摘 要 近年来，建筑物的加固改造已受到国内外极大的重视，并已发展成为一门新兴的学科。然而，在对既有建筑物的功能改造、加层改造，或在出现下沉、开裂、倾斜等建筑病害的治理中，大都涉及既有建筑物地基基础的加固处理。实践证明，预压桩基础托换法确为一种行之有效的地基基础加固方法，尤其在西北湿陷性黄土地区更为适用。但目前在本项技术的理论研究方面还较欠缺，施工工法还需改进，其中预压托换桩的压桩机理和压桩阻力是一个急需解决的研究课题。 本文基于孔扩张理论、采用数值模拟方法及工程实例验证，对预压托换桩的压桩机理和压桩阻力进行了较为系统的研究分析，取得了五个方面的研究成果：1.压桩过程中桩侧土体受到剪切和侧向挤压作用，桩尖下部土体被向下和向两侧压缩挤开，贴近桩周的土体结构发生破坏，使桩体连续压入土体成为可能，且周围土体亦受较大的扰动影响；2.基于孔扩张理论，考虑初始附加应力，推导了预压桩压桩过程中地基土的弹塑性区应力、变形、塑性区半径和桩端阻力的解析解，进一步给出桩端阻力的简化理论计算方法，并由工程实例进行了验证；3.距离桩体（5-6）d之内的土体是桩体压桩过程作用的主要区域；压桩过程中，桩周土体将会发生向上隆起，且在径向上衰减较快，随着深度增加，隆起值呈线性减小趋势；4.压桩阻力的主要影响因素为上部既有建筑基底附加压力、地基土黏聚力和桩-土接触面法向刚度，桩体垂直度偏差应控制在0.003Ll（各接桩桩段长度）内为宜；5.针对西安地区土层，给出了采用系数调节法计算压桩阻力的修正系数取值范围。 上述研究成果对于进一步研究预压托换桩的工作原理、承载力及压桩力和改进施工工法有较好的参考价值，并可供工程实践参考。

ABSTRACT In recent years, great attentions have been attached to the building reinforcement and reconstruction in and out, and it has developed into a new subject. However, in treating most of existing buildings which needs function reconstruction，story-adding reconstruction, or appearing some problems like settling, cracking, leaning etc are related to the treatment and reinforcement of the ground and foundation. Practices showed that the pretest underpinning piles is obviously an effective reinforcing method for ground and foundation, and it is especially available in northwest collapsible loess areas of China. But in present, there are still many shortcomings in theory researches, and many construction technology need to be improved, but the mechanism and the resistance in the process of driving pretest piles is a problem needs solved badly among them. The mechanism and resistance in the process of driving pretest piles have been systemic researched and analyzed according to Hole expansion theory, adopting numerical analysis method and verified by field test, and obtained five conclusions: 1.Soils beside the pile were suffered shearing stresses and side extrusion effect in the process of driving pretest piles, soils below pile tip were squeezed to downwards and around at the same time, and the soil structure beside the pile were deadly damaged. This mechanism made it is possible to keep the pile driving downwards, in addition, soils around the pile were affected by disturbance greatly; 2.According to Hole expansion theory and considering to the initially additional stress, obtained formula for foundation soil’s elastic and plastic stress, deformation, radius of plastic zone and pile tip resistance in the process of driving protest pile. More over, rendered simplified theoretic method for calculating pile tip resistance, and verified by field test; 3.Soils around the pile between (5-6)d is the main affected region in the process of driving pretest pile. In the process of driving pile, the soils around the pile uplifted, and weakened rapidly along the radial direction, also the amount of uplifted soils declined slowly with depth; 4.The main effective factors on pile tip resistance are additional pressure under the basement of existing building, soil cohesion, and the normal stiffness of the pile-soil interface. The pile vertical deviation should be controlled in 0.003L1(L1 represent the length of each pile extension); 5.Rendered value range of correction coefficient ,using adjustment coefficient method to calculate the resistance of driving pretest pile in xi’an area. The research results above have some reference value for further study on mechanism of pretest underpinning pile, bearing capacity, the resistance of driving pretest pile, and construction technology. Moreover, it could be referenced in engineering practices.