2. 5 Design for Buckling

Lateral restraint of the ground is sufficient in most practical cases to prevent buckling failure of fully embedded piles. When referring to driven piles, BS 8004 requires that buckling need only be considered for piles through soil with a shear strength less than 20 kN/m

2• At shear strengths greater than this, buckling is said to be unlikely and piles

need not be designed in accordance with BS 8110. Where buckling is a consideration, the work of Francis et al (1962) is referenced: this describes a series of laboratory and field tests in Melbourne on long thin steel piles driven into soft soils.

Hollow, rectangular (110mm x 150mm) piles 28m long were driven through soils with shear strengths between 1 and 16 psi (7 to 110 kN/m

2). It was shown that even for these

extreme dimensions failure was due to squashing of the pile and not buckling. Tests were also carried out on prestressed octagonal concrete piles 710mm across, 28m long. These were not loaded to failure but carried more load than the short column failure load without buckling.

From this research, Francis .concluded that only for cases where L/1 I < 11(2)'h, should consideration be given to buckling of the pile. (L = length of pile in soft soil, 1 I = length of half sine wave deflection of pile generated by buckling load and described in Appendix 2).

A theory for calculating buckling resistance was presented by Francis based on the Winkler spring system. Where buckling is a possible failure mechanism, the Winkler approach may be used to calculate the failure load. A summary of this method is given in Appendix 2. Structural frame analysis or finite element software may also be used to determine the resistance to buckling.

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5 .2. 6 Early Thermal Cracking

Early thermal cracking is a phenomenon well known in concrete design. It occurs as a result of temperature changes generated in the concrete during the curing process setting up tensile strains and stresses. CIRIA Report 91 (1981) produced a detailed report on the subject as it relates to standard building design and the same principles can be applied to piles.

Thermal cracking can be divided into two types, externally restrained and internally restrained. Externally restrained cracking results from the concrete section being restrained from movement during its cooling phase by external factors such as adjacent wall sections, a base slab or, in the case of piles, the surrounding ground. Internally restrained cracking, however, is caused by differential temperature gradients set up within a concrete section whereby the outer edge cools faster than the core.

Fully embedded piles, restrained along their outer edges can suffer both externally and internally restrained cracking. Soil, being a good insulator, increases the peak curing temperature of the concrete but reduces the temperature gradient across the concrete section. Internally restrained cracking is dependent on temperature gradient and is therefore reduced in piles whilst externally restrained cracking is governed by peak temperature rise. Externally restrained cracking is also dependent on the soil adhesion and is therefore likely to be more marked in granular soils or stiff clays. In all cases, except at the pile head where additional restraint is provided, thermal cracks occur across rather than along the length of the pile. Externally restrained cracking penetrates through the entire concrete section whilst internally restrained cracks are localised at the outer edges.

Concerns relating to thermal cracking are based on structural integrity and durability. The durability aspects are discussed in Section 5.2.7 below.

The structural integrity of a pile suffering cracks across its section needs to be assured if the pile is subjected to lateral forces at the point of cracking or if the crack is sufficiently near vertical to reduce the axial capacity of the pile. Thermal cracking in piles is critically dependent on the concrete mix design as discussed in detail in CIRIA Report 91. Where externally restrained cracking is expected to occur, it is necessary to ensure that the pile remains serviceable after cracking. One possible method of achieving this would be to provide longitudinal reinforcement for a sufficient length of pile over which lateral stresses exceeded the bearing capacity of the soil. The use of factored soil strength parameters in the calculation would ensure that a reasonable safety factor was achieved. Alternatively, various pile lengths could be analysed to simulate cracking at different depths.

Reinforcement would then be provided to the depth at which it was shown that a crack would not affect the ultimate or serviceability limit state performance of the pile. More research, however, is required into this aspect of pile design before recommendations can be made.

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5. 2. 7 Corrosion and Durability

Durability and corrosion of steel reinforcement in concrete structures has long been a concern. Corrosion of steel below ground has, however, been observed principally in buried metal pipes and for, instance, the upper parts of steel piles in marine conditions just below the mud line.

Extensive research has been undertaken into the corrosion of steel below ground and it is generally accepted that, except in extreme exposure situations such as chemically aggressive ground, buried steel below a standing water table is not subject to any significant corrosive activity. Debate continues, however, regarding the corrosivity of soil above the water table and the degree of protection afforded to steel reinforcement by a cracked concrete section.

Corrosion of steel reinforcement underground relies on a current being set up between the soil and the metal. One area of steel forms the anode (negative potential) and another the cathode (positive potential). The electrical current causes metal ions at the anode to be lost to the electrolyte causing corrosion at that point. As part of this process, the metal ions lost at the anode travel to the cathode to complete the electrical circuit. These ions then combine with oxygen and are re-deposited at the cathode.

For electrolytic corrosion to be continuous, a bare metal face must be constantly exposed at the anode, oxygen must be readily available at the cathode and an electrolyte must be present to carry the current. If the environment surrounding the anode is alkaline, oxidised solids, hydroxides or basic salts can be formed and deposited on the metal at the anode inhibiting the corrosion process.

For the general case of reinforcement within a fully embedded pile, ready access to oxygen is restricted to perhaps the upper metre or so from the ground surface through shrinkage cracks, worm holes etc. A cathodic region can only exist in these upper layers where oxygen is present. An anode may be formed below ground when cracked concrete exposes bare metal. The further the anode is from the cathode area, the longer the path that the ions must follow and the slower the rate of corrosion. Unless the concrete has been heavily carbonated, conditions around the reinforcement remain strongly alkaline and protecting solids are deposited at the anode. Corrosion below about lm below ground is therefore likely to be initially slow and, once started, quickly stopped by the deposition of solids.

