Primary Firm Secant Pile Concrete Specification
Geotechnical EngineeringProceedings of the Institution of Civil EngineersPrimary firm secant pile 80/jgeen.15.00038Paper 1500038Received 05/02/2015Accepted 14/12/2015Keywords: concrete technology & manufacture/piles & piling/retaining wallsICE Publishing: All rights reservedPrimary firm secant pileconcrete specificationJohn Gannon BSc, MSc, CEng, MICEDirector, Byland Engineering Limited, Clifton Moor, UK(corresponding author: john.gannon@bylandengineering.com)The minimum compressive strength of concrete at age 28 d is the main performance criterion for the acceptability ofan approved mix design. In secant pile wall construction, it is necessary for the secondary male pile bore to be cutinto the concrete of the primary female pile concrete to produce a water-resistant pile interlock. The accuracy andefficiency of the cut and the pile verticality that can be achieved are influenced, among other things, by the strengthof the primary pile concrete at the time the cut is made. Minimum characteristic primary pile concrete strengthdepends also on the long-term function of the pile and can vary widely from about 0·5 MPa for ‘soft’ non-structuralpiles to about 40 MPa for ‘hard’ structural piles. Thus, there are potentially conflicting concrete requirements for earlyand long-term strength. This paper reviews current practice for concrete used in secant piling, identifies that there areareas of uncertainty in standard specifications, that over-specification and over-provision of strength is probablycommonplace and shows how a window of allowable compressive strength would be a superior method to controlpile concrete cu(2)fcu(7)fcu(56)fcu(t)k0lMSstuvvcinterlock dimension between male secondary pilespile diameterextreme fibre stresscompressive strength of concretedesign value of compressive strength of concretecharacteristic compressive cylinder strength ofconcrete at 28 dmean value of concrete cylinder compressivestrengthmean compressive cube strength of concrete atage 2 dmean compressive cube strength of concrete atage 28 dcharacteristic value of cube strength concrete atage 28 dcompressive cube strength of concrete at age 2 dcompressive cube strength of concrete at age 7 dcompressive cube strength of concrete at age 56 dcompressive cube strength of concrete at age t dayscoefficient of horizontal earth pressure at resthorizontal span between adjacent secondary secantpilesbending momentpile spacingcoefficient in calculation of βcc linked to cementgradetime being considered (age of concrete (d))pore water pressuredesign shear stressdesign concrete shear stressvuwzβccγFγMσh′σv′1.ultimate concrete shear stressdistributed load on equivalent beamsection moduluscoefficient linking strength of concrete at differentages to strength at 28 d agepartial safety factor for loadpartial material factor for concrete in sheareffective horizontal ground stresseffective vertical ground stressIntroductionThis paper presents a review of current practice for the specification of concrete used to construct unreinforced primaryfemale firm concrete piles which form part of a secant piledretaining wall. Secant pile walls are used in preference to contiguous piled walls where a substantially water-resistant barrieris required. Therefore the primary pile must interlock with thesecondary male pile either side of it. This is achieved by boringthe secondary pile down through part of the primary pile sothat one pile interlocks or secants with its neighbour, theoverlap at each interface being up to 30% of the diameter ofthe primary pile. It is common and preferred practice for theprimary pile to be of plain unreinforced concrete. Typical spacings of primary and secondary piles are shown in Figure 1(taken from Ciria C580 (Ciria, 2003)).Where the primary and secondary piles are both reinforced,there is a need for the concrete of both piles to achieve acharacteristic compressive strength normal for that of structural concrete. Where only the secondary pile is reinforced, therequired characteristic compressive strength of the primary pile1
