Undrained shear strength of Pleistocene clay in Osaka Bay

This study presents the undrained shear characteristics of Holocene and Pleistocene clay samples from depths of 20-200 m under the seabed in Osaka Bay. Automated triaxial K-0 consolidation tests and anisotropically consolidated-undrained triaxial compression and extension tests are conducted using the recompression method. The average undrained strength ratio (s(u)/sigma'(v0)) is 0.33 (SD=0.03) when the extension strength is defined as the peak strength or the strength at an axial strain of 15%, while s(u)/sigma'(v0) is 0.29 (SD=0.04) when the extension strength is defined as the shear stress at the axial strain corresponding to the peak compression strength. Circular are stability analyses are carried out with the modified Fellenius and Bishop methods for the design cross section of the seawall structure of the Kansai International Airport to study the effects of different definitions of shear strength. The seawall is founded on 19 m of soft Holocene clay and 10 m of Pleistocene sand overlying the Pleistocene clay. The stability analyses show that the factor of safety and depth of the critical circle (i.e., above versus below the sand layer) are sharply affected by both the value of s(u)/sigma'(v0) (0.33 versus 0.29) and the method of slices (Fellenius versus Bishop). The marginal stability calls for careful monitoring of construction with field instrumentation.