Undrained shear strength of a glacial clay overconsolidated by desiccation

Abstract

Block samples of a Boston Blue clay crust were used in a detailed study of the relationship between undrained shear strength and consolidation pressure in the compression and extension shear modes. The glacial silty clay sample that was mainly composed of quartz, feldspar, illite and chlorite has developed an overconsolidation ratio near to 3·0 by desiccation. The in situ coefficient of earth pressure at rest was near to 0·9 as compared to K0p = 0·56 of the normally consolidated young clay in the compression range beyond the preconsolidation pressure. In the compression range, for each consolidation-shear mode, undrained shear strength to vertical consolidation pressure ratio as well as shearing-induced pore water pressure to vertical consolidation pressure ratio, friction angle mobilized at yield, and the coefficient of earth pressure at rest, were constant and independent of consolidation pressure. The mobilized undrained shear strength of the normally consolidated young clay obtained by consolidating samples to the compression range was 0·22σ′vc. However, mobilized undrained shear strength from specimens that were subjected to the in situ effective stress condition before undrained shear was 0·17σ′p. This value of in situ strength from the laboratory tests as well as the increase in undrained shear strength in the recompression range from the effective overburden pressure to the preconsolidation pressure are probably related to the microfissures in the desiccated stiff clay. Undrained shear strength of the desiccated stiff clay tended towards zero as the vertical consolidation pressure decreased starting from the in situ condition.