基于自钻式旁压仪的残积土原位力学特性试验研究

残积土属区域性特殊土,准确获得其力学性质指标是各类工程场地地基处治及承载力评价的必要条件。针对典型花岗岩残积土层,在20 m深度范围内开展自钻式旁压试验,获得表征土体原始水平应力、强度与刚度的相关指标,类比分析深度与水平方向土体力学性质的差异性,并论证了土体刚度在不同应变水平下的衰减规律。结果表明,该场地浅层土体刚强度指标差异性较大,存在明显的风化壳,导致剪切模量及变形参数沿深度呈先减小后增大的趋势;土体割线剪切模量Gs,切线剪切模量Gt随剪应变变化关系具有明显的非线性特性,在小应变范围(γc≤0.3%)内Gs,Gt快速衰减,在γc=2.0%~3.0%时,趋于平稳,采用非线性分析方法得到的模量值较为合理,认为在该类场地工程设计中,剪切模量的选取宜基于自钻式旁压试验结果,根据不同应变水平确定,采用线性割线法得到的剪切模量Gur偏于冒险。同时发现,小应变时剪切模量值相对于土体差异较为敏感,为准确获得土性参数,可适当增加浅层土体测试数量。

Experimental study on in-situ properties of residual soil by self-boring pressuremeter

Residual soil is a regional and special soil, and accurate evaluation of its mechanical properties and bearing capacity is a necessary condition for the treatment and foundation for any types of engineering projects. The residual soil in the studied area within the range 20 min depth is tested by using the self-boring pressuremeter. The strength and stiffness parameters of the soil are evaluated through theoretical analysis. The variations of mechanical properties with respect to depth and horizontal direction are also analyzed. The decay law of stiffness characteristics and its applicability under different strain levels are evaluated. The results show that the values of the stiffness and strength of the soil are much different in the site, and the residual soil has obvious weathering crust. Initially the shearing and deformation parameters decrease up to a certain depth and then increase gradually with the increasing depth. Within the test site, the mechanical parameters of soil at shallow depths are quite different and converge after reaching a certain depth. G_s and G_t show non-linear relationship with γ_c. In a small strain range, G_s and G_t decay rapidly when the strain is 2% to 3% and tend to be stable. The non-linear analysis method is more reasonable for determining the shear modulus. It is believed that in the engineering design of such site, shear modulus should be chosen based on the strain level of self-drilling lateral pressure test results, and the linear secant shear modulus G_ur is somewhat risky. Moreover in the small strain, shear modulus of the soil is more sensitive to difference of the soil, and in order to ensure the representative parameters, the test density of soil at shallow depth should be increased.