针对基坑开挖卸荷会导致桩基承载力损失和引起桩身拉力作用所带来的安全问题，研制了一套可控制复杂应力状态变化的桩土接触面三轴模拟试验仪．该试验仪采用三轴围压室的气压加载控制接触面法向应力，利用围压室顶部的双套活塞实现对顺接触面向土体应力状态和桩土接触面剪切加载的分别控制，通过设置土样上下表面孔压来控制接触面的渗流状态和有效应力变化．其测试系统可对接触面总摩擦阻力、桩土相对位移和土样分层变形进行量测，实现对复杂应力状态下的桩土接触面相互作用特性的研究．通过等向加卸载应力作用下的接触面试验验证了该试验仪的有效性．试验结果表明，桩土接触面的极限摩擦阻力和摩擦系数随着法向应力的增加而增大，土层的变形也随着法向应力的增加而变大．

Unloading effect of the deep excavation would decrease the pile's bearing capacity and increase the tensile in pile shaft. A novel test system which can control the soil stress state was developed to study the pile-soil interactive behavior. In this novel system, the normal stress on the pile-soil interface is applied by the air pressure in the triaxial cell. The soil stress along the pile axis and the movement of model pile are controlled by the double piston. The pore pressure and water flow in the soil sample are adjusted by the two water cannels on the test cell. With the monitoring part of the system, the total shear force on the interface, the relative movement of the pile, and the deformation of the soil layer can be measured during tests. The triaxial interface tests show that the limit friction resistance and friction coefficient increase with the increase of the confining pressure, and the displacement of soil also increases with the increase of the confininig pressure.