湛江黏土动剪切模量的结构损伤效应与定量表征

湛江黏土是一种具有高结构强度的灵敏性黏土,其强度包络线在固结围压达到结构屈服应力时存在明显转折,为了考察小应变条件下该黏土的动剪切模量随围压水平演化规律,开展了原状样与重塑样在不同围压水平下的共振柱试验与三轴CU试验,探讨考虑土体结构性损伤影响的动剪切模量表征方法。结果表明,重塑土最大动剪切模量G_(max)随有效围压的变化可用Hardin公式很好表征,而原状土G_(max)随有效围压的变化呈现先增大后减小特征,其转折点对应围压大于结构屈服应力,但最大动剪切模量与不排水剪切强度比值或经孔隙比函数归一化后的最大剪切模量随有效围压变化的转折特征点与结构屈服应力相当。出现上述现象的缘由在于该黏土G_(max)同时受土体压硬性的正效应与结构损伤的负效应双重影响,当固结压力小于结构屈服压力时,正效应占主导,反之则相反。针对Hardin公式未考虑结构性损伤的影响与难以延伸极端应力水平的不足,提出具有更广适宜性的描述模式。基于不同固结压力下该黏土的静刚度与微观结构演变性状,阐明了结构性损伤对其动剪切模量影响的物理机制,并间接印证了提出表征公式的合理性。

Structural damage effect on dynamic shear modulus of Zhanjiang clay and quantitative characterization

The Zhanjiang clay is a kind of sensitive clay with high structural strength, whose failure envelope has a significant breakpoint when the confining pressure reaches the structural yield stress. The resonant column tests and CU triaxial tests are performed on undisturbed and remoulded specimens to investigate the evolution rules of dynamic shear modulus with confining pressure under small strain, and the characterization method for dynamic shear modulus considering the structural damage effect is also discussed. The results show that the values of G_max of remoulded and undisturbed specimens with the confining pressure are different: the former can be well described by the formula of Hardin, while the latter increases initially then decreases; and confining pressure at the the turning points are higher than the yield stress, however, the turning points of the ratio of shear modulus to shear strength or the normalized shear modulus by the void ratio function with confining pressure are close to the yield stress. The reason is that the values of G_max of the structured clay are influenced by both the positive effect of compressive hardening and the negative effect of structural damage, and the former is predominant when the confining pressures are less than the yield stress, otherwise it is opposite. Focusing on the deficiencies of the formula of Hardin without considering the structural damage effect and difficultly extending to the extreme stress level, a widely suitable expression mode is proposed. Based on the change of static rigidity and microstructure of Zhanjiang clay with consolidation pressure, the physical mechanism of influence of structural damage on dynamic shear modulus is illustrated, and the rationality of the proposed formula is confirmed indirectly.