上海地区软黏土的OCR及地质成因

OCR of Shanghai soft clay and its geological causes

通过覆盖上海地区两种沉积相(滨海平原和湖沼平原)19个工程的原位十字板强度Su FV,结合国内外研究成果,对上海3个软黏土层的超固结比OCR及其地质成因进行了分析。根据强度比S_(uFV)/σ'_(v0)(σ'_(v0)为有效上覆压力)计算OCR,给出了OCR随深度的变化规律。上海软土整体上处于超固结状态,浅部具有较大的OCR且较为离散(在1.3~5.0),随深度的增加OCR迅速减小,超过约13 m后维持在一稳定的数值(平均为1.6)。过去采用室内压缩试验给出的结果显著低估了上海软黏土的OCR。结合Hanzawa和Tanaka给出的模型对上海软黏土的强度及超固结地质成因进行了分析,认为基本符合浅部以胶结作用为主、深部以次压缩为主的特征。浅部的胶结作用变化范围较大而深部的次压缩作用则相对稳定,并初步给出了正常压缩、胶结作用和次压缩作用对总强度的贡献。

Through the field vane strengths Su FV of 19 projects involving two sedimentary facies of coastal plain and lacustrine plain in Shanghai and combined with the worldwide research findings, the overconsolidation ratio(OCR) and its geological causes of three Shanghai soft clay layers are analyzed. The OCR is calculated according to strength ratio S_(uFV)/σ'_(v0)(σ'_(v0) is the effective overburden pressure), and then its variation with depth is given. The results show that the Shanghai soft clay is overconsolidated in general. The OCR of shallow layers is larger and more discrete(from 1.3 to 5.0). The OCR decreases rapidly with the increase of depth and almost maintains a stable value(1.6 in averages) after 13 m deep. The OCR of Shanghai soft clay is significantly underestimated according to the laboratory compression tests in the past. Based on the model given by Hanzawa and Tanaka, the strength and geological causes of overconsolidation of Shanghai soft clay are analyzed. It is conformed that for the shallow layers of the clay the main cause is the cement effect varying in a wide range, and for deep layers the main cause is the secondary compression effect being in a relatively stable range. The contribution of normal compression, cement effect and secondary compression effect to the total strength of the soft clay is also given.