Sampling disturbance effects in soft laminated clays

Abstract

Sample disturbance effects in the laminated facies at the Bothkennar soft clay research site are examined by comparing the results of laboratory tests on specimens obtained from three fixed piston tube samplers. A significant feature of the results is that the sampling process associated with the conventional piston tube used in the UK and Ireland appears to increase the post-yield stiffness and the CAUC triaxial undrained strength, reduce the preconsolidation stress and the pore pressure coefficient at failure, and induce a strong tendency for dilatant behaviour post-peak when compared with tests on specimens from a sampler with a modified (sharper) cutting edge angle. It is reasoned that these findings are caused by material destructuration during the sampling process with the conventional tubes. Increased densification of these specimens caused both by sampling and during anisotropic consolidation may also contribute to these effects. The results are supported by reference to recent analytical work and data for three other soils. It is recommended that the use of these conventional thin-walled tubes with a crude cutting edge is abandoned and that some with improved cutting edge geometry replace them.