Geotechnical EngineeringPrimary firm secant pile concretespecificationGannonPrimary female pile, unreinforcedBDiameter, dSecondary male pile (reinforced)Pile spacing, SFigure 1. Typical pile spacings in a secant wallcan be lower as the unreinforced concrete is easily capableof transferring the soil and/or groundwater pressures ontothe closely adjacent secondary reinforced piles. The decisionon what strength of concrete is required depends on severalfactors, for example strength, durability, permeability anddegree of water-tightness. The latter may be influenced by theamount the concrete may shrink during drying or crack duringdrilling.The characteristic compressive strength of material formingsecant piles (and therefore the piles themselves) is commonlyreferred to as either hard, firm or soft. The secondary pile isusually a hard pile, so there are three possible combinations ofpile type: hard/hard, hard/firm and hard/soft.Soft piles utilise unreinforced weakly cemented materialssuch as cement–bentonite–sand slurries with a characteristiccompressive cube strength at 28 d of up to 3 MPa. They aretypically used in temporary works applications and are notconsidered further here.Firm piles utilise concrete with a characteristic compressivecube strength at 28 or 56 d of about 10 MPa to 20 MPa whilethe characteristic compressive cube strength of hard pile concrete at 28 d is greater than 25 MPa.The specification of the strength and the rate of strength gainof concrete in firm primary piles is of particular concern andis the subject of this review. It is argued that, provided permeability, shrinkage and cracking is controlled and long-termdurability provided, a lower than normal concrete strengthshould be adequate for most applications.The benefits of carefully controlling the properties of a firmprimary secant pile concrete are to enable less powerful pilingrigs to be used to achieve the specified pile verticality andinterlock. In practice this means that the uncased continuousflight auger (CFA) method of rotary bored piling, the fastestand most cost-effective form of replacement secant piling, canbe applied with greater confidence. Where concrete strengthdevelopment is poorly predicted and controlled and primary2Figure 2. Out-of-position continuous flight auger bored secantpilespiles are cut when they have developed excessive strength, pilewall alignment and interlock suffer, see for example Figure 2.This paper makes reference to compressive strength of concreteestablished by tests on cylinders and on cubes. Practice inthe UK is generally to test cubes, not cylinders, and unlessthe context specifically requires, compressive strengths shouldbe taken to mean those determined on cubes.2.2.1Current practiceICE specification for piling and embeddedretaining wallsThe de facto standard specification in the UK for secantpile wall construction is the second edition of the ICESpecification for Piling and Embedded Retaining Walls(Sperw2) (FPS, 2007). For both hard/hard and hard/firm walls,the guidance provided in section 9 of this document is toutilise low-strength high-cement replacement prescribed concrete mixes with characteristic compressive strength measuredat 56 d. The control of early-age strength gain is stated to be
Geotechnical Engineeringcritical to the secant pile construction. In practice, either a prescribed concrete (for which the composition and constituentmaterials are specified) or a designed concrete (in which theproperties are specified) is used.Primary firm secant pile concretespecificationGannonConcreteCEM1 concreteStrength at7 da: %Strength gainfrom 28 to 90 da: %805–10Section 19 of Sperw2 deals with the requirements for concrete.Structural concrete is defined in table C19.1 as that which hasa compressive strength class of C16/20 or higher, the 16 referring to cylinder strength and the 20 referring to cube strength.This grade of concrete is used to form a hard pile. Nonstructural concrete used for ‘infill piles’ is of lower strength class.30% fly ash concrete50% GGBS concrete50–6010–2050% fly ash concrete70% GGBS concrete40–5015–30Compressive strength conformity testing and acceptance criteria in Sperw2 follow the requirements of BS 5328 Part 4(BSI, 1990). For structural concrete, this may be considered tobe appropriate even though BS 8500 (BSI, 2006a, 2006b) wasextant at the time Sperw2 was produced. For non-structuralconcrete it is not.Table 1. Rate of strength gain of different concretesIn order for trial concrete mixes to be acceptable, the averagestrength of two 28 d cubes is required to exceed the characteristic strength by not less than 11·5 MPa. It is assumed that fornormal grades of structural concrete with a characteristicstrength in the region of 40 MPa, a margin of 11·5 MPa mightrepresent 2 standard deviations between the mean value andthe value at which 95% of results lie. Such a large finite marginis not appropriate for lower-strength firm primary secant pileconcrete.In order for works concrete to be acceptable, Sperw2 requiresthe strength of 28 d cubes to exceed the characteristic strengthby not less than 1 