Detecting Piston Effect on Drilled Shafts Side Resistance Using Instrumented Pile Load Tests

Abstract

The capacity of drilled shafts is strongly related to the method and adequacy of drilling practices. Despite the accumulated knowledge and experience, there is significant variability in the measured performance of drilled shafts. This paper describes the effect of unfavorable drilling practice, not detected with routine checks, on the shaft side resistance after loading. Large diameter drilling buckets create suction inside the drilled hole which is called the piston effect. The paper presents the results of 3 pile load tests, two of which are instrumented. The analysis of results outlines how piston effect can be detected by the examination of measured skin friction coefficients and comparison with calculated bias in an Egyptian pile test database. The paper also presents a numerical simulation of the drilling process and an approximate representation of the piston effect in this simulation. The finite elements model uses coupled analysis to calculate the straining actions in the surrounding soil due to drilling and piston effect. The repeated application of suction leads to hysteresis cycles of deviatoric stresses at large portions of the hole, weakening the soil. Additional cone penetration test (CPT) measurements in the field after performing the test on the piston affected pile prove that the shear applied to the soil around the pile dissipates its peak strength. Hence, side friction with the pile after construction mobilizes only residual strength for a considerable distance above and below the piston affected layer. The paper outlines the indicators needed to detect piston effect in pile load test results.