EVALUATION OF UNDRAINED SHEAR STRENGTH OF SOFT CLAY WITH CONSIDERATION OF SAMPLE QUALITY

 TAKASHI TSUCHIDA

ABSTRACT: In Japan, the undrained strength of soft clay deposits is determined by half of the average unconfined compressive strength q_u. One problem with the q_u method is that the validity of the method depends largely on the quality of the "undisturbed" soil sample, while there exists no accepted method to evaluate the sample quality. The strengths from the q_u method, s_u(qu), were compared with those from the two proposed methods, SHANSEP and Recompression. For most "undisturbed" samples taken with a fixed piston sampler, the strengths obtained by the q_u method were almost the same as the s_u(M.B.) that was obtained by the Modified Bjerrum's method (Recompression method). However, when the samples were disturbed for some reason, s_u(qu) was smaller than s_u(M.B.). The strength by the SHANSEP method is dependent on the consolidation yield stress, p_c. When p_c was obtained by conventional consolidation test, the strengths by the SHANSEP method were usually smaller than those by the q_u method. The q_u method seems to be valid for most Japanese marine clays, when the undisturbed samples are taken with a fixed piston sampler of Japanese Industrial Standard. For samples taken with a Shelby-tube, the q_u method seriously underestimates the undrained strength. A new practical method named "Advanced q_u method" is proposed to determine the quality of the sample and the strength to be used for design at a reasonable cost. The effectiveness of the new method has been ascertained in field test.

Key words: cohesive soil, sample disturbance, shear strength, triaxial compression test, unconfined compression test (IGC : D6)