The Study Of Predicting Method For Lateral Spreading Of Gentle Slope Induced By Liquefaction During Earthquake

Seismic liquefaction causes various damages,and the calculating of lateral spreading of gentle slope induced by liquefaction is of interest to the Geotechnical earthquake engineering.The existing calculation methods are mainly empirical methods,which considers the dynamic input,site topography and the parameters of soil layer,thus the mechanism of lateral spreading induced by liquefaction could not be reflected in the empirical methods.Newmark sliding block method,a classical method for calculating the dynamic displacement of slope,embankment and landfills,needs to be studied systematically in the calculation of lateral spreading induced by liquefaction based on the relevant research.This paper studied the calculation method of lateral spreading induced by seismic liquefaction based on the case histories,and proposed different calculation methods.By comparing the calculated lateral spreading to the recorded one,the proposed methods were validated.The paper provides scientific basis for the earthquake engineering design,and the research was carried out from the following aspects:The literature reporting lateral spreading were collected and the first database of lateral spreading was established.The database of liquefaction included seismic input,soil profile,soil layers,soil parameters,the standard penetration test values and fines content of the liquefiable soil,the residual shear strength of liquefiable soil and the effective vertical stress.The database of lateral spreading was used in the Newmark sliding block method analysis of lateral spreading,the deconvolution analysis based on the case history of lateral spreading,the numerical analysis of lateral spreading based on PM4 sand constitutive model and the calculation method based on nonlinear effective stress analysis of lateral spreading.Systematic Newmark displacement analyses of lateral spreading induced by liquefaction were conducted using a database of 23 documented case histories for the first time.The mechanism of lateral spreading induced by liquefaction is: the sliding of soil above the liquefied soil induces the accumulation of the displacement,the lateral spreading begins to occur when the liquefaction is triggered and ceased once the earthquake stopped.In the analysis,to reflect the mechanism of the lateral spreading induced by liquefaction,Site stratigraphy,including representative values for the Standard Penetration Test(SPT)blow counts of the liquefiable soil layers,was based upon the case history documentation.The postliquefaction residual shear strength of the liquefiable soil layer was estimated based on SPT blow count using three available methods.The yield acceleration was estimated using limit equilibrium analysis and the residual shear strength.Representative acceleration time histories recorded in the earthquake were employed in the displacement analysis.To obtain the calculation accuracy of Newmark sliding block method,the calculated lateral spreading was analyzed statistically.The mean value for the ratio of predicted to observed displacement varied depending on what residual shear strength relationship was used,with a mean value of0.80 for the Kramer and Wang relationship,a mean value of 1.80 for the Idriss and Boulanger relationship,and a mean value of 1.96 for the Olson and Johnson relationship.The Newmark sliding block method based on the yield acceleration using Olson and Johnson relationship can be regarded as the maximum calculated lateral spreading.The accuracy(confidence level)for Newmark sliding block method based on the yield acceleration using different residual shear strength was obtained by comparing the calculated lateral spreading to the recorded lateral spreading.Probabilistic analysis using a truncated normal distribution indicated that the observed displacement can be assumed to be less than twice calculated displacement with a confidence of 97% if the Olson and Johnson relationship was used,94% of the Idriss and Boulanger relationship was used,and 85% if the Kramer and Wang relationship was used.When the lateral spreading is calculated with Newmark sliding block method based on the yield acceleration using correspondent residual shear strength,the deconvolution analysis is conducted to obtain the dynamic input if the outcrop motion is available.The applicability,advantages and shortcomings of four equivalent linear analysis programs was evaluated in the paper.A historical site was selected analyzed using the four programs,the outcrop motions were used as input motions,the shear wave velocity and density of the soils were used to calculated the initial shear modulus,and the modulus reduction curves and damping ratio curves were assigned to each soil layer.The ground surface motions,the corresponded acceleration response spectrum,and the Fourier amplitude spectrum were obtained using the four programs,i.e.SHAKE 2000,DEEPSOIL,EERA and Strata.The maximum shear strain and peak ground acceleration versus depths for the site were plotted.The results showed that the ground surface motion,acceleration response spectrum,and Fourier spectrum amplitude were identical based upon the results by the four programs.The differences between four programs were summarized as well,the random vibration theory in Strata could be used for site response analysis.SHAKE 2000 can be used for the deconvolution analysis in the analysis of lateral spreading induced by liquefaction.A plasticity sand constitutive model was introduced in the prediction of lateral