高填方夯实地基桩侧负摩阻力受力性状试验研究

对原为沟壑的场地，经回填全风化泥质粉砂岩形成高填方地基。对高填方地基采用3000kN·m能级强夯预处理后，打设钻孔灌注桩，通过在桩身钢筋笼主筋上安装应力计，在桩身截面和桩周土层分别埋设沉降杆、分层沉降仪，测试桩身轴力、桩身及桩周土层沉降变化情况，得到高填方夯实地基未处理填土层桩侧负摩阻力变化规律。试验结果表明，未处理填土层桩侧摩阻力沿深度呈现“负-正”变化的现象，随着固结时间的增加，端承桩负摩阻力区段大于摩擦桩。端承桩桩侧土层提供的最大负摩阻力约是摩擦桩的1.18~2.56倍，桩周土层密实度对桩侧最大负摩阻力有影响。采用一阶负指数函数拟合得到桩身下拉荷载预测模型，随着固结时间的增加，作用于桩身的下拉荷载趋于定值，作用于端承桩的下拉荷载比摩擦桩高41.2%~55.4%，从控制负摩阻力角度推导出高填方夯实地基摩擦桩桩长设计计算方法。桩身中性点位置均随固结时间增加而逐渐下移，端承桩中性点深度较摩擦桩平均大0.7m。

Experimental study on mechanical behavior ofnegative skin friction on piles in high filledembankment after dynamic compaction

High filled embankment is formed by filling the strongly weathering muddy siltstone. The reinforced concrete piles were installed after the foundation was improved by dynamic compaction of which the energy is 3000kN·m. The axial force of pile shaft and settlement of pile and adjacent soil were acquired by installing stress gauges in main reinforcements, and burying settlement rods in different pile shafts and layered settlement meters in adjacent soil, respectively. In this study,the changing law of negative skin friction (NSF) was obtained from the field test in non-improved depth in high filled embankment after dynamic compaction. The test results indicate that the skin friction changes from negative to positive with depth in non-improved depth and the scale of NSF of end bearing pile is more than that of the floating pile with the increase of consolidation time. The maximum NSF supplied with adjacent soil in end bearing pile is about 1.18-2.56 times that of floating pile. The degree of compaction of adjacent soil have impact on the maximum NSF to some extent. The drag load prediction model was acquired by data fitting, and the values of drag load are larger by about 41.2%-55.4% in end bearing pile than that of floating pile and tend to be stable with the increase of consolidation time. The calculation method of length of floating pile in high filled embankment after dynamic compaction was developed considering the control of NSF. The location of neutral point followed with the increase of consolidation time and the depth of neutral point in end bearing pile is 0.7m more than that of floating pile on average.