Observations on the Design of a Typical Anchored Sheet Pile in Cohesionless Media using the Conventional CECP2 and the BS8002 Design Methods

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1 J. Bsic. Appl. Sci. Res., 2(5) , , TextRod Publiction ISSN Journl of Bsic nd Applied Scientific Reserch Observtions on the Design of Typicl Anchored Sheet Pile in Cohesionless Medi using the Conventionl CECP2 nd the BS8002 Design Methods Shodolpo Oluyemi Frnklin * nd Abiodun Luqmn Olopde b Deprtment of Civil Engineering, University of Botswn, Gborone, Botswn b Formerly, Deprtment of Civil Engineering, University of Ibdn, Ibdn, Nigeri ABSTRACT In the present study comprison of two lterntive designs of typicl nchored sheet pile in cohesionless medi whose soil design prmeters hd been previously estblished through pproprite site nd lbortory investigtions is crried out. The designs hve been executed using the conventionl Civil Engineering Code of Prctice CECP2 - Erth Retining Structures nd the Code of Prctice for Erth Retining Structures - BS8002 bsed on limit stte procedures. It ws found tht the BS8002 method yielded higher depth of penetrtion nd lrger nchor rod force compred to the CECP2 pproch. This ws felt to be due to the BS8002 procedure which stipultes n dditionl surchrge lod of 10 KN/m 2 s well s n dditionl unplnned excvtion of pproximtely 10% of the height between the dredge level nd the position of the nchor rod. Keywords: Anchor, pile, cohesionless, design, embedment, depth. INTRODUCTION The use of sheet piles hs grown in populrity over the pst six decdes or more due to the gret technologicl strides tht hve tken plce coupled with the ever incresing economic, socil, urbn nd developmentl needs tht hve risen. Cntilever sheet piles re generlly used s temporry structures in civil engineering construction, nd re employed for moderte heights only. Anchored sheet piles on the other hnd re extensively used in wter- front or mrine construction, where the wter tble is high or the soils encountered re soft. The stbility of such wlls is ctered for by the pssive resistnce developed in front of the wll s well s the nchor rod forces. The ltter ssists in reducing the embedment depth, lterl deflection nd bending moments in the wll. Vrious methods of nlysis nd design of cntilevered nd nchored sheet piles hve evolved nd n overview of the more significnt developments is provided by Crig [1]. One of the more populr conventionl methods tht hve been pplied extensively with some mesure of success is tht represented by the conventionl Civil Engineering Code of Prctice CECP2 code [2]. The method employs lumped prmeter pproch whereby the fctor of sfety is expressed in terms of the rtio of restoring moments to overturning moments. The depth of penetrtion is determined from stbility ssessment bsed on the limiting equilibrium method of nlysis in which conditions of filure re postulted nd fctor of sfety pplied to ensure tht such filure does not occur. This is done by using multiplying fctor to increse the depth of penetrtion from tht required by limiting equilibrium. A more recent pproch is tht contined in the Code of Prctice for Erth Retining Structures BS8002 [3] which incorportes limit stte design principles. The BS8002 method sets out to determine the most unfvourble or the most onerous loding conditions. In order to summrize the mjor chnges incorported in BS8002 in comprison to CECP2 it should be noted tht retining wll design to BS8002 is to be bsed on n effective stress nlysis if the wll is to be permnent structure, however for temporry wlls n undrined nlysis hs to be undertken. Secondly, lthough BS8002 dopts limit stte philosophy it voids the use of prtil sfety fctors. In contrst CECP2 uses fctors of sfety. Thirdly design bsed on BS8002 is for servicebility. This fct ccording to Akroyd [4] is necessry becuse the mximum erth pressures occur during working conditions when there is minimum movement of the wll. CECP2 *Corresponding Author: Shodolpo Oluyemi Frnklin, Deprtment of Civil Engineering, University of Botswn, P. Bg 0061, Gborone, Botswn. Emil: frnklinso@mopipi.ub.bw 4571

