The,Transport Research Laboratory is the largest and most comprehensive centre for the study of roadtransport in the United Kingdom. For more than 60 years it has provided information that has helpedframe transport policy, set standards and save lives.

TRL provides research-based technical help which enables its Government Customers to set standardsfor highway and vehicle design, formulate policies on road safety, transport and the environment, andencourage good traffic engineering practice.

As a national research laboratory TRL has developedclose working links with many other internationaltransport centres.

It also sells its services to other customers inthe UK and overseas, providing fundamental and appliedresearch, working as a contractor, consultant or providing facilities and staff. TWS customers includelocal and regional authorities, major civil engineering contractors, transport consultants, industry, foreigngovernments and international aid agencies.

TRL employs around 300 technical specialists - among them mathematicians, physicists, psychologists,engineers, geologists, computer experts, statisticians - most of whom are based at Crowthorne, Berkshire.Facilities include a state of the art driving simulator, a new indoor impact test facility, a 3.8krn test track,a separate self-contained road network, a structures hall, an indoor facility that can dynamically testroads and advanced computer prc)gramswhich are used to develop sophisticated traffic control systems.

TRL also has a facility in Scotland, based in Livingston, near Edinburgh, that looks after the specialneeds of road transport in Scotland. ,,

The laboratory’s primary objective is to carry out commissioned research, investigations, studies andtests to the highest levels of quali~, reliability and impartiality. TRL carries out its work in such a wayas to ensure that customers receive results that not only meet the project specification or requirement butare also geared to rapid and effective implementation. In doing this, TRL recognises the need of thecustomer to be able to generate maximum value from the investment it has placed with the laboratory.

TRL covers all major aspects of road transport, and is able to offer a wide range of expertise ranging fromdetailed specialist analysis to complex multi-disciplinary programmed and from basic research to advancedconsultancy.

TRL with its breadth of expertise and facilities can provide customers with a research and consultancycapability matched to the complex problems arising across the whole transport field. Mess such assafety, congestion, environment and the’infrastructure require a multi-disciplinary approach and TRL isideally structured to deliver effective solutions.

TRL prides itself on its record for delivering projects that meet customers’ quality, delivery and costtargets. The laboratory has, however, instigated a programme of continuous improvement and continuallyreviews customers satisfaction to ensure that its performance stays in line with the increasing expectationsof its customers.

Quality control systems have been introduced across all major areas of TRL activity and TRL is workingtowards full compliance with BS EN 9001:1994.

.

TRANSPORT RESEARCH LABORATORYAn Executive Agency of the Department of Transport

TRL REPORT 144

DESIGN OF REINFORCEMENT ~ PILES

by J P ~son (~afalgar House Twhnology Limited)

This report describes work commissioned by the Bridges Engineering Division of theHighways Agency under E553C~G, Reinforcement in Piles @esk Study)

Crown Copyright 195. me contentsoftisreportare tieresponsibility oftie authors mdthe ChiefExecutiveof~.~ey do not necessarily represent tie views or policies of the Department of Transport.

Transport Research LaboratoryOld Wokingham Road

Crowthorne, Berkshire,RG456AU

1995ISSN 0968-4107

Highways Agency

St Christopher HouseSouthwark Street, London SE1 OTE

.... ... . . . .. . . . . .. .. . . . . . . . ...... .... .. . ------- ------- -

CONTENTS

ABSTRACT

1.1 Scow of Study1.2 R=rh Strategy

2.0 DATA COW~ON

2.1 General2.2 Data SO-

3.1 current UK codes3.2 Historic/SuPrcedti S~/Codes3.3 Non UK Standards/Codes

5.0 DISCUSSION

5.1 Changing Design & Cons@ction Practice5.2 Pfle Reinfoument Design

5.2.1 Concrete Strength & Stiffness5.2.2 Steel Reinforcement Strength5.2.3 Design for Bending5.2.4 Design for Shear5.2.5 Design for BucMing5.2.6 Early Thermal Cractig5.2.7 Corrosion and Durability5.2.8 Nominal Reinforcement5.2.9 Curtaihnent of Reinforcement

i

PAGE

1

2

2

23

3

34

4

41012

13

16

1619192020212222232526

CONTENTS (cent’d)

PAGE

5.3 Design Relating to Free StandingLen- of Pfles

5.4 Design Relating to Pfied Retaining Wtis5.4.1 Concrete Strength and Stiffness5.4.2 Steel Reinforcement Strength5.4.3 Design for Bending5.4.4 Design of Shear5.4.5 Design for Buctiing5.4.6 Thermal Cracking5.4.7 Corrosion and Durability5.4.8 Nominal Reinforcement5.4.9 Curtailment of Steel

6.1 summary6.1.1 Fully Embedded Piles6.1.2 Free Standing Lengths of Piles&

Pile Retaining Walls6.2 Recommendations6.3 _ for Further Study

7.0 A~OWLEDGEMENTS

8.0 REFERENC~

APPENDIC~

26

27272727282828282929

30

3030

313134

Appendk 1- Data Sources for R~h StudyAppendk 2- BucHing Resistance of FuUy Embedded PflesAppendti 3- _ple Method of Calculating Spachg of Links to Prevent

H BucMg of Embedded PfieAppendk 4- Crdculation of Depth to Pie Ffiv of Free Standing Len@ of

Ptie

ii

. . .. . . . . .... .. .. .. ......

Feedback obtained from two construction sites in the ~ has suggested that currentdesign practices for pile reinforcement may be overconservative.

This report investigates the development of the design of reinforcement in piles andassesses the applicability of current design codes to pile design. It also givesrecommendations for amendments to the Standard BD 32/88 (DMW 2.1) for piledfoundations and suggestions for clari~ing existing British Standard requirements. Areasfor further research are hig~ighted.

It is shown that developments in the understanding of concrete, steel and structuraldesign, together with the development of geotechnics have led to an increase in the useof vertical piles to resist lateral loads. This in turn has resulted in a parallel requirementfor increased steel quantities to resist lateral forces. Computer design techniques havealso developed rapidly, allowing the effects of temporary loadings and deflections to beincorporated 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 pilereinforcement design, consideration must be given to factors unique to the pilingsituation. In particular, the supporting effect of the surrounding ground and protectionprovided against corrosion are significant factors in determining reinforcementrequirements.

For fully embedded piles nomiml requirements for links, minimum numbers of bars andcrack control steel can be ignored.

Crack control steel need ody be applied to the control of early thermal cracking andthen ody if this is required to ensure the serviceability of the pile. Some evidencesuggests that crack control steel may not be effective in reducing long term corrosion ofsteel. Dense concrete, resistant to carbomtion, should be used with external sleeving orsteel 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 ody down to a point of fmity below groundlevel. A method for determining the point of fixity is suggested.

1

., ,.

ABS~CT

The quantity of reinforcement installed in concrete piles appears to have increasedsignificantly over the years. Recent case histories have suggested that overlyconservative designs may be generated when current design standards are applied. Thisreport considers the historical development of the design of reinforcement in piles andreviews the reasons behind the increase in pile reinforcement quantities. The report alsoresearches the concepts underlying current design requirements and their applicability topile design.

Advice is given on the implementation of existing British Standards andrecommendations given for amendments to the Standard for piled foundations BD 32/88(DMRB 2. 1).

1.1 scow of Study

In July 1994, the Transport Research hboratory (TW) commissioned Trafalgar HouseTechnology to undertake a desk study into the design of reinforcement in piles. Thespecific requirements were to identify reasons for the increase in pile reinforcement inrecent years and to establish whether the present high levels of reinforcement arejustified.

The catilyst for this work is feedback from two completed projects. The first was anunpublished study, commissioned by the DOT, into the design of the Holmesdale andBell Common Tunnel retaining walls. This reviewed various methods of deriving thelateral forces applied to the walls and considered the implications for quantities ofreinforcement. For the diaphragm walls of Hohesdale tunnel, one of the findings wasthat the application of crack control criteria significantly increased the steelreinforcement requirements.

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

This study researches the current and historic methods of the design of the reinforcementin 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 tothe pile and to provide for a durable pile. me derivation of such forces, however, is notincluded within this study. Pre-cast concrete piles are excluded as the reinforcement forthese is generally controlled by the handling and insertion forces and not the in-serviceforces.

2

1.2 Research Strate~

To consider as many aspects of pile reinforcement design as possible in the timeavailable, a research strategy was devised which incorporated a review of current andsuperseded design codes, published literature and consultation with externalorganisations.

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

2.0 DATA COW~ON

2.1 Gened

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

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

o Changing design and construction practiceo Concrete strength and E valueso Steel reinforcement strength and E valueso Design for bendingo Design for shearo Design for buctiingo Thermal crackingo Corrosion and durabilityo Nomiml reinforcemento Curtailment of steel at depth

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

External consultees were as follows:-

Consultants Contractors

Mott MacDonald Group Bachy LimitedOve Amp and Partners Cementation PilingRendel Palmer and Tritton Ltd Foundations Ltd

Keller Foundations

&

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

2.2 Data So-s

Design codes and published literature were generally obtained using standard librarydatabase searches. Some unpublished data was obtained from ~ and externalconsultees.

