刍议相关规范中关于单桩承载力计算的几点疑惑

国内相关规范中单桩抗压承载力设计计算方法分为特征值法及极限值法两类，单桩承载力均通过桩侧阻力与桩端阻力直接相加的方法得到，在概念及理论上均存在着不完善之处，主要原因是桩侧阻力与桩端阻力的力学特性及对变形的要求不同、不能同步发挥所致。相对来说，极限值设计法更直接、更可靠、更准确，概念更明确，且易于与国际接轨，建议规范优先采用。不同规范根据抗拔试验结果求取单桩抗拔承载力的计算方法存在矛盾，建议求取时桩的自重按天然重度计算、不考虑水的浮力。建议相关规范对以下两种现象进行深入研究：桩长超过有效长度后，单位长度侧摩阻力将会降低；部分工程中得到的抗拔桩的抗拔系数远小于相关规范推荐值，尤其是桩长较长时。

Discussions on some controversies about pile bearing capacity calculation in the codes

The designing and calculating methods for compressive bearing capacity of a single pile can be divided into two types, i.e. characteristic value method and ultimate value method, in the Chinese codes related to pile foundations. The bearing capacity of a single pile is obtained by directly summarizing the skin friction and the tip resistance. These methods have some conceptual and theoretical disadvantages, since the skin friction and tip resistance cannot be simultaneously mobilized due to different mechanical responses and deformation requirements at the pile skin and the pile tip. The ultimate value method is more direct, reliable, accurate and clear in concept, and is more ready to be used in parallel to international standards. Hence it is suggested here that this method is preferred in the standards. A contradiction may occur in determining the uplift bearing capacity of a single pile according to the results of pull-out tests using the different standards. Because of this, the natural unit weight of pile should be used in calculation, instead of the buoyant unit weight of pile. Further research should be conducted to explore the following two phenomena in revising the relevant codes: one is that the unit skin friction decreases when exceeding the effective length of the pile, and the other is that tension coefficients for pull-out pile in some engineering is far less than the recommended values in the relevant codes, especially when pile length is longer.