MPa to 3 MPa, the actual value dependingon the number of cubes tested.Sperw2 also requires trial cubes and works cubes to be testedat age 7 d, but no acceptability criteria are specified. The compressive strength test results at 7 d are thus taken to be earlyindicators of the likely 28 d strength. Concrete’s early strengthdevelopment is dependent primarily on its Portland cementcontent and Table 1, taken from The Concrete Centre guidance(Specifying Sustainable Concrete, 2011), provides an indicationfor concretes made with varying proportions of OrdinaryPortland Cement (OPC CEM1) with partial replacementby fly ash (PFA) or ground granulated blast-furnace slag(GGBS).aStrength as a proportion of 28 d strength.Table 1 of say 40%, Figure 3 presents a chart of required cubestrength against cube age for two typical non-structural firmpile concretes.BS EN 1992-1-1:2004 (BSI, 2004) section 3 provides equationsto estimate the compressive strength at a given age, thestrength being dependent on grade of CEM1 cement, temperature and curing conditions. It should be noted that the concrete strength is measured on cylinders, not cubes, althoughthe equation holds for cube strengths. For uniform curing conditions, the compressive strength of concrete at age t days isgiven by1:fcm ðtÞ ¼βcc ðtÞ fcmin which βcc(t) en and n s(1 (28/t)0·5)The coefficient s depends on cement grade and the values inTable 2 are given in the code.It is noted that cements of class S contain between 66 and80% of GGBS and as such constitute cement group CEMIII.(BS EN 197-1 (BSI, 2000a)). Cements of class N are CEM1cements with ordinary early gain of compressive strength,whereas cements of class R are CEM1 cements with highearly strength gain. For concretes made with high CEM1 substitution and requiring a low rate of strength gain, it is thuslogical to expect cement of class N and grade 42·5 to be mixedwith GGBS and/or PFA for firm pile concrete.Clearly, the focus of Sperw2 is to ensure that the characteristiccompressive strength of 28 d cubes comfortably exceeds thespecified characteristic cube strength. It is considered that thisis not appropriate for firm secant pile concrete.For the prediction of the rate of strength gain of high-cementreplacement mixes, it might therefore be appropriate to adoptan s coefficient of not less than 0·25 and probably more,perhaps substantially more, than 0·38.2.2Strength developmentThe use of GGBS and/or PFA to replace CEM1 is commonpractice, desirable and to be encouraged. Using the 7 dstrength indicator for a 70% GGBS replacement mix onFigure 4 shows theoretical strength development curves for C8/10 and C16/20 concretes made with class N CEM1 cementcured in accordance with BS EN 12390 (BSI, 2000c) at 20 Cand assuming s 0·25. At age 7 d, some 78% of the 28 d3
Geotechnical EngineeringPrimary firm secant pile concretespecificationGannon35Compressive cube strength: MPa30252015105005C8/10 trial mix1015Age: dC8/10 single works cube20C16/20 trial mix2530C16/20 single works cubeFigure 3. Typical minimum compressive cube strength requiredby Sperw2Cement classCement gradesSNR32·5 N32·5 R, 42·5 N42·5 R, 52·5 N, 52·5 Rs0·380·250·20Table 2. Strength gain coefficient s for of different cementgradesstrength is predicted to have been achieved and the differencebetween the 28 and 56 d compressive strength is only some8%. The Table 1 figures for a substantial amount of CEM1replacement suggest that, on average, approximately 50% ofthe 28 d strength is predicted to be achieved at age 7 d andthe difference between the 28 and 56 d compressive strengthis predicted to be some 20%. There is thus significant variancebetween the Table 1 guidance (the basis of which is notknown) and the Eurocode 2 (BS EN 1992-1-1:2004 (BSI,2004)) predictive equation when used with recommendeds values. The s value required to approximately produce theTable 1 gain percentages is about 0·6.Wharmby (2010) comments that in a number of secant pilingprojects in New Zealand there was considerable variance in theconcrete strength at any given age. The details of the concretemix are not known but may be assumed to have involvedconsiderable CEM1 substitution. The standard deviation ofsample strength at 3 to 56 d is given as 0·65 MPa to 2·99 MPafor a specified concrete cylinder strength of 6 MPa at 28 d.Approximately 48% of the mean 28 d cylinder strength is4shown to have been reached at age 7 d and the differencebetween the mean 28 and mean 56 d compressive cylinderstrength is shown to be 27%.2.3Cement replacementThe replacement of OPC with GGBS or PFA has a successfultrack record for use in primary