NORMALIZED UNDRAINED STRENGTH OF CLAY IN THE NORMALLY CONSOLIDATED STATE AND IN THE FIELD

抄録

Undrained strength, S_u normalized with the effective overburden stress, σ'_vo or the consolidation stress in the laboratory tests is an important index in evaluating engineering properties of a clay. Extensive studies have been made on this subject and the results mostly suggest that there is a correlation between the normalized undrained shear strength and plasticity index, I_p. In this paper, undrained strength in the normally consolidated state, S_un normalized with the vertical consolidation stress, σ'_vc obtained from K₀-consolidated triaxial compression and extension, and direct shear tests is first presented for various clays with I_p=5 to 100 together with discussions on previous studies on this subject. Undrained strength in the field, S_uf normalized with σ'_vo and the consolidation yielding stress, σ'_y investigated by field vane, direct shear and oedometer tests is then presented for 7 clays in Arabian Gulf, Indonesia and Japan. Further, a strain rate effect on S_u is also investigated. Conclusive points obtained from this study are : -1) Normalized undrained shear strengths, S_un/σ'_vc and S_uf/σ'_vo as well as strength anisotropy are not correlated to I_p, 2) S_uf of clays investigated is expressed by S_un+k₁ to a certain σ'_vo and S_un× k₂ beyond this σ'_vo where k₁ and k₂ are constants, 3) S_uf/σ'_y shows a constant value being close to S_un/σ'_vc irrespective of overconsolidation ratio and I_p, and 4) the effect of strain rate on S_u is independent on I_p.