Strength Decreasing due to Dynamic Stress of Earthquake

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This research consists of three parts, that is, 1) pore water pressure and liquefaction of saturated sands due earthquake, 2) strength characteristics of clayy soil to which a cyclic shear stress have applied, 3) development of a simple shear apparatus in which arbitrary stress conditions can be controlled. The cyclic shear tests for the three kinds of soils were carried out using the triaxial apparatus. Amplitudes of strain of both compression and extesion sides become nearly same by the proposed method of giving cyclic shear stress. Resistance of silty soil of OCR=5 for liquefaction corresponds to one of sand of 40% relative density. An increase of pore water pressure in one cycle of shear stress can be expressed as the function of the ratio of the amplitude of the shear stress to the effective stress and it is possible to calculate the pore water pressure due to a random shear stress wave. The resistance for liquefaction have good correlation to a dilatancy coefficient. The pore water pressure of a clayy soil due to cyclic shear is also expressed by the same function for sand. The strength parameters in terms of effective stress is constant regardless of disturbance and history of cyclic shear, but the effective stress path is influenced by the history of cyclic shear. The coefficient of pore water pressure at failure and the effective stress path are determined by the proposed parameter b for the soil having the history of cyclic shear. Magnitude of shear strength of the specimens which have same water content is in order of over consolidated, having cyclic shear and remolded reconsolidation. The strength parameters from the simple shear apparatus is nearly same one from the triaxial test.