Study On Prediction Method Of Large-scale Liquefaction Slip Distance In Loess Deposit

The liquefaction slip of loess strata caused by earthquake is a serious geotechnical earthquake disaster under the coupling effect of ground motion and water.The loess plateau is an earthquake prone area.With the rapid development of economy and society in loessland,large and medium-sized cities and towns is expanding rapidly to thick loess covering sites and slopes.Due to the radiation of water supply and drainage system,the improvement of agricultural irrigation conditions,the increase of rainfall and the change of groundwater level,the water content of surface loess is increasing,which increases the risk of large-scale liquefaction and slip disasters in loess strata at low angles.The mechanism of this formation slip disaster is different from that of unsaturated soil landslide,so it is necessary to establish a new method to predict the slip distance in order to fully evaluate the risk of liquefaction slip in loess strata.In this dissertation,the loess layer of Guyuan Shibeiyuan tableland in Ningxia was taken as an example to study the low angle liquefaction slip disaster caused by Haiyuan earthquake in 1920.Through field investigation,the loess tableland soil profile before slip was restored.The liquefaction slip surface and mechanism were determined by laboratory test,on-site trench excavation,shear wave velocity test and numerical simulation.On this basis,from the perspective of predicting the maximum slip distance,the calculation model of soil liquefaction slip and the calculation formula of slip distance are established.The main innovations are as follows :1.Through field investigation,drilling exploration and trench excavation,the stratigraphic distribution characteristics of the southern liquefaction slip area of the Shibeiyuan tabletland are obtained,and the two-dimensional stratigraphic profile model of the southern slip area of the Shibeiyuan tabletland along the slip direction before the seismic liquefaction slip is restored,and the physical parameters of each soil layer are given.2.Based on the dynamic triaxial test,cyclic torsional shear test,shear wave velocity test and groundwater level analysis,the liquefaction discrimination of each stratum in the liquefaction slip area of the south of the Shibeiyuan tabletland was carried out.Combined with the field investigation results,the saturated sandy loess layer under the first paleosol layer was determined.3.On the basis of dynamic triaxial liquefaction test,undrained shear test was carried out on the liquefied loess sample,and the variation laws of apparent viscosity,pore pressure and strain with time and vibration times were given.It was confirmed that the flow characteristics of the original sample after liquefaction changed from‘shear thickening'to‘shear thinning',and it was found that the flow characteristics of the remolded sample had always presented the phenomenon of‘shear thickening',thus revealing the motion mechanism in the liquefaction-sliding process of loess strata.4.According to the profile model,slip surface and migration characteristics of loess liquefaction in Shibeiyuan,the slip soil model is simplified from the perspective of maximum slip distance prediction.Through the quasi-static method,the calculation formula of maximum slip distance of loess liquefaction was established based on the equivalent seismic load and the law of conservation of energy.The slip formula was verified by the liquefaction and slip disasters in Shibeiyuan of Guyuan in Ningxia and Xunbu in Shanxi,and the results were consistent with the field investigation results.