《高等土力学》课程教学资源（书籍文献）United States Steel - Steel Sheet Piling Design Manual.pdf_P16-P20

SURCHARGELOADSThe function of a sheet pile structure is often to retain various surface loadings as well asthe soil behind it.These surface loads,or surcharge,also exert lateral pressures on thewall which contribute to the active pressure tending to move the wall outward. Typicalsurchargeloadings arerailroads,highways,buildings,orepiles,cranes,etc.The loading casesof particular interest in the determination of lateral soil pressuresare:1.Uniform Surcharge2.Point Loads3.Line Loads Parallel to the Wall4.Strip Loads Parallel tothe WallFor the case of a uniform surcharge loading, the conventional theories of earth pressurecan be effectively utilized.On the other hand, for point, line and strip loads the theory ofelasticity (Boussinesq Analysis) modified by experiment provides the most accuratesolutions. These solutions are summarized in Foundation Design by Wayne C. Teng' and"Anchored Bulkheads"by Karl Terzaghi.22Uniform Surcharge-When a uniformly distributed surcharge is applied at the surface,the vertical pressuresat all depths in the soil are increased equally.Without the surchargethe vertical pressure at any depth h would be h, where is the unit weight of the soil.When a surcharge of intensity q (force/area) is added, the vertical pressures at depth hbecomeYh+q.The lateral pressure, OH, due to the uniform surcharge q. is equal to Kq, as shown inFigure 7below.q lb/ft*OH (due to q) =qK (lb/ft2)Fig.7- Lateral pressure due to uniform surchargeThe K value is either the active coefficient Ka or the passive coefficient K,dependingupon whether thewall tendstofromortowardthesurcharge area.Theawayuniformlateralthen added to the lateral dead weightpresscnaroeearthpresstas desFor the caseiovementoftheplaneonwhichofthehorizontalaccountbyconsidering thattheentire"activefailure.Onthe other hand,computationslimited area(point,line andstriploads)is complicatedoachto the distribution of shearstresses inthe soil adjaceTherefore,semi-empiricalmethodsofanalysis have beendevelopedelastic theory and experiments on rigidunyieldingwalls. The lateral pressurescomputedbythesemethodsareconservativeforsheet pilewalls since,asthewall deflects,soil shearresistance ismobilizedand the lateralpressure on the wall in reduced

SURCHARGE LOADS The function of a sheet pile structure is often to retain various surface loadings as well as the soil behind it. These surface loads, or surcharge, also exert lateral pressures on the wall which contribute to the active pressure tending to move the wall outward. Typical surcharge loadings are railroads, highways, buildings, ore piles, cranes, etc. The loading cases of particular interest in the determination of lateral soil pressures are: 1. Uniform Surcharge 2. Point Loads 3. Line Loads Parallel to the Wall 4. Strip Loads Parallel to the Wall For the case of a uniform surcharge loading, the conventional theories of earth pressure can be effectively utilized. On the other hand, for point, line and strip loads the theory of elasticity (Boussinesq Analysis) modified by experiment provides the most accurate solutions. These solutions are summarized in Foundation Design by Wayne C. Teng1 and “Anchored Bulkheads” by Karl Terzaghi.22 Uniform Surcharge - When a uniformly distributed surcharge is applied at the surface, the vertical pressures at all depths in the soil are increased equally. Without the surcharge the vertical pressure at any depth h would be where is the unit weight of the soil. When a surcharge of intensity q (force/area) is added, the vertical pressures at depth h become The lateral pressure, due to the uniform surcharge q, is equal to Kq, as shown in Figure 7 below. Fig. 7 - Lateral pressure due to uniform surcharge The K value is either the active coefficient Ka or the passive coefficient Kp depending upon whether the wall tends to move away from or toward the surcharge area. The uniform lateral pressure due to the surcharge is then added to the lateral dead weight earth pressures as described in previous sections. For the case of a uniform surcharge loading, lateral movement of the plane on which the horizontal stresses are being computed is taken into account by considering that the entire “active wedge” of soil is in a state of impending shear failure. On the other hand, computations of lateral stresses due to surcharge applied on a limited area (point, line and strip loads) is complicated by the lack of a rational approach to the distribution of shear stresses in the soil adjacent a yielding vertical plane. Therefore, semi-empirical methods of analysis have been developed based upon elastic theory and experiments on rigid unyielding walls. The lateral pressures computed by these methods are conservative for sheet pile walls since, as the wall deflects, soil shear resistance is mobilized and the lateral pressure on the wall in reduced

