Numerical Simulation Of Seismic Response In Non-saturated Free Field Soil Layer

Both the experience and theoretical study of earthquake damage show that local site conditions have a significant effect on ground motion.Using site seismic response analysis technology is an effective way and an important method to estimate the site effect of ground motion.Its qualitative and quantitative conclusions have been adopted by the current seismic design code,and have been widely concerned by the seismological and engineering circles.However,in the seismic safety evaluation of practical projects and seismic response analysis of soil layers,the local site is usually simplified into a one-dimensional one-dimensional uniform medium model with layered distribution,without considering the three-phase and unsaturated characteristics of natural soil layers.This is inconsistent with the fact that superficial unsaturated soil is generally developed,and will lead to large deviations and uncertainties in the seismic response analysis results of the site.It brings potential risk of earthquake disaster to the project site.Focusing on the scientific problem of the influence of unsaturated soil on the ground motion site effect,a large finite difference software FLAC 3D was developed.Taking unsaturated sand layer and red clay as examples,a complex dynamic free field calculation model of unsaturated-saturated soil layer-basement was established,and numerical simulation of free field seismic response analysis was carried out.The influence of soil saturation,unsaturated layer thickness and other site conditions on the peak acceleration and spectral characteristics of ground motion and the response law are discussed and preliminarily revealed.The main work and research achievements are as follows:(1)Taking the Kobe wave and EI Centro wave as the base input in the form of horizontal and vertical,there are certain differences in the surface peak acceleration and acceleration response spectrum between saturated and unsaturated sites.The difference of peak amplification coefficient and acceleration response spectrum is obvious for different types of ground motion input.(2)Because the soil shear modulus and volume modulus are affected by saturation,the soil shear modulus is obviously affected by saturation when the seismic wave is input horizontally,thus affecting the surface peak acceleration.In the case of vertical input seismic wave,the volume modulus of completely saturated soil is much larger than that of unsaturated soil,so the vertical input seismic wave is amplified less in saturated field.In terms of acceleration response spectrum,overlying soil saturation affects the peak value of acceleration response spectrum.The base input form affects the peak arrival period of the acceleration response spectrum.(3)The calculation results of unsaturated layer thickness show that there is a linear correlation between surface peak acceleration and unsaturated layer thickness,with positive correlation when Kobe wave input and negative correlation when EI Centro wave input.Moreover,the input form of the base affects the peak period of the acceleration response spectrum.With the increase of the thickness of the unsaturated layer,the peak value of the acceleration response spectrum increases,and there is a hysteric phenomenon in the peak period.(4)The unsaturated red clay presents the phenomenon of "upper hard and lower soft".The numerical calculation for different thickness ratios of soft and hard layers shows that the variation trend of peak acceleration of red clay with depth is significantly influenced by the ratio of soft and hard layers.The input wave type and soft-hard thickness affect the amplification effect of the field.According to the results of the acceleration response spectrum,the soft soil layer has a great influence on the ground motion in the short period range.