Characteristics of one-dimensional compressibility of Shanghai clay

Although the compressibility of Shanghai clay has been widely investigated over the years, few attempts have been made to compare the characteristics of the clay with those of other natural clays worldwide, in particular its long-term behaviours. A number of series of conventional and long-term consolidation tests up to 70 days on natural undisturbed samples are described and reported. These samples are taken from different soil layers in a basement excavation in Puxi area in Shanghai. The main objectives of these tests are to investigate the compressibility, especially the secondary compression of Shanghai clay and to compare the measured results with those of other typical natural clays worldwide. Experimental results reveal that the compression index Cc of Shanghai clay is relatively small ranging from 0.43 to 0.50. The coefficient of the secondary compression C? is found to be almost constant when the vertical consolidation pressure is smaller than or close to the preconsolidation pressure, while C????decreases significantly with time when the consolidation pressure is beyond the preconsolidation pressure. By comparing the measured C??/Cc ratios of Shanghai clay with those of other typical natural soils worldwide, the ratio is found to fall within the lower range of those for inorganic clay and silt. Based on the classifications by the coefficient of the secondary compression suggested by Mesri (1973), Shanghai clay can be classified as a soil with low to medium secondary compressibility.