Study On The Mechanism Of The Loess Landslide Under The Action Of The Earthquake And Snowmelt In The Ili Valley

The landslide has become a significant geological disaster for wide distribution,high frequency and large loss.As many as 1800 landslides occur annually in China which causing more than 400 deaths and direct economic losses of up to 4.6 billion RMB,landslides seriously threaten the lives and property of people and cause huge loss to the country.In recent years,with the proposal of China's One Belt And One Road strategy,development and construction along the route have flourished,the strategy advanced the process of economic globalization and opening up a new model of regional cooperation,but landslides seriously threaten the economic construction of the One Belt And One Road.According to statistics,from 1900 to 2015,there were more than 330 landslides along areas of the One Belt And One Road,which killed more than20,000 people.The Ili valley is an important hub for China to connect Central Asia,Eastern Europe and other countries.It is also a significant node of One Belt And One Road strategy,so geographical location of Ili valley is very important.However,the loess in the Ili valley is widely distributed and Ili valley has complex structures,frequent earthquakes,and strong snowmelt seasons.Due to the deposition of ice water from the Tianshan Mountains,the formation of the Ili valley has a diadactic structure,with ice water depositing pebbles at the bottom and loess at the top.Therefore,under the influence of earthquakes,snowmelt and other factors,it is easy to cause loess landslides and threatens implementation of the One Belt And One Road strategy.Therefore,this paper systematically studied loess landslides in Ili valley through the cooperation with Xinjiang Geological Environment Monitoring Institute and Xinjiang Earthquake Administration.Basing on data collection,field geological survey,laboratory test and numerical simulation,this paper researched the distribution law and influencing factors of the loess landslieds in Ili valley and a geological model was established.In the case of Xiaomohuer landslide,this paper investigated the tensile strength of loess in the trailing edge of the landslide,the evolution process of the tensile cracks,the law of water migration and the strength deterioration mechanism of the loess-pebble interface during snowmelt.Finally,the formation mechanism of the loess landslieds under the action of earthquake and snowmelt was explored by numerical simulation,which had great significance for the early identification,early warning and prevention of loess landslides in Ili valley.The main research results and conclusions of the paper were as follows:(1)Through collecting the basic data of loess landslides in Ili valley,the temporal and spatial distribution characteristics of loess landslides were analyzed by using Arc GIS software and GA-BP neural network,meanwhile,the primary and secondary influencing factors of loess landslides were obtained.The loess landslides in Ili valley mainly occurred from March to May every year,which was the snowmelting period;the elevations of loess landslides mainly distributed in the range of 1000-2000m above sea level;the loess landslides were mainly distributed near faults and earthquakes as groups or landslide chains,the main strike was basically consistent with the trend of the fault;the loess landslides were affected by the snow depth in the range of 14.8cm-15.7cm;the loess landslides were mainly distributed in a slope gradient of 15°-25°and the direction were mainly south(S),Southwest(SW)and southeast(SE),the main type of the landslides was convex.Comprehensive analysis showed that elevation,landslide structure were the controlling factors for the loess landslides in Ili valley,what's more,earthquakes and snowmelt were the main external causes.The loess landslides in the Ili valley could be divided into two categories:homogeneous loess landslide and loess interface landslide.The homogeneous loess landslides were mainly shallow landslides,and there were a few deep landslides.the sliding surface of some homogeneous loess landslides were between the roots of plants and loess.In terms of plane shape,the front edge of the landslide was mostly dustpan-shaped and fan-shaped,the trailing edge of the landslide were in the shape of a ring chair which often developed tensile cracks.The vertical section of landslides was mostly arc-shaped.The homogeneous loess landslides were greatly affected by rainfall and snowmelt.On the other hand,loess interface landslides were mostly formed in the transition zone between low mountain and mid-mountain terrain.The sliding surface was mainly between loess and pebble(some loess and bedrock contact surfaces also existed).The landslides were the tongue shape on the plane and stepped on vertical section.Springs were often exposed at the front edge of the landslide and there were obvious tensile cracks at the trailing edge.The landslides were mainly affected by snowmelt and earthquakes.