Heavy corrosion can therefore only occur where there is one of the following:

a) rapid flow of oxygen or carbon dioxide rich groundwater

b) highly acidic groundwater

c) heavy carbonation of the concrete.

Protection of reinforcement in piles from corrosion under all conditions is best achieved by good initial site investigation and the provision of dense, durable concrete. Control of crack widths in concrete, even in corrosive environments, as shown by Beeby (1978), has little effect on the corrosion of reinforcement.

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Where highly corrosive environments are identified such as highly acidic groundwaters, sulphuric ground or rapidly flowing oxygenated groundwater, protection may be best achieved using concrete resistant to carbonation attack. In addition, a protective cover to the reinforcement or protection with a sleeve of a non corrosive or sacrificial material may be considered.

Crack control steel, where not required to prevent thermal cracking (See Section 5.4.6) appears to be merely cosmetic in its function producing a crazing of tight, closely spaced cracks not visible except on close inspection. The tightness of the cracks may also inhibit the leakage of unsightly rust stains onto the concrete surface. Such considerations are rarely of significance in foundation design.

5 .2. 8 Nominal Reinforcement

Reinforced concrete columns in air under compressive loading are required by BS 8110 and BS 5400 Pt 4 to contain nominal reinforcement even when design loads indicate no reinforcement requirement. Nominal reinforcement takes the form of minimum numbers and diameters of longitudinal and transverse bars with maximum allowable spacings (see Table 3.1.1).

The requirements for the provision of nominal reinforcement are somewhat empirical but appear to be based on the following.

a) As a safety measure to contain the core of columns

b) To prevent buckling during a fire

c) To prevent catastropic shear failure from strong impacts such as collisions and earthquakes

d) To cater for unforeseen lateral and vertical loads

Nominal transverse reinforcement is also provided to:

a) maintain longitudinal bars straight and in position until concrete has set

b) strengthen columns where they could otherwise conceivably buckle

For fully embedded piles, most of the above are inapplicable. Fire is not an issue below ground except in exceptional circumstances (ie spontaneous combustion of domestic waste or colliery spoil). Catastrophic shear failure of a pile is not generally critical to the safety of a structure. Unforeseen lateral loads are rarely applicable to piles and embedded columns rarely fail in buckling. Restraint of the core of a column is demonstrated in Appendix 3 to be of minor relevance in most situations.

Of the remaining reasons for providing nominal reinforcement, design for earthquake forces is a specialised subject for which reinforcement is specifically provided. Only provision of lateral ties to provide a rigid cage for handling and installation remains as a

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valid argument for nominal reinforcement. It is interesting to note that ACI report 543R-74 contains no requirements for nominal reinforcement.

Unless, therefore, there is some overriding concern, the provision of nominal reinforcement other than for reasons of handling and insertion, seems unnecessary and overly conservative for fully embedded piles.

5 .2. 9 Curtailment of Reinforcement

An economic pile design will ensure that the minimum reinforcement is provided to resist the applied load and that this is curtailed as quickly as possible.

For most piled foundations, maximum bending and shear forces will occur close to ground level. Current understanding of shear failure suggests that shear links should be provided wherever the applied shear stress is greater than about half the design shear strength of the concrete. Clarke and Birjandi (1993) has suggested that the full concrete strength may be allowed: however further research is required before this is adopted.

Longitudinal steel to resist bending and lateral loads should be continued until no tensile stresses are present in the concrete section. This may most easily be done by resolving the applied bending moment at any section into the applied vertical load on the pile acting at an eccentricity from the centre of the pile. If this eccentricity is less than 118 of the pile diameter, no tension can exist in the pile section and reinforcement may be stopped. Sudden curtailment of all longitudinal steel may, however, encourage a horizontal crack at that level. As given in ACI Report 543R-74, no more than two bars should be stopped off at a particular depth and a 1m overlap, say, should be provided before curtailment of the next pair of bars.

5. 3 Design Relating to Free Standing Lengths of Piles

The free standing length of a pile refers to any portion which projects above ground level and is therefore not subject to support and protection by the surrounding ground. For the purposes of this report it is also taken to refer to the upper parts of piles which are submerged under water, for instance jetty piles above the level of the sea bed.

Free standing lengths of piles are mentioned, for example, in BS 8004 (clauses 7.3.3.3 and 7.3.3.4) and the American ACI Committee report 543.

BS 8004 requires that the upper part of the pile be designed as a column in accordance with BS 8110 or CP114 and that the length ·over which this applies extends beneath ground level down to the point of contraflexure. This is said to vary from between lm below ground level in firm soil to approximately 3m in soft soil. CIRIA Report 103 (Elson, 1984) is also referred to for design of laterally loaded piles and free standing lengths.

The ACI code gives a simple formula for reducing allowable design loads for a laterally supported pile to account for the free standing section.

Sensibly, both the above consider the pile to behave as an unsupported column only above the point of fixity of the pile. Where complex soil interaction analysis models are

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not applicable to a particular pile design and the soil profile is simple, the point of fixity can be determined from a formula relating pile stiffness to pile head fixity and modulus of subgrade reaction (k) in a similar manner to the ACI code (see Appendix 4). If typical k values are given, these should be conservative to ensure no accidental overstressing of the pile.

5. 4 Design Relating to Piled Retaining Wails

Piled retaining walls in many ways form a hybrid structure between a superstructure and a foundation. They also have many aspects unique to themselves. For the purposes of this report it has been convenient to divide the wall into two parts:

i) the lower part of the wall, fully embedded below ground level

ii) the upper part projecting upwards from the base of the retained section

Each of the key issues discussed above for fully embedded piles are considered below for a piled retaining wall bearing in mind the dual nature of the wall.

5. 4.1 Concrete strength and stiffness

For the fully embedded portion of the wall, the comments in Section 5.2.1 regarding concrete strength and stiffness above may be applied to piled retaining walls.

