常州地区大直径钻孔灌注桩承载性状及尺寸效应试验研究

基于常州高架一期工程2根大直径钻孔灌注桩静载荷试验及桩身力学测试结果,研究常州地区大直径钻孔灌注桩加载过程中侧摩阻力和端阻力的发挥特点及荷载传递规律,得到极限状态下桩侧摩阻力和桩端阻力尺寸效应系数,对成孔卸载造成桩承载力降低及其他影响桩承载力的因素进行分析。试验研究结果表明:试桩的荷载-沉降(Q-s)曲线为陡降型,极限承载状态下,2根大直径钻孔灌注桩(长径比为36～43)端阻力分担总荷载的比例较小,分别为11.4%和18.1%,属于摩擦型桩;极限荷载时浅层黏性土发生侧阻软化,桩-土位移为3～7mm,砂土中为4～7mm;桩侧摩阻力与端阻力尺寸效应明显,大直径钻孔灌注桩侧摩阻力及端阻力尺寸效应系数与土的种类有很大关系,较《建筑桩基技术规范》(JGJ94-2008)计算值偏小,侧阻尺寸效应系数平均值大约为0.85(砂土)和0.88(黏性土),且具有沿深度减小的趋势,砂土和黏性土中桩端阻力尺寸效应系数分别为0.74和0.75。

EXPERIMENTAL RESEARCH ON BEARING BEHAVIOR AND SIZE EFFECT OF LARGE DIAMETER BORED CAST-IN-SITU PILES IN CHANGZHOU AREA

Based on the static load tests and mechanical data of two large diameter bored cast-in-situ piles,the working character of lateral friction resistance and end resistance of large diameter bored cast-in-situ piles are studied as well as load transfer mechanism in Changzhou area. Size effect coefficients of lateral friction resistance and end resistance in ultimate limit state of two testing piles are derived. The factors which weaken bearing capacity of large diameter piles are analyzed especially for the unloading of the hole. The test results show that the Q-s curves of the testing piles are of steep drop type. In ultimate state,the proportions of end resistance in total resistance are 11.4% and 18.1% respectively,which prove that the two piles(slenderness ratios range from 36-43) are of friction pile. Under ultimate load,the lateral friction resistance of shallow clay can be degraded;the relative displacements between pile and soil are 3-7 mm in clay and 4-7 mm in sand. Researches show that size effects of lateral friction resistance and end resistance are obvious and related to the property of soil. The average value of size effect coefficient of lateral friction resistance is 0.85 in sand while it is 0.88 in clay. Those are smaller than calculating values according to "Technical code for building pile foundations"(JGJ 94-2008). It is also shown that the size effect of lateral friction resistance in the same kind of soil will decrease with increase of depth. The size effect coefficients of end resistance are 0.74 in sand and 0.75 in clay respectively.