Study On The Instability Of Loess Slope Under Earthquake And Rainfall Coupling Action Based On Large Shaking Table Model Test

Loess is a kind of soil of weakly consolidated and porous, and its engineering properties are very poor. Loess have water sensitivity and dynamic vulnerability and when it was wetted with water or encountered strong earthquake would induce serious geological disasters and engineering problems. If the landslide disaster was induced by the coupled action of the earthquake and rainfall, damage will be greater. In recent years, people’s demand for seismic safety is also growing day by day with the rapid development of social economy in the Loess Plateau, the inner demand of seismic safety and the real possibility of slope failure induced by multi- field coupling effect has fully shown that the research of slope failure under the coupling action of earthquake and rainfall is necessary. In this paper, the study of loess slope instability under earthquake and rainfall coupling was carried out with shaking table as the main research means based on the loess slope of low angle. Firstly, a natural undisturbed loess slope in Lanzhou New Area was selected as test site and artificial rainfall penetration test was implemented, the Infiltration law and The change rule of soil strength under different rainfall conditions was studied, and shaking table model test was carried out based on the results of the rainfall penetration test. In this paper, we designed and implemented shaking table model test of natural loess slope, loess slope of rainfall before earthquake and loess slope of rainfall after earthquake under different coupling modes of earthquake and rainfall, differences of self- vibration characteristics of loess slope models, dynamic response rules under different intensity earthquakes and processes of different deformation evolution and characteristics of instability destruction were analysed systematically. Instability criterion of loess slope under earthquake and rainfall coupling action was determined by shaking table test, the prediction evaluation models of loess slope stability and instability damage under earthquake and rainfall coupling action was proposed, and the mechanism of loess slope instability was discussed based on the study mentioned above. The main conclusions and innovation points are as follows:1.The field artificial rainfall infiltratio n test was implemented by the artificial rainfall equipment developed independently. Experimental results show that soil saturation degree of loess slope under the natural state is low and would appear great differences between different parts of slope after a rainfall infiltration, soil saturation degree within rainfall infiltration wetting front changes between 30% ~ 70% and little more than 70%, the deeper penetration depth and the greater upper soil saturation degree. Due to the difference of soil saturation degree in different part of the slope, soil strength will also appears very different. Soil strength declines 42% from 10 cm above the wetting front to the surface of slope; Soil strength decrease less in the upper 10 cm range of the wetting front; Soil strength returns to the natural state below the wetting front.2.The change of the weight and the shear strength of slope soil under different rainfall conditions gives rise to dynamic stability of slope changes dramatically. Through the shaking table test, visually verify the low angle of loess slope the critical instability acceleration caused by earthquakes under different rainfall conditions. The critical instability acceleration of slope reaches 1000 gal under the natural state( low water content), pull crack damage of slope shoulder is main modes; When the rainfall reaches to a certain extent, such as 10 mm, its critical acceleration of earthquake instability would rapidly reduce, the upper slope will appear instability phenomenon when earthquake intensity reaches 700 gal, seismic subsidence of slope crest is main damage form; With the increase of rainfall, the critical acce leration value gradually reduce, loess liquefaction and slip phenomenon will appear when the rainfall reaches 100 mm and earthquake intensity reaches 600 gal.3.Loess slope of low angle after strong earthquake, upper part of slope generated several small cracks; At the beginning of the rainfall, the original cracks expanded scale rapidly with the increase of rainfall, fracture number increased rapidly; But with the continuous increase of rainfall, slope soil water erosion rate speeded up; When there was surface runoff occurred, surface runoff with loess silt filled fissure and fissure was “welding”. The result shows that low angle of loess slope has a low probability of landslides under normal rainfall conditions if only produce small cracks in the surface of slope after medium- strong earthquakes action, the fracture of loess slope after earthquake action heals by water effect step by step, loess slope gradually restore stability.4.The water content of loess slope surface under different rainfall conditions was very different, and slope has different failure modes under earthquake action, and the internal mechanism is very different. Soil shear strength of natural loess slope under low water content is relatively high, slope will produce instability failure over a certain intensity of earthquake action, the mechanism of instability is that the overlap effect of the seismic inertia force and gravity cause s the upper soil of slope to produce rip failure. For loess slope after small rainfall, saturation degree of surface soil within a certain range increase and mechanical properties decrease, soil microstructure destruction and seismic subsidence Under the action of a strong earthquake, slope produce dislocation of ladder type and fracture failure under the action of subsequent ground motion. For loess slope of long duration and large rainfall, the collapse of pore structure under earthquake action and the liquefaction slip of loess are the basic failure mechanism.5.Based on shaking table test results, the prediction evaluation model of slope stability and damage of slope instability under the coupled action of earthquake and rainfall was put forward in this paper.