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

At present, specific references in ~ codes to the design of reinforcement in piles islimited to two documenfi, BS 8004:1986 “Foundations” and BD 32/88 “PiledFoundations” (DM~ 2. 1). This latter document is mandatory on DOT jobs ordy. Asummary 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 columnsufless part of the pile extends above ground level (Cl 7.3.3.3). For this lattercase, ody the upper portion of the pile need be considered as a column down to apoint of fwity. Para 2 Cl 7.3.3.3 staks:-

“where part of the finished pile projects above ground, that length should bedesigned as a column in accordance with BS 8110, CP114 or BS 449. Theeffective length to be tien in the calculation is dependent on the lateral loadingif any and on the degree of fmity provided by the ground, by the structure whichthe pile supports and by any bracing. The depth below the ground surface to thepoint of contraflexure varies with the type of soil. In fm ground it maybetien as about lm below the ground surface; in we~ ground, such as sofi clayor silt, it may be as much as one half of the depth of-penetration into the stratumbut not necessarily more than 3m. The degree of fuity, the position andinclination of the pile top and the restraint supplied by any bracing should beestimated as in normal structural calculations”.

4

(b)

(c)

(d)

(e)

(0

ii)

(a)

(b)

All forces acting on the pile are to be determined and the pile reinforcedaccordingly (Cl 7.3.3.4, Cl 7.3.3.6, and Cl 7.4.4.3.2). Some or all of the pilelength may be unreinforced (Cl 7.4.4.3.2). Pre-cast concrete piles are to bedesigned to BS 8110 (or CP 116).

Where tensile forces are to be resisted by the pile, adequate reinforcement isrequired to resist the entire tension stresses. The reinforcement should beprovided for the full length of the pile or where temile forces are small, to adepth at which the tensile forces have been filly transmitted to the ground (Cl7.3.3.7 and Cl 7.4.5.3.2).

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

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

Durability and protection of reinforcement against corrosion is provided by denseimpermeable concrete free from defects (Cl 10.4. 7). Nomiml cover for variousexposure conditions should be as BS 8110.

Standard BD 32/88: 1988 “Piled Foundations” (DM~ 2. 1).

This standard applies to both the design of driven and bored piles (Cl 2. 1) and ismandatory on all DOT projects.

Pile caps are to be designed to BS 8004 but Cl 3.1 states that the structural designof 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 carryingaxial load ordy need not be reinforced. Some rules are provided regarding calculation ofaxial forces which may be accommodated without reinforcement but no guidance isgiven on calculation of shear capacity. Where reinforcement is required, BS 8110 (andCP116 and CP1 14) is mentioned for design but is not specifically invoked, except forpre-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 stringentdesign standard than the general civil engineering concrete code BS 8110. Additiomlforces are imposed on a bridge structure such as impact and braking forces and abutmentearth pressures. Also the often exposed and relatively long and flexible mture ofbridges leads to high wind and themal expansion forces. The difficulties in deterrnitingthe magnitude of these forces and their effect on the structure has required a moreconservative design approach which is reflected in the bridge code.

The requirements of BS 8004, BS 54Q Pt 4 and BS 8110 are summarised in Table:3.1.1and3.1 .2.

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

5

Table 3.1.l summaq of Code Re~irements

u

Code/Standard Cover to Longitudinal Reinforcement Transverse Reinforcement

(Date )Crack Control

Reinforcement Min Min. Max Min Max Min Max Crack WidthNo. Dia. Spacing % Spacing Diameter

BS 8110 7Smm 4 Tension: 12 times

Pt 1 (Concrete cast‘~ times Only checked if

(Rectangular) 0.0% smallest largest main N C 0.2fm.&

(1985) against ground) 6 12mm (mild main bar bar size or

(Circular)Then max. width

(steel) size 6mm = 0.3mm0.45%(highyield)

~BS 5400 45MM 4 1% or 12 times ~htimesPt 1 (Buried c30

0.25mm(Rectangular) 0.15 N~ smallest main largest main (Buried concrete)

(1990) Concrete) 6 12mm 300mm bar or 0.74

(Circular)

bar size O.lmm iftimes effective groundwaterdepth pH C 4.5

BS 8004 As BS811O ---- AS BS 8110 ----- ----- - ------As BS8110-- -----”--------but add 40mm

Not mentioned

fourconcretecast against

ground I

4

Table 3.1.1 cent/d Summag of Code Re~irements

L

Zode/Standard Permissible Stre9ses Maximum Axial Load[Date) Concrete Steel Without Reinforcement With Reinforcement

)s 8110 Compression

at 1 0.67 fc./ym fy/ym

[1985)

Shear (Tension & 0.4 fc..b o.4.fc..& + o.75.hc.fyand compression)

Vc + 0.6.NVh-h.M

3s 5400 Comfiression Compression

Pt 4

(1990) 0.67.f.U/Ym fy/ (ym+fy/2ooo) 0.4 fc..b.dc 0.4.fcu.b-.dc+ fw.A’!l + f,l.h~

Shear Tension

o.5.fm fy/ym ~

(Triangular

StressDistribution)

o.38.fm

(UniformStress

Distrubtion)

IS 8004 A9 ES 8110 As BS 8110 As BS 8110 As BS 8110

(1986)

k - area of concrete d, - depth of concrete in compression

A’,1

M - applied design moment

- area of compression fa - characteristic concrete cube strength

&2 - area of reinf. in

N - applied design axial loadf12 - stress in reinf. in other face v - applied design shear

other face fy - characteristic strength of reinforcement Vc - desiqn concrete shear stress

b - area of vertical ‘ h - overall depth of section y. - partial safety factor for strenqh of

reinf.‘b -

materialwidth of section

Table 3.1.2 S~ of tient ~ Design Codes

Global Reinforcement BD 32/88Design Requirements BS8004 BS811O (BS5400 Pt 4)

oAxial Compression No guidance Detailed DetailedRefer to design design rulesBS 8110 rules for for

structural structuraldesign design(See (See TableTable 3.1.1) 3.1.1)

o Axial Tension General II IIguidance givenon length ofpile to bereinforced

o Flexure No guidance It 11

Refer toBS 8110

0 Shear No guidance II IIRefer toBS 8110

0 BucNing Check for No checks No checksBuc~ing if required requiredC. C 20kN/m’

Rules for:

o Minimum No minimum II IIReinforcement reinforcement

required

o Links Minimumspacing 150mm “ 1!

) Calculation of Refer to II IIRebar BS811O. Ordy

upper part ofpile aboveground levelto be designedas a column

8

. ..

Table 3.1.2 cent/d Summqof Current ~Design Cties

BD 32188Global Reinforcement BS8004 BS811O (BS5400 Pt 4)Design Requirements

o Cover As BS811O 75mm 45rnm maxplus 4ornrn +40rnm in

(Concrete accordance witicast against BS8004 Cl 2.4.5ground)

o Crack Control Not Ody O.lrnm tomentioned required 0.25mm

for dependingN< O.2f.” on exposure

conditions

o Durability Some Detailed Detailedguidance reqmts requirementsgiven for for concrete

concrete design anddesign nominal coverand tonominal reinforcementcover toreinforcement

9

specifically with the design of embedded retiining walls and bridge abutments. There isa 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 (BS5400 Pt 4 or BS 8110).

3.2 Historic/Superseded Standards/Codes

Historically, there are very few codes relating specifically to design of piles. The otiydocuments dealing with this subject are British Standards documents CP2: 1951 “EarthRetaiting Structures”, CP4: 1954 “Foundations” and CP101: 1972” Foundations andSubstructures 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 codeswhich specifically dealt with it, and therefore the general design standards for reinforcedconcrete were used instead. These included CP1 14:1948 “Reinforced Concrete forBuildings”, CP1 10:1972 “The Structural use on Concrete” and BE1/73: 1973“Reinforced Concrete for Highway Structures”. A summary of the various requirementsof 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 forcesacting on piles and therefore the required reinforcement was based on key referencedocuments, such as:-

0

0

0

0

0

0

0

0

0

Caquot and.Kerisel (1948)“Tables for tie calculation of passive pressure, active pressure and bearingcapacity of foundations”.

Temaghi (1955)“Evaluation of coefficients of subgrade reaction”.

Rowe (1957)“Sheet Pile Walls in Clay”.

British Steel Piling Handbook (1963)

Broms (1964)“The Uteral Resistance of Piles in Cohesive Soils”

Tomlinson (1977)“Pile Design and Construction Practice”

Harnbly (1979)“Bridge Foundations and Substructures”

Randolph (1981) “The response of flexible piles to lateral loading”.

