| The northern section of China North South seismic belt runs through the central and western regions of the loess plateau,where strong earthquake often happens,and rainfall is abundant during the rainy season.In addition,due to the special water sensitivity and dynamic vulnerability of loess,the seismic induced geological disasters in this region are faced with severe situation.There are a lot of researches on seismic landslide and rainfall landslide in loess region.However,there are few researches on seismic landslide after rainfall,especially on the influence of rainfall coupling effect on loess landslide.On July 22,2013,after continuous rainfall,an earthquake with Ms 6.6 occurred in the loess covered area of Dingxi city,Gansu province,which induced the instability of loess slope and caused serious casualties and losses.It has been paid great attention to by geotechnical earthquake engineering,which makes it vital urgent to study the mechanism of landslide instability after rainfall and prediction method.This thesis is aimed at studying the earthquake-induced mechanics mechanism of loess slope and its stability under the influence of rainfallRegarding the 'two landslides on a hillslope' induced by the Min-Zhang earthquake in the Wamuchi hillslope as the breakthrough point,plenty of field and laboratory tests were completed based on scientific investigation of landslide.Under the same earthquake and rainfall conditions,the two landslides showed great differences in sliding accumulation characteristics.Then the geological environment characteristics and sliding features of the two landslides were compared and analyzed under the earthquake and rainfall conditions.At same time,the influence of rainfall infiltration on physical and mechanical properties of slope soil was analyzed by in-situ infiltration test of artificial rainfall.The influence of slope site conditions on seismic ground motion characteristics was analyzed by numerical simulation test.The influence factors of seismic liquefaction of loess were analyzed by full-scale shaking table test.Finally,based on the above research results,a slope stability analysis method which can take the influence of rainfall and earthquake into account is proposed to clarify the control mechanism of rainfall on loess slope seismic instability.The innovative achievements obtained through the above research work are summarized as follows:(1)Based on the emergency investigation of seismic geological disaster after the earthquake,unmanned aerial vehicle photographic mapping,engineering drilling,high-density electric method testing,laboratory test,etc.,the paper puts forward that rainfall before the earthquake has a controlling effect on the instability of loess seismic landslide in Wamuchi slope and the differential instability mechanism of 'two landslides on a hillslope'.It is found that the instability mechanism of the eastern landslide in Wamuchi slope is induced by seismic inertia force,while that the western landslide is induced by loess seismic liquefaction.Based on instability mechanism analysis of the two landslide,three key issues on instability analysis of the loess seismic landslide under the rainfall was proposed,such as physical and mechanical characteristic of soil was changed by the cumulative effect of rainfall infiltration,the difference of slope site conditions directly affects the seismic dynamic response,coupling effects rainfall and earthquake controls the liquefaction mechanism of loess slopes.(2)Based on the in-situ infiltration experiment of artificial rainfall,the spatial and temporal regularity of rainwater infiltration,the model of water transport and the influence mechanism of cracks on the infiltration range of natural loess slope were established.It is found that the rainfall infiltration depth of natural loess slope is limited,and the uneven infiltration of slope body is closely related to the physical state of soil mass.Based on theoretical analysis,an infinite slope model for water transport under rainfall infiltration is proposed,which reveals the influence law of saturated water content depth characteristics of loess slope.The relationship between the water content of intact loess and the characteristic parameters of dynamic and static shear strength is presented through laboratory geotechnical tests.The influence mechanism of rainfall infiltration on collapsing strength reduction of loess slope was revealed by water content.(3)Historical earthquake damage data and co-earthquake landslide disaster was statistical analyzed,based on the calculation of ground motion response,the estimation method of ground motion amplification effects of loess slope and the ground motion characteristics of different parts of loess slope are presented.It is found that loess slope is the main geomorphologic type that causes seismic intensity anomaly.Seismic amplification is the main factor that induces loess slope instability and disaster.Based on the spectral analysis of the typical strong earthquake records in loess region,it is proposed that the loess overburden thickness and topographic characteristic parameters can be used as the main parameters to determine the amplification factor of slope.The homogeneous loess slope,the layered slope and terrace slope was analyzed by parameterize seismic response calculation.The influence patterns of on the characteristic parameters of ground motion in different parts of the slope is proposed.The influencing factors include mechanical parameters of slope and terrain geometry feature,stratigraphic structure,seismic input for seismic characteristic parameters.A method for estimating the amplifying effect of ground motion on loess slopes is proposed.(4)A shaking table testing on seismic liquefaction of large size intact loess was carried out.The effect of initial water content of undisturbed loess on seismic liquefaction of loess is revealed.The determining method of water content in seismic liquefaction of loess and the influencing patterns of pore pressure growth are presented.It is found that 85%saturation is the critical value for complete liquefaction of loess which is like sandy soil.A shaking table test for saturated loess liquefaction on inclined bedrock was carried out.It was found that seismic liquefaction was prone to occur on loess slopes on inclined strata,and the higher the dip Angle,the higher the degree of liquefaction.The mechanism of the influence of inclined bedrock on the growth pattern of pore pressure in saturated loess under dynamic conditions is revealed.(5)Based on the infinite slope analysis model,a stability analysis method considering the coupling effect of rainfall infiltration and seismic action is proposed.The influence of seepage direction,seismic inertia force direction,seismic coefficient,amplification coefficient,vibration cycles,water content,underground water level and overburden thickness on the stability coefficient of infinite slope is analyzed.Combined with the above research results,the process of inducing and destabilizing of 'two landslides on a hillslope' was analyzed.The control process and mechanical mechanism of rainfall on seismic instability of loess slope are clarified. |
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