(2)Aiming at the ubiquitous tensile cracks in the trailing edge of the loess landslide in the Ili valley.This paper took the Xiaomohuer landslide as an example.A set of unsaturated soil tensile instruments combined with particle image velocimetry(PIV)was designed and nuclear magnetic resonance experiment and scanning electron microscope test were explored to study the loess tensile strength and the evolution process of the tensile cracks in aspects of water occurrence state and content,microstructure of the loess.The water content had a great influence on the tensile strength of the loess,and there was a limit moisture content w_c,which approached to18%(near the plastic limit water content).When the water content was greater than w_c,the tensile strength was smaller.With the water content decreasing,the tensile strength of loess rose slowly;when the water content was less than w_c,the tensile strength increased rapidly with the water content decreasing.In addition,with the dry density increasing,the overall tensile strength increased gradually.When the water content was greater than w_c,the sample exhibited plastic failure,on the other hand,when the water content was less than w_c,the sample exhibited brittle failure.The brittle failure could be divided into two stages:stage I was the structural adjustment stage,the tensile stress increased with the increase of tensile strain until the peak strength,the sample had no cracks;stage II was the stage of rapid penetration of cracks,tensile stress quickly dropped from the peak to 0,the crack connected instantly.Plastic failure could be divided into four stages:stage I was the structural adjustment stage,no cracks were formed on the sample surface and the tensile stress increased slowly;stage II was the stage of microcrack development,microcracks were formed on the surafce of the sample,the tensile stress increased gradually to the peak.Stage III was the stage of crack formation,the macro-cracks formed on the surface of the sample,the tensile stress decreased rapidly;Stage IV was the stage of crack penetration,the crack of the sample connected downward and the tensile stress decreased to zero.When the water content of the sample was high(w=26%),there was capillary free water in the pores of the loess,the capillary free water softened soil particles,meawhile,the clay was wet and dispersed which evenly covered above the particles,the inter-particle cementation was poor.Thus,the overall structure of the soil sample was instability and the tensile strength was very low.With the decrease of water content(26%>w>18%),the capillary free water was partially lost,the dispersed clay was partially aggregated,cemented and filled above the loess particles and pores.In addition,with unsaturation increasing,the matrix suction increased.So,the tensile strength increased slowly to a certain extent.With the water content decreasing continuously(w<18%),the capillary free water content was very low but the the bound water began to decrease rapidly,a large amount of clay aggregated and cemented above the loess particles,the clay filled in the pores and enhanced the structure of the loess.In addition,with the thinning of the bound water film,the physical and chemical forces bewteen the water film and the soil particles increased,the original cohesion enhanced rapidly.The recovery of the structure improved the tensile strength.(3)In the condition of snowmelt,for the loess-pebble interface in the loess landslides.A one-dimensional seepage soil column system model test was carried out to derive the water migration equation,the water migration of the loess-pebble interface was simulated by ABAQUS software,then the strength deterioration mechanism of the loess-pebble interface was studied by large direct shear test.Affected by snowmelt,the water in the saturated pebble layer migrated to the loess layer.The water content of the soil column model increased gradually from the bottom to the top,with volume water content decreasing,the matrix suction decreased,finally became basically stable.The curve that volumetric water content varied with time showed an S shape,and which could be divided into an initial stable stage,a fast rising stage,a slow rising stage and a final stable stage.The maximum volumetric water content was 34.1%at the 0cm section 1 and the minimum volumetric water content was 16.3%at the 75cm section 5.Respectively,the maximum matrix suction was-2 k Pa at 0 cm section 1 and the minimum matrix suction was-169.5 k Pa at 75 cm section 5.The Maximum height of the capillary free water rise could be fitted by an exponential function and the speed of capillary free water rise could be fitted by an inverse proportional function.The maximum rise height of capillary free water above the loess-pebble interface was 2.3m by ABAQUS numerical simulation.The numerical simulation showed that the saturation of each section was suited with the one-dimensional seepage soil column test results.On the other hand,the large direct shear test of loess-pebble interface were conducted,with the water content increasing,the occlusal force c of the loess-pebble interface and the internal friction angle?all decreased.When the water content is 2%,occlusal force c was the largest at 54.5 k Pa and the internal friction angle?was the largest at 24.2°.Respectively,when the water content is 26%,the occlusal force c was the smallest at 25.1k Pa and the internal friction angle?was the smallest at 14.3°.When the water content was low(less than the plastic limit water content),the hard loess particles rolled and pushed in the uneven pebble interface and the matrix suction in the soil was large.Therefore,the strength of the loess-pebble interface was high.When the water content increased by snowmelt,bound water and capillary free water existed in the interface,the occlusal force bewteen the soften loess particles was very week.Meanwhile,the matrix suction was low,so the strength of the loess-pebble interface was low.