2 Frnklin nd Olopde 2012 in comprison bses design on the ultimte limit stte whereby fctors of sfety re dopted to gurd ginst filure t this stte. Fourthly, for design, BS8002 uses frction of the pek soil strength for erth pressure clcultions when deling with soil prmeters such s cohesion c or ngle of shering resistnce ϕ. Akroyd [4] hs opined tht due to the need to chieve stisfctory wll in service the movement of the wll must be reltively smll. Since strins in the soil mss re smll, the design procedure tkes ccount of this by using frction of the pek strength. In contrst the CECP2 pproch recommends the use of the ctul strength. Furthermore in BS8002, minimum surchrge of 10 KN/m 2 is stipulted t ground level behind the wll. Finlly, required 0.5 m over-dig level or not less thn 10% of the totl height retined is specified in order to provide for ccidentl nd unforeseen events. Aprt from the methods nd design procedures outlined in CECP2 nd BS8002, severl prominent investigtions nd developments in design methods s well s studies on the stbility performnce of erth retining structures in service hve occurred over the pst six decdes [5 16]. Powrie [14, 15] investigted embedded retining wlls nlysed using the limit equilibrium pproch nd delt with prmeters such s soil strength, soil-wll friction nd sfety fctors, ll of which hd generted controversies in the pst. The im of his reserch ws to expound rtionl bsis for the selection of soil prmeters nd fctors of sfety for use in conventionl limit equilibrium nlysis of embedded retining wlls. He concluded tht criticl fctors were the identifiction of the most onerous loding condition nd considertion of groundwter effect s well s the selection of pproprite soil prmeters nd fctors of sfety. It is cler from review of recent developments [8, 14, 16] tht some uncertinties nd controversy still remin in respect of the nlysis nd design of embedded wlls, nd the use of the vrious fctors referred to erlier with reference to the conventionl CECP2 method nd the limit stte BS8002 pproch. The present study hs consequently been directed t the design of typicl nchored sheet pile wll in mrine conditions using rel soil design prmeters whose vlues hve been determined through relevnt site nd lbortory investigtions. Designs hve been performed using both the conventionl CECP2 nd the limit stte BS8002 methods nd the results of both pproches hve been compred. METHODOLOGY A schemtic for the nchored sheet pile nd the surrounding soil medium is shown in Figure 1. The position of the nchor rod force hs been pre-set t 0.5 m from the top off the sheet pile in ccordnce with the dicttes of locl consultncy. The wter tble is locted t 2.8 m below ground level nd the dredge line is 4.4 m below the wter tble. It is required to determine the embedment depth, the totl length of pile nd the force in the nchor rod. A surchrge of 25 KN/m 2 behind the retined soil mss t ground level hs lso been stipulted by the locl consultncy. The idelized pressure distribution digrm bsed on the conventionl CECP2 method in the presence of wter in the bckfill is shown in Figure 2. Following Bowles [17] nd Murthy [18], the depth of penetrtion of the sheet pile below the point of zero pressure is given by 2X 3 3X 2 6R h 0 3 G k y (1) Also the force in the nchor prop is given by the expression F R R p (2) 4572

3 J. Bsic. Appl. Sci. Res., 2(5) , m 2.3 m 4.4 m Sheet pile 25KN/m 2 surchrge ϕ = 33 o γ = 18.5 KN/m 3 ϕ = 33 o γ = 11.5 KN/m 3 Ground level F = Anchor force W.T. Dredge line D ϕ = 33 o γ = 10 KN/m 3 Figure 1. Schemtic of the nchored sheet pile nd the surrounding soil Ground level 2.8 m F 4.4 m y h 3 Dredge line y R W.T. O X R p Figure 2. Pressure distribution digrm using the conventionl CECP2 method If the BS8002 design pproch is used, the pressure distribution digrm is shown in Figure 3. The moment equilibrium reltionship is obtined by tking moments bout the nchor rod position, thus yielding R y R y 0 p (3) 4573

