| Historically strong and large earthquakes around the world have resulted in numerous instances of liquefaction-induced landslides in loess regions,causing significant loss of life and property damage.The devastating large-scale liquefaction landslide event caused by the 7.5 magnitude earthquake in Palu,Indonesia on 28 September 2018 has raised a warning about the urgency and necessity of research on liquefaction triggered landslides in loess regions.In addition,with the improvement of agricultural irrigation conditions and the increase of extreme rainfall events due to global warming in the Loess Plateau region of China,the risk of large-scale loess liquefaction triggered landslides has increased substantially.Therefore,it is of great theoretical significance and practical engineering value to carry out systematic and in-depth research on the mechanism of loess liquefaction triggered landslides and their hazard assessment methods for the seismic safety of urban and rural areas and critical infrastructure in the loess plateau.This doctoral dissertation adopts field investigation,field tests,unmanned aerial surveys,large shaking table tests and numerical simulation to systematically study eight historical loess liquefaction triggered landslides,and propose the definition and classification of loess liquefaction triggered landslides.The study carry out the shaking table model tests of liquefaction slip at the bottom of loess plateau,liquefaction mud flow at the surface of loess slope,and bottom-surface joint liquefaction slip at the bottom of loess slope based on the historical liquefaction triggered landslides examples.Based on the test results,Arias intensity analysis,HHT transform analysis and energy analysis are used to systematically study the mechanism of different types of loess liquefaction triggered landslides from temporal,frequency and energy perspectives.Considering multiple factors such as topography,geomorphology,stratigraphic lithology,hydrometeorology,depositional age and thickness of the loess layer,and ground vibration intensity,ten factors and their weights affecting the liquefaction triggered landslides of loess are selected,and a method of evaluating the risk of liquefaction triggered landslides of loess plateau based on the weighted analysis method of GIS platform(ESRI ArcMap).This method is then used to compile a hazard zoning map showing the liquefaction triggered landslides on the Loess Plateau with different probabilities of exceedance.The main results of this study can be summarized as follows:1.The classification,characteristics and screening methods of loess seismic liquefaction landslide triggering mechanism are proposed.Based on the location of the liquefied soil layer and the movement characteristics of the slippery soil after liquefaction,the loess liquefaction triggered landslides are classified into three types:bottom saturated liquefaction slip type,surface saturated liquefaction mudflow type,and bottom-surface combined liquefaction slip-flow type,and the characteristics and screening methods of each type of loess liquefaction landslides are described.2.The study reveals the mechanism,process and characteristics of large-scale liquefaction slip of loess plateau.The simulated shaking table tests were carried out to study the liquefaction slip pattern of the loess plateau saturated at the bottom of the low angle,and The mechanism of liquefaction slip and the formation mechanism of wave undulating geomorphic features of saturated loess at the bottom of Shibei Plateau were quantitatively analyzed in terms of acceleration amplification effect,Arias intensity variation,frequency and time-holding variation of pore water pressure response of ground shaking,revealing the mechanism of liquefaction-slip of large-scale strata in loess plateau landscape.3.The study reveals the mechanism,process and characteristics of surface saturated liquefaction type loess mudflow.Taking the Yongguang Village East landslide induced by the combined effect of earthquake and rainfall in Yongguang Village,Min County in 2013 as a prototype,shaking table model tests of in-situ soil and remodeled soil were carried out to quantitatively analyze the liquefaction characteristics and differences between in-situ soil and remodeled soil from the perspectives of acceleration dynamic response peak variation,pore water pressure HHT spectrum characteristics,and dynamic soil pressure time-frequency characteristics.The deformation evolution process of surface liquefaction mudflow type landslide is found to be:structural damage→soil loose rupture deformation→saturated soil liquefaction→ mudflow formation during the sliding of liquefied soil.The low-frequency energy response of ground shaking is analyzed by HHT transformation,which confirms that liquefaction starts from the high water content surface soil at the back wall of the model,and the soil forms mudflow in the sliding process after surface liquefaction.The comprehensive analysis of field investigation,shaking table test and numerical simulation results reveals the slip-flow mechanism of liquefied mudflow-type landslide of surface loess.4.The study reveals the initiation mechanism,process and characteristics of the bottom-surface joint liquefaction-slip type loess landslide,and simulates the maximum sliding velocity,thickness of the slip body and slip range of the landslide.Taking the west landslide of Yongguang Village induced by the 2013 Minxian-Zhangxian 6.6 magnitude earthquake in Minxian,Gansu Province as a prototype,a large shaking table test of bottom-surface joint liquefaction-slip type loess landslide was carried out to study the destabilization process of the slope body from local deformation,local liquefaction to overall slip under the action of low,medium and high intensity ground shaking and the two-stage slip flow mechanism of bottom liquefaction triggering landslide and surface liquefaction forming mudflow;by The kinetic characteristics of bottom-surface joint liquefaction landslide type landslide are given by the time-frequency analysis of model acceleration,pore water pressure and soil pressure before and after liquefaction slip,and the maximum sliding velocity,slip thickness and slip range of the landslide are given by the simulation study combined with 3D numerical model.Thus,the mechanism of bottom-surface joint liquefaction slip-flow type loess landslide is revealed.5.The index system,index weights and hazard zoning method are proposed to evaluate the risk of loess liquefaction triggered landslides.The hazard zoning map of liquefaction triggered landslides on the Loess Plateau is prepared.Based on the above-mentioned mechanism of loess liquefaction triggered landslides,five types of influencing factors,including seismic action,topography and geomorphology,stratigraphic lithology,meteorological and hydrological conditions,and geological structure characteristics,are screened out,and a hazard evaluation index system and the weighting of each factor are established.Based on the minimum disaster-causing acceleration method,a hazard assessment zoning method for liquefaction triggered landslides was proposed.Then,using the weighted analysis method of ArcGIS geographic information computing platform,the hazard zoning maps of liquefaction triggered landslides on Loess Plateau under different probabilities of exceedance ground motion was prepared. |
PDF Full Text Request