Development of a simplified geophysical exploration system for seismic hazard evaluations

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An improved version of Rayleigh wave method is presented for the determination of shear wave velocity (V_s) profiles of near surface soils. A dispersion curve and particle orbits of Rayleigh waves can readily be determined in the field through measurements of ground motions induced by an exciter using sensors placed on the ground surface. To determine Vs-profile from the dispersion curve, an inverse analysis is presented in which higher modes and amplitude ratios of particle orbits of Rayleigh waves are taken into account. To increase the effectiveness of the proposed method, desirable distances among the exciter and sensors are indicated. Comparative field tests are made at several sites using the proposed method and the conventional down hole method. The V_s-profiles from the two methods are in good agreement with each other, indicating the effectiveness of the proposed method.Undrained cyclic triaxial tests are conducted using various sands under different confining pressures, to establish a relation between liquefaction resistance and shear wave velocity or elastic shear modulus. The test results together with those of previous studies indicate that the correlation depends on such factors as soil type and confining pressure. Elastic shear modulus is normalized with respect to minimum void ratio and confining pressure, and the test results are re-examined with this value. The normalized shear modulus has a good correlation with liquefaction resistance, irrespective of soil type and confining pressure.Based on the above findings a simplified method is presented for evaluation liquefaction potential of sand deposits using shear wave velocity. Effectiveness of the proposed method is evaluated through field tests at various sites where field performance during earthquakes is known. The estimated results are reasonably consistent with the actual field behavior during the earthquakes, indicating that the proposed method is effective.