Effects of sample disturbance and consolidation procedures on measured shear strength of soft marine Norwegian clays

Abstract

After many decades of research, the issue of sample disturbance is still important as regards to determining reliable and representative soil parameters for foundation design in soft clays. Parallel laboratory tests have been carried out on high-quality block samples and ordinary piston tube samples from 12 deposits of soft Norwegian marine clays. Undrained triaxial and direct simple shear (DSS) tests on samples reconsolidated to the in situ effective stresses show that sample disturbance has a significant effect on the measured stress–strain–strength behaviour: the more disturbed the sample, the lower the shear stress at small strains and the higher the shear stress at large strains. Breakdown of the clay structure, including cementation bonds, is the assumed cause of lower shear resistance at small strains, whereas at large strains the shear resistance is governed mainly by the water content, which for soft clay samples, reconsolidated to the in situ effective stresses, will be lower, and the strength thereby higher, the more disturbed the sample. The work described herein also includes the effects of the consolidation procedure; in addition to the reconsolidation technique, both stress history and normalized soil engineering properties (SHANSEP) and delayed consolidation tests have been carried out.Key words: soft clays, sample disturbance, consolidation procedures, stress–strain–strength behaviour, stress–strain–time behaviour.