Unconfined compression tests are performed in this study to evaluate the effect of matric suction on the strength and the deformation characteristics of dynamically and statically compacted silt soils. In order to estimate the suction stress, the change in suction and the volume change of the soil samples are measured, and the relationship between suction stress and unconfined strength is established. Under different conditions of the initial degree of saturation and the dry density in soils, the relationship between suction and the degree of saturation at failure can be expressed by a unique soil-water characteristic curve. This result shows a similar tendency to that of a predicted equation using the same parameters for the soil-water characteristic curves. The unconfined compressive strength is also affected by the suction stress which seems to work as a confining pressure for unsaturated conditions. These results demonstrate that the suction stress can be used for estimating the relationship between suction and the unconfined compression test.

Prior to the huge reclamation work for constructing Kobe Airport, the sea-wall structure resting on a 30m-thick soft Holocene clay layer in the 16m-deep sea was constructed at stages over the period of three years. Regarding the design of short-term stability of the sea-wall structure, the ratio of undrained shear strength of the normally-consolidated seabed clay to in-situ yield stress was estimated to take the value of 0.35 in order to satisfy the factor of safety in excess of 1.3. This design strength in the clay foundation was checked with the unconfined compression strength of the fresh clay samples retrieved from boreholes during the construction. In this paper, the short-term stability of the embankment in this well-documented case history was re-evaluated after-the-event with the results of comprehensive series of constant-volume direct shear box tests in which the effects of strength anisotropy as well as the shearing rate on undrained shear strength were both examined. Discussion is made how to estimate the short-term stability of the embankment by considering the effects of strength anisotropy and shearing speed on undrained shear strength and how it is linked with the current design method in Japan in which a half of the undrained compression from unconfined compression test is employed.

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Shear behavior obtained by direct shear tests is dependent on shear box and boundary condition. The objective of this study is to analyze problems of conventional direct shear test (type-A) and provide the reliable results by developing type-C direct shear apparatus. Experimental tests are carried out for Ulleung sand by using type-A and -C direct shear devices. The soil specimens, which are prepared at the relative density of 60%, and are applied to vertical confining stresses of 50, 100, 200, 300, and 400 kPa, are sheared at a constant shear strain rate of 0.

The case record is reported, each in which the landslide took place along the fissured zone of Osaka soft rock group in the residential land development area at Sakai city of Osaka. A series of investigation including in-situ and laboratory direct shear tests, the analysis of diatom content etc. was carried out in order to examine the failure mechanism by considering the geological background. It was manifested that i) the basic properties were significantly different between the marine clay and the freshwater formed clay, ii) the shear strength of marine clay was smaller than that of the freshwater formed clay, iii) the fissured zone in the marine clay is likely to be formed, and iv) the shear strength of the fissured clay subjected to cyclic shear in direct shear box test was close to the back-analyzed shear strength of the slope.