secant piles. The advantagesare several: reduced rate of strength gain; inherent resistance toacid and sulfate attack; sustainable use of waste/industrial byproduct; and slightly improved workability for a given cementcontent. Up to 95% of CEM1 may be replaced by GGBS andBRE Information Paper IP17/05 (Quillan et al., 2005) presentsthe case for such concrete, the CEM1 replacement level ofwhich exceeds the 80% value covered by the guidance on thechemical attack of concrete given in BRE Special Digest 1(SD1 (BRE, 2005)). The maximum replacement level forCEM1 by PFA in SD1 is 55%. There appears to be no reasonwhy this cannot be safely increased without unduly compromising long-term strength or durability and further confirmatory research in this area is desirable.SD1 appears to limit the amount of CEM1 replacement byGGBS to 80% to ensure that the long-term strength and durability of the concrete is satisfactory for the majority of structural applications. For unreinforced firm primary female pileswhich are not considered to be formed of structural concrete,long-term strength is not a critical design consideration. Whatis crucially important is very slow early rate of strength gain,the mean early strength and its variation. The concrete mustbe strong enough not to be damaged during boring but not sostrong that it cannot be bored through accurately and
Geotechnical EngineeringPrimary firm secant pile concretespecificationGannonCompressive cube strength: MPa3025201510500102030Age: dC8/10405060C16/20Figure 4. Concrete strength development curves for class Ncement from section 3 of BS EN 1992-1-1: 2004 (BSI, 2004)efficiently. Usually the secondary piles are cut into the primarypiles when the concrete is about 2 to 7 d in age and has a compressive cube strength of about 2 to 7 MPa.Also of importance to the designer are durability andresistance to cracking. Durability with respect to aggressiveground and groundwater is offered by the CEM1 substitution.Cracking is caused primarily by shrinkage and is best controlled by maintaining as low a water–cement ratio as possible.However, concretes made with CEM1 and with a low water–cement ratio typically achieve high characteristic strengthsand possess poor workability. High-workability concretesare essential for constructing primary female piles by theCFA method, which require the concrete to pumped down thecentral stem of the auger in order to form the concrete pilecylinder ‘bottom-up’ as the auger is withdrawn. Hence, a highproportion of CEM1 substitution by GGBS and/or fly ash inconjunction with the use of water-reducing admixtures to keepthe water–cement ratio low, offer a practical compromise.2.4BS EN 206-1 concreteSection 7·2 of BS EN 206-1 (BSI, 2000b) requires the strengthdevelopment of a designed concrete to be stated by the concrete producer in terms of table 12 (reproduced here asTable 3) or by a strength development curve at 20 C between 2and 28 d.Firm secant pile concrete will require a very slow rateof strength development. It is noted that the ratio of meanstrength at age 2 d and mean strength at 28 d depends onactual test data so that if the mean strength at 28 d substantially exceeds the specified minimum characteristicstrength, then the mean strength at a particular time may besubstantially in excess of that which enables a primary secantStrength developmentEstimate of strength ratio fcm(2)/fcm(28)RapidMediumSlowVery slow 0·5 0·3 to 0·5 0·15 to 0·3 0·15Table 3. Strength development of concretepile to be accurately cut. For example, if the specified strengthclass is C15/20, the minimum works compressive strength permitted by Sperw2 for a single cube would be 23 MPa and themaximum compressive strength at 2 d would be 3·45 MPa.However, if the actual compressive strength of cubes at 28 dwas, say, 40 MPa, then the maximum compressive strength at2 d would be 6 MPa and such a concrete would probably haveto be cut at 2 or 3 d age for CFA piles to be straight, verticaland well interlocked.3.Case historiesCompressive cube strength test results have been obtainedfor three firm secant pile concretes provided by UK Namasaccredited ready-mix concrete suppliers; a summary of themix designs and the retaining wall construction is given inTable 4. All concretes were pumped mixes, class DC2,maximum aggregate size 20 mm and the CEM1 cement wasgrade 52·5N.Figures 5–7 show the compressive strength test results for workcubes and, where available, trial cubes. The solid curve represents a logarithmic fit to the data. The dashed line representsthe best available fit to the data using the Eurocode 2 prediction equation for the stated value of s.5
