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

可破碎砂土在世界各地都可以找到，其特点是压缩性很强。在这种地基中存在着承载力极低、桩基沉降大的工程问题。为了建立合理的桩基承载力评价方法，本文分析了桩基位移承载力的产生机理。在不同定应力水平下对不同破碎性的砂土进行单粒破碎试验、三轴试验、环剪试验和模型桩加载试验的基础上，分析了应力条件和土体性质对砂土承载力的影响。得出的主要结论是：(1)与标准土工材料相比，可破碎土颗粒一般呈棱角状，易破碎；可破碎土具有高压缩性、大剪切模量和大内摩擦角等材料特性。为了获得足够的土体强度，需要进行较大的变形。(2)对于此类土，桩抗沉降动员承载力较低的机理是明确的。桩端承载力是由可破碎土的材料特性和桩周边界应力的影响决定的。(3)光滑表面的桩在发生滑移破坏时，桩与土体之间通常发生表皮摩擦。在这种情况下，它可以用库仑定律来表示，库仑定律是摩擦角和法向压力在不同可破碎性下的分量。另一方面，注意在大应力比情况下的表面摩擦的估计。(4)从(1)~(3)的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).