Engineering Approach to the Evaluation of Bearing Capacity of Pile Considering the Ground Crushability

可压碎的桑迪土壤在世界各地都有，其特征是压缩性很强。在此类地基中，桩基存在承载力低、沉降大的工程问题。为了建立合理的桩基承载力评价方法，本文对桩基承载力的发挥机理进行了研究。在此基础上，根据单颗粒破碎试验、三轴试验、环剪试验和模型桩载荷试验的结果，讨论了应力条件和土的性质对砂土承载力的影响，得到的主要结论是：（1）可压碎土颗粒一般有棱角，与标准土工材料相比容易破碎。可压碎土具有高压缩性、大剪切模量和大内摩擦角等材料特性。而要获得足够的土体强度，需要较大的变形。(2)对于这种土，桩的抗沉降动员承载力低得多的机制是清楚的。桩端承载力是由可压碎土的材料特性和桩周边界应力的影响引起的。(3)对于表面光滑的桩，在发生滑动破坏时，桩与土之间通常会产生摩阻力。在这种情况下，它可以表示为库仑定律，这是摩擦角和法向压力的分量在不同的破碎性的沙子。另一方面，注意在大应力比的情况下，估算表面摩擦力。(4)根据（1）~（3）的计算结果，可以得出一种合理的承载力评价方法。

Crushable sandy soils can be found throughout the world and are characterized very compressible. It have engineering problems of very low bearing capacity and large settlement of the pile foundation in such grounds. In this study, in order to develop a rational geotechnical method for the evaluation of the bearing capacity of pile, it showed the mechanisms of the mobilized bearing capacity. And then the influence of the stress condition and the soil properties on the bearing capacity in sands is discussed based on the theoretical considerations and the experimental results of the individual particle fragmentation tests, the triaxial tests, the ring shear tests and the loading tests of model pile in some types of sand with different crushabilities at various constant stress levels.The main conclusions obtained are :(1) Crushable soil particles are generally angular and easy to break as compared with standard geomaterials. The crushable soils have material properties such as high compressibility, large shear modulus and large internal friction angle. And a large deformation is needed to get enough soil strength.(2) For such soils, the mechanism for the much lower mobilizing bearing capacity of the pile against settlement is clear. The end bearing capacity is caused by the material properties of crushable soils and by the effects of boundary stress around the pile.(3) The skin friction of the pile with a smooth surface is usually occurred between the pile and the soil on slip failure. At that case it can be expressed by Coulomb's law which is the component of the friction angle and the normal pressure in different crushabilities of sands. On the other hands, it pays attention to estimate the skin friction in the case of large stress ratio.(4) It can be shown a rational method to evaluate the bearing capacity from the results of(1)-(3).