(4)Through the ABAQUS numerical simulation,the deformation,failure characteristics and safety factor of the Xiaomohuer landslide were analyzed in the condition of no snowmelt and no earthquake,snowmelt c and earthquake with snowmelt.In the condition of no snowmelt and no earthquake,the Xiaomohuer landslide only had relatively large displacement at the trailing edge of the landslide,the displacement at the leading edge of the landslide was relatively small.The trailing edge of the landslide was in a tensile environment and the stress in the landslide distributed relatively uniform.There was no obvious plastic failure region in the landslide but only partial tensile failure occurred at the trailing edge of the landslide.The safety factor of the landslide was 1.21 and the landslide was basically stable.In the condition of no snowmelt,the pebble layer was saturated for the snowmelt,the water migrated into the loess layer and water content of the bottom loess layer increased.The pore pressure was evenly distributed along the loess-pebble interface in the vertical direction,when loess was below the saturation line,the pore pressure was positive,conversely,the pore pressure was negative,the displacement of the landslide increased.The trailing edge of the landslide was stretched and tensile stress area was increased,the tensile cracks were further expanded,there were obvious plastic failure region along the trailing edge of landslide and the loess-pebble interface.The safety factor of the landslide was 1.09 and landslide was in a critical stable state.In the condition of earthquake and snowmelt,the loess on shoulder,surface and the trailing edge of the landslide had a magnifying effect on the seismic acceleration and there was a clear boundary near the loess-pebble interface.The displacement of the overall landslide increased,especially,the largest displacement was 8.4m at the foot of the landslide.Under the action of earthquakes,the stress of the landslide greatly changed,it partially adjusted from a compressed state to a tensile state.The sliding surface was connected from the tensile cracks in the trailing edge to the loess-pebble interface.The The safety factor of the landslide was 0.89 and the landslide was very likely to slide.(5)Taking Xiaomohuer landslide as a typical example,the deformation and failure stages of loess landslides under the action of earthquake and snowmelt in Ili valley could be divided into progressive creep-tensil cracking failure stage,and drastic collapse-shearing failure stage.(1)Progressive creep-tensil cracking failure stage,during the snowmelt period of each year,the pebble layer was gradually saturated by the infiltration of snowmelt,the spring was exposed at the foot of the landslide,the water gradually migrated from the pebble layer into the loess layer.The migration of the water caused the bottom of the loess saturated and the strength of the loess-pebble interface decreased.The stress in the landslide was adjusted and crept along the loess-pebble interface.With the landslide moved forward,the trailing edge of the landslide was under an extensional environment.When the tensile stress was greater than the strength of the loess,tensile cracks were formed on the trailing edge of the landslide.With the landslide crept slowly,the width of cracks increased gradually and spread to the deep part of the landslide,finally,landslide was in the critical stable state.After the snowmelt period,the water supply in the pebble layer decreased,the saturation line dropped from the bottom of loess.the loess-pebble inetrface changed from saturated to unsaturated,the surface strength increased,the stress in the landslide adjusted and the displacement was reduced,so the landslide became stable gradually.The landslide changed from a critical stable state to a stable state by the cyclical snowmelt.The creeping displacement of the landslide accumulated gradually.The landslide was in the progressive failure stage and the failure form was creeping-tensil cracking mode.When the progressive creeping displacement was large enough,the landslide could also slide.(2)In the stage of dramatic landslide-shear failure,under the action of the earthquake and snowmelt,the seismic force was superimposed on the landslide and the stress in the landslide changed,due to seismic force,the loess was subjected to the tensile and shear stress,The structure of the loess was collapsed and pore pressure of the saturated loess-pebble interface increased,the strength was quickly lost in a short time for the liquefaction.The landslide quickly collapsed and sheared along the loess-pebble interface.The failure form was collapse-shearing failure model.