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

10

Table 3.2.l Summaq of Superseded Codes Re~irements

:ode/Standard

(Date) Minimum Cover Minimum Concrete

to Reinforcement (mm) Strength (Nmm2) Concrete (N/mm2) ●High Yield Steel (N/mm2) Notes

PERMISSIBLE STRESSES

ZP114

(1948)

3E1/73

38

40

25.0 Direct 5.50 Tension 190 Limit stateBending 10.70 Compression 143 design notShear 0.70 introduced

22,5 Direct 5.70 Tension 230 Limit state(1973) Bending 7.50 Compression 175 design not

Shear 0.72 introduced

I ~TI~TE LIMIT STATE

:Pllo

(1972) 40 20.0 Direct 8.90 Tension 400 Limit stateBending 8.90 Compression 333 designShear 0.60 introduced

● Mild Steel Grade is also allowed.

,. .,..

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

3.3 Non ~ Stan@ds/Codes

A review of codes from Europe and America was undertaken to look at current designrequirements outside the ~.

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

The following non-~ codes are discussed below in relation to pile design: DIN 4014,ACI 336.3 R-72, ACI 318:1992, ACI 318.19992, ACI Committee Report 543, ENV1992.

i) DIN 4014 “Bored Piles”

Reinforcement in piles is designed to structural Code DIN 1045. Piles over 0.5mdiameter need not be reinforced tiess required for structural reasons. Piles lessthan 0.5 m diameter may be unreinforced if there is no structural requirementand 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 “lPierFoundations”

This document deals specifically with bored concrete piles over 0.76m indiameter. Design of plain concrete piers (piles) are to ACI 318.1 and reinforcedconcrete to ACI 318, both structural codes. Where the soil SPT N value exceeds2, sufficient lateral support is provided by the soil to prevent buctiing of the pile.

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

Design of piles is specifically excluded and reference made to ACI Committeereport 543 “Recommendations for Design, Manufacture and Installation ofConcrete Piles”.

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

Unreinforced concrete piles continuously supported by soil are dealt within ACI318.1 provided compression occurs across the entire cross section under allloading conditions. The tensile strength of the concrete is allowed in designproviding structural failure is not induced by uncontrolled cracting.

A minimum concrete strength of 2500 psi (17.5 N/mm*) is specified forunreinforced concrete. Shear in the concrete for any section shape is calculatedusing a simple formula. When calculating stresses, the cross section of theconcrete is reduced by 2” (50rnrn) for concrete cast against soil.

12

. . .-. ,..

v) The ACICommittee Report 543(1973) “RecommendationsforDesign, Manufacture and Installation of Concrete Piles”

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

hteral support of ground, lateral capacities of piles, uplift, tension and shearstresses, 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 ACI543R-74 are summarised in Table 3.3.1

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

For a conventional design situation (non DMRB), a consulting engineer generallyproduces a design which is then put out to tender to various Contractors. Using BS8004, the engineer is not required to design the pile as a column in free air and insteadmust, to a large extent, select elements of the existing structural codes which areappropriate to complete the design. Alternatively a specialist piling contractor may berequested to take responsibility for the pile design. In either case, there is significantscope for variations to occur in the quantities of the fiml pile reinforcement dependingupon, 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 externaldesign office is used for checking the design.

Since the issue of BD 32/88 (DMRB 2.1), it is mandatory in highway projects for thestructural design of the concrete elements of piles to be designed in accordance with BS5400 Pt 4. Current design practice therefore requires piles to be treated as columns inair for the purposes of reinforcement design.

It is now normal practice in large civil engineering schemes to incorporate the fullrequirements of the structural codes when determining the reinforcement requirementsfor 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 bemade for nomiml reinforcement links, mtium numbers of bars, maximum barspacings and minimum bar diameters. Checks for allowable crack widths are alsorequired. Many of these may be inapplicable to pile reinforcement.

13

Table 3.3.1 Summary of hnt Non-m Design Codes

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

(R$~:~ (1992)Revised

o Axial Compression No guidance Refer to Detailed Detailedspecifically structural guidance guidancefor rebar code

o Axial Tension II VI

o Flexure Methods of II It

determininglateral loadsbut noguidance onrebar

o Shear Not mentioned II Notmentioned

o BucNing Ordy Otiyconsidered consideredif C. ifN > 2< 15 ~/m2 (N=SPT N

value)

Rules for:

o Minimum General Refer to No NoReinforcement comments Struct. reqmts reqmts

Ody code

o Li~ Minimum “ II To bediameter & providedmaximum wherespacing loadsgiven indicate

a reqmt

14

Table 3.3.1 cent/d S~ of ~nt Non-m Design

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

(R~~jf 1995)

o Calculation of Limited It II Refer toRebar comment structural

Refer DIN code.1045

0 Cover 7omm fl 40-75mrn

o Crack Control Not II 11 Notmentioned mentioned

o Durabili~ Concrete 1! 1! Litiewith high guidance.chemical Aggressiveresistance environmentsrequired requireif ground protectiveis coatings,aggressive sleeving or

cathodicprotection

15

IDuring construction, reinforcement cages are assembled in accordance with the detaildrawings. For bored piles ordriven cast-in-place piles, tiereMorcement istiefiedinto 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 difficultiesin ensuring a satisfactory construction of the pile. For concrete pumped around thereinforcement, it may be difficult to provide proper compaction around the steel inheavily reinforced piles which may result in defects in the pile. Where reinforcement isinserted through concrete, it may not be possible to achieve sufficient penetration of thesteel. Excessive vibration of the cage to aid penetration may damage the cage and causesegregation of the concrete.

IIt is therefore necessary for the designer to speci~ the minimum reinforcement to satisfythe structural requirements of the pile. If this is not done, erstwhile economical pilingtechniques may be excluded through the specification of excessive reinforcement.

I 5.0 DISCUSSION

5.1 Changing Design and Co-ction hctim

Over many years structural reinforced concrete design and concrete foundation designhas undergone a continuing development. Advances have been made in theunderstanding of the behaviour of materials, mechanisms of failure, magnitude of forcesapplied to structures and methods of ultimate and serviceability limit state design. Thishas resulted in a greater knowledge of the nature and interaction of materials and forces.Developments in structural and foundation analyses have continued along essentiallyparallel but often separate paths. Superstructures including in many cases pile caps andbasement constructions, have been designed in accordance with the relevant structuralcodes 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 cotildence, the forces, deflections and reactions generated by and applied tothe structure. Over the same period, the science of geotechnics developed significantly,making it possible to determine, at least theoretically, the response of the groundsurrounding the foundatio]~. hndmark publications giving methods of deriving thelateral resistance of piles (Broms, 19@) and coefficients of subgrade reaction (Temghi1955) were significant in moving pde design forward. Techniques therefore becameavailable for estimating the bending moments and shear forces which a laterally loadedpile must resist.

This led to a significant change in the way piles were used. bteral forces on bridgedecks, for instance, were historically resisted by raking piles, passive pressure on thepile caps or abutment keys. As the theoretical understanding and analytical toolsadvanced it became more popular to resist lateral forces on vertical piles in flexure,albeit with increased steel reinforcement. Publication of the BSC Steel DesignersHandbook (1963) greatiy aided the engineer’s tisk in assigning steel reinforcementquantities for piled retiining structures. Over the same period, the introduction of morerapid concrete piling tec~ques, (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~rther encouraging the use of vertical piles to resist lateral loads.

The advances in tec~ques and reduced costs also led to an increased use of concretepiles in retaining wall applications. Where, prior to the early 1970’s, the majority ofcantilever retaining walls would be designed using steel sheet piling, it is now commonfor concrete piles to be a viable alternative. The larger cross-sections and thereforegreater stiffnesses achievable with concrete has also meant that larger and deeperexcavations in difficult ground can be completed using concrete retaining walls withminimum prop requirements. Such uses obviously generate higher pile shaft forceswhich must be resisted by increased quantities of steel reinforcement.

It has therefore become necessary for designers to develop procedures for designing pilereinforcement to resist the induced pile forces. As previously stated, foundation designcodes offered otiy limited guidance and the designer was forced to turn to structuralcodes. The applicability of the structural codes to the design of fully embedded pileshas, however, been a subject for debate amongst designers. For example, it isquestiomble whether a filly embedded pile should be provided with hoops or links inaccordance with the column codes. This issue was the subject of an adjudication on theQEII bridge, a DOT project. The pile designer argued that the surrounding soilsprovided sufficient restraint to prevent bar buctiing, and detailed widely spaced hoopsfor cage rigidity ody. The checker called for hoops in accordance with BS 5400 (Part4, Clause 5.8.4.3). The level of curtaihent of the main axial reinforcement was also asubject of adjudication.

The adjudication concluded that BS 5400 does not address itself to pile design and BS8004 ody addressed piles as a structural member when in free air. It was alsoconsidered that “the subject of the design and specification of steel reinforcement cagesfor bored cast-in-place piles is not addressed adequately in current Codes of Practice”.Furthermore that it is therefore necessary to rely on traditioml practice for pile designand the experience of specialist piling contractors in relation to their particular types ofpiles. Steel reinforcement cages could be required in the upper portions of pile shafts toresist flexural stresses from lateral loads or eccentricities of loads, but the remainder ofpile shafts, subjected ody to axial compressive stresses, may be unreinforced orprovided with nominal cages designed primarily to resist handling and insertion forces.

