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.

钻孔轴的能力与钻孔方法和钻孔实践的充分性密切相关。尽管有积累的知识和经验，但钻探轴的测量性能存在很大差异。本文介绍了不利的钻孔操作对常规载荷检查后未发现的影响，该载荷对钻孔后的轴侧阻力有影响。大直径钻铲斗在钻孔内产生吸力，这称为活塞效应。本文介绍了3个桩载荷测试的结果，其中两个已进行了仪器测试。结果分析概述了如何通过检查测得的皮肤摩擦系数并与埃及桩测试数据库中的计算偏差进行比较来检测活塞效应。本文还提供了钻孔过程的数值模拟，并在此模拟中近似表示了活塞效应。有限元模型使用耦合分析来计算由于钻孔和活塞效应而在周围土壤中产生的应变作用。反复施加吸力会在孔的大部分区域产生偏应力的滞后循环，从而削弱土壤。在对受活塞影响的桩进行测试之后，在现场进行的其他圆锥体渗透测试（CPT）测量证明，施加在桩周围土壤上的剪切力消除了其峰值强度。因此，在构造之后，与桩的侧摩擦仅在活塞受影响的层的上方和下方相当大的距离上动员了残余强度。