2001 Volume 41 Issue 5 Pages 47-59
This paper describes a geotechnical survey evaluating the shear strength of Pleistocene clay in Osaka Bay. In the stability evaluation for a seawall structure of Kansai International Airport, as the slip circle giving the minimum safety factor possibly passes through the Pleistocene clay layer, the stability could be greatly affected by the shear strength profile of the Pleistocene clay. The result of the stability analysis is very sensitively affected by the undrained shear strength profile, which is roughly estimated based on a ratio of undrained shear strength to consolidation yield stress (su / py) of natural clay deposit (Mesri, 1975 ; Tanaka and Tanaka, 1994) and the average apparent overconsolidation ratio (OCR=py / σ'v0) at this site (Horie et al., 1984). In this study, in order to evaluate the undrained shear strength obtained by typical testing methods, UC (unconfined compression) test, UU, CU, K0CU compression and extension triaxial tests and DS (direct shear) test were carried out for undisturbed samples of the Pleistocene clays collected for the second phase construction of Kansai International Airport. Mechanical behaviors and shear strength profiles obtained by each test were compared and discussed. The undrained shear strength and its scattering depend on the testing methods. The recompression method, in which a specimen is consolidated under the in-situ stress, gives many reliable test results. The UC and UU tests, in which a specimen is not consolidated, give very scattered and unreliable test results. From the comparison between undrained shear strengths for design determined by each test with correction factors, it is found that the strength of the DS test is smaller than those of K0CU and CU tests. This fact is derived from the relatively larger strength anisotropy of the Pleistocene clay. The undrained shear strength increase ratio with depth Δsu / Δz for design is obtained as 2.09kPa / m, considering strength anisotropy.