| Strong earthquakes would not only induce a large number of secondary disasters such as landslides and collapses,but also produce a large number of earthquakedamaged slopes.Therefore,under the action of subsequent strong aftershocks and rainfall,new landslides and collapses would be induced.The Loess Plateau is one of the regions with the strongest seismic activity in China.Landslides and collapses have always been the most prominent geotechnical disasters in the Loess Plateau due to seismic tectonic environment with frequent strong earthquakes,the topography of the ravines,the weak mechanical properties of loess,multi-factor coupling and multicausal cycle.Damaged loess slopes after earthquake have a high risk of instability under aftershocks(subsequent earthquakes),rainfall and their coupling effects.However,there are few studies on the stability evolution mechanism of earthquake-damaged loess slopes and the emergency evaluation method system has not yet been formed.Therefore,it is of great significance to carry out relevant research to improve the emergency rescue and disaster defense capabilities and ensure the safety of life and property in the Loess Plateau.In this paper,the seismic damage mechanism of loess slope is revealed by collecting and studying the seismic damage data,combining with geological survey and mapping,geotechnical exploration,triaxial test and shaking table model test,and the seismic damage grade evaluation standard of loess slope is established.Based on the artificial rainfall infiltration test and the physical parameter test of slope loess,the rainfall infiltration law of loess slope was studied.Through shaking table model test,numerical simulation and theoretical calculation analysis,the seismic damage characteristics of loess slopes under natural conditions and different rainfall conditions were studied,and the stability evolution mechanism of earthquake-damaged loess slopes under subsequent earthquakes,rainfall and their interactions was revealed.Based on the above research results and the latest research results at home and abroad,an emergency evaluation method system for earthquake-damaged loess slopes is established,which includes site ground motion prediction,slope amplification effect estimation,geomechanical model construction and stability coefficient calculation of post-earthquake slope.Suggestions on emergency disposal measures for loess slopes with different earthquake damage levels are given.The main results are as follows :(1)Through geological survey,mapping,geotechnical investigation,triaxial shear test of deformation and strength of earthquake-damaged loess,shaking table model test of earthquake-damaged slope,the seismic damage mechanism of loess slope is revealed,the evaluation standard of seismic damage grade of loess slope is established,and the main types and characteristics of earthquake damaged loess slopes are given.In a broad sense,earthquake-damaged loess slopes could be divided into two major categories:landslide and collapse accumulation,and seismic damage slope with cracks.The first category includes two sub categories: loose accumulation mass and bulk landslide mass,and the second major category includes three sub categories: steep slope at landslide trailing edge,earthquake-damaged slope without downward dislocation,and earthquake-damaged slope with downward dislocation;In a narrow sense,earthquakedamaged loess slope only include the second major category;According to the main damage characteristics of the second major category,and considering the need for rapid landslide risk assessment during the emergency period after an earthquake,the earthquake-damaged slopes are divided into three grades: light,moderate,and severe based on indexes such as crack length,width,and depth;As a special type of earthquake-damage slope,the landslide and collapse accumulation classifies as severe earthquake-damaged slope.(2)Based on the shaking table model test,combined with numerical simulation and theoretical calculation,the stability evolution law and mechanism of earthquakedamaged loess slopes under subsequent earthquake actions were studied and revealed.For natural loess slopes with slight and moderate earthquake damage,when they are subjected to later earthquakes whose intensity is less than or equal to that of the previous period,the scale of slope cracks and the degree of earthquake damage would not change significantly,and the stability of the earthquake-damaged slope would not significantly decrease;When they are subjected to later earthquakes whose intensity is stronger than that of the previous period,the slope damage level would increase,the stability would greatly decrease,and even the instability and damage would occur.The severe earthquake-damaged slope of instability and downward dislocation but without landslide would maintain its basic stability due to friction and anti-sliding resistance under post-earthquake static condition,but the subsequent smaller ground motion would trigger the sliding of the earthquake-damaged slope.Therefore,the landslide hazard of such earthquake-damaged slope under aftershocks is very high,and so it should be the focus of attention after earthquake of the main shock and aftershock type.(3)By testing the physical parameters of soil in different parts of the slope and combining them with in-situ rainfall infiltration tests on the slope,the permeability characteristics of loess and the rainfall infiltration law of the slope were revealed.Due to long-term rainfall infiltration and dry-wet cycling,a low permeability zone is formed at a depth of 55-75 cm of loess slope,which has a certain barrier effect on rainfall infiltration.In the process of slope rainfall infiltration,the infiltration rate would decrease significantly with the increase of rainwater infiltration depth,so the slope rainfall infiltration depth is usually within 1 meter under normal rainfall conditions.If there are extreme events such as long duration and great rainfall,rainfall infiltration would reach a depth of about 2 meters.When there are special infiltration channels such as joints and cracks,rainwater would directly infiltrate into the interior of slope.Therefore,the focus of geological survey of earthquake-damaged slopes is the depth and distribution range of seismic cracks.(4)Through shaking table model tests with different coupling modes of earthquake and rainfall,the stability evolution mechanism of earthquake damaged loess slopes under the coupling interaction of earthquake and rainfall was clarified.The loess slope after initial rainfall,under the action of previous and subsequent gradually increasing seismic ground motion,the cracks gradually increase,deepen and enlarge,the wetting front moves down,the damage range continues to expand,and the damage degree continues to intensify.If there is subsequent rainfall after the previous earthquake,the rainwater would penetrate into the deep area of the slope along the cracks,so the critical instability acceleration of the subsequent earthquake of the earthquake-damaged slope would be significantly decreased,and the instability sliding range would be larger.(5)Based on this article and the latest research results,a system of emergency evaluation methods for earthquake damaged loess slopes has been established,which includes prediction of site seismic motion,estimation of slope amplification effect,construction of post earthquake damaged slope geomechanical model,and simplified calculation of seismic stability coefficient.The potential sliding surface of loess slope damaged by earthquake could be assumed to be composed of three sections: tensile fracture section,shear section and anti-sliding section.For seismically damaged slopes with tension cracks at the top platform of the slope,the potential sliding body can be simplified into a three-block model.For the earthquake-damaged slopes with main tensile cracks on the slope surface,the potential sliding body can be simplified into a two-block model.Based on the above assumptions and rigid body limit equilibrium theory,the emergency evaluation method system of earthquake-damaged slope are established.(6)Drawing on the research results of landslide prevention and control,taking into account the seismic damage characteristics of loess slopes,emergency response measures for loess slopes with different seismic damage levels were proposed.The hazard of loess slope with slight and moderate earthquake damage is low,the main measures are to fill cracks and compact loose soil;Severe earthquake-damaged slopes(no penetrating rupture surfaces)should consider the possibility of landslide induced by the coupling of aftershocks and rainfall.On the basis of filling cracks and tamping loose loess,it is necessary to set up special personnel to carry out regular inspections and timely research and judge the slope stability.For severe earthquake-damaged slopes with penetrating rupture surfaces,the possibility of aftershock triggering landslide is very high.People in the dangerous area need to evacuate urgently at the first time,and should set up professional monitoring equipment and assign special teams to carry out deformation monitoring,and need to dynamically predict and evaluate landslide hazard. |
