厚层沉渣嵌岩桩承载性状研究

Research on bearing behavior of rock-socketed piles in thick sediments

首先对嵌岩桩进行了现场钻孔抽芯试验和静力载荷试验,试验结果表明:当采用冲击成孔工法施工且桩侧存在弱胶结土层时,桩底沉渣厚度普遍较大,远厚于规范允许值;当荷载达一定值时,呈现突发性下沉且下沉量较大,主要与厚层沉渣导致端阻力未发挥作用有关。试验结果说明:当采用冲击成孔工法施工且桩侧存在弱胶结土层时,现有嵌岩桩设计方法同时考虑端阻和侧阻作用是不妥的,嵌岩桩单桩承载力宜仅按土层和嵌岩段侧阻力确定。进一步基于数值分析对嵌岩段摩阻力分布及变形等承载性状进行研究。在施加荷载一定条件下,嵌岩深度与基岩强度越大,下沉量越小,且厚层沉渣嵌岩桩沉降存在嵌岩深度效应。随着桩顶荷载增大,嵌岩深度与基岩强度差异导致的下沉量的差值增大。嵌岩段桩侧摩阻力沿深度呈马鞍型分布,嵌岩深度与基岩强度较小时,嵌岩段下部峰值比上部峰值大;嵌岩深度与基岩强度较大时,嵌岩段下部峰值则比上部峰值小。在桩顶荷载一定条件下,嵌岩深度越小,下沉量越大,越有利于侧摩阻力发挥,嵌岩段桩侧摩阻力越大。由于厚层沉渣存在,嵌岩段桩身轴力沿深度方向从桩顶荷载逐渐衰减为0,嵌岩深度越小,衰减速度越快。

In situ core-drilling tests and static loading tests are carried out on rock-socketed pile.The test results show that when using the impact pore-forming construction and the upper part is a weakly cemented soil,pile bottom sediment thickness is generally larger,which is much thicker than the specified value by the foundation code.When the load reached a certain value,sudden and large subsidence is observed,because the end resistance does not take effect due to the existence of a thick layer of sediments.The test results indicate that the current design method of rock-socketed pile which considering both of the end and side resistance is not appropriate when impact pore-forming construction is used and there is weakly cemented soil around the pile.The bearing capacity of single rock-socketed pile should be determined by side resistance in soil and the socketed segment in rock.Numerical analysis is carried out on the distribution of side resistance of the rock-socketed segment and deformation and load carrying capacity of the pile.Under a given applied load,smaller settlement is observed when socketed thickness and rock strength is greater.With the increase of the applied load on pile top,settlement difference caused by the variation of socketed thickness and rock strength increases.The distribution of side resistance of the socketed segment along the vertical direction shows a double-peak saddle shape.When the socketed thickness and rock strength are smaller,the lower peak is higher than the upper peak.When the socketed thickness and rock strength are larger,the lower peak is lower than the upper peak.Under a given applied load on the pile top,smaller socketed thickness would result in larger settlement and side resistance.Due to the thick layer of sediment,the axial force of the rock-socketed segment of the pile gradually decreases from the applied load on pile top to zero along vertical direction,the smaller the socketed thichness,the faster the decrease rate.