spreading.In the finite element analysis,a stress-ratio controlled,critical state compatible,bounding surface plasticity model for sand was used to simulate the liquefiable soil,and a case history of lateral spreading induced by liquefaction was analyzed and predicted.Based upon the previous investigation,the input motions and parameters for liquefiable soil were obtained using SPT blow count value of the liquefiable sand,the non-liquefiable soil were set as MohrCoulomb constitutive model.The horizontal displacement at the free face was monitored as well as the pore water pressure at the bottom of liquefied soil.The peak acceleration of ground surface motion and input motion was compared.By analyzing the results,the site liquefied under the shaking and the lateral spreading was observed,the predicted value is less than the observed lateral spreading,the input motion was amplified by the site and the plasticity sand model could be used in the lateral spreading and the calculated lateral spreading is conservative with a certain safety reserve.The approach considering liquefaction characteristic of site liquefaction is proposed to reflect the mechanism of lateral spreading.The approach first uses the advanced plasticity model to calculate the time to triggering of liquefaction and then uses the free field ground surface motion,the free field ground surface motion after the triggering time of liquefaction,the motion beneath the liquefiable soil after triggering time of liquefaction are used in the Newmark-type displacement analysis to predict the magnitude of lateral spreading.The yield acceleration is obtained using the residual shear strength relationship and limit equilibrium method.The approach is applied to five well-documented case histories of lateral spreading,Comparison of the calculated and reported displacements suggests that the proposed calculation method can be used in the calculation of lateral spreading.The approach that calculates lateral spreading with a Newmark analysis that employs the free field ground surface motions starting from the time at which liquefaction is triggered was,on the average,the most accurate approach,which is the minimum calculated lateral spreading,with a greater standard deviation for the lateral spreading ratio(the calculated lateral spreading to the recorded lateral spreading).The additional analysis using finite difference analysis was applied to calculate the lateral spreading of the five case histories,based upon the five case histories analyzed herein,the two-dimensional finite difference analysis can be used with good confidence to predict a maximum value for the magnitude of liquefaction-induced lateral spreading.A simplified method based upon Newmark sliding block method and one-dimensional effective stress analysis was proposed.The triggering time of liquefaction was predicted by effective stress analysis which was implemented in D-mod 2000.The post-liquefaction yield acceleration based upon the residual shear strength of liquefiable soil correlating with equivalent clean sand SPT blow count was calculated with Morgenstern-Price method.The predicted lateral spreading was calculated with Newmark displacement method using the motion beneath the liquefiable soil that started at the triggering time of liquefaction.Furthermore,the lateral spreading prediction based upon Newmark displacement method and numerical difference analysis using advanced sand plasticity model were reviewed and compared in the paper.To evaluate the proposed simplified method,five well-documented cases were analyzed and compared to the predicted lateral spreading to reported displacements.It is concluded that the simplified method captured the liquefaction characteristic of the site and predicts the lateral spreading induced by liquefaction at a reasonable level.The uncertainty of prediction was contributed by site investigation,input motions,and dynamic properties of the soil in the effective stress analysis.In order to consider the influence of the thick liquefied layer on the lateral spreading,based on the characteristics of lateral spreading,the liquefied soil is discretized,and the mechanical balance is established and the horizontal slice method is used to calculate the yield acceleration of the lateral spreading site.In order to calculate the lateral displacement of the liquefiable site,the transfer and dissipation of seismic energy at the site is considered based on the energy method,and the lateral displacement of the site is obtained.The method is verified according to the shaking table test result,and the results of the shaking table test are further analyzed for the mechanism of lateral spreading.Results showed that: the energy method can be used for lateral spreading calculation.The liquefaction will amplify the low frequency components in the earthquake,and several sliding surfaces will be formed in the lateral spreading,and then the overlying soil will move laterally along the sliding surface,the pore redistribution will reduce the residual shear strength of the liquefied soil.The effects of different calculation methods on lateral spreading were analyzed.The principle of Newmark sliding method and numerical calculation method were compared.Different constitutive models were used to analyze WLA liquefaction array.The results show that the calculation mechanism of Newmark slider method and numerical calculation method is different in essence.The liquefaction time has a significant influence on the calculation of lateral spreading,while the Newmark slider method needs to consider the influence of residual shear strength.