Geotechnical EngineeringMix123GradeC8/10C12/15C8/10Primary firm secant pile concretespecificationGannonTotal bindercontent: kg/m3280280340Proportion of bindercontent: %CEM1GGBSPFA191026819000074Water–cementratioMale pile diameterand spacing: mmWall retainedheight: Table 4. Primary pile concrete and construction details(RC rotary cased, casing outer diameter (OD) 880 mm)Compressive cube strength: MPa302520y 8·3255ln(x) –7·5757R² 0·8624151050010Works cubes2030Age: ds 0·25 prediction405060Log(works cubes)Figure 5. Compressive cube strength plotted against age – mix 1(C8/10)Compressive cube strength: MPa353025y 8·9944ln(x) – 4·6507R² 0·8986201510500Works cubes1020Trial cubesFigure 6. Compressive cube strength plotted against age – mix 2(C12/15)630Age: d40s 0·5 prediction5060Log(works cubes)PiletypeCFACFARC & CFA
Geotechnical EngineeringPrimary firm secant pile concretespecificationGannonCompressive cube strength: MPa201510y 3·2566ln(x) – 0·9493R² 0·63550010Works cubes20Trial cubes30Age: d40s 0·5 prediction5060Log(works cubes)Figure 7. Compressive cube strength plotted against age – mix 3(C8/10)Table 5 presents the indicated strength development coefficientused to generate the dashed line in Figures 5–7 and a series ofstrength ratio values that may be used to characterise the testresults. Where there are no test data at 2 and 56 d, the best-fitequation has been used, where possible, to estimate the valuethat would have been 1·23Table 5. Primary pile concrete strength ratios3.1Mix 1 (GGBS replacement)Compressive strength achieved at 28 d ranged from 17 MPa to23 MPa, up to more than double the characteristic strength of10 MPa. There was no trial mix and compressive strength wasdetermined on works cubes at only 7 and 28 d. There is a widespread of test results. The mean fit to the data is reasonablywell correlated but there is a complete mismatch between thisand the predicted strength development curve constructedusing a lower bound s value of 0·25. At 7 d, only some 35% ofthe 28 d strength is achieved and the extrapolated meanstrength at 56 d is 30% higher than the mean 28 d strength.3.2Mix 2 (GGBS replacement)Compressive strength achieved at 28 d ranged from 18 MPato 30 MPa, up to more than double the characteristicstrength of 15 MPa. A trial mix was carried out with cubescrushed at 1, 2, 3, 4, 7, 14 and 28 d. The predicted strengthdevelopment curve constructed using an s value of 0·5 fits thetrial mix test results tolerably well. The works cubes test resultsare very different from the trial mix results. At 7 d, about 50%of the mean 28 d strength is achieved and the extrapolatedmean strength at 56 d is 32 MPa, some 28% higher thanthe mean 28 d strength of 25 MPa. The strength gain indexfcu(2)/fcu is below the Table 3 value of 0·15 for a very slow rateof gain.3.3Mix 3 (PFA replacement)Compressive strength achieved at 28 d ranged from 5 MPa to15 MPa, between half and one and a half times the characteristic strength of 10 MPa. A trial mix was carried out withcubes crushed at 2, 3, 5, 7, 14 and 28 d. The predicted strengthdevelopment curve constructed using an s value of 0·5 fitsthe trial mix test results fairly well and is quite similar to theworks cube results trend line, although due to variation in theresults, the correlation coefficient is low. At 7 d, about 54% ofthe mean 28 d strength is achieved and the extrapolated meanstrength at 56 d is 12 MPa, some 20% higher than the mean28 d strength of 10 MPa. The strength gain index fcu(2)/fcu isbelow the Table 3 value of 0·15 for a very slow rate of gain.4.DiscussionThe overriding observations from the three sets of case historyresults for concrete made with CEM1 substitution by 70–90%GGBS or PFA are given below.&&&Strength at any age is very variable, much more so than isdesirable.Works concrete appears to be stronger than trial mixconcrete.A strength development curve s value of about 0·5 mightbe typical for these high-CEM1 replacement mixes.7