The upper part of the wall in many ways is affected similarly to the lower part, but concrete strength and stiffness is generally governed by requirements to limit deflections of the wall or to resist prop forces. Adjustments to steel quantities due to changes in concrete design are therefore usually insignificant when compared to the overall design requirements.

5 .4. 2 Steel reinforcement strength

See Section 5 .2.2 above.

5.4. 3 Design for Bending

Bending on a piled retaining wall is usually largely generated by lateral forces from the retained soil. The interaction between the structure and. the ~oil is therefore more critical than when designing fully embedded axially loaded piles. Steel quantities required to resist bending will be governed by the assumptions made for soil parameters, the analysis method and the soil/structure interaction model used for the design. These issues are covered elsewhere. However, recent experience on the Jubilee Line extension suggests that many designers are overly conservative in their design requirements, requiring at rest earth pressures to be considered for reinforcement design.

Designers are, rightly, asking for reinforcement to resist long term serviceability loads which in many cases are the most critical for retaining walls. Consideration must be given, however, to the response of the soil to wall moments. It can be envisaged that in

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stiff, overconsolidated soil, earth pressures in the long term may return to their at rest (Ko) values due to softening, swelling and creep of the clay. For granular soils, however, Ko conditions are unlikely to re-establish themselves since only a small movement of the wall will return earth pressures to the active (K.) case. In this case, the use of a partial safety factor on the friction angle of the soil may be sufficient to account for any uncertainty in the long term K.. value. Other soils such as soft clays or silts may have long term pressures intermediate between K.. and Ko conditions. The choice of value used will have a significant effect on the required steel reinforcement.

5.4.4 Design for Shear

Similar comments to the above relate to design for shear in piled retaining walls. Comments given in Section 5.2.4 also apply. The upper part of the retaining wall may, however, be subject to some cyclic loading such as thermal effects, impact and braking forces etc. The additional extra capacity requirements of BS 5400 may, therefore, be applicable. In certain cases, however, where the shear force is distributed along the wall, as a result, for instance, of earth pressure rather than prop forces, concrete has an increased shear capacity, as demonstrated by Clarke and Birjandi(1993). This suggests that, for these cases, the increased safety factor of 1.15 in BS 5400 is perhaps unnecessary. Further researchs needed to provide firm data for development of design procedures.

5 .4. 5 Design for Buckling

Buckling is important only when a retaining wall is subjected to an axial load such as in a bridge abutment. Where this is the case, the upper part of the wall should be treated as a column in air and designed in accordance with BS 5400 Part 4. The lower portion may be considered as a fully embedded pile.

5 .4. 6 Thermal cracking

Comments in Section 5.2.6 on thermal cracking apply equally to piled retaining walls. The consequence, however, of a horizontal crack in the piles may be much more serious due to the high lateral forces to be resisted. No recorded instance of failure of a wall due to thermal cracking has, however, been identified. This may be due to the general practice of overdesigning such piles as if they were columns in air even below ground level.

Further research is needed into the formation of these cracks below ground, their depth, location and conditions under which they form. Until the completion of such research it is difficult to provide guidance on the correct design approach to shear in these walls. However, due to the large lateral forces to be resisted by these piles, checks using piles of various lengths to simulate cracks at different depths using a finite element or other soil/structure interaction model appears to be the only sensible method of ensuring a safe design for this condition. Alternatively the full requirements of BD 28/87 "Early Thermal Cracking" (DMRB 1.3.2) may be employed.

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5. 4. 7 Corrosion and Durability

The discussion on corrosion and durability in Section 5.2.7 applies equally to piled retaining walls. There are, however, significant differences in the geometry of a retaining wall which directly influence the rate of corrosion. Principal among these are the exposure of one or both faces of the upper part of the wall to oxygen and the seepage of water around the wall system.

Oxygen can access either side of the retaining wall depending on the design of the drainage system behind the wall and the facing units in front. Conditions may therefore exist which allow the onset of corrosion once cracking has occurred. Corrosion can only

. continue, however, if deposited solids at the anodic and cathodic regions are removed or prevented from forming. This may most easily be envisaged where deflection of the pile results in a crack parallel to longitudinal reinforcement or where thermal effects result in cracks along transverse reinforcement. Under these conditions, sufficient area of reinforcement may be exposed to prevent chemical deposits from the corrosion process from inhibiting further corrosion. Sufficient water may also be able to penetrate and pond within the crack to aid the corrosion process.

Seepage of water around the retaining wall may also provide a ready source of oxygen and carbon dioxide for corrosion. The flow of water may also be sufficient to prevent the build up of a protective layer. Conditions can therefore exist which would allow corrosion to continue.

It has been demonstrated that the width of a crack perpendicular to reinforcement has little effect on the rate of corrosion. The major factors appear to be the corrosivity of the environment, the flexure of the pile, the resistance of the concrete to carbonation and the cover to the reinforcement. In the absence of alternative design methods for controlling corrosion, it may be prudent to follow the recommendations of Beeby (1978) and to provide;

i) a minimum cover of 3 times the bar diameter, ii) a dense durable concrete.

In severe conditions a protective coating to the reinforcement or skewing of the piles may be necessary.

5 .4. 8 Nominal Reinforcement

For the purposes of nominal·reinforcement it would ·seem sensible to follow existing codes for the upper portion of the pile wall but take account of the comments in Section 5.2.8 above for the fully embedded portion.

5 .4. 9 Curtailment of Steel

The comments regarding curtailment given in Section 5.2.9 for fully embedded piles apply equally to piled retaining walls except the distribution of forces will be modified by the geometry, strutting etc.

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6.0 SUMMARY AND RECOMMENDATIONS

This study has provided a review of the fundamentals of pile reinforcement design. It has also reviewed design and construction practice and the relationship of these to design code requirements. Conclusions have been drawn regarding reinforcement design which are now summarised. Recommendations are given later in this section for amendments to codes and areas requiring further study.

