湛江结构性黏土自钻旁压试验的加载速率效应

Effects of loading rate on SBPT responses of Zhanjiang structured clay

作为一种具有高结构屈服强度的灵敏性黏土,湛江黏土力学性质的加载速率效应值得关注。为了研究湛江黏土原位力学性质的加载速率效应,开展了不同加载速率的自钻旁压试验,分析应力速率对湛江黏土原位不排水抗剪强度、线性刚度以及剪切模量非线性衰减特性的影响。结果表明:应力速率越大,不排水抗剪强度就越大,二者之间具有良好的线性关系,这一变化关系不同于文献中控制应变速率的研究结果;线性割线剪切模量G_(ur)随应力速率的增大而增大,经相应速率下G_(ur)归一化后的不排水抗剪强度落于较窄的数值区间内,表明应力速率对峰值强度对应的应变影响很小;加载速率越大,切线剪切模量G_(t)衰减幅度越大、衰减越迅速、G_(t)-γ衰减曲线越陡,但对于各应力速率,G_(t)均约在g=0.1%时衰减至小于G_(ur);加载速率影响了各应变区间内G_(t)的衰减幅度和速度,加载速率越小,衰减集中发生于越低的应变水平。研究指出湛江黏土特殊的微观结构是控制其力学性质加载速率效应的主导因素,并通过开展原状土样和重塑土样的室内超微型贯入试验予以初步印证。认为应当重视土体原位力学性质的加载速率效应,同时关注加载速率历史对土体蠕变性质的影响。

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.