An Improved Method for Estimating In-Situ Undrained Shear Strength of Natural Deposits

An equation for estimating in-situ undrained shear strength (q_u(I)) of natural deposits is derived as q_u(I)/2c_u(I)=1.0-0.285 ln p_m/S₀ through the unconfined compression test (UCT) and K₀ consolidated-undrained triaxial compression test (CK₀UC). The q_u(I) of natural clay deposits can be estimated from the q_u value multiplied by the reciprocal number of q_u(I)/2c_u(I) of the equation using the suction (S₀) and q_u obtained from UCT for a specimen, where c_u(I) is in-situ shear strength measured from CK₀UC and p_m is two times the effective overburden pressure divided by three. The q_u(I)/2c_u(I) values were unrelated to I_p, q_u and p_m/S₀ and the mean value of these ratios was 0.98 in the range of I_p=26~110 and q_u=12~178 kPa. The mean values of the ratios for q_u, q*_u(I) and q_u(I) to 2c_u(I) were 0.629, 0.998 and 0.977 and the standard deviation of those ratios were 0.14, 0.10 and 0.16, respectively. Therefore, it can be seen that the improved method is appropriate as well as Shogaki's basic method (q*_u(I)). The mean values of q_u(I)/2c_u(I) were 0.94, 0.99 and 0.91 for Iwai organic and soft clay plus Kahokugata clay, respectively. The coefficient of variations of the q_u and q_u(I) values were (13~14)% and unrelated to soils, q_u or q_u(I) values. Therefore, the applicability of the improved method newly developed in this study can be confirmed for Kahokugata and Iwai clays as well as Iwai organic soils. The proposed method is a simple and easy one for practical engineering usage.