Sample disturbance effects on medium plasticity clay/silt

Practising engineers designing civil engineering works on soft clay usually assume that laboratory tests on normal piston tube sampling will at worst give conservative design parameters. Indeed most research into sampling disturbance effects on soft clays has proven that use of poor-quality tube samplers ultimately leads to lower undrained strength, stiffness and preconsolidation stress than actually exist in situ. There is some evidence to suggest that this may not hold true for ‘intermediate’ clay/silts or laminated soils. These issues were investigated for the Athlone laminated clay/silt, and it was found that tube sampling disturbance leads to increase in stiffness and undrained strength, and induces a strong tendency for dilatant behaviour post peak. These findings were made by comparing tube sample data with data from high-quality Sherbrooke block sampling and full-scale field trials. From the results of this investigation, and from comparisons with other soils, it is concluded that undrained tube sampling strains have only a minor influence, and that the behaviour of these medium plasticity clay/silts is due mainly to partially drained tube insertion process. Engineers dealing with soft ‘intermediate’ or laminated soils need to assess the laboratory-derived parameters carefully, especially when the clay content is less than about 40% and the plasticity index is less than 20%.