Geotechnical EngineeringPrimary firm secant pile concretespecificationGannon&where σh′ is the effective horizontal soil pressure and u is thepore water pressure (hydrostatic)&The 7 d strength may be 35 to 55% of the 28 d strength.56 d strength is indicated to be 20 to 30% higher than28 d strength, which is broadly as predicted by Table 1.There is thus some merit is establishing characteristiccompressive strength fcu at 56 d rather than 28 d.The variance of strength appears to be a common problem,see for example Wharmby (2011) where 28 d cylinder strengthsvaried from 3 MPa to 9 MPa, 100% either side of the mean.The variability could be related to the fact that small variationsin moisture content in the aggregates have a disproportionatelylarge effect on the water–cement ratio when relatively smallquantities of binder, 280 to 340 kg/m3, are being used. It istherefore important that the moisture content of the aggregatesand the weights of all batched constituents are carefully measured and the added free water content of the mix adjustedaccordingly. Wharmby (2010) makes similar observations andhis remarks also may be interpreted to suggest that the batching tolerances at ready-mix plants can be too large.It is also possible that contractual responsibilities to customarilyprovide concrete for general structural applications get in theway – low early strength is not a normal requirement for readymix concrete suppliers and there is an understandable tendencyfor them to supply a product with a compressive strength thatcomfortably exceeds the characteristic strength. This may be thereason why works cube strengths are significantly higher thantrial mix cube results. Clear communication of the permissiblelower and upper compressive strength limits for use in primarysecant pile applications therefore would be beneficial.4.1Minimum compressive strengthThe following considers how the minimum compressivestrength of the firm primary pile concrete may be established.A structural design procedure using compressive cube strength,as set out in BS 8110 part 1 (BSI, 1997), is followed for simplicity. The secant piled retaining wall shown in Figure 1 isinstalled in soil with a unit weight, γ 20 kN/m3 and withgroundwater level at surface in the retained soil. Piles are900 mm in diameter (d) at 1200 mm centres (S) with the interlock dimension between male secondary piles of B 300 mm.The dimension A is given by2:4:σ 0h ¼ k0 σ 0vLet k0 1·0, σv′ σh′ 15 20 15 10 150 kPau ¼ 15 10 ¼ 150 kPaHence v (150 150) 0·3/(2 0·849) 53 kPa.4.3Flexural stress in primary pileThe maximum fibre stress, f, due to horizontal bending of theunreinforced primary pile will be negligible in most instances.Consider the following in which d is the diameter of primarypile, l is the effective horizontal span of unreinforced primarypile, and w is the uniformly distributed load on one side of theprimary pilez¼6:M¼7:f ¼8B2AMzM¼" #0 5d 2B 2A¼2 ¼ 849 mm22v ¼ ðσ 0h þ uÞwl 28Let w 300 kN/m, l 0·3 m (B of Figure 1), d 0·90 m300 0 32¼ 3 4 kN m8z¼22 0 753 ¼ 0 041 m3732f ¼3 4¼ 83 kPa0 0414.2Shear stress in pileThe shear stress (v) acting across a concrete section 1 m longand with the width A at an average depth of 15 m is given by3:πd 3325:
Geotechnical EngineeringPrimary firm secant pile concretespecificationGannonThe mean tensile strength of concrete in flexure fctm is givenby BS EN 1992-1-1 (BSI, 2004) asmethods should prove adequate and economical. Therefore theapproach to follow generally should be to specify low concretestrength and to achieve good interlock. Increased concretestrength should not be specified in the mistaken belief that amore durable and/or stronger concrete structure necessarilywill be produced.8:ð2 3Þf ctm ¼ 0 3 f ckHence, the required mean characteristic cylinder strength inthe above example is 0·1 MPa and the characteristic cubestrength is 0·13 MPa, that is, negligible.4.4Design shear strengthSteel shear reinforcement is provided when the design concreteshear stress (vc) exceeds 0·4 MPa. The ultimate concrete shearstress may be related to the unconfined compression strengthin the usual way9:vu ¼f cu2Hence there appears to be no necessity to adopt a characteristic concrete compressive cube strength of more than 0·8 MPaif the concrete is not reinforced, provided the piles are fullyinterlocked.By adopting a concrete material factor for concrete in shearγM 1·5 and a partial safety factor for load γF 1·35, theminimum required characteristic compressive strength of concrete is given by10:f cu 2v γF γMTherefore in this example, fcu 0·21 MPa.Allowing for an imperfect interlock in which the bearing areais reduced, the required characteristic compressive strengthof the female pile concrete is unlikely to exceed 1 MPa.Arching in planar walls, and also the force transference incircular construction where hoop compression forces are transmitted circumferentially from pile to pile, are therefore bestfacilitated by consistent good pile interlock and the provisionof even pile to pile bearing. In circular construction, the pile topile bearing stress may well exceed 1 MPa but seldom will itneed to be more than 10 MPa, except in very deep shaftswhere the soil and groundwater pressures are large and/or