6.1 Summary

6.1.1 Fully embedded piles

Many factors over the years have contributed to an increase in steel reinforcement for bridge foundations and pile retaining walls. Such factors include:-

i) Increased use of vertical piles for resisting lateral loads.

ii) The increased use of computer design software for modelling structural systems leading to higher design forces on foundations.

iii) An increased concern for corrosion of reinforcement leading to the use of crack control steel.

iv) Increased use of quality assurance and checking procedures and increasing fear of litigation.

v) An increase in professional indemnity premiums leading to more reliance on structural codes for reinforcement design.

Only (i) above, however, provides a sensible argument for increasing the quantities of steel reinforcement. It has also been shown that existing UK design codes do not provide an adequate or coherent method of design for many aspects of pile reinforcement.

Treating a pile as a column in air in accordance with BS 5400 part 4 orBS 8110 generally leads to another conservative design. Conventional structural theory can, however, be used to design a pile as a column provided the supporting effect of the ground is considered when calculating forces applied to the pile. The overall structural strength of the soil/pile system should also be taken into account. "Piles designed in this manner may have significantly less reinforcement than those complying with BS 8110 or BS 5400 yet will still perform satisfactorily.

For a design method to approach reality, a computer analysis is required which models both the pile and the soil reactions. Such analyses may use p-y curves or finite elements or, where applicable, elastic continuum or non-linear spring models. Except where the ground is sufficiently soft that buckling is a possible failure mechanism, nominal reinforcement need not be provided except where it is needed for stability during insertion of the reinforcing cage. Where buckling is important, nominal reinforcement may be required.

30

Design for shear in a fully embedded pile should similarly take account of the supporting effect of the ground and the distributed loading effects. Some concrete strength should therefore be allowed when determining shear reinforcement and providing some redundancy exists in the design layout of the piles, no additional shear capacity need be catered for in the provisions of reinforcement. Except where the pile acts as a column in air or in the upper exposed parts of retaining walls, nominal shear reinforcement need not be applied to pile design.

Corrosion of reinforcement in a fully embedded pile is unlikely to be significant below about 1m depth even where reinforcement is fully exposed to the ground. Even where corrosion may be significant, such as in highly acidic (pH < 4.5) groundwaters, or where groundwaters containing high levels of oxygen or carbon dioxide continuously flow past the pile, crack control steel is of dubious use. Of more importance in controlling corrosion is provision of a dense, well compacted concrete, resistant to carbonation and with adequate cover to the reinforcement. In this respect crack control steel may in fact be counter productive since it can interfere with the placement and compaction of the concrete.

Reinforcement should always be kept to a minimum. Where it can be demonstrated that no tension exists in the pile section and that applied stresses are less than those permissible, reinforcement need not be provided. Where curtailment of steel is required, it should be gradual to prevent the formation of a plane of weakness in the pile.

The effects of early thermal cracking should be considered particularly for laterally loaded piles. In these situations, reinforcement should be provided for a sufficient length to ensure that the pile performs satisfactorily under ultimate and limit state conditions. It is considered adequate to curtail this reinforcement at the point at which the applied horizontal stresses equal the bearing capacity of the soil calculated using factored shear strength parameters.

6.1.2 Free Standing Lengths of Piles and Pile Retaining Walls

In contrast to fully embedded piles, free standing lengths of piles and the exposed portions of pile retaining walls approximate in varying degrees to structural columns. Design according to structural codes BS 5400 Pt 4 and BS 8110 is therefore generally applicable. Reductions in reinforcement quantities may still be made by ignoring crack width requirements in relation to corrosion and exposure conditions. As previously mentioned, these have little effect on durability.

Below the point of fixity the pile may be treated as fully embedded for the design of reinforcement.

6.2 Recommendations

The results of this study suggest that there is a need to provide clearer guidance on the design and specification of reinforcement in piles. General comments are made on the use of British Standard BS 8004 with suggestions for updating or clarifying certain

31

areas. Detailed amendments are also suggested for the BD 32/88 (DMRB 2.1) and BA 25/88 (DMRB 2.1) which deal specifically with piled foundations.

BS 8004:1986 Foundations

From consideration of the comments made in Section 3.1 there appear to be a number of areas where BS 8004 could be amended to provide clearer and more complete guidance on pile reinforcement. In particular, better guidance could be provided on the following:

i) Depth to fixity of a free standing pile.

ii) Conditions under which steel reinforcement may be omitted/curtailed.

iii) Minimum concrete strengths. (Those given in BS 8004 do not appear compatible with BS 8110 orBS 5400).

iv) Calculation method for shear capacity of piles.

v) Corrosion of reinforcement in buried concrete.

vi) Effects of early thermal cracking on pile design.

BD 32/88 Piled Foundations IDMRB 2.1)

As an alternative to changes to BS8004, guidance could be implemented by amendments to BD 32/88 (DMRB 2.1) as follows. These amendments would need to be accompanied by a revision to BA25/88 (DMRB 2.1) (referred to as BA25/88).

After clause 3.1 (a) of BD32/88 insert "with the following modifications", (Then insert the following:)

Bored and Driven Cast in Place Piles

A. General

i) Loads, moments and forces acting on piled foundations shall be calculated using a suitable method which adequately models the supporting effect of the surrounding ground. Guidance on available methods is given in BA 25/88*.

ii) Reinforcement of concrete piles need only be provided where tension exists in the concrete section. Where tension is generated by lateral loadings or applied moments, the moment within the pile may be resolved into the axial load on the pile acting at an eccentricity. Where the eccentricity of loading is less than 118 of the diameter of the pile, no tension exists in the section and no reinforcement need be provided.

iii) Where reinforcement is required, it should be designed to resist the applied forces and reactions of the supported structure and surrounding ground.

