小型桩基竖向循环加载模型试验系统研制与应用

介绍了一种小型桩基竖向循环加载模型试验系统,包括加载系统、压力室、起吊装置、模型桩、数据采集系统。该系统通过伺服电机驱动联轴器带动滚柱丝杠转动,进而带动加载板对模型桩施加竖向位移荷载;通过水压加载法对土样进行围压加载,模拟不同深度土层的应力状态及固结情况。可针对不同固结状态的地基土和多种桩基型式,开展不同荷载组合下桩基竖向循环加载模型试验研究,适用于饱和软土、一般黏性土、粉土、砂土等均质或非均质地基。应用该设备进行了单桩竖向循环加载模型试验,并与数值模拟进行对比。研究结果表明,动荷载幅值、地基土固结压力对桩基承载力特性影响很大;动荷载幅值较小时,桩基承载力几乎没有弱化;地基土固结压力升高时,桩基承载力提高显著,桩基承载力弱化速度减慢;随着振次及振幅的增加,桩顶轴力逐渐弱化至残余值;位移循环荷载作用下,桩周土的弱化导致桩基产生了负摩阻力,进一步降低了桩基承载力。模型试验结果与数值模拟结果吻合较好。通过初步应用,证明了该试验系统弥补了常规1g小比例尺模型试验中低围压的不足,可以较好地反应出桩顶荷载和位移的非线性关系和承载力循环弱化现象。同时该系统输出荷载波形精确,量测系统灵敏度高、稳定性好,可用于多工况下桩基的竖向循环加载特性研究。

Development and application of a small-scale vertical cyclic loading system for model tests of pile foundations

A small-scale vertical cyclic loading system for model tests of pile foundations,consisting of a loading system,a pressure chamber,a lifting device,model piles and a data acquisition system,is introduced.The model pile top is vertically loaded through a rotating roller screw driven by a coupler which is controlled by a servo motor,and soils samples are exerted with the confining pressure by hydraulic loading in order to simulate the in suit stress and consolidation of soil layers at different depths.The small-scale vertical cyclic loading system can be applied to carry out model tests of different pile foundations under vertical cycle loading in saturated soft soil,general clay,silty soil,sandy soil and other homogeneous or heterogeneous soils.Model tests of single pile under vertical cyclic loading are carried out and numerical simulation is performed for verification.Results show that the dynamic load amplitude and the consolidation pressure have great influence on the bearing capacity of the pile foundation.When the dynamic load amplitude is small,the bearing capacity of the pile foundation hardly weakens.When the consolidation pressure of the foundation soil increases,the bearing capacity of the pile foundation will be significantly enhanced,and the weakening will slow down.With increasing the cycle number and the amplitude,the axial force of the pile top gradually weakens to the residual value.During the cyclic loading,the degradation of the soil around the pile leads to a negative friction resistance of the pile foundation,and the bearing capacity of the pile foundation is weakened significantly.The test results are in good agreement with those obtained by the numerical simulation.Preliminary application proves that the test system can make up for the deficiency of low confining pressure in conventional 1 g small scale model test and that the nonlinear relationship between the load and the displacement of the pile top and that the cyclic degradation of the bearing capacity can be well reflected.This test system can output accurate load waveform,has high sensitivity and good stability in measurement and can be used to study the vertical cyclic loading characteristics of pile foundations under multiple working conditions.