Geotechnical Characterization of Deep Shanghai Clays

Until present, the investigations on Shanghai ground mainly focused on upper clays, namely Layers 2~6, generally within 30~40 m depth. The knowledges on deep clays of Layer 8 and Layer 10, approximately 50 m and 100 m depth respectively, are very limited. In this paper, detailed geotechnical properties of deep Shanghai clays of these two layers are investigated by a series of field and laboratory tests. The physical properties tests show that both clays can be classified as low plasticity silty clay. For the first time in Shanghai, high quality intact clay samples were retrieved from deep ground using a double tube rotary sampler. High pressure oedometer tests and consolidated undrained (CU) triaxial tests were conducted to evaluate the compressibility and shear strength of intact samples. The oedometer tests indicate that Layer 8 is an over-consolidated clay with OCR=2~3 and Layer 10 is a normally consolidated clay with a light degree of structure. CU triaxial tests furtherly confirmed above findings. The compressibility of Layer 8 clay is smaller, and its undrained shear strength and effective friction angle are larger, comparing with those of Layer 10, although the latter one is deeper. The sensitivity framework, based on Burland’ ICL and SCL, is applicable to characterize the overconsolidation / structure nature of deep Shanghai clays. It is also found that the strain energy method is more suitable than the Casagrande method for determine the pre-consolidation pressure of deep Shanghai clays