4 Frnklin nd Olopde 2012 where y = 0.67D + h, nd D represents the minimum computed embedment depth. A similr reltionship to eqution (2) is used for horizontl equilibrium. Ground level 2.8 m F W.T. 5.1 m = (4.4 m m) h y Dredge line R D R p Figure 3. Pressure distribution digrm using the BS8002 design method RESULTS AND DISCUSSION A comprison of the results of the design of the nchored sheet pile wll using the two design codes is shown in Tble 1. For this purpose the CECP2 computed depth hs been incresed by 30% to obtin the embedment depth shown. In conventionl CECP2 design n embedment fctor of between 1.20 nd 1.40 is considered norml, so the 30% increse pplied in Tble 1 is not out of plce. It is obvious tht higher depth of penetrtion, by mrgin of bout 21%, is obtined using the BS8002 method compred to the conventionl CECP2 pproch. Frnklin nd Olopde [19, 20] in erlier studies on cntilever nd nchored sheet piles in both cohesive nd cohesionless soils found tht greter embedment depths by mrgins of bout 12% were obtined for grnulr soils when using BS8002 s opposed to the conventionl CECP2. For cohesive soils, differences between the two design methods were found to be well bove 20% in some cses. The differences in the current investigtion, s observed lso in the previous studies mentioned, re most probbly due to the BS8002 design specifiction of n dditionl surchrge lod of 10 KN/m 2 thereby incresing the design surchrge to 35 KN/m 2. Another likely contributory fctor in the present cse is the dditionl unplnned excvtion of 0.7 m which represents bout 10% of the height between the dredge line nd the nchor rod (6.7 m for the present cse). All these ccording to BS8002 represents the most unfvourble or onerous loding condition. It should be noted t this juncture tht Crig [1] does not recommend the use of n embedment fctor s pplied in CECP2 on the grounds tht the introduction of such fctor is somewht empiricl. If such fctor were to be utilized it is recommended tht the CECP2 design should be checked by n lterntive method. The force in the nchor rod ccording to BS8002 is pproximtely 75% to 80% lrger thn the conventionl CECP2 estimte. In ddition the totl length of the pile ccording to BS8002 is bout 14% in excess of the CECP2 vlue. Agin both these differences cn be ttributed to the rdiclly different pproch dopted by BS8002 for embedded wlls compred to CECP2. As finl remrk it should be emphsized tht no ttempt is being mde here to generlize the findings of the present investigtion. The ltter should be viewed s being 4574

5 J. Bsic. Appl. Sci. Res., 2(5) , 2012 somewht relted to the problem t hnd nd the results my not be specificlly pplicble to other design scenrios with different soil prmeters nd vrying groundwter conditions. Nevertheless it is still generlly true tht the BS8002 estimtes of both embedment depths nd nchor rod forces re likely to be in excess of those recommended by CECP2 on the premise tht BS8002 sets out delibertely to bse designs on the most severe nd unfvourble loding conditions rther thn for exmple, the utiliztion of fctors of sfety in mnner s suggested by CECP2 which ultimtely bers little reltionship to service lod behviour [4]. Tble 1. Comprison of results obtined using the conventionl CECP2 nd the BS8002 methods Design detils Conventionl CECP2 BS8002 design Active force R (KN/m run) Pssive force R p (KN/m run) Computed depth (m) Embedment depth (m) Overturning moment (KNm/m) Restoring moment (KNm/m) Fctor of sfety Anchor force (KN) Pile length (m) CONCLUSIONS From the results of the present study which involves the design of n nchored sheet pile in cohesionless medium bsed on rel soil design prmeters, the following conclusions my be drwn. (1) The BS8110 design method produces higher penetrtion or embedment depth thn the conventionl CECP2 pproch by mrgin of bout 21%. (2) The BS8002 estimte of force in the nchor rod is bout 75% to 80% lrger thn tht given by CECP2. (3) The observed differences between the results of BS8002 nd CECP2 in respect of embedment depth nd nchor rod force re felt to be due to the more stringent loding conditions stipulted by BS8002 for the design of embedded wlls. REFERENCES 1. Crig, R.F., Crig s Soil Mechnics, 7 th Edition, Tylor nd Frncis, London, pp Civil Engineering Codes of Prctice Joint Committee, Erth Retining Structures (CECP2), Institution of Structurl Engineers, London. 3. British Stndrds Institution, Code of Prctice for Erth Retining Structures (BS8002), British Stndrds Institution, London. 4. Akroyd, T.N.W., Erth Retining Structures: Introduction to the Code of Prctice, The Structurl Engineer, Vol. 74, No. 21, pp Tschebotrioff, G.P., Lrge Scle Erth Pressure Tests with Model Flexible Bulkheds, Finl Report to Bureu of Yrds nd Docks U.S. Nvy, Princeton University, 112 pp. 6. Rowe, P.W., Anchored Sheet Pile Wlls, Proceedings Institution of Civil Engineers London, Prt1, Vol. 1, pp Terzghi, K., Anchored Bulkheds, Trnsctions Americn Society of Civil Engineers, Vol. 119, pp