32

Nominal reinforcement is not required other than for the purposes of ensuring a rigid cage during handling and insertion.

iv) Design charts such as those in CPllO parts 2 and 3 may be used for design of symmetrically reinforced rectangular or circular sections subject to bending moments with appropriate modifications for the value of ym. Alternatively the methods for short columns given in BD 44/90 (DMRB 3.4.4) may be used to calculate reinforcement requirements.

v) Design for shear in piles should follow the recommendations for shear in columns given in BP 44/90 (DMRB 3.4.4).

vi) Buckling of a pile need only be considered where L/1' < 11(2)'11

where Lis the length of pile in soft soil and I' is the length of half sine wave deflection of the pile generated by the buckling load and described in BA 25/88*. Where buckling is a consideration, provision of additional links may be calculated as follows.

vii) Where piles are subjected to lateral forces, a check should be made on the possible effects of a horizontal crack at depth. In the absence of more sophisticated modelling techniques, longitudinal reinforcement should be provided at least to a depth at which the bearing capacity of the surrounding ground exceeds the lateral applied stresses. Factored soil shear parameters should be used in the bearing capacity calculation.

viii) Except where otherwise indicated by the site investigation, fully embedded piles below a depth of about lm may be considered to be within a non aggressive environment with respect to steel reinforcement. Examples of where corrosive underground environments may exist are given in BA 25/88*.

xi) Regardless of the exposure environment, reinforcement solely for controlling crack widths need not be provided.

x) Curtailment of longitudinal reinforcement should occur gradually to prevent a plane of weakness developing in the pile. No more than two bars should be stopped off at one level and a minimum distance of lm provided between subsequent curtailments.

B. Free Standing Lengths of Piles

i) The free standing length of pile shall be taken as that section of pile extending above the point of fixity. The point of fixity may be determined in accordance with the procedures set out in BA 25/88*.

ii) Except for the modification provided for in (iii) below, structural design of free standing concrete piles shall be designed as columns in accordance with BS 5400 pt 4.

33

iii) Except where requirements other than the control of corrosion dictate, reinforcement solely for controlling crack widths need not be provided.

C. Pile Retaining Walls

i) Pile retaining walls should be designed as free standing piles over their retained height down to the point of fixity as given in B(i). Below this level piles should be designed as fully embedded.

ii) Durability of the full length of the pile should be carefully considered taking account of any likelihood of seepages being set up around the pile system. Provided the pile is embedded in clay soils, seepage is likely to be slow and corrosion of steel insignificant. For granular soils however, seepage may be rapid and the corrosive environment may approach severe conditions.

iii) Where shear forces are distributed along the pile length rather than concentrated at a point, the allowable shear strength of the pile may be increased by 15% above that provided for in BS 5400 Pt 4.

iv) Buckling of the upper part of the pile is accommodated within the overall design of the pile. The lower, fully embedded portion of the pile should be checked as for B(iii).

v) The effects of early thermal cracking in pile retaining walls should be checked in accordance with BD 28/87 (DMRB 1.3 .2) for the upper part of each pile. Effects of a horizontal crack through the fully embedded portion of the pile should be checked for using a suitable soil! structure interaction model as described in BA 25/88*.

6. 3 Areas for further research

During this study it has become clear that certain aspects of the behaviour of embedded piles require further investigation. These are identified below.

i) The formation, orientation and extent of early thermal cracking in concrete fully and partially embedded in the ground.

ii) The distribution of shear forces along the length of a fully or partially embedded pile or retaining -wall and the effect of the ground in modifying the failure mode of the pile.

iii) The effect of computer aided design of concrete structures and foundations on calculated design forces and the relevance of existing partial load and material factors for these design cases.

iv) The long term development of lateral earth pressures.

34

It is also clear that although this study deals specifically with piles and therefore piled retaining walls, many of the issues raised relate equally to diaphragm wall and basement constructions. It would be useful, therefore, to extend the scope of this study to include these structures.

35

7.0 ACKNOWLEDGEMENTS

The work described in this report fonns part of the research programme of the Civil Engineering Resource Centre at TRL. The Project Officer at TRL was Mr P Darley and the work is published by pennission of the Chief Executive.

Grateful thanks are due to all the various contributors and respondents who significantly helped with the production of this report. Particular thanks go to Dr W G K Fleming and Mr R Fernie (Cementation Piling & Foundations), Mr W P Raies (Trafalgar House Technology), Mr C Raison (Keller Foundations), Mr D Headman (Bachy Group), Mr J Barr (Rendel Geotechnics), Mr A Powderham and Mr J Robb (Mott MacDonald Ltd) and Mr D Nicholson (Ove Arup & Partners). Thanks are also due to the respective companies of the above for cooperation in providing resources for this project.

8.0 REFERENCES

BANERJEE, PJ and DAVIES TG. Analysis of pile groups embedded in Gibson soil. Proc. 9th Int. Conf. Soil Mech. and Fnd. Eng., Vol. 1, Tokyo 1977.

BARTHOLOMEW, RF. The protection of concrete piles in aggressive ground conditions. Conf. on Recent Developments in the Design and Construction of Piles, ICE, London 1979.

BEEBY, A W. Corrosion of reinforcing steel in concrete and its relation to cracking. The Structural Engineer, Vol. 56A, No. 3, 1978.

BOOTH, GH, COOPER A W, COOPER PM, and W AKERLEY DS. Criteria of soil aggressiveness towards buried metals. British Corrosion Journal, Vol. 2, 1967.

BROMS, B. The lateral resistance of piles in cohesive soils. J. Soil Mech. Div. ASCE, V89, No. SM2, 1964a.