During the adjudication period BD 32/88 (DM~ 2. 1) was issued and calls for fullcompliance with BS 5400 Pt 4, including :

0 nomiml links throughout the length of the main reinforcemento minimum vertical steel bar numbers, and maximum and minimum bar spacingso 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 includea greater undersmnding of the effects of lateral earth pressures, increased use of designsoftware and the routine use of pile integrity testing techniques.

17

r“” ““”. ........ ... ..

I

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

The recent increased availability of complex and comprehensive structural andgeotechnical 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 beeasily 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 designwill be over conservative. In the past, soil/structure interaction effects would have beenignored due to the complexity of the analyses or lack of design tools. bwer calculateddesign forces may have been compensated for by the use of overall safety factors priorto CP1 10:1972. Existing partial safe~ factors are based on CINA Report No. 63(1977), into safety and serviceability factors in structural codes and were introduced toachieve roughly similar effects to the previous overall factors but allow greaterflexibility in design. There may now, however, be a case for reviewing the partial loadand material safety factors used in current design to allow for the increasedsophistication of the design process.

I

For example, designers have recently become more aware, through the use of computerprograms, of the sensitivity of lateral forces to the design model chosen and parameters,particularly geotechnical hput into the model. Sensitivity analyses are thereforefrequently run and designers may then perhaps use the more conservative analyses intheir final design.

The routine use of non destructive integrity testing of piles has lead to the discovery thatmany piles suffer significant cracking after installation. This has lead to concerns overdurability and a desire to limit crack widths. Checks for crack widths as required in BS5400 can have significant effects on reinforcement quantities. Examples for embeddedretaining walls are found h cases such as the A406 North Circular contiguous piledwalls and the Holmesdale Tunnel diaphragm walls. Although both these involvereinforcement design, where checking for crack widths may be applicable (see Section5.4), steel requirements are often applied over the full length of the wall. This wascertairdy the case for the A406 piles where provision of crack control steel was thegoverning design critera. Crack width checks are also often specified for fillyembedded piles.

At Holmesdale, the ground comprised up to 6m of Terrace Gravels overlying another6m of the basement beds of the bndon Clay with Woolwich and Reading Beds below.The tunnel structure was formed by reinforced concrete diaphragm walls. The roofseined to prop the walls and form a cut and cover tunnel. A study was made into theeffects of designing the diaphragm retaining walls using C~A report 104, BS 5400 Pt 4and the then draft Standard BD 42 (DMRB 2.1). Under certain conditions, it was foundthat 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 couldbe made if cracking were discounted.

18

BS 8110 has a slightly less onerous crack width requirement than BS 5400 and istherefore less severe in its requirements for additioml steel. Further discussion on therelevance of structural codes including crack control requirements for buried reinforcedconcrete may be found in Section 5.2 of this report.

Relatively recently, developments affecting the wider field of civil engineering have alsoaffected the design of piles. An increase in professioml liability, larger and morefrequent claims for negligence and the introduction of widespread internal mandatorychecking procedures under BS 5750, or similar quality assurance systems have allaffected engineers’ attitudes to design. The increased threat of litigation has meant thatcompanies, both consultants and contractors, are less willing to amend design coderequirements to fit their needs and since there is no comprehensive code dealing withpile design, it is often easiest to invoke a structural code as a basis for design to speedboth internal QA and external checking.

5.2 Pfie Retiorcement Design

The design of pile reinforcement has been discussed in Section 5.1 in terms of existingand historic design practice. The following discussion, however, deals with the designof 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 designhighlighted.

These key issues are listed below:

o00000000

Concrete strength and stiffnessSteel reinforcement strengthDesign for bendingDesign for shearDesign for buc~ingThermal crackingCorrosion and durabilityNominal reinforcementCurtailment 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 Conc~te Strength& S-s

The concrete used to construct a pile obviously has a profound effect on pile capacityand the forces attracted to it. Concrete design for foundations is a subject in itself and isthe subject of an ongoing T~ study. For pile reinforcement, however, ifi effect canessentially be reduced to two elements, strength and stiffness.

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

19

Over the years, minimum concrete strength for use in foundations has increased (seeFig. 2). This has been driven by a desire for denser, more durable concrete and has alsohad the effect of increasing the load capacity of piles. As a result the pile stiffness hasincreased resulting in an increase in the relative differences between soil and pilestiffnesses. This in turn has increased the magnitude of shear forces and bendingmoments 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 increasein strength directly reduces reinforcement requirements. Stiffness ody plays a rolewhere small groups of piles require design for moments resulting from nominal designeccentricities. For laterally loaded piles and retaining walls, however, stiffness is alsoimportant in the design for providing lateral resistance. Retaining wall design inparticular is largely governed by requirements for stiffness rather than axial loadcarrying capacity.

Increased concrete strength, although allowing greater axial and lateral loads to becarried, also has drawbacks since it produces a greater tendency for tierrnal cracking tooccur. The greater the concrete strength, the higher the curing temperature due toincreased cement content. This leads to higher thermal strains and larger concreteshrinkage. The concrete shrinkage is resisted by the surrounding soil thus generatingtensile 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 thestructural integrity of the pile. A more detailed discussion of this is given in Section5.2.6.

5.2.2 Steel reinfoument 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 incost of high yield steel. This has meant that fewer and smaller diameter bars are used toaccommodate larger bending moments and shear forces allowing for a reduction inoverall steel quantities for axially loaded piles and greater resistance to lateral forces forlaterally loaded piles.

5.2.3 Design for bending

bteral forces when applied to a pile setup bending moments within the pile.Rcentricities of loading also apply moments to the pile. Where these moments exceedthe design bending resistance of the pile, reinforcement is required to strengthen theconcrete section.

Design charts for calculating the required area of steel for a given rectangular concretesection and given bending moment are provided in structural design code BS 8110 Part3. BS 8110 does not provide design charts for circular sections and design for these isofien based on an equivalent rectangular area. BD 44/90 (DMRB 3.4.4) allows circularcolumns to be assessed using the design charts for circular sections given in CP1 10.Dedicated computer design software is also available for design of circular sections.

20

For design of reinforcement to resist bending, therefore, the major issue is not themethod of calculating steel quantities but the method of determining the magnitude anddistribution 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 bythe ground and the pile to resist the load. The bending moment in the pile, will dependon the strength and stiffness of the ground and the stiffness of the pile. A stiff soil willbe able to mobilise greater resistance against the pile resulting in less deflection of thepile for a given load. Bending moments will therefore be large. Soft soil will have thereverse effect with a trade off between decreased bending moment and increaseddeflection.

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

The effect of lateral deflections of the pile can be assessed using dedicated designsoftware using either non linear elastic spring model, p-y curves or ftite elements tomodel the soil response. Soil parameters and factors of safety must be carefully chosentaking account of the type of amlysis to be used. BA 25/88 (DM~ 2.1) provides arecommended method of determining additioml forces on piles from soil movementsrelated to loading of adjacent ground. CIWA Technical Note 109 provides advice onassessing the forces on a laterally loaded pile.

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

5.2.4 Design for Shmr

Until recently, design for shear in circular sections was one of the least defined aspectsof column and therefore pile design. Both BS 8110 and BS 5400 Pt 4 require columnsto be treated as beams for the purposes of shear. This requires that an equivalentrecmngular area be derived from the circular section and the shear capacity determinedaccordingly. BD 44/90 (DM~ 3.4.4) and BA 44/90 (DMM 3.4.4) now specifi adesign method for circular sections based on the ACI code published in 1983 andcotilrrned 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 columnsplus provision of an increase of 0.4 N/mm* 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 safetyfactor and

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

21

., ...- - . .. ... . .. . . . .. . . ..-.

BS 8110 has neither of these requirements.

For piles not subject to repeated loading, such as filly embedded piles and manyretaining walls, Clarke suggests that the additional 15% load factor in BS 5400 appearsoverly conservative. Clarke also suggests that for distributed loading, as applies alongthe length of a pile, shear capacity of a given section is approximately twice that for aconcentrated load and suggested that the addition of 0.4 N/mm* extra capacity thereforeseems unnecessary. Fimlly, Clarke also suggests that the full shear resistance of theconcrete can be taken into account when calculating the necessary reinforcement butrecommends further research to confii this view. Clarke’s findings confii a generalimpression that BS 5400 is overly conservative in its requirements for design for shearparticularly for foundation work.

The case ofien made for allowing for significant conservatism h shear design is thebrittle mode and possible catastrophic consequences of such a failure. Uflike the case ofbeams and columns in air, buried foundations have the support of the ground to modifytheir failure mode. Additionally, considerable redundancy is often incorporated into pilegroup designs such that tie failure of a single pile is not catastrophic to the wholestructure. As an example many elastic computer analysis models of pile groups generatelarge design forces in tie comer piles of a group. The resultant large steel requirementsare therefore, for simplicity, often provided for all piles in the group.

