Load Testing of Instrumented 225-Foot-Long Prestressed Concrete Piles

Abstract

Piles driven through 62 to 77 m (200 to 250 ft) of low-strength clay penetrated up to 23 m (75 ft) into compact fine sand with little increase in driving resistance. At other locations, the piles met refusal after only about 1 m (3.3 ft) of penetration into the sand. Borings indicated a consistently compact bearing stratum. This test program was developed to establish whether piles driven to very low driving resistances at moderate penetration into the lower compact sands would support the 91-Mg (100-ton) design load. The overlying clays provided no permanent pile support, as in some areas they are underconsolidated as a result of site filling. The test program included casting eight electric strain gages and a 7.6-cm (3-in.) diameter hole in 36-cm (14-in.) square prestressed concrete piles. The piles were driven in three sections to low driving resistances with a continuous record of driving stresses. Anchors were installed in the center hole, with rods extending to the surface. The top of the piles were loaded in increments to 3½ times the design load to overcome the loss of load into the thick clay layer. A procedure of performing the load test a short time after the pile was driven and cyclic loading after reaching maximum test load was used to increase the load reaching the lower compact sand stratum. Full-scale laboratory load tests were preformed on a section of pile to establish the variation in elastic modulus to permit accurate estimates of pile load at various depths. The pile load tests demonstrated that the deep compact sand layer was capable of supporting the design loads, even though penetration was terminated at low driving resistances. Substantial portions of the load during the short-term tests were resisted by adhesion in the thick clay layer. The amount of clay adhesion was directly related to the time after driving of the piles.