BROMS, B. The lateral resistance of piles in cohesionless soils. J. Soil Mech. Div. ASCE, V90, No. SM3, 1964b.

BURLAND, JB, POTTS DM, and WALSH NM. The overall stability of free and propped embedded cantilever retaining walls. Ground Engineering 1981.

CAQUOT, A. and KERISEL, J. Tables for the calculation of passive pressure, active pressure and the learning capacity of foundations. Gauthier, Villay, Paris 1948.

CLARK, JL and FK BIRJANDI. The behaviour of Reinforced Concrete Circular Section in Shear. The Structural Engineer, Vol. 71 No. 5/2, 1993.

CIRIA Report PG2. Review of problems associated with the construction of cast-in-place concrete piles. Thorburn and Thorburn JQ. 1977.

CIRIA Report 63. Rationalisation of safety and serviceability factors in structural codes 1977.

CIRIA Report 91. Early-age thermal crack control in concrete. Harrison TA. 1981.

CIRIA Report 103. Design of Laterally Loaded Piles. Elson WK. 1985.

CIRIA Technical Note 14. An experimental investigation into the effects of shear and tension on the flexural behaviour of reinforced concrete beams. Lownds P. and Parnell FN 1971.

CIRIA Technical Note 36. Elimination of Shrinkage and Thermal Cracking in a water retaining structure, Hughes BP. 1971.

CIRIA Technical Note 109. The Performance of a piled bridge abutment at Newhaven. Reddaway ALand Elson WK. 1982.

C & CA Technical Report 559. The Effects of autogenous healing upon the leakage of water through cracks in concrete. Clear CA Cement and Concrete Association 1985.

DAVIDSON MT and ROBINSON KE. Bending and Buckling of Partially Embedded Piles, Proc 6th Int. Conf. Soil. Mech. and Found. Eng. Montreal V 2 : 1965.

FLEMING, WGK and ENGLAND, MG. Some recent insights into Foundation Behaviour. Ground Board ofinst. Civ. Eng. Informal Discussion, July 1993.

FLEMING, WELTMAN, RANDOLPH and ELSON; Piling Engineering. Surrey University Press 1985.

FRANCIS, AJ, SAVOURY NR, STEVENS LK, and TROLLOP£ DH. The Behaviour of Slender Point-Bearing Piles in Soft Soil. Proc. Univ. Hong Kong Golden Jubilee Congress 1962.

GOURLEY, JT and BIENIAK DT. Diffusion of Chloride into Reinforced Concrete Piles. Symp. on Concrete Perth 1983.

HAMBLY, EC and BURLAND JB. Bridge Foundations and Substructures. Building Research Establishment 1979.

JOEN, PH and PARK R. Flexural Strength and Ductility Analysis of Spirally Reinforced Prestressed Concrete piles. PCI Journal Aug. 1990.

KRAMER, SL and HEAVY EJ. Analysis of laterally loaded piles with non-linear bending behaviour. Transport Research Record 1169.

LEEK, DS. The Passivity of Steel in Concrete. QJEG V24, 1991.

Manual of Contract Documents for Highway Works Vols 1 to 6. Department of Transport 1992.

NORTH-LEWIS JP and SCOTT ID. Constructional Control affecting the behaviour of piles with particular reference to small diameter bored cast in situ piles. ICE Conference, Behaviour of Piles, 1970.

Piling Handbook, First Edition, British Steel Corporation 1963.

Recommendations for an International Code of Practice for Reinforced Concrete ACI and CCA.

REESE, LC, COX WR and KOOP KD. Analysis of laterally Loaded Piles in Sand. Offshore Technology Conference, Dallas, Texas, 1974.

REESE, LC, COX WR and KOOP KD. Field Testing and Analysis of Laterally Loaded Piles in Stiff Clay. Offshore Technology Conference. Dallas, Texas 1975.

RANDOLPH, MF. The response of flexible piles to lateral loading Geotechnique V31 No.2 1981.

ROMANOFF, M. Corrosion of Steel Pilings in Soils. J. of Research of National Bureau of Standards V66C No. 3 1962.

ROMANOFF, M. Underground Corrosion. Nat Bureau of Standards Circular579, 1957.

ROWE, PW. Sheet pile walls in clay. Proc-Int. Civ. Eng. V7 1957.

SASTRY, VVRN and MEYERHOF GG. Behaviour of flexible piles under inclined loads. Can Geotech. J. V27 1990.

Specification for Piling, Contract Documentation and Measurement, ICE 1988.

TERZAGHI, K. Evaluation of coefficients of subgrade reaction. Geotechnique, V5 No.4, 1955.

TOMLINSON, MJ. Pile Design and Construction Practice. First edition, Palladian Publications 1977.

TRL Research Report 359. Design of Embedded retaining walls in stiff clay. Symons I. F. Tranport Research Laboratory 1992.

TRL Project Report 23. Behaviour of a propped contiguous bored pile wall in stiff clay at Rayleight Weir. Darley P, Carder D.R. and Alderman G.H. Transport Research Laboratory 1994.

TRL Project Report 113. Advice on integrity testing of piles Turner M. J. 1994.

Transport Research Record 1211. Concrete Bridge Design and Maintenance : Steel Corrosion in Concrete. Transport Research Board, National Research Council 1989.

WATSON, GVR and CARDER, DR. Comparison of the measured and computed performance of a propped bored pile retaining wall at Walthamstow. Proc-Inst. Civ. Eng. Geotech. Eng., V107, 127-133, 1994.

WOOD, JH and PHILLIPS, MH. Lateral stiffness of Bridge Foundations: Load Tests on Newmans Bridge. Structures Committee Road Research Unit. National Roads Board. Report No. ST 87/2 1987.

