Investigation into Skin Friction of Bored Pile Including Influence of Soil Strength at Pile Base

Traditional pile design methods of separately assessing the shaft and base capacities have been doubted by some model pile tests and numerical analyses. In this study, two case histories in China are investigated to look through the influence of the material strength beneath the pile base on the shaft resistance mobilized under working load and at failure. The case studies involve a series of full-scale load tests on long bored piles instrumented with strain gauges along the shafts. Comparative parameters include the formation of end-bearing soils and the craftwork of base-grouting, all attributing to one unified factor of material strength at the pile base. The field observations reveal that an increase in the material strength at the pile base puts significant and positive impact on mobilizing the shaft resistance. A simple analysis procedure based on soil mechanics principles is likely to capture the mechanism that the slip displacement over the whole shaft and the radial stress acting on the shaft at deep depth are essentially responsible for the phenomena. This study is an attempt to initiate design approaches with optimistic evaluation on the shaft capacity given that special techniques of enhancing the pile base strength have been made.