inshafts in which the diameter to wall thickness ratio is large andthe hoop buckling stress is low.For circular construction, hard/hard secant piles constructedusing cased bored piling methods are often used. However,where it can be shown that a lower-strength concrete can beused, firm primary piles constructed using CFA piling4.5Crack widthThe specification of maximum crack width usually arises outof concerns about durability with respect to reinforcement corrosion and water-tightness. Reinforcement corrosion is not relevant to the design of unreinforced primary female piles. Thelocation and orientation of a crack is more important than itssize. Any size of crack that passes through a section may let inwater. However, wall flexure will normally cause a compressionzone that will maintain the crack tightly closed and preventwater passage.5.Proposed specificationThe use of the strength development curve, as opposed to the2:28 d strength ratio value method, offers a better possibilityof communicating and controlling concrete strength development, particularly in the critical first 7 d following placementwhen strength develops rapidly. However, for firm primarysecant piles, a strength development window, rather than acurved line which represents the minimum required strength, ispreferred. The consequences of over-providing strength aremore severe than those of under-providing strength. It is recommended that calculations are carried out to determine theminimum required compressive strength of the primary pile.The requirement to achieve the characteristic compressivestrength should be set at 56 d age.Equation 1 may be used with an s value of 0·5 to generatethe lower curve of the strength development window. Theupper curve of the window should reflect the inconsistencyof strength production evidently provided in practice andvalues twice the minimum are suggested as being appropriate.The proposed strength development window is shown, for
an approved mix design. In secant pile wall construction, it is necessary for the secondary male pile bore to be cut into the concrete of the primary female pile concrete to produce a water-resistant pile interlock. The accuracy and efficiency of the cut and the pile verticality that can b
of cantilevered, braced, and/or anchored secant and tangent pile retaining wall systems. Secant and/or tangent pile walls may be constructed using various technologies including . A reinforced concrete template that is formed in the ground prior to the start of the construction of the secant or tangent pile walls. The guidewall provides .
This paper summarizes critical design considerations for secant pile shoring systems, detailing recent advances in installation procedures and verification techniques that allow secant piles to be used for an expanding range of project conditions. Technically- and commercially-viable secant pile shoring designs
project: a permanent structural secant pile wall system. The installation of the secant pile wall system is expensive due to its uniqueness, difficulty of installation, and long schedule. For this analysis, an alternate system is designed to replace the secant pile wall while maintaining
To observe the design capacity, a test pile is constructed and estimated load is given upon the designed pile. There are three kinds of static pile load testing. 1. Compression pile load test. 2. Tension pile load test. 3. Lateral pile load test. A lobal Journal of Researches in Engineering V olume XVI Issue IV Ve rsion I 41 Year 201 E
PHC pile, with a diameter of 1000mm and a wall thickness of 130mm , is adopted for the wall pile frame structure of this project. The pipe pile prefabrication is carried out by dalian prefabrication factory. The parameters of the PHC prestressed concrete pipe pile are as follows: Table 1 parameters of PHC pile of wall pile frame structure .
DEEPEX EXAMPLE: SECANT PILES WITH TIEBACKS DEEP EXCAVATION 2 A. Project description In this example we will design an anchored secant pile wall with 3 tieback rows, supporting a 12m excavation. The Figure below presents the project model. Tables 1 and 2 present the soil properties and the stratigraphy respectively. Table 3 presents the external .
becomes corresponding to secant i.e. cosecant. Cosecant with angle Ө secant (hypotenuse) / Opposite side (cotangent) cosec Ө AC / AB The defination of Cosecant : - it is the ratio of hypotenus i.e.secant to the opposite side of given triangle. The reason for main function sine , secant , tangent :- Sine is only use for chord which is .
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