利用弯曲元测量上海原状软黏土各向异性剪切模量的试验研究

Measurements of small-strain inherent stiffness anisotropy of intact Shanghai soft clay using bender elements

在许多发达国家例如英国,土体的小应变特性已被广泛应用于预测地下建筑物施工引起的地表变形。尽管上海的城市地下基础、基坑与隧道开挖日益增多,但是关于上海原状软黏土小应变剪切模量的研究几近空白。利用装有霍尔局部应变传感器与弯曲元测试系统的三轴仪对上海软黏土剪切刚度的固有各向异性进行了研究。在等向应力状态下对方形原状上海软黏土试样进行了两组试验。试验结果表明:较之传统的初达波法和峰-峰法,互相关法一定程度上提高了弯曲元试验中确定剪切波速的客观性。由于土体水平向层理结构与颗粒间胶结作用的影响,上海软黏土呈现出明显的各向异性,实测最大剪切刚度的固有各向异性比(G0(hh)/G0(hv))约为1.21。利用孔隙比函数F(e)=e-1.3能够很好的描述不同平面内最大剪切模量与土体应力状态的相互关系。

Measurements of small-strain stiffness are widely used for predicting ground movements in many developed countries such as the UK. Although an increasing number of foundations, excavations and tunnels are constructed in Shanghai, the measurements of the small-strain shear modulus of Shanghai soft clay have rarely been reported. In this study, the degree of inherent stiffness anisotropy of intact Shanghai soft clay is investigated using a triaxial apparatus equipped with Hall-effect local strain transducers and a bender element testing system. Two series of tests are carried out on intact prismatic soil specimens under an isotropic stress state. The experimental results reveal that the cross-correlation method using two received signals gives rise to more objective and repeatable results than the conventional first-arrival-time and peak-to-peak methods. The intact Shanghai soft clay clearly exhibits inherent stiffness anisotropy, as demonstrated by its elastic shear modulus ratio （G0（hh）/G0（hv）） of about 1.21, due to the stronger layered structure in the horizontal plane and a bonding effect. A unique relationship is found and established between the normalized shear modulus and the stress state in each plane by incorporating a void ratio function in the form of F（e）=e^-1.3.