Design of' ReinforceMent In Piles

DQ tQ CollQ tion

I I I

Published UK Design Non UK Design end LlterQture Codes Design Construction

Codes Prcctlce

I I I I I I I

LibrQry DQtQbQSe Current· Superceded ConsultQnts• ContrQctors: I SeQrches SeQrches Design Design Mott MQcDonQic:l BQChy

Codes Codes Ove Arup CeMentQ tlon Piling Qnd f" ounciQ tlons

I Rende! _peotechnlc Keller

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Design Issues

I I I I I

Bending of' Sheer In Buckling of' CrQck Durcblllty Piles Piles Piles Control In of' Piles

Piles -~--- ------- -- - ---- ---- ------ ~-

tlgure 1• Flow ChQrt Showing DQ tc ColiQ tlon Procedure.

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Appendix 1 : Data Sources for Research Study

A1.1 Published Literature

Design Codes

The following current and withdrawn design codes were examined.

UK CP2:1951

CP4:1954

CP101:1972

CP110:1972

CP114:1957

CP114:1957

BS8002: 1994

BS8004: 1986

BS8110: 1985

BS 5328:1991

BS 5493: 1977

BE 1173: 1973

BA 24/87:1987 (DMRB 1.3.2)

Earth Retaining Structures (Withdrawn)

Foundations (Withdrawn)

Foundations and Substructures for non industrial buildings of not more than four storeys (Withdrawn)

Parts 1 & 2 Foundations and substructures for non industrial buildings of not more than four storeys (Withdrawn)

The structural use of Concrete (Withdrawn)

Reinforced Concrete in Buildings (Withdrawn) Code of Practice for design of concrete Bridges Part 4.

Earth Retaining Structures

Foundations

The Structural use of Concrete Parts 1 to 3

Concrete. Guide to Specifying Concrete Part 1

Protective Coating of Iron and Steel Structures Against Corrosion

Technical Memorandum (Bridges; Reinforced Concrete for Highway Structures) (Withdrawn)

Department Advice Note; Early Thermal Cracking in Concrete

UK BD 28/87:1987 Department Standard; Backfilled Retaining (cont) (DMRB 1.3.2) Walls and Bridge Abutments

BD 30/87:1987 Departmental Standard; Backfilled Retaining (DMRB 2.1) Walls and Bridge Abutments

BA 25/88:1988 Departmental Advice Note; Piled Foundations (DMRB 2.1)

BD 32/88:1988 Departmental Standard; Piled Foundations (DMRB 2.1)

BD 24/92:1992 Departmental Standard; Design of Concrete (DMRB 1.3.1) Highway Bridges and Structures

BD 42/94 Design of Embedded Retaining Walls and (DMRB 2.1) Bridge Abutments (Unpropped or Propped at

the Top

Europe Eurocode 2 Design of Concrete Structures. Concrete ENV 1992-1-1:1992 Bridges.

ENV 1994-1-1:1992 Design of Composite Steel and Concrete Structures

ENV 1997-1 Geotechnical Design

Germany DIN 1945:1988 Structural use of Concrete. Design & Construction

DIN 4014:1990 Bored cast-in place piles

DIN 4128:1983 Small diameter injection piles

DIN 4026:1975 Driven Piles

France Pll-212,DTU 13.2 1992 Deep Foundations for Buildings

A05-251: 1990 Ground Corrosion

Holland NEN 7053 Concrete Piles

U.S.A. AC1 318-83 Building Code Requirements for reinforced concrete

AC 1 318-1 : 1992 Building Code Requirements for Structural plain concrete

AC1 336.1-79 Standard Specification for the construction of end bearing drilled piers

ACI 336-3R-72:1972 Suggested Design & Construction Procedures for Pier Foundations

ACI 543R-74:1974 Recommendations for Design, Manufacture and Installation of Concrete Piles (Reaffirmed 1980)

NAVFACDM-7.2:1982 Foundations and Earth Structures Design Manual, Department of the Navy

UK Design Publications

The following UK design reports were considered as part of this research study.

UK CIRIA

CIRIA

CIRIA

Early-age Thermal Crack Control in Concrete (Report No. 91: 1981)

Design of Laterally Loaded Piles (Report No. 193:1984)

Review of problems associated with the construction of cast-in-place concrete piles

In addition to the above, the following draft design code was consulted:

British Standards

BS 8110 Part 1

Draft Amendment No. 5

Appendix 1 cont/d : Data Sources for Research Study

Al.2 Practicing Companies

Consultation of External Organisations

A consultation process was set up with a number of UK based contractors and consultants. This attempted to obtain a broad view of current design and construction practices operating in the UK today.

Due to the limited time period available for the study, the consultation was restricted to three consultants and three contractors. The choice of each company was essentially arbitrary but was intended to encompass a cross section of companies involved in pile design.

The consultation involved a two stage process. Firstly the companies were approached for their agreement to participate and then sent an initial questionnaire canvassing their views. The questionnaire asked for general comments on past and current practices employed by the company in its day to day pile design work. Following receipt of the various replies to the questionnaire, all responses were compiled, summarised and subsequently re-circulated to the respondents for further comments.

Companies invited to contribute to study:

Consultants

1.

2.

3.

Mott MacDonald Group St Anne House 20-26 Wellesley Road Croydon CR92UL

Ove Arup & Partners 13 Fitzroy Street London SE1 1SA

Pendel Palmer and Tritton Ltd 61 Southwark Street London SE1 1SA

Contractors

Bachy Ltd Foundation Court Godalming Business Centre Cattleshall Lane Godalming Surrey GU71XW

Cementation Piling & Foundations Ltd Maple Cross House Denham Way Maple Cross Rickmansworth Herts WD3 2SW

Keller Foundations Oxford Road Ryton on Dunsmore Coventry CV83EG

Appendix 2 : Buckling Resistance of Fully Embedded Piles

(After Francis et al, 1965)

1. Buckling load, of a pile, Per, can be found from:

2 2 .fer = (n + P In ) PE

where n = number of half sine waves caused by buckling load in pile

PE = Buckling load of pin ended strut in air.

