DESIGNOFSHEETPILERETAININGWALLSGENERALCONSIDERATIONSThe design of sheet pile retaining walls requires several successive operations: (a)evaluation of the forces and lateral pressures that act on the wall, (b) determination of thecomputationofthemaximumbendingmomentsrequired depth of piling penetration,(c)inthepiling,(d)computationof theinthe wall and selection of the appropriateTESpiling sectionand(e)thedesignoftheanchorage system.Before these opera-walingnctions can be initiated,howeverinformationmust be obtained.Inoreliminalparticular,the controlling dimensionshese include the elevation of the topof the wall,the elevation of thof the wall (commonlycalled theordredge line),the maximumtide levelor normal pool elevation andaterDVEnearthelowwaterlevel.Atopographicalarea is also helpful.Earthpressuretheorieshavedevelopedtothepointwhereitispossibletoobtainreliableestimates oftheforcespilewailsexertedbyhomogeneouslayersof soilsheetwithknownphysicalconstantsuncertaintiess involved inthe design of sheet pilestructures no longer result frominadequateknowledge of the fundamentals involved.They are caused bythefactnatural soil deposits is usually quiteOsfrucfurecomplex,whereasthetheoriesbulkheadesiqrinevitablypresupposehomogeneousmaterials.Becauseessential that a subsurface investigation beoftheseconditionsnrperformedwithexploratorylaboratonytestsof representativesamples.Onboringancanthis basis,asoilprofilecanbedrawnand the engineering properties of the different soilstrata canbedeterminedproperties should reflect the field conditionsRaccuratelyTheseunder which the wall is expected to operate. Only after these preliminary steps are takenshould the final design be undertaken.There are two basic types of steel sheet pile walls: cantilevered walls and anchoredwalls.The design of each type for various subsurface conditions will be discussed in thefollowing sections.CANTILEVERWALLSIn the case of a cantilevered wall,sheet piling is driven to.a sufficient depth into theground to become fixed as a verticalin resisting the lateral active earthcantileverpressure. This type of wall is suitable for moderate height. Walls designed as cantileversF.usually undergo large lateral deflectionsandare readily affected by scour and erosion inFfront of the wall..Since thelateralforacantileveredwall comesfrom passivesupportpressure exertedontheembeddedportion,penetration depths can be quite high,Therefore, cantilevered walls using steelresultingis excessive stressesandsevereyield.sheet piling are restricted to aheight of approximately 15 feet.maximumEarth pressure against a cantileveredwall is illustratedinFigure14.Whenthelateralactive pressure (P) is applied to the top of the wall, the piling rotates about the pivotpoint, b, mobilizing passive pressure above and below the pivot point. The term (Pp-Pa) isthe net passive pressure,Pp,minus the active pressure,Pa-(Since both are exerting pres-sure upon the wall.)(po-(a)(b)Fig. 14 - Earth pressure on cantilever sheet piling (after Teng')19
DESIGN OF SHEET PILE RETAINING WALLS GENERAL CONSIDERATIONS The design of sheet pile retaining walls requires several successive operations: (a) evaluation of the forces and lateral pressures that act on the wall, (b) determination of the required depth of piling penetration, (c) computation of the maximum bending moments in the piling, (d) computation of the stresses in the wall and selection of the appropriate piling section and (e) the design of the waling and anchorage system. Before these operations can be initiated, however, certain preliminary information must be obtained. In particular, the controlling dimensions must be set. These include the elevation of the top of the wall, the elevation of the ground surface in front of the wall (commonly called the dredge line), the maximum water level, the mean tide level or normal pool elevation and the low water level. A topographical survey of the area is also helpful. Earth pressure theories have developed to the point where it is possible to obtain reliable estimates of the forces on sheet pile wails exerted by homogeneous layers of soil with known physical constants. The uncertainties involved in the design of sheet pile structures no longer result from an inadequate knowledge of the fundamentals involved. They are caused by the fact that the structure of natural soil deposits is usually quite complex, whereas the theories of bulkhead design inevitably presuppose homogeneous materials. Because of these conditions, it is essential that a subsurface investigation be performed with exploratory borings and laboratory tests of representative samples. On this basis, a soil profile can be drawn and the engineering properties of the different soil strata can be accurately determined. These properties should reflect the field conditions under which the wall is expected to operate. Only after these preliminary steps are taken should the final design be undertaken. There are two