成层地基土中群桩挤土效应现场试验研究

Field test research on compaction effects during installation of group piles in layered soils

为探究沉桩挤土效应的空间分布规律,开展了预制方桩群桩锤击施工的现场试验研究.针对群桩贯入产生的超孔隙水压力与土体深层水平位移进行测试分析,并结合深厚淤泥软土层下卧粉砂粉土互层的上软下硬地层条件,研究了土层性质、沉桩速率、结构损伤等要素对成层地基土中挤土效应的影响机理.结果表明,深厚淤泥土层中超孔隙水压力随深度近似线性增大,可达有效上覆压力的1.61倍,孔压消散速率极为缓慢,休止30 d后仍残余50%以上.下卧粉砂粉土互层中超孔隙水压力峰值明显较低,随深度变化无明显线性规律,且孔压消散迅速,休止2 d后即可消散95%以上.由于上软下硬的地层分布形式,土体深层水平位移曲线表现出上大下小的特征,在软硬土层交界面处变形易发生突变.群桩锤击施工对土体结构的扰动损伤导致休止期土体的侧向回移速率明显滞后于孔压的消散速率.

To explore the spatial distribution law of the compaction effects during pile installation,field tests were carried out on hammering installation of group precast square piles.The excess pore water pressure(EPWP)as well as the lateral soil deformation induced by installation of precast piles were documented and analyzed.In terms of the particular soil conditions where silty-sandy layers underlie deep muddy clay,the effects of the soil properties,the pile installation speed and the disturbance of the soil structure on the compaction effects in layered soil foundation were clarified.The results show that the EPWP increases linearly with the increase of the depth in deep mucky clay with the maximum value of 1.61 times the effective vertical stress.The dissipation rate of the EPWP is very slow with more than a half of residual EPWP after 30 d rest.In contrast,the peak value of the EPWP is obviously lower in the silty-sandy layers.The linear law cannot be observed in deep muddy clay.The EPWP dissipates fast in the silty-sandy layers with the dissipation ratio of more than 95%after 2 d rest.With vertical soil distribution featured by upper-soft and lower-hard,the curve of lateral soil deformation along the depth shows large upward and small downward.A sudden change of lateral deformation tends to occur at the interface between the soft layers and the hard layers.The rate of the lateral return deflection of soils is obviously slower than that of the EPWP dissipation because of the disturbance of the soil structure induced by pile driving.