Abstract:

The study on influence of loading rate on mechanical behavior of Zhanjiang clay, which is a kind of sensitive clay with high yield strength, could be noteworthy. In order to study the loading rate effects on intact mechanical properties of Zhanjiang clay, the self-boring pressuremeter tests (SBPT) under five different loading rates have been carried out. The influence of stress rate on undrained shear strength, linear stiffness and non-linear degradation of shear modulus of the soil have been discussed. It is shown that the greater stress rate results in higher undrained shear strength, proposing strong linear correlation between two variables, and that behaves differently from the results of tests controlling strain rate given in literature. The linear secant shear modulus G_ur increases with stress rate and the undrained shear strengths varies within a class with small width after normalized by corresponding G_ur, which indicates little effect caused by loading rate on the strain corresponding to the peak strength. The significant degradation of tangential shear modulus G_t, at higher gradient and the steeper G_t-γ decay curve have been observed when at higher stress rate. The value of shear strain where G_t decreases to less than G_ur is about 0.1% for all stress rate conditions. The loading rate significantly affects the amplitude and the gradient of the decay of G_t in the corresponding class. The smaller the stress rate leads to the lower strain level where the decay of G_t mainly occurs. It is recognized that the special microstructure could account for the loading rate effects of Zhanjiang clay and this comment is preliminarily proved by super-mini-penetration tests conducted on both undisturbed and remolded samples. It is recommended that special attention should be paid to both the loading rate effects on in-situ mechanical properties and the influence of “loading rate history” on creep behavior of soils.

Key words:  structured clay, self-boring pressuremeter test, super mini-penetration test, rate effects, shear modulus, G-γ  decay curves