Development of a simplified geophysical exploration system for seismic hazard evaluations

提出了一种改进的瑞利波方法，用于测定近表层土壤的横波速度(V_S)剖面。通过安装在地面上的传感器测量激励器引起的地面运动，可以很容易地在野外确定瑞利波的频散曲线和粒子轨道。为了从频散曲线确定VS分布，提出了一种反分析方法，其中考虑了瑞利波粒子轨道的较高模式和振幅比。为了提高该方法的有效性，提出了激励器和传感器之间的合理距离。利用该方法和常规井下方法在多个现场进行了对比试验。通过对不同砂土在不同围压下的不排水循环三轴试验，建立了液化阻力与剪切波速或弹性剪切模数之间的关系。试验结果和前人的研究结果表明，这种相关性取决于土的类型和围压等因素。将弹性剪切模量值与最小孔隙率和围压进行归一化，并用该值对试验结果进行重新检验。在此基础上，提出了一种用剪切波速评价砂土液化势的简化方法。通过在已知地震期间现场表现的不同地点进行现场测试，对所提出方法的有效性进行了评估。估计结果与现场实际情况吻合较好，说明该方法是有效的。

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.