5.2.5 Design for BucMing

hteral restraint of the ground is sufficient in most practical cases to prevent buc~ingfailure of fully embedded piles. When referring to driven piles, BS 8004 requires thatbuctiing need otiy be considered for piles through soil with a shear stren~ less than 20kN/m2. At shear strengths greater than this, buc~ing is said to be u~ikely and pilesneed not be designed in accordance with BS 8110. Where buc~ing is a consideration,the work of Francis et al (1962) is referenced: this describes a series of laboratory andfield tests in Melbourne on long thin steel piles driven into sofi soils.

Hollow, rectangular (110mm x 150mm) piles 28m long were driven through soils withshear strengths between 1 and 16 psi (7 to 110 ~/m2). It was shown that even for theseextreme dimensions failure was due to squashing of the pile and not buc~ing. Testswere also carried out on prestressed octagonal concrete piles 710mm across, 28m long.These were not loaded to fadure but carried more load than the short column failure loadwithout buc~ing.

From this research, Francis concluded that ody for cases where L/l’ < 1/(2)’, shouldconsideration be given to buc~ing of the pile. (L = lengti of pile h Sofi soil, 1’ =

length of half sine wave deflection of pile generated by buc~ing load and described inAppendix 2).

A theory for calculating bucNing resistance was presented by Francis based on theWitier spring system. Where buc~ing is a possible failure mechanism, the Wtierapproach may be used to calculate tie failure load. A summary of tis method is givenin Appendix 2. Structural frame analysis or finite element sotiare may also be used todetermine the resistance to buc~ing.

22

5.2.6 ~ly Them Cractig

Early thermal cracking is a phenomenon well known in concrete design. It occurs as aresult of temperature changes generated in the concrete during the curing process settingup tensile strains and stresses. CIHA Report 91 (1981) produced a detailed report onthe subject as it relates to standard building design and the same principles can beapplied to piles.

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

Fully embedded piles, restrained along their outer edges can suffer both externally andinternally restrained cracking. Soil, being a good insulator, increases the peak curingtemperature of the concrete but reduces the temperature gradient across the concretesection. Internally restrained cracking is dependent on temperature gradient and istherefore reduced in piles whilst externally restrained cracking is governed by peaktemperature rise. Externally restrained cracking is also dependent on the soil adhesionand 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 occuracross rather than along the length of the pile. Externally restrained cracking penetratesthrough the entire concrete section whilst internally restrained cracks are localised at theouter 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 ifthe pile is subjected to lateral forces at the point of cracking or if the crack is sufficientlynear vertical to reduce the axial capacity of the pile. Thermal cracking in piles iscritically dependent on the concrete mix design as discussed in detail in CIWA Report91. Where externally restrained cracking is expected to occur, it is necessary to ensurethat the pile remains serviceable afier cracking. One possible method of achieving thiswould be to provide longitudinal reinforcement for a sufficient length of pile over whichlateral stresses exceeded the bearing capacity of the soil. The use of factored soilstrength parameters in the calculation would ensure that a reasomble safety factor wasachieved. Alternatively, various pile lengths could be amlysed to simulate cracking atdifferent depths.

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

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-“ ““““-”””““’”’““”‘“”“-5.2.7 Corrosion and Durabfiity

Durability and corrosiorl of steel reinforcement in concrete structures has long been aconcern. Corrosion of steel below ground has, however, been observed principally inburied metal pipes and for, instance, the upper parts of steel piles in marine conditionsjust below the mud line.

Extensive resmrch has been undertaken into the corrosion of steel below ground and it isgenerally accepted that, except in extreme exposure situations such as chemicallyaggressive ground, buried steel below a standing water table is not subject to any

significant corrosive activity. Debate continues, however, regarding the corrosivity ofsoil above the water table and the degree of protection afforded to steel reinforcement bya cracked concrete section.

Corrosion of steel reinforcement underground relies on a current being setup betweenthe soil and the metal. C)nearea of steel forms the anode (negative potential) andanother the cathode (positive potential). The electrical current causes metal ions at theanode to be lost to the electrolyte causing corrosion at that point. As part of thisprocess, the metal ions lost at the anode travel to the cathode to complete the electricalcircuit. 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 exposedat the anode, oxygen must be readily available at the cathode and an electrolyte must bepresent to carry the current. If the environment surrounding the anode is akaline,oxidised solids, hydroxides or basic salts can be formed and deposited on the metal atthe anode inhibiting the corrosion process.

For the general case of reinforcement within a fully embedded pile, ready access tooxygen is restricted to perhaps the upper metre or so from the ground surface throughshrinkage cracks, worm holes etc. A cathodic region can ofly exist in these upper layerswhere oxygen is present. An anode may be formed below ground when crackedconcrete exposes bare metil. The further the anode is from the cathode area, the longerthe path that the ions must follow and the slower the rate of corrosion. Udess theconcrete has been heavily carbonated, conditions around the reinforcement remainstrongly alkaline and protecting solids are deposited at the anode. Corrosion belowabout lm below ground is therefore likely to be initially slow and, once started, quic~ystopped by the deposition of solids.

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

a) rapid flow of oxygen or carbon dioxide rich groundwater

b) higMy acidic groundwater

c) heavy carbomtion of the concrete.

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

24

. . ..- .,. . .. . . . . . . . . .- .’.

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

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

I 5.2.8 Noti Reinfomment

Reinforced concrete columns in air under compressive loading are required by BS 8110and BS 5400 Pt 4 to contain nominal reinforcement even when design loads indicate noreinforcement requirement. Nominal reinforcement takes the form of minimum numbersand diameters of longitudiml and transverse bars with maximum allowable spacings (seeTable 3.1.1).

The requirements for the provision of nomiml reinforcement are somewhat empirical butappear to be based on the following.

I a) As a safe~ measure to contain the core of columns

I b) To prevent buc~ing during a fire

I c) To prevent catastrophicshear failure from strong impacts such as collisions andearthquakes

d) To cater for unforeseen lateral and vertical loads

I Nominal transverse reinforcement is also provided to:

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

I b) strengthen columns where they could otherwise conceivably buc~e

For filly embedded piles, most of the above are inapplicable. Fire is not an issue belowground except in exceptioml circumstances (ie spontaneous combustion of domesticwaste or colliery spoil). Catastrophic shear failure of a pile is not generally critical tothe safety of a structure. Unforeseen lateral loads are rarely applicable to piles andembedded columns rarely fail in buc~ing. Restrakt of the core of a column isdemonstrated in Appendix 3 to be of minor relevance in most situations.

IOf the remaining reasons for providing nominal reinforcement, design for earthquakeforces is a specialised subject for which reinforcement is specifically provided. Oflyprovision of lateral ties to provide a rigid cage for handling and installation remains as a

I25

valid argument for nominal reinforcement. It is interesting to note hat ACI report543R-74 contains no requirements for nomiml reinforcement.

Urdess, therefore, there is some overriding concern, the provision of nominalreinforcement other than for reasons of handling and insertion, seems unnecessaryoverly conservative for fully embedded piles.

5.2.9 Mtient of Reinfo~ent

and

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

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

hngitudinal steel to resist bending and lateral loads should be continued until no tensilestresses are present in the concrete section. This may most easily be done by resolvingthe applied bending moment at any section into the applied vertical load on the pileacting at an eccentricity from the centre of the pile. If this eccentricity is less than 1/8of the pile diameter, no tension can exist in the pile section and reinforcement may bestopped. Sudden curtailment of all longitudinal steel may, however, encourage ahorizontal crack at that level. As given in ACI Report 543R-74, no more than two barsshould be stopped off at a particular depth and a lm overlap, say, should be providedbefore curtailment of the next pair of bars.

5.3 Design Relating to F= Standing hn~ of Pties

The free standing length of a pile refers to any portion which projects above gr,oundlevel 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 whichare 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 7.3.3.3and 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 accordancewith BS 8110 or CP114 and that the length over which this applies extends beneathground level down to the point of contraflexure. This is said to vary from between lmbelow ground level in fim soil to approximately 3m in soft soil. CIWA Report 103(Elson, 1984) is also referred to for design of laterally loaded piles and free standinglengths.

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

Sensibly, both the above consider the pile to behave as an unsupported COIU odyabove the point of f~ity of the pile. Where complex soil interaction analysis models are

26

not applicable to a particular pile design and the soil profile is simple, the point of fixitycan be determined from a formula relating pile stiffness to pile head fixity and modulusof subgrade reaction (k) in a similar manner to the ACI code (see Appendix 4). Iftypical k values are given, these should be conservative to efiure no accidentaloverstressing of the pile.

