Undrained shear strength of a glacial clay overconsolidated by desiccation

Block samples of a Boston Blue clay crust were used in a detailed study of the relationship between undrained shear strength and consolidation pressur e in the compression and extension shear modes. The glacial silty clay samp le that was mainly composed of quartz, feldspar, illite and chlorite has de veloped 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 K-0p = 0.56 of the normally consolidated young clay in the compression range beyo nd the preconsolidation pressure. In the compression range, for each consol idation-shear mode, undrained shear strength to vertical consolidation pres sure ratio as well as shearing-induced pore water pressure to vertical cons olidation pressure ratio, friction angle mobilized at yield, and the coeffi cient of earth pressure at rest, were constant and independent of consolida tion pressure. The mobilized undrained shear strength of the normally conso lidated young clay obtained by consolidating samples to the compression ran ge was 0.22 sigma(vc)'. However, mobilized undrained shear strength from sp ecimens that were subjected to the in situ effective stress condition befor e undrained shear was 0.17 sigma(p)'. This value of in situ strength from t he laboratory tests as well as the increase in undrained shear strength in the recompression range from the effective overburden pressure to the preco nsolidation pressure are probably related to the microfissures in the desic cated stiff clay. Undrained shear strength of the desiccated stiff clay ten ded towards zero as the vertical consolidation pressure decreased starting from the in situ condition.