EFFECTSOFUNBALANCEDHYDROSTATICANDSEEPAGEFORCESSheet pile structures built today in connection with waterfront facilities are subjected tomaximumearthpressurewhenthetide orriverlevel is atitsloweststage.Arecedingtidereceding high water,or heavy rainstorm may causeahigherwaterlevel behind a sheetpilewall than infront of it,dependinggon the type of backfill used. If the backfill is fine orsilty sand, the height of water behind the sheet pile wall may be several feet. If the soilbehind the wall is silt or clay, full hydrostatic pressure in back of the wall should be as-sumed up to the highest position of the previous water level.The difference in watereithersideof the wall introduces (1)additionallevelonpressureonthe backof thewallhydrostatic load and (2)reduction in the unitduetoweight of the soil in front of the piling (thus, a reduction of passive resistance).Thedistribution of the unbalanced waterpressureonthetwofacesofthestructurecorresponding to a hydraulic head,Hu,can be determined by meansof the flow netmethod as illustrated in Figure 12(a).If a sheet. pile structure is driven in granular soilwithfairlyuniformpermeability,theunbalancedwaterpressuremay beapproximatedbythe trapezoid in Figure 12(b). If the permeability of the soil varies greatly in the verticaldirection,aflow netshould beusedtodeterminethe unbalancedpressure.Hu2.5H,(a)(b)Fig.12-Hydrostatic and seepage pressures (after Terzaghi)The upward seepage pressure exerted by the rising ground water in front of the outerface of a sheet pile wall reduces the submerged unit weight in front of the wall byapproximately the amount:Hu4=20Dwhere=reduction in submergedunitweightof soil,pcf.Hence,the effectiveunitweightto be used in the computation of passive pressureisHu-4"=-20DwhereH=unbalanced water head, feetD = as shown in Figure 12The relationship between and Hu/D is given in Figure 1310.40.60.810vaofgFig.13-Average reduction of effective unit weight of passive wedge due to seepage pressure exertedbytheupwardflowofwater(afterTerzaghi17

EFFECTS OF UNBALANCED HYDROSTATIC AND SEEPAGE FORCES Sheet pile structures built today in connection with waterfront facilities are subjected to maximum earth pressure when the tide or river level is at its lowest stage. A receding tide, receding high water, or heavy rainstorm may cause a higher water level behind a sheet pile wall than in front of it, depending on the type of backfill used. If the backfill is fine or silty sand, the height of water behind the sheet pile wall may be several feet. If the soil behind the wall is silt or clay, full hydrostatic pressure in back of the wall should be assumed up to the highest position of the previous water level. The difference in water level on either side of the wall introduces (1) additional pressure on the back of the wall due to hydrostatic load and (2) reduction in the unit weight of the soil in front of the piling (thus, a reduction of passive resistance). The distribution of the unbalanced water pressure on the two faces of the structure corresponding to a hydraulic head, HU, can be determined by means of the flow net method as illustrated in Figure 12(a). If a sheet. pile structure is driven in granular soil with fairly uniform permeability, the unbalanced water pressure may be approximated by the trapezoid in Figure 12(b). If the permeability of the soil varies greatly in the vertical direction, a flow net should be used to determine the unbalanced pressure. Fig. 12 - Hydrostatic and seepage pressures (after Terzaghi22) The upward seepage pressure exerted by the rising ground water in front of the outer face of a sheet pile wall reduces the submerged unit weight in front of the wall by approximately the amount: where = reduction in submerged unit weight of soil, pcf. Hence, the effective unit weight to be used in the computation of passive pressure is where H u = unbalanced water head, feet D = as shown in Figure 12 The relationship between and HU/D is given in Figure 13. Fig. 13 - Average reduction of effective unit weight of passive wedge due to seepage pressure exerted by the upward flow of water (after Terzaghi22) 17