5.4 Design Relating to Pfled Retig WWS

Piled retaining walls in many ways form a hybrid structure between a superstructure anda foundation. They also have many aspects unique to themselves. For the purposes ofthis 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 retainedsection

fich of the key issues discussed above for fully embedded piles are considered belowfor a piled retaining wall bearing in mind the dual mture of the wall.

5.4.1 Concrete s-n~ and stiffness

For the fully embedded portion of the wall, the comments in Section 5.2.1 regardingconcrete 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, butconcrete strength and stiffness is generally governed by requirements to limit deflectionsof the wall or to resist prop forces. Adjustments to steel quantities due to changes inconcrete design are therefore usually insignificant when compared to the overall designrequirements. ..

5.4.2 Steel reinfomment stren~

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 theretained soil. The interaction between the structure and the soil is therefore more criticalthan when designing fully embedded axially loaded piles. Steel quantities required toresist bending will be governed by the assumptions made for soil parameters, theanalysis method and the soil/structure interaction model used for the design. Theseissues are covered elsewhere. However, recent experience on the Jubilee Line extensionsuggests 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 loadswhich in many cases are the most critical for retaining walls. Consideration must begiven, however, to the response of the soil to wall moments. It can be envisaged that in

27

~“-””’’””” ~‘“““””“““-““““““stiff, overconsolidated s~>il,earth pressures in the long term may return to their at rest~) values due to softening, swelling and creep of the clay. For granular soils,however, K conditions are urdikely to re-establish themselves since ordy a smallmovement of the wall will return earth pressures to the active (K) case. In this case, theuse of a partial safety factor on the friction angle of the soil may be sufficient to accountfor any uncertainty in the long term L value. Other soils such as soft clays or silts mayhave long term pressures intermediate between K and h conditions. The choice ofvalue 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 brakingforces etc. The additional extra capacity requirements of BS 5400 may, therefore, beapplicable. In ce~in cases, however, where the shear force is distributed along thewall, as a result, for instince, of earth pressure rather than prop forces, concrete has anincreased shear capacity, as demonstrated by Clarke and Birjandi(1993). This suggeststhat, for these cases, the increased safety factor of 1.15 in BS 5400 is perhapsunnecessary. Further researchs needed to provide firm data for development of designprocedures.

5.4.5 Design for BucMing

IBuckling is important ordy when a retaining wall is subjected to an axial load such as ina bridge abutment. mere this is the case, the upper part of the wall should be treatedas a column in air and designed in accordance with BS 5400 Part 4. The lower portionmay be considered as a fully embedded pile.

I 5.4.6 Theti cracking

Comments in Section 5.2.6 on themal cracking apply equally to piled retaining walls.The consequence, however, of a hotiontal crack in the piles may be much more seriousdue to the high lateral forces to be resisted. No recorded instance of failure of a walldue to thermal cracking has, however, been identified. This may be due to the generalpractice of overdesigning such piles as if they were columns in air even below groundlevel.

Further research is needed into the formation of these cracks below ground, their depth,location and conditions under which they fem. Until the completion of such research itis 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 pilesof various lengths to simulate cracks at different depths using a ftite element or othersoil/structure interaction model appears to be the ody sensible method of ensuring a safedesign for this condition. Alternatively the full requirements of BD 28/87 “~lyThermal Cracking” (DM~ 1.3.2) maybe employed.

28

5.4.7 Corrosion and bbtiity

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

Oxygen can access either side of the retaining wall depending on the design of thedrainage system behind the wall and the facing units in front. Conditions may thereforeexist which allow the onset of corrosion once cracking has occurred. Corrosion can ordycontinue, however, if deposited solids at the anodic and cathodic regions are removed orprevented from forming. This may most easily be envisaged where deflection of the pileresults in a crack parallel to longitudinal reinforcement or where thermal effects result incracks along transverse reinforcement. Under these conditions, sufficient area ofreinforcement may be exposed to prevent chemical deposits from the corrosion processfrom inhibiting further corrosion. Sufficient water may also be able to penetrate andpond within the crack to aid the corrosion process.

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

It has been demonstrated that the width of a crack perpendicular to reinforcement haslittle effect on the rate of corrosion. The major factors appear to be the corrosivity ofthe environment, the flexure of the pile, the resistance of the concrete to carbomtion andthe cover to the reinforcement. In the absence of alternative design methods forcontrolling corrosion, it may be prudent to follow the recornmendatiom 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 pilesmay be necessary.

5.4.8 No- Winforcement

For the purposes of nomiml reinforcement it would seem sensible to follow existingcodes for the upper portion of the pile wall but take account of the comments in Section5.2.8 above for the fully embedded portion.

5.4.9 ~tient of Skl

The comments regarding curtaihnent given in Section 5.2.9 for fully embedded pilesapply equally to piled retiining walls except the distribution of forces will be modifiedby the geometry, strutting etc.

29

. . . . . .. . . . . . . .-. . ... . .. .. ., ... . .

6.0 S~Y AND RECOMMENDAmONS

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

6.1 summary

6.1.1 Fully em~ded pfies

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

i)

ii)

iii)

iv)

v)

Increased use of vertical piles for resisting lateral loads.

The increased use of computer design sotiare for modelling structural systemsleading to higher design forces on foundations.

An increased concern for corrosion of reinforcement leading to the use of crackcontrol steel.

Increased use of quality assurance and checking procedures and increasing fear oflitigation.

An increase in professional indemnity premiums leading to more reliance onstructural codes for reinforcement design.

Ody (i) above, however, provides a sensible argument for increasing the quantities ofsteel reinforcement. It has also been shown that existing W design codes do notprovide an adequate or coherent method of design for many aspects of pilereinforcement.

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

For a design method to approach reality, a computer analysis is required which modelsboth the pile and the soil reactions. Such analyses may use p-y curves or ftite elementsor, where applicable, elastic continuum or non-linear spring models. Except where theground is sufficiently soft that buctiing is a possible failure mechanism, nomimlreinforcement need not be provided except where it is needed for stability duringinsertion of the reinforcing cage. Where buctiing is important, nominal reinforcementmay be required.

30

Design for shear in a fully embedded pile should similarly take account of the supportingeffect of the ground and tie distributed loading effects. Some concrete strength shouldtherefore be allowed when determining shear reinforcement and providing someredundancy exists in the design layout of the piles, no additioml shear capacity need becatered for in the provisions of reinforcement. Except where the pile acts as a column inair or in the upper exposed parts of retaining walls, nomiml shear reinforcement neednot be applied to pile design.

Corrosion of reinforcement in a fully embedded pile is urdikely to be significant belowabout lm depth even where reinforcement is fully exposed to the ground. Even wherecorrosion may be significant, such as in hig~y acidic (pH < 4.5) groundwaters, orwhere groundwaters containing high levels of oxygen or carbon dioxide continuouslyflow past the pile, crack control steel is of dubious use. Of more importance incontrolling corrosion is provision of a dense, well compacted concrete, resistant tocarbonation and with adequate cover to the reinforcement. In this respect crack controlsteel may in fact be counter productive since it can interfere with the placement andcompaction of the concrete.

Reinforcement should always be kept to a minimum. Where it can be demonstrated thatno tension exists in the pile section and that applied stresses are less than thosepermissible, reinforcement need not be provided. Where curtaihent of steel isrequired, it should be gradual to prevent the formation of a plane of weakness in thepile.

The effects of early thermal cracking should be considered particularly for laterallyloaded piles. In these si~ations, reinforcement should be provided for a sufficientlength to ensure that the pile performs satisfactorily under ultimate and limit stateconditions. It is considered adequate to cu~il this reinforcement at the point at whichthe applied horizontal stresses equal the bearing capacity of the soil calculated usingfactored shear strength parameters. ..

6.1.2 Free Standing Un@ of Pfies and Pfie Retaining WWS

In contrast to filly embedded piles, free standing lengths of piles and the exposedportions 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 generallyapplicable. Reductions in reinforcement quantities may still be made by ignoring crackwidth requirements in relation to corrosion and exposure conditions. As previouslymentioned, these have little effect on durability.

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

6.2 Recommendations

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

31

. .,, ,. ,. ... ---- .

areas. Detailed amendments are also suggested for the BD 32/88 (DMRB 2.1) and BA25/88 (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 ofareas where BS 8004 could be amended to provide clearer and more complete guidanceon 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 compatiblewith BS 8110 or BS 5400).

iv) Calculation method for shear capacity of piles.

v) Corrosion of reitiorcement in buried concrete.

vi) Effects of early thermal cracking on pile design.

BD 32/88 Piled Foundations ~MRB 2. 1~

As an alternative to changes to BS8004, guidance could be implemented by amendmentsto BD 32/88 (DMRB 2.1) as follows. These amendments would need to beaccompanied 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 insertthe following:)

Bored and Driven Cast in Place Piles

A. General

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

ii) Reinforcement of concrete piles need ody be provided where tension existsin the concrete section. Where tension is generated by lateral loadings orapplied moments, the moment within the pile may be resolved into the axialload on the pile acting at an eccentrici~. Where the eccentricity of loadingis less than 1/8 of the diameter of the pile, no tension exisfi in the sectionand no reinforcement need be provided.