PE = 1t 2

E11e

p = (LII/

L = Length of pile

I' = Length of half sine wave If I. is the effective length of the pile considered as a pin-ended strut

ie Per = 1t 2 EIII/

Then

1/(lell') = 1/(L/n.l'/ + (L/n.l'/

Equation (2) is plotted as Fig. A.2.2.

From Fig. A.3.2 it is found that:

for Lll' < 11( 2 )'/: ie L < 0.71.1'

for L/l' > 112 ie L > 0.71.1'

it can be assumed that the soil offers no support to the pile and le = L

it can be assumed that 1. = I' I ( 2 ) •;, and Per is twice the buckling load of a pin-ended column in air.

and Per = (Elk)'/:

1' is governed by the soil properties and may be given by

1' = ( r(EI/k) '-' (uniform soil) or

I' = (2 n 4

Ellk) 115

(soil stiffness proportional to depth

k = coefficient of lateral displacement of soil and may be taken as

k = 8nE{l-J.!)/(3-4J.!){l +}.!)(1 +2 log.(2llb))

where I = length and b = breadth of pile.

2. Example

2.1 For a typical uniform soft soil of undrained shear stren9th, Cu = 10kN/m2

The modulus of elasticity Es = 500 x Cu = 5000 kN/m, J.! = 0.4 and a 15m long, 0.5m diameter pile

K = 8 n 5000 {1-0.4) = 4186 kN/m3

(3-4x0.4)(1 +0.4)(1 +2 log. 2x15/0.5)

I' = (1t 4

EI/4186)'-' = 6.5m (E = 25 x 106 kN/m2,1=n D4/64)

L > 0.711' therefore Per = (EIK)'h

= 118 MN which is equivalent to an applied stress of 600 MN/m2

which is well in excess of the 28 day concrete characteristic strength of, say 40 MN/m

2

2.2 Axial loading of the pile can be considered by dividing 1. by

(1 - PIPer),, where P is the axial load.

In the above example,

ifP =50 MN, Per= 118 MN

and (1 - PIPer),, = 0. 76

1. = 1'/(2),, = 6.5/(2)'11 = 4.6m

Adjusted 1. = 4.6/0.76 = 6.0m.

but 0.71.1. = 4.26

which is still < L ( = 15m) so

Per remains 118 MN.

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Appendix 3 : Example method of calculating spacing of links to prevent local buckling of embedded pile.

1. Consider a 450mm diameter pile with four 32mm diameter longitudinal reinforcing bars carrying an axial load of 400 kN.

Stress in each bar = (400x103/4) I (n x 3il4) = 124 N/mm

2

To prevent buckling of the bar, a lateral restraining force of 2.5% of the axial load on the bar is required.

For a 1m length of bar, required lateral stress to resist buckling

= 124 X 103 X 0.032 X 2 X 0.025

200kN

Total earth pressure required on pile = 200/0.45 = 445 kN/m2

Assuming K.o = 1.0 andy = 20 kN/m3

, the depth at which a lateral earth pressure of 445 kN/m

2 is achieved is

445/20 =22m

At 1m depth, the lateral soil pressure is 1 x 20 = 20 kN/m2

Force on pile = 20 x 0.45 = 9 kN

Force required from lateral ties = 200-9 = 191 kN

From BS 8110 cl 3.4.5.6 eq" 4

Vb = Ab (0.87 fy.-) (cos a + sina cot p ((d-d')/Sb)

for a = 90° and p = 90°

vb = Ab (0.87 fy.-) x ((d-d')/Sb)

If Vb = 164 kN/m2

, d = 450mm, d' = 75mm

Then Sb = .A.b {0.87 fvv) X 0.375) 191

= (0.03i X 7t )/4 X 0.87 X 460 X 103 X 0.375

191

= 0.625m

This is the maximum spacing of links to prevent local buckling at lm depth ignoring the contribution of the concrete.

Nominal link requirements of BS 8110 are for maximum spacings of 12 x d = 12 x 0.032 = 0.38m.

Provision of nominal links below about 1m is evidently an overconservative design.

Above 1m, flexure of the pile may remove any lateral support to the pile and nominal links could therefore be considered.

Appendix 4 : Method of calculation of depth to fixity of free standing length of pile

(After ACI 543R-74)

The structural length (L) of an unsupported pile is defined as the length between points of fixity or between hinged ends. For a pile fixed at some depth (L) below ground level, the structural length would be equal to the length of pile above ground (L) plus the depth (L).

L can be estimated as follows

For overconsolidated clays

L = 1.4S where S = (EI/k)14

For granular soils and normally consolidated clays and silts

L = 1.8T where T = (EI/Ilhf'5

k is the modulus of horizontal subgrade reaction for overconsolidated clay. As a guide this may be taken as 67 times the undrained shear strength of the clay (see Fleming, Weltman, Randolph and Elson 1985 for guidance on k values).

Ilh is the modulus of horizontal subgrade reaction for granular soils and normally consolidated clays and silts. For clays this may be taken as 67 times the undrained shear strength divided by the depth averaged over the top 3m or 5m (ie it is the slope of the k vs depth plot for the upper layers of the soft soil).

Ilh values for other soils are given in table below :

Soil Type Ilh

Sand and inorganic silt Loose 1.5 Medium 10 Dense 30

Organic Silt 0.4 to 3.0

Peat 0.2

If the embedded length L < 4S or 4T then this analysis is not valid and analyses such as those presented by Broms (1964a and 1964b) for short piles should be undertaken. Alternatively, more detailed p-y curve or finite element analyses may be performed to obtain the point of fixity.

' i ,_

J