The effect of downward seepage in the soil behind the piling is very small and may beneglected.It must be anticipated that some seepage will occur through interlocks, although theamount is difficult to predict. As an approximation, the seepage should be assumed toequalatleast0.025gallonsperminutepersquarefootofwall perfootofnetheadacrossthe wall for installations in moderately to highly permeable soils.OTHERLATERALLOADSIn addition tothe lateral pressures described previously,sheet pile structures maybesubjectedtosomeofthelateralloadsdescribedbelow.Ice Thrust -Lateral thrusts can be caused by the volume expansion of ice infine-grained soils (very fine sand, silt and clay). The possibility of lateral thrust from iceorfrozen ground should be eliminated by placingfree-draining coarse granular soil abovethe frost line behind a sheet pile wall. Steel sheet piling also offers the advantage that itcanyield laterallyto relieve anythrust loaddue toice.Wave Forces - There are many theories concerning wave pressure against a verticalsurface. In general, wave pressure is a function of wave height, length, velocity and manyotherfactors.The reader is directed to the following references for a detailed explanationof methods of analysis.Design and Construction of Ports and Marine StructuresbyAlonzo DeF. Quinn, Substructure Analysis and Design by Paul Anderson, PileFoundations by Robert D. Chellis and Shore Protection, Planning and Design, TR No. 4Dept.oftheArmy,CorpsofEngineers.Ship impact - Sheet pile dock and waterfront structures may often be subjected to thedirect impact of a moving ship. Fender systems should be used in this case to spread outthe reaction and reduce the impact to a minimum.Allowance for the effect of a shipsimpact is sometimes made by the inclusion of an arbitrary horizontal force such as 50 to100 tons.The reader is directed to the above mentioned references for further discussion.Mooring Pull -Sheet pile dock and water front structures generally provide mooringposts for anchoring and docking ships. The magnitude of the mooring pull in thedirection of the ship may be taken as the winch capacity used on the ship. When thespacing of themooringposts is known,an evaluationof moorpostpull on the structurecan be made.Earthquake Forces-During an earthquake the vibration of the ground maytemporarily increase the lateral pressure against a retaining structure.This increase is aresult of a number of factors including inertia force, direction, horizontal accelerationand period. For the design of retaining walls of moderate height, the lateral pressure fordesign may be increased by about 10 per cent. In the case of high retaining structures, thetrial wedge method of analysis should beused.The trial sliding wedge is assumed to beacted upon by a horizontal force in additional to all other forces.Some engineers assumethatthe horizontal force isequal to 18to 33percent of the weight of the slidingwedge.Thedesigner,of course,should consider the location of the structure relative to previousearthquakehistory

The effect of downward seepage in the soil behind the piling is very small and may be neglected. It must be anticipated that some seepage will occur through interlocks, although the amount is difficult to predict. As an approximation, the seepage should be assumed to equal at least 0.025 gallons per minute per square foot of wall per foot of net head across the wall for installations in moderately to highly permeable soils. OTHER LATERAL LOADS In addition to the lateral pressures described previously, sheet pile structures may be subjected to some of the lateral loads described below. Ice Thrust - Lateral thrusts can be caused by the volume expansion of ice in fine-grained soils (very fine sand, silt and clay). The possibility of lateral thrust from ice or frozen ground should be eliminated by placing free-draining coarse granular soil above the frost line behind a sheet pile wall. Steel sheet piling also offers the advantage that it can yield laterally to relieve any thrust load due to ice. Wave Forces - There are many theories concerning wave pressure against a vertical surface. In general, wave pressure is a function of wave height, length, velocity and many other factors. The reader is directed to the following references for a detailed explanation of methods of analysis. Design and Construction of Ports and Marine Structures by Alonzo DeF. Quinn, Substructure Analysis and Design by Paul Anderson, Pile Foundations by Robert D. Chellis and Shore Protection, Planning and Design, TR No. 4 Dept. of the Army, Corps of Engineers. Ship impact - Sheet pile dock and waterfront structures may often be subjected to the direct impact of a moving ship. Fender systems should be used in this case to spread out the reaction and reduce the impact to a minimum. Allowance for the effect of a ships’ impact is sometimes made by the inclusion of an arbitrary horizontal force such as 50 to 100 tons. The reader is directed to the above mentioned references for further discussion. Mooring Pull - Sheet pile dock and water front structures generally provide mooring posts for anchoring and docking ships. The magnitude of the mooring pull in the direction of the ship may be taken as the winch capacity used on the ship. When the spacing of the mooring posts is known, an evaluation of moor post pull on the structure can be made. Earthquake Forces - During an earthquake the vibration of the ground may temporarily increase the lateral pressure against a retaining structure. This increase is a result of a number of factors including inertia force, direction, horizontal acceleration and period. For the design of retaining walls of moderate height, the lateral pressure for design may be increased by about 10 per cent. In the case of high retaining structures, the trial wedge method of analysis should be used. The trial sliding wedge is assumed to be acted upon by a horizontal force in additional to all other forces. Some engineers assume that the horizontal force is equal to 18 to 33 percent of the weight of the sliding wedge. The designer, of course, should consider the location of the structure relative to previous earthquake history