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

32

iv)

v)

vi)

vii)

Nomiml reinforcement is not required other than for the purposes ofensuring a rigid cage during handling and insertion.

Design charts such as those inCP110 parts 2 and 3 may be used for designof symmetrically reinforced rectangular or circular sections subject tobending moments with appropriate modifications for the value of y..Alternatively the methods for short columns given in BD 44/90 (DMRB3.4.4) may be used to calculate reinforcement requirements.

Design for shear in piles should follow the recommendations for shear incolumns given in BD 44/90 (DMRB 3.4.4).

Buctiing of a pile need ordy be considered where L/l’ < l/(2)’h where L isthe length of pile in soft soil and 1’ is the length of half sine wave deflectionof the pile generated by the buctiing load and described in BA 25/88*.Where buc~ing is a consideration, provision of additioml links may becalculated as follows.

Where piles are subjected to lateral forces, a check should be made on thepossible effects of a horhontal crack at depth. In the absence of moresophisticated modelling tec~ques, longitudinal reinforcement should beprovided at least to a depth at which the bearing capacity of the surroundingground exceeds the lateral applied stresses. Factored soil shear parametersshould be used in the bearing capacity calculation.

viii) Except where otherwise indicated by the site investigation, fully embeddedpiles below a depth of about lm may be considered to be within a nonaggressive environment with respect to steel reinforcement. Examples ofwhere corrosive underground environments may exist are given in BA25188*.

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

x) Curtailment of longitudinal reinforcement should occur gradually to preventa plane of weakness developing in the pile. No more than two bars shouldbe stopped off at one level and a minimum distance of lm provided betweensubsequent curtaihnents.

B. Free Standing hngths of Piles

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

ii) Except for the modification provided for in (iii) below, structural design offree standing concrete piles shall be designed as columns in accordance withBS 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.

I

Pile Retaining Walls

Pile retaining walls should be designed as free standing piles over theirretained height down to the point of f~ity as given in B(i). Below this levelpiles should be designed as fully embedded.

Durability of the fill length of the pile should be carefully considered takingaccount of any likelihood of seepages being set up around the pile system.Provided the ptie is embedded in clay soils, seepage is likely to be slow andcorrosion of steel insignificant. For granular soils however, seepage may berapid and the corrosive environment may approach severe conditions.

iii)

iv)

v)

Where shear forces are distributed along the pile length rather thanconcentrated at a point, the allowable shear strength of the pile may beincreased by 15% above that provided for in BS 54~ Pt 4.

BucHing of the upper part of the pile is accommodated within the overalldesign of the pile. The lower, fully embedded portion of the pile should bechecked as for B(iii).

The effects of early thermal cracking in pile retaining walls should bechecked in accordance with BD 28/87 (DMRB 1.3.2) for the upper part ofeach pile. Effects of a hotiontal crack through the fully embedded portionof the pile should be checked for using a suitable soil/structure interactionmodel as described in BA 25/88*.

6.3 Areas for tier research

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

i) The formation, orientation and extent of early themal cracking in concretefully and partially embedded in the ground.

ii) The distribution of shear forces along the length of a filly or partiallyembedded pile or retaining wall and the effect of the ground in modifying thefailure mode of the pile.

iii) The effect of computer aided design of concrete structures and foundations oncalculated design forces and the relevance of existing partial load and materialfactors 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 piledretaining walls, many of the issues raised relate equally to diaphragm wall and basementconstructions. It would be useful, therefore, to extend the scope of this study to includethese structures.

35

. ,,.. .. . . .. . . .... .... . . . . ...

7.0 ACKNOWDG~S

The work described in this report forms part of the research programme of the CivilEngineering Resource Centieat TW. The Project Officer at TWwas MrPDarley andthe work is published by permission of the Chief Executive.

Grateful thanks are due to all the various contributors and respondents who significantlyhelped with the production of this report. Particular thanks go to Dr W G K Flemingand Mr R Fernie (Cementation Piling & Foundations), Mr W P Raies (Trafalgar HouseTechnology), Mr C Raison Keller Foundations), Mr D Beadman @achy Group), MrJ Barr (Rendel Geotectics), Mr A Powderharn and Mr J Robb (Mott MacDonald Ltd)and Mr D Nicholson (Ove Amp & Partners). Thanks are also due to the respectivecompanies of the above for cooperation in providing resources for this project.

8.0 ~cEs

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

BARTHOLOMEW, ~. The protection of concrete piles in aggressive groundconditions. Conf. on Recent Developments in the Design and Construction of Piles,ICE, bndon 1979.

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

BOOTH, GH, COOPER AW, COOPER PM, and WAKERLEY DS. Criteria of soilaggressiveness 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 cohesiodess soils. J. Soil Mech. Div.ASCE, V90, No. SM3, 1964b.

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

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

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

CIWA Report PG2. Review of problems associated with the construction ofcast-in-place concrete piles. Thorbum and Thorbum JQ. 1977.

CIWA Report 63. Ratiomlisation of safety and serviceability factors in structural codes1977.

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

CIWA Report 103. Design of Laterally baded Piles. Elson WK. 1985,

CIWA Technical Note 14. An experimental investigation into the effects of shear andtension on the flexural behaviour of reinforced concrete beams. bwnds P. and PamellFN 1971.

CIWA Technical Note 36. Elimination of Shrinkage and Thermal Cracking in a waterretaining structure, Hughes BP. 1971.

CIWA Tectical Note 109. The Performance of a piled bridge abutment at Newhaven.Reddaway AL and Elson WK. 1982.

I

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

I

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

I

FLEMING, WGK and ENGLAND, MG. Some recent insights into FoundationBehaviour. Ground Board of Inst. Civ. Eng. Informal Discussion, July 1993.

I

FLEMING, WELTMAN, RANDOLPH and ENON; Piling Engineering. SurreyUniversity Press 1985.

I

FRANCIS, AJ, SAVOURY NR, STEVENS LK, and TROLLOPE DH. The Behaviourof Slender Point-Bearing Piles in Soft Soil. Proc. Univ. Hong Kong Golden JubileeCongress 1962.

I GOURLEY, JT and BIENIAK DT. Diffusion of Ctioride into Reinforced ConcretePiles. Symp. on Concrete Perth 1983.

[

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

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

I KRAMER, SL and HEAVY EJ. Amlysis of laterally loaded piles with non-linearbending behaviour. Trmsport Research Record 1169.

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

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

INORTH-LEWS JP and SCOTT ID. Constructional Control affecting the behaviour ofpiles with particular reference to small diameter bored cast in situ piles. ICEConference, Behaviour of Piles, 1970.

I Piling Handbook, First Edition, British Steel Corporation 1963.

IRecommendations for an Internatioml Code of Practice for Reinforced Concrete ACIand CCA.

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

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

WNDOLPH, MF. The response of flexible piles to lateral loading Geotec~que V31No. 21981.

ROMANOFF, M. Corrosion of Steel Pilings in Soils. J. of Research of NationalBureau of Standards V66C No. 31962.

ROMANOFF, M. Underground Corrosion. Nat Bureau of Standards Circular 579,1957.

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

SASTRY, WRN and MEYERHOF GG. Behaviour of flexible piles under inclinedloads. Can Geotech. J. V27 1990.

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

TEWAGHI, K. Evaluation of coefficients of subgrade reaction. Geotechnique, V5No. 4, 1955.

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

TRL Research Report 359. Design of Embedded retaining walls in stiff clay. SymonsI.F. Tranport Research hboratory 1992.

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

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

Transport Research Record 1211. Concrete Bridge Design and Maintemnce : SteelCorrosion in Concrete. Transport Research Board, Natioml Research Council 1989.

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

WOOD, JH and PHILLIPS, MH. hteral stiffness of Bridge Foundations : Load Testson Newmans Bridge. Structures Committee Road Research Unit. National RoadsBoard. Report No. ST 87/2 1987.

.

1

I I I

r

I II

I

ILibrarySearches

II , 1 I

mm

aDesignIssues

m I::octors’“ ICenentatlon Pillng Qnd foundations

I II

1

mmmt J t I 1 J I I I I

figure 11 F(OW Chart Showing Data Col(atlon Procedure.

r=6

. . . . . . . . . . . . . . .....,..- . . . ..

-.. ,- . . .. -.. .,. . .. .

Appendix 1: Data So-s for Research Study

Al. 1 Publishti Litera~

Design Codes

The following current and withdrawn design codes were examined.

CP2:1951

CP4: 1954

CP101:1972

CP11O:1972

CP114:1957

CP114:1957

BS8002: 1994

BS8004: 1986

BS811O:1985

BS 5328:1991

BS 5493:1977

BE 1/73: 1973

BA 24/87: 1987(DMRB 1.3.2)

Earth Retaining Structures ~ithdrawn)

Foundations withdrawn)

Foundations and Substructures for nonindustrial buildings of not more than fourstoreys ~ithdrawn)

Parts 1 & 2 Foundations and substructuresfor non industrial buildings of not morethan four storeys ~ithdrawn)

The structural use of Concrete ~ithdrawn)

Reinforced Concrete in Buildings withdrawn)Code of Practice for design of concrete Bridges Part4.

