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COMPARATIVE STUDY ON UNDRAINED SHEAR STRENGTH OF
OSAKA BAY PLEISTOCENE CLAY DETERMINED BY SEVERAL KINDS OF LABORATORY
TEST
YOICHI WATABE and TAKASHI TSUCHIDA
ABSTRACT :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
/s'vo)
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, KOCU
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 KOCU
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 Dsu
/Dz for design is
obtained as 2.09 kPa/m, considering strength anisotropy.
Key words: direct shear test, Pleistocene clay, stability
analysis, triaxial test, unconfined compression test, un- disturbed
sample (IGC: D6)
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