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Click to edit Master title style,*,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,Screw Piles:,Use and Design,Kristen M.Tappenden,November 2006,Objectives,What are screw piles?,geometry,fabrication,installation,common uses,Why use screw piles?,advantages over conventional pile types,How do we design screw piles?,axial failure models,direct pile design approach:LCPC method,empirical approach:correlates installation effort to axial capacity,What are Screw Piles?,Deep foundations:carry tensile,compressive,and lateral loads,Constructed of helical plates welded to hollow steel pipe,Emergence of Screw Piles,No related engineering literature exists prior to 1950s/1960s,First use of screw piles:,Maplin,Sands light house in the Thames estuary in 1838,Screw Pile Geometries,Terminology,Inter-Helix Spacing Ratio,=,S,/,D,18 cm diameter shaft,35 cm diameter helix,5 meter length,Shaft diameters:11 cm to 32 cm(4 to 12 inches),Helix diameters:,Commonly 2-3 times the shaft diameter,30 cm to 91 cm(12 to 36 inches),Installation,Turning moment applied to the head of screw pile shaft,and pile“twisted”into the ground,Desirable rate of penetration is one helix pitch per revolution,Video Clip:courtesy of ALMITA Manufacturing,Installation Equipment,Screw Pile Advantages,Rapid installation(typ.30 min per pile),Little installation noise or vibration,No casing or dewatering required,Lightweight installation equipment:,soft terrain,areas of restricted access,Sustain load immediately after installation,May be removed and re-used,temporary structures,Resistant to frost heave,Screw Pile Limitations,Not for use in very hard or rocky soils,may sustain damage to the helical plates,piles may be removed and helices checked,Lack of acceptance/understanding in the engineering community,Typical Screw Pile Uses:,Tower foundations,Ft.McMurray,Alberta:27 cm(10 in)shaft,one or two 76 cm(30 in)helices,6 m length,Pipeline foundations,Earth retention systems,Guy wire anchors,Building Foundations:,Warehouses,Multi-family Housing,Commercial Buildings,Modular Homes,Hythe,Alberta:22 cm(8 5/8 in)shaft,single 40 cm(16 in)helix,8 m length,Oil Field Foundations,Temporary Buildings,Pump Jacks,Compressors,Tanks,Typically 18 cm(7 in)shaft,single 40 cm(16 in)helix,7.5 m deep,Screw Pile Failure Models,Cylindrical Shear Model,Individual Plate-Bearing Model,Choice of the most representative model depends on the screw pile geometry,in particular the,Inter-Helix Spacing Ratio(S/D),Cylindrical Shear Model,After,Narasimha,Rao,et al.(1991),Effect of Inter-Helix Spacing Ratio(S/D),1:S/D,1.5,Cylindrical surface fully forms,2:S/D,2,Cylindrical surface begins to deteriorate,3:S/D 4.5,Cylindrical surface nearly non-existent,1,2,3,After,Narasimha,Rao,et al.(1991),Individual Plate Bearing Model,Summary:Failure Models,Cylindrical Shear Model:,Multi-helix screw piles,generally most representative for S/D 2,Axial Capacity Prediction,Theoretical Design Methods,Application of relevant soil strength parameters(,s,u,N,q,N,qu,),Direct Design Approach:LCPC Method,Directly relates results of cone penetration test to ultimate axial screw pile capacity,with no intermediate determination of soil strength parameters,Empirical Approach,Directly correlates measured installation torque to ultimate axial screw pile capacity,Direct Design:LCPC Method,Established design method for predicting the axial capacity of conventional piles,based on site-specific CPT,LCPC method developed in France by the,L,aboratoire,C,entral des,P,onts,et,C,hausees,based on results of many full-scale pile load tests(Bustamante and,Gianeselli,1982),Use of the CPT is advantageous because the test is fast,repeatable,and provides continuous profile of soil information,Direct Design:LCPC Method,Basic premise of LCPC method is to apply scaling(reduction)factors to CPT profile of tip resistance to calculate appropriate components of bearing resistance and friction/adhesion,Q,total,=,Q,bearing,+,Q,shaft,+,Q,cylinder,Direct Design:LCPC Method,LCPC Calculation,Two 36 cm helices,Spacing=3D,21 cm shaft,q,s,=35,kPa,q,b1,=811,kPa,q,b2,=990,kPa,Calculated Capacity in Compression:,Cylindrical Shear:188,kN,Individual Plate Bearing:209,kN,Calculated Capacity in Tension:,Cylindrical Shear:160,kN,Individual Plate Bearing:180,kN,Measured Capacity:210,kN,in both tension and compression,after Zhang(1999),LCPC MethodCompression,LCPC Method,26 axial load tests,7 test sites:clay,sand,clay shale,glacial till,LCPC Method,Empirical Torque Correlation,Direct empirical relationship between torque required to install a given screw pile and the piles ultimate axial capacity,Q,ultimate,=K,t,T,(after Hoyt and,Clemence,1989),Analogous to relationship between pile driving effort and pile capacity used for driven steel piles,Can only predict capacity once pile is installed best used for field-level verification of expected design capacities,Torque Correlation,Torque Correlation,29 screw pile axial load tests,10 test sites:san
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