Eafi Retaining Structures

Foundations

The Structural use of Concrete Parts 1 to 3

Concrete. Guide to Specifying ConcretePart 1

Protective Coating of Iron and SteelStructures Against Corrosion

Technical Memorandum (Bridges; ReinforcedConcrete for Highway Structures) withdrawn)

Department Advice Note; Wly Thermal Cractigin Concrete

m(cent)

Europe

Germany

France

Holland

BD 28187:1987(DMRB 1.3.2)

BD 30/87: 1987(DMRB 2.1)

BA 25188:1988(DMRB 2.1)

BD 32/88: 1988(DMRB 2.1)

BD 24/92: 1992(DMRB 1.3.1)

BD 42/94(DMRB 2.1)

Eurocode2ENV 1992-1-1:1992

ENV 1994-1-1:1992

ENV 1997-1

DIN 1945:1988

DIN 4014:1990

DIN 4128:1983

DIN 4026:1975

P1l-212,DTU 13.21992

A05-251 :1990

NEN 7053

Department Standard ; Backfilled RetainingWalls and Bridge Abutments

Departmental Standard ; Backfilled RetainingWalls and Bridge Abutments

Departmental Advice Note; Piled Foundation

Departmental Standard ;Piled Foundation

Departmental Standard ;Design ofConcreteHighway Bridges and Structures

DesignofEmbedded Retaining Walls andBridge Abutments (Uncropped or Propped atthe Top

Design of Concrete Structures. ConcreteBridges.

Design of Composite Steel and ConcreteStructures

Geotechnical Design

Structural use of Concrete. Design&Construction

Bored cast-in place piles

Small diameter injection piles

Driven Piles

Deep Foundations for Buildings

Ground Corrosion

Concrete Piles

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

AC1 318-1:1992 Building Code Requirements for Structuralplain concrete

AC1 336.1-79 Stidard Specification for tie constructionof end bearing drilled piers

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

ACI 543R-74: 1974 Recomrnendatiom for Design, Manufactureand Installation of Concrete Piles(Reaffirmed 1980)

NAVFACD~-7.2: 1982 Foundations and H Structures DesignManual, Department of tie NaW

~=. . ... . ..- -. ..... . . . .. .-.

~ Design Publications

The following ~ design reports were considered as part of this research study,

m CINA firly-age Thermal Crack Control in Concrete(Report No. 91:1981)

CIMA Design of hterally baded Piles(Report No. 193:19M)

CINA Review of problems associated wifi theconstruction of cast-in-place concrete piles

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

British Standards

BS 8110 Draft Amendment No. 5Part 1

Appendix 1 contid : Dam So-s for Research Study

Al.2 Practictig Companies

Cogitation of Exteti Orgardsations

A consultation process was setup with a number of UK based contractors andconsultants. This attempted to obtain a broad view of current design and constructionpractices operating in the ~ today.

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

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

Companies invited to contribute to study:

Consultants

1. Mott MacDonald GroupSt Anne House20-26 Wellesley RoadCroydonCR92UL

2. Ove Amp & Partners13 Fitiroy StreetbndonSE1 lSA

3. Pendel Palmer and Tritton Ltd61 Southwark StreetLondonSE1 lSA

Contractors

Bachy LtdFoundation CourtG.odalming Business CentreCattleshall bneGodalmingSurreyGU7 lXW

Cementation Piling & Foundations LtdMaple Cross HouseDenharn WayMaple CrossRickrnansworthHertsWD32SW

Keller FoundationsOxford RoadRyton on DunsmoreCoventryCV83EG

Appendk 2: BucMingResistanceof Ftiy EmbeddedPties

(After Francis et al, 1965)

1. BucNing load, of a pile, P~,,can be found from:

_. = (n’ + ~ /n*)PPE

where n = number of half sine waves caused by buc~ing load in pile

PE = BucNing load of pin ended strut in air.

PE = n 2 EI/L2

L = Length of pile

1’ = Length of half sine waveIf 1, is the effective length of the pile considered as a pin-ended strut

ie Pcr = n 2 EI/le2

Then

1/(1,/1’) = l/(L/n.l’)2 + (L/n.l’)2

~uation (2) is plotied as Fig. A.2.2.

From Fig. A.3.2 it is found that:

for L/l’ < 1/(2)’ie L < 0.71.1’ it can be assumed that the soil offers

no support to the pile and 1. = L

for L/l’ > 1/2ie L > 0.71.1’ it can be assumed that le = 1’/( 2 )fi

and Pa is twice the buc~ing load ofa pin-ended column in air.

and P. = (EIk)”

1’ is governed by the soil properties and maybe given by

1’ = ( n4.EI/k)U (unifom soil) or

1’ = (2 n’ EI/k)”5 (soil stiffness proportioml to depth

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

k = 8xE(l-y)/(34~~)(1 +v)(l +2 log,(21/b))

where 1 = length and b = breadth of pile.

2. Example

2.1 For a typical uniform soft soil of undrained shwr stren~th, Cu = 10kN/rn2The modulus of elasticity E = 500x C. = 5000 kN/m , p = 0.4and 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/O.5)

1’ = (n 4EI/4186)” = 6.5m (E = 25x 106kN/m2,1=n D4/64)

L > 0.711’ therefore Pa = (EK)%

= 118 MN which is equivalent to an applied stress of 600 MN/m2which is well in excess of the 28 day concretecharacteristic strength of, say 40 MN/m2

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

(1 - P/Per)” where P is the axial load.

In the above example,

ifP= 50 MN, Pa = 118 MN

and (1 - P/P~)x = 0.76

1c = 1’/(2)” = 6.5/(2)” = 4.6m

Adjusted 1, = 4.6/0.76 = 6.Om.

but 0.71.1, = 4.26

which is still < L (= 15m) so

Pa remains 118 MN.

s——

\

. . .t. . . . . . . . .

Appendix 3: _ple method of Mcdating spacing of W to preventId bulcUhg of embeddedptie.

1. Consider a 450mm diameter pile with four 32mm diameter longitudinalreinforcing bars cawing an axial load of 400 W.

Stress in each bar = (400x103/4)/ (n X 322/4) = 124 N/mm2

To prevent buc~ing of the bar, a lateral restraining force of 2.5% of the axialload on the bar is required.

For a lm length of bar, required lateral stress to resist buc~ing

= 124 X 103X0.032X 2 X 0.025

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

Assuming k = 1.0 and y = 20 ~/m3, the depth at which a lateral earthpressure of 445 ~/m2 is achieved is

445/20 = 22m

At lm depth, the lateral soil pressure is 1 x 20 = 20 ~/m2

Force on pile =20 X0.45=9N

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

From BS 8110 c1 3.4.5.6 eq” 4 ““

Vb = ~b (0.87 f~) (Cosa + Sins cot ~ ((d-d’)/Sb)

for a = 90° and ~ = 90°

Vb = ~b (0.87 fw)X ((d-d’)/Sb)

If Vb = 164 W/m2, d = 450rnm, d’ = 75mm

. -

men Sb = ~b (0.87 fv) X 0.375)191

= (0.0322Xn )/4 X 0.87 X 460X 103X0.375191

= 0.625m

~is is tie maximum spacing of lti to prevent local buc~ing at lm deptiignoring tie contribution of tie concrete.

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

Provision of nominal ltis below about lm is evidently an overconsemativedesign.

Above lm, flexure of tie pile may remove any lateral support to tie pile andnomiml Ii* could tierefore be considered.

12

Appendix4: Method of dctiation of depth to Wty of & standinglength of pfle

(After ACI 543R-74)

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

L can be estimated as follows

For overconsolidated clays

L = 1.4S where S = (EI/k)”

For granular soils and normally consolidated clays and silts

L = 1.8T where T = (EI/m)’”

k is the modulus of hotiontal subgrade reaction for overconsolidated clay. As aguide this may be taken as 67 times the undrained shear strength of the clay (seeFleming, Weltrnan, Randolph and Elson 1985 for guidance on k values).

m is the modulus of horbontal subgrade reaction for granular soils and normallyconsolidated clays and silts. For clays this may be taken as 67 times theundrained shear strength divided by the depth averaged over the top 3m or 5m (ieit is the slope of the k vs depth plot for the upper layers of the soft soil).

m values for other soils are given in table below :

.

Soil Type

Sand and inorganic silthoseMediumDense

Organic Silt

Peat

am1.51030

0.4 to 3.0 I0.2 I

If the embedded length L < 4S or 4T then this amlysis is not valid and analysessuch as hose presented by Broms (1964a and 1964b) for short piles should beundertaken. Nternatively, more detailed p-y curve or ftite element analysesmay be performed to obtain the point of fmity.