basic types of steel sheet pile walls: cantilevered walls and anchored walls. The design of each type for various subsurface conditions will be discussed in the following sections. CANTILEVER WALLS In the case of a cantilevered wall, sheet piling is driven to. a sufficient depth into the ground to become fixed as a vertical cantilever in resisting the lateral active earth pressure. This type of wall is suitable for moderate height. Walls designed as cantilevers usually undergo large lateral deflections and are readily affected by scour and erosion in front of the wall. Since the lateral support for a cantilevered wall comes from passive pressure exerted on the embedded portion, penetration depths can be quite high, resulting is excessive stresses and severe yield. Therefore, cantilevered walls using steel sheet piling are restricted to a maximum height of approximately 15 feet. Earth pressure against a cantilevered wall is illustrated in Figure 14. When the lateral active pressure (P) is applied to the top of the wall, the piling rotates about the pivot point, b, mobilizing passive pressure above and below the pivot point. The term (pp -pa ) is the net passive pressure, pP, minus the active pressure, pa . (Since both are exerting pressure upon the wall.) Fig. 14 - Earth pressure on cantilever sheet piling (after Teng1 ) 19
Atpointbthe pilingdoes not move and would be subjectedto equal and opposite at-restearth pressures with a net pressure equal to zero. The resulting earth pressure isrepresented bythe diagramoabc.Forthepurpose of design,thecurveabcisreplaced bya straight linedc.Thepoint d is located so as to make thesheet piling in a state of staticequilibrium.Although the assumed pressure distribution is in error,it is sufficient fordesign purposes.The distribution of earth pressure is different for sheet piling in granular soils andsheet piling in cohesive soils. Also, the pressure distribution in clays in likely to changewith time.Therefore,the design procedures for steel sheetpiling in both types of soils arediscussedseparatelyCantileverSheet Piling inGranular Soils-Acantilevered sheetpilewall may bedesigned in accordance with the principles and assumptions just discussed or by anapproximate method based on further simplifying assumptions shown in Figure 15.Backfill3Backfill量t1HDredgeOredigeActivepressureActivepressurelin789Passive earthoressur001-0PpPaPaDoNet passiveresistancPp-Pa(b)-Pa(a)Fig. 15 - Design of cantilever sheet piling in granular soils: (a) conventional method:(b) simplified method. (after Teng')For casesof two or more layersof soil,the earth pressure distributions would besomewhat different due to the different soil properties;however,the design concept isexactly the same.Lateral pressures should be calculated usingthe curvedfailure surface(log spiral)method asshown in Figure5 (a).Conventional Method-The conventional design procedure for granular soils is asfollows:1.Assumeatrial depthof penetration,D.This may beestimatedfromthe followingapproximatecorrelation.Standard PenetrationDepth ofRelative DensityResistance,NBlows/Footof Soil, DaPenetration*0-42.0 HVeryloose5-10Loose1.5 H1.25 H11-30Medium denseDense1.0 H31-50+500.75 HVery dense*H = height of piling above dredge line.2.Determine the active and passive lateral pressures using appropriate coefficients oflateral earthpressure.If the Coulomb method isused,it should be usedfortheThe resultingconservativelypassivecase.earthpressure diagram forahomogeneousgranular soil is shown in Figure16Swheretheactiveandpassiveareoverlaintopictoriallydescribetheresulting soil reactions.pressures20
The distribution of earth pressure is different for sheet piling in granular soils and sheet piling in cohesive soils. Also, the pressure distribution in clays in likely to change with time. Therefore, the design procedures for steel sheet piling in both types of soils are discussed separately. Cantilever Sheet Piling in Granular Soils - A cantilevered sheet pile wall may be designed in accordance with the principles and assumptions just discussed or by an approximate method based on further simplifying assumptions shown in Figure 15. At point b the piling does not move and would be subjected to equal and opposite at-rest earth pressures with a net pressure equal to zero. The resulting earth pressure is represented by the diagram oabc. For the purpose of design, the curve abc is replaced by a straight line dc. The point d is located so as to make the sheet piling in a state of static equilibrium. Although the assumed pressure distribution is in error, it is sufficient for design purposes. (b) Fig. 15 - Design of cantilever sheet piling in granular soils: (a) conventional method; (b) simplified method. (after Teng1 ) For cases of two or more layers of soil, the earth pressure distributions would be somewhat different due to the different soil properties; however, the design concept is exactly the same. Lateral pressures should