ESTIMATION OF IN-SITU UNDRAINED STRENGTH OF SOFT SOIL DEPOSITS BY USE OF UNCONFlNED COMPRESSION TEST WITH SUCTION MEASUREMENT

 TOSHIYUKI MITACHl , YUTAKA KUDOH and MASAKI TSUSHIMA

 ABSTRACT: Unconfined compression tests have been widely used in Japan for the purpose of determining undrained strengths f clay samples, but the unconfined compressive strengths are usually scattered even if specimens tested seem to have been subjected to the same stress history. This is due to the characteristic of the test itself, which is performed under unstable confining stress given by capillary pressure of the sample itself. In order to make clear the cause of scattering f measured strengths, three series of tests simulating the process from sampling to unconfined and triaxial compression tests on saturated soil samples are performed with three remolded and three undisturbed clays together with an undisturbed peat, and the influence of stress release and mechanical disturbance of the test specimen on the undrained strength is examined. Based on the test results, a practical method for estimating the in-situ undrained strength of soft oils from the results of a routine unconfined compression (UC) test together with suction measurement is proposed. From comparisons of the undrained strength profiles estimated by the proposed method with the strength profiles obtained by in-situ sounding test, it is found that the strength obtained by UC test can be reasonably corrected to estiate the in-situ undrained strength of soft soil deposits.

 key words: clay, highly organic soil, overconsolidation, residual stress, sample disturbance, shear strength, suction, test procedure, unconfined compression test (IGC: D6/D5/C6)