既有建筑物的功能改造、加层改造，或在下沉、开裂、倾斜等工程病害处理中大都涉及地基基础的加固处理。预压桩基础托换法在西北黄土地区地基加固处理中已被大量应用，并取得了很多研究与应用成果，但该方法在理论和技术方面的研究和应用中还存在不少问题亟待解决，如预压托换桩承载力确定，桩合理间距的确定，桩体回弹的控制和施工工艺作法的改进等。本文对预压托换桩单桩承载力进行了较为系统地分析和研究。 本文采用理论和工程试验结合的方法对预压托换桩的工作机理及承载力进行了研究分析，并用工程实践进行了验证，得到以下结论：1预压托换桩的承载力随桩尖入土深度的增加而增加，它们之间存在一定的比例关系，但非线性；2针对西安地区和周边地区地基土特性，提出确定预压托换桩单桩承载力的经验公式；3对规范中关于预压托换桩单桩承载力的理论计算公式提出了修改建议。 上述研究成果已在工程应用中取得一定的成效，对进一步研究预压托换桩的工作原理，桩的合理间距，改进工艺作法和指导工程实践具有很好的参考价值。

Functional remodeling, story-adding reconstruction of original building or transmutation, cracked, inclination and other aspects in engineering disease treatment, and most of the techniques involve ground and basement treatment and reinforce. Foundation underpinning of pretest piles have made a lot of achievements, applying to loess in northwest, but there are still a number of issues that must be resolved. For instance, determination on bearing capacity, rational layout of the pretest piles space, rebound mechanism and controlling and improvement of construction techniques. In this paper, determination of pretest piles on bearing capacity is systematically studied and analyzed. This paper analyses the mechanism and bearing capacity of pretest piles by combining theories with engineering tests, and verifies the availability, finally, achieved the following results：1 The bearing capacity of pretest piles has a proportional relation to the depth of the pile’s tip in the soil, but nonlinear, and the deeper the depth is, the higher the bearing capacity is；2 Obtain an empirical formula of bearing capacity in xi’an region and peripheral areas；3 Offer suggestion for modification of the theoretical formula in the standard. Above research results have been applied to engineering with good impression, and provide important reference value for further study on mechanism of pretest underpinning pile, rational space of pile and construction improvement.