be calculated using the curved failure surface (log spiral) method as shown in Figure 5 (a). Conventional Method - The conventional design procedure for granular soils is as follows: 1. Assume a trial depth of penetration, D. This may be estimated from the following approximate correlation. Standard Penetration Resistance, N Blows/Foot Relative Density of Soil, Dd Depth of Penetration* l 0-4 Very loose 5-10 Loose 1l-30 Medium dense 31-50 Dense +50 Very dense 2.0 H 1.5 H 1.25 H l.0 H 0.75 H *H = height of piling above dredge line. 2. Determine the active and passive lateral pressures using appropriate coefficients of lateral earth pressure. If the Coulomb method is used, it should be used conservatively for the passive case. The resulting earth pressure diagram for a homogeneous granular soil is shown in Figure 16 where the active and passive pressures are overlain to pictorially describe the resulting soil reactions. 20
OPOFGROUNDPY(H+D(H+D)KYOKp(H+D)KoKDFig. 16 -Resultant earth-pressure diagram3Satisfythe requirements of static equilibrium:thesum of the forces in thehorizontal direction must be zero and the sum of the moments about any pointmustbezero.The sumof thehorizontal forcesmaybe writtenintermsofpressureareas:△(EA1 A2) -△(FBA2) -△(ECJ) = 0Solve the above equation for the distance, Z. For a uniform granular soil,Z - KpD’-Ka(H+D)2(Kp-Ka) (H+2D)Takemoments about thepoint Fand checkto determineif thesum of themoments isequaltozero,as itmustbe.Readjustthedepthofpenetration,D,andrepeat until convergence is reached; i.e.,the sum of the moments about F is zero4.Add20to4o percenttothecalculateddepthofpenetration.Thiswillgiveasafety factor of approximately 1.5 to 2.0. An alternate and more desirablemethod isthe use of a reduced value of thepassive earthpressure coefficient fordesign. The maximum allowable earth pressure should be limited to 50 to 75percenttotheultimatepassiveresistance5.Computethe maximum bending moment,which occurs at the point-ofzero shearpriortoincreasingthedepthby20to40percent.A roughestimateof thelateral displacementmaybeobtainedbyconsideringthewallto be rigidly held at an embedment of /D and subjected to a triangular load distributionapproximating the actual applied active loading. The displacement at any distance y fromthetop of the pile is thengivenbythe following expression:Pt: (y5-504y+405)Ae60EIQ221
Fig. 16 - Resultant earth-pressure diagram 3. Satisfy the requirements of static equilibrium: the sum of the forces in the horizontal direction must be zero and the sum of the moments about any point must be zero. The sum of the horizontal forces may be written in terms of pressure areas: Solve the above equation for the distance, Z. For a uniform granular soil, Take moments about the point F and check to determine if the sum of the moments is equal to zero, as it must be. Readjust the depth of penetration, D, and repeat until convergence is reached; i.e., the sum of the moments about F is zero. 4. Add 20 to 40 percent to the calculated depth of penetration. This will give a safety factor of approximately 1.5 to 2.0. An alternate and more desirable method is the use of a reduced value of the passive earth pressure coefficient for design. The maximum allowable earth pressure should be limited to 50 to 75 percent to the ultimate passive resistance. 5. Compute the maximum bending moment, which occurs at the point-of zero shear, prior to increasing the depth by 20 to 40 percent. A rough estimate of the lateral displacement may be obtained by considering the wall to be rigidly held at an embedment of D and subjected to a triangular load distribution approximating the actual applied active loading. The displacement at any distance y from the top of the pile is then given by the following expression: 21
MOMENTRATIOPERFT.WIDTH,Mmax"K,H9'0O1S'l02gz00SEo0010608OL'LMLM INDAM:=SNEONElos10:RMIONS NI NAANS8==N':8::...A...4：8E3DENESEE2Dx/dy00SS30608O7654=0.7500=03Sa0.50SL0-0a3090-000=042509'0S02091oDEPTHRATIO,D/H(INCREASEBY20%TO40%FORS.F.)22
MOMENT RATIO PER FT. WIDTH, Mmax/ KaH 3 DEPTH RATIO, D/H (INCREASE BY 20% TO 40% FOR S.F.) 22
Ag=displacement in incheswherePt =total applied load over length in poundsQ =H+%Din inchesH=exposedlength of sheeting in inchesin whichandD=penetration of sheeting insurface stratum,plus one-half ofpenetration in any lower, more dense, coarse grainedstratum. Neglect any penetration in rock (inches).Simplified Method-A simplified method of design isillustrated in Figure 15(b).Thepassive resistancesare simplifiedbyassuminga righttriangularpressureon theleft sideofthe piling and by substitution of a concentrated force C for the net passive resistance onthe right side of the piling. This method results in some error but saves greatly in thecomputations.The distance,Do,must satisfy both the requirements of equilibrium.Thecalculated value of D。 