6 Frnklin nd Olopde Burlnd, J.B., D. M. Potts nd N.M. Wlsh, The Overll Stbility of Free nd Propped Cntilever Retining Wlls, Ground Engineering, Vol. 14, No. 5, pp Potts, D.M. nd J.B. Burlnd, A Numericl Investigtion of the Retining Wlls of the Bell Common Tunnel, Supplementry Report 783, Trnsport nd Rod Reserch Lbortory (TRRL), Crowthorne, Englnd. 10. Pdfield, C.J. nd R.J. Mir, Design of Retining Wlls Embedded in Stiff Cly, Report 104, Construction Industry Reserch nd Informtion Assocition (CIRIA), London. 11. Potts, D.M. nd R.A. Dy, Use of Sheet Pile Retining Wlls for Deep Excvtions in Stiff Cly, Proceedings Institution of Civil Engineers London, Vol. 88, Prt 1, pp Clyton, C.R. I. nd A.V.D. Bic, Discussion of Limit Equilibrium Methods for Free Embedded Cntilever Wlls in Grnulr Mterils by Symons, I.F., Proceedings Institution of Civil Engineers London, Vol. 90, Prt 1, pp Bolton, M.D., Geotechnicl Design of Retining Wlls, The Structurl Engineer, Institution of Structurl Engineers London, Vol. 74, No. 21, pp Powrie, W., Limit Equilibrium Anlysis of Embedded Retining Wlls, Geotechnique, Vol. 46, No. 4, pp Powrie, W., Soil Mechnics Concepts nd Applictions, 2 nd Edition, Spon Press, Oxon, pp nd pp Tomlinson, M.J., Foundtion Design nd Construction, 7 th Edition, Person Prentice Hll, Hrlow, pp Bowles, J.E., Foundtion Anlysis nd Design, 4 th Edition, McGrw-Hill, New York, pp Murthy, V.N.S., Geotechnicl Engineering: Principles nd Prctice of Soil Mechnics nd Foundtion Engineering, 1 st Edition, Mrcel Dekker Inc., New York, pp Frnklin, S.O. nd A.L. Olopde, 2012, A Commentry on the Use of the CECP2 nd BS8002 Methods for the Anlysis nd Design of Cntilever Sheet Piles, Journl of Bsic nd Applied Scientific Reserch, TextRod Publiction, Vol. 2, No. 2, pp Frnklin, S.O. nd A.L. Olopde, 2012, Some Remrks on the Appliction of the CECP2 nd BS8002 Methods for the Anlysis nd Design of Anchored Sheet Pile Wlls, Interntionl Journl of Reserch nd Reviews in Applied Sciences, Acdemic Reserch Publishing Agency, Vol. 11, Issue 1, pp

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