饱和砂土中地下水对标准贯入试验击数影响分析研究

Study on the influence of groundwater on blow counts of standard penetration tests in saturated confined sand

南昌赣江(南支)与抚河(故道)入湖河流尾闾段河道之间的湖积冲积平原,地形平坦开阔,区域内预制管桩施工普遍采用锤击沉桩。在桩基施工过程中,出现大范围贯入度已达到控制贯入度或锤击总数过大而沉桩还远不到位的现象,经分析,是地下水压力影响造成了砂层标贯试验(SPT)击数偏低,依据SPT击数选取桩的极限侧阻力标准值q_(sik)和极限端阻力标准值q_(pk)偏小,导致计算的单桩承载力数值偏低,设计桩长偏于保守造成的。通过对研究区和地下水漏斗区两个区域内砂土的SPT击数进行对比分析,得出地下水对各类砂土密实度的影响程度,建立了不同深度段各类砂土天然和饱和两种状态下SPT击数的经验关系式。同时还得到两点认识:地下水对细颗粒砂土SPT击数的影响明显高于粗颗粒砂土;地下水对浅部砂土的SPT击数影响较大,随着砂土埋深增大影响逐渐减小,研究区域内影响的临界深度大约在地表以下15m。

The construction of precast pipe piles generally adopts hammer sinking piles in the flat lacustrine alluvial plain,where is between Ganjiang River(the Southern Branch)and Fuhe River(the ancient watercourse)in Nanchang.In the process of pile foundation construction,a wide range of pile sinking cannot reach the desired depth,although the control penetration has been achieved or the total number of hammering is out of normal.Through the analysis,it is the influence of the groundwater pressure caused a low SPT blow count of sand layer.The ultimate shaft resistance(q_(sik))and ultimate tip resistance(q_(pk))of piles are relatively small according to the SPT blow count,which leads to a low numerical calculation of the ultimate vertical bearing capacity of a single pile and the conservative design pile length.The influence degree of groundwater on the compactness of all kinds of sand is obtained by comparing and analyzing the SPT blow counts of sand in the study area and the groundwater funnel area.It is established the empirical relation formula of SPT blow counts of all kinds of sand in different depths under natural and saturated conditions.Meanwhile,there are two conclusions:the influence of groundwater on SPT blow counts of fine-grained sand is significantly higher than that of coarse-grained sand;the groundwater has a great influence on the SPT number of shallow sand,and the influence gradually decreases with the increase of sand burial depth,and the critical depth of influence in the study area is about 15m below the surface.