The Study on End Bearing Capacity of Deep Foundation in Highly Compressible sandy Soils

为了弄清砂土特别是碳酸盐砂等高压缩性砂土的变形和强度特性与桩端承载力的关系，采用3种不同相对密度的可压缩性砂土进行了一系列的三轴压缩和模型桩试验。在试验和理论分析的基础上，提出了将空腔扩张压力与桩端极限承载力联系起来的破坏机制。得到了以下主要结果。1)三轴试验结果表明，尽管碳酸盐砂的内摩擦割线角较大，但与两种硅质砂相比，碳酸盐砂的可压缩性明显提高，刚性明显降低。2)研究发现，在高压缩性砂土的情况下，即使获得较大的内摩擦角，桩端承载力仍然较低。这一结果表明，在评价桩端承载力时应考虑各种砂土的可压缩性。3)利用所提出的破坏机理进行分析，可以合理估算高压缩性砂土的桩端承载力，估算材料的可压缩性、刚度和破坏包络曲线。

In order to clarify the relationship between the deformation and strength properties and the pile end bearing capacity of sands,especially highly compressible sands such as carbonate sands,a series of triaxial compression and model pile tests were performed using three kinds of different compressible sands with various relative density.Further-more, a failure mechanism for linking the cavity expansion pressure to the ultimate pile end bearing capacity was proposed on the basis of the experimental and theoretical considerations. The following main results were obtained.1)It was recognized in the triaxial test results that despite the high secant angle of internal friction,the carbonate sand used was markedly higher compressible and less stiff, comparing with two kinds of siliceous sands.2)It was found that,even if much higher internal friction angle was obtained,the low pile end bearing capacity could be observed in the case of highly compressible sands.This result indicates that the compressibility of each sand should be considered in the evaluation of pile end bearing capacity.3)The analysis using the failure mechanism presented can reasonably estimate the pile end bearing capacity of highly compressible sands,estimating the material compressibility, stiffness and failure envelope curveture.