should be increased by 20 to 40 percent to get the total designdepthof penetration.Figure 18 gives a useful method to design cantilever sheet piling in homogeneousgranular soil,analyzed by the conventional method.This chart allows the designer toobtain directly the depth ratio, D/H, and the maximum moment ratio, Mmax/'KaH3asa function of the ratio of passive to active pressure coefficients, Kp/Ka.for variouspositions of water level. It is, therefore, independent of the method of obtaining K orKa.The chartwas developed for a wet unitweight,,equal to twice the submerged unitweight, ,.To use Figure 18, one may determine and from Table 2, from Table4and K/K and K, from Figure 3 (a). A design example is given at the end of problemNo. 1 (pages 86-90).Cantilever Sheet Piling in Cohesive Soils-Two cases of cantilevered walls in cohesivesoils are of interest:(1)sheet pile walls entirely in clay and (2)walls driven in clay andbackfilledwithsand.Differentlateralearthpressuresdevelopforeachcase.Wall Entirely in Cohesive Soil-Designof sheetpiling incohesivesoils iscomplicatedbythefactthatthestrengthof claywithtime and,accordingly.thelateral earthchanpressures also change withtimepenetration and the size of pilingmustsatisfy the pressureafter installation and the long-termcondition0edlateconditions after the strengtImmediatelyafterthe sheetpiling isangeinstalled,earthpressureassumptionthatundrained strengthof2the clay prevails.derives all its strength from cohesionLhatisclavand no strength from internal frictionanalysis is usually carried out interms of totalhestress using a cohesion value,to one-half the unconfined compressive strength,eoualqu. The method is usually referred to as a "β= O"analysis.Figure 19 illustrates the initial pressure conditions for sheet piling embedded incohesive soil for its entire depth.Originafunconfined compdustrength of clayground2c:Y,=unit weight (effective)of soilIzPassivepressur.DredgelineT,H-quActive pressureActivexrePa-eZ-quPa=Ta(Z-H)-apressureP,-7e(Z-H)+q,Y.H2au,H(c)(a)(b)Fig. 19 - Initial earth pressure for design of cantilever sheet piling entirely on cohesive soil (after Teng')23
Simplified Method - A simplified method of design isillustrated in Figure 15(b). The passive resistances are simplified by assuming a right triangular pressure on the left side of the piling and by substitution of a concentrated force C for the net passive resistance on the right side of the piling. This method results in some error but saves greatly in the computations. The distance, Do, must satisfy both the requirements of equilibrium. The calculated value of Do should be increased by 20 to 40 percent to get the total design depth of penetration. Figure 18 gives a useful method to design cantilever sheet piling in homogeneous granular soil, analyzed by the conventional method. This chart allows the designer to obtain directly the depth ratio, D/H, and the maximum moment ratio, as a function of the ratio of passive to active pressure coefficients, Kp /Ka , for various positions of water level. It is, therefore, independent of the method of obtaining Kp or Ka . The chart was developed for a wet unit weight, equal to twice the submerged unit weight, . To use Figure 18, one may determine and from Table 2, from Table 4 and KP/Ka and Ka from Figure 3 (a). A design example is given at the end of problem No. 1 (pages 86-90). Cantilever Sheet Piling in Cohesive Soils - Two cases of cantilevered walls in cohesive soils are of interest: (1) sheet pile walls entirely in clay and (2) walls driven in clay and backfilled with sand. Different lateral earth pressures develop for each case. Figure 19 illustrates the initial pressure conditions for sheet piling embedded in cohesive soil for its entire depth. Wall Entirely in Cohesive Soil - Design of sheet piling in cohesive soils is complicated by the fact that the strength of clay changes with time and, accordingly, the lateral earth pressures also change with time. The depth of penetration and the size of piling must satisfy the pressure conditions that exist immediately after installation and the long-term conditions after the strength of the clay has changed. Immediately after the sheet piling is installed, earth pressure may be calculated on the assumption that undrained strength of the clay prevails. That is, it is assumed that the clay derives all its strength from cohesion and no strength from internal friction. The analysis is usually carried out in terms of total stress using a cohesion value, c, equal to one-half the unconfined compressive strength, qu . The method is usually referred to as a = analysis. where = displacement in inches = total applied load over length in pounds = H + D in inches in which = exposed length of sheeting in inches and D = penetration of sheeting in surface stratum, plus one-half of penetration in any lower, more dense, coarse grained stratum. Neglect any penetration in rock (inches). Fig. 19 - Initial earth pressure for design of cantilever sheet piling entirely on cohesive soil (after Teng1 ) 23