| Rainfall induced landslides is one of the most widely distributed geological disasters,many researchers have used different methods from formation mechanism,the control factors,the evolution of the landslide movement process and evaluation method system,etc.A lot of research on various aspects,but the development features of landslide and run mode is controlled by complex and the influence factors of the mutual coupling,the complexity of landslide hazard and risk assessment mechanism of still have been posing a great challenge to the community.Shallow landslides induced by heavy rainfall were extensively distributed in the Ren river catchment at southern Qinba Mountains,colluvium and intense weathered zone present apparent heterogeneous structure,coupled with variable slope structure types and complicated meteorological conditions,unsaturated-saturated hydrological response process are more complicated,largely affect the deformation and strength characteristics of the shallow soil and landslide failure mode.In this paper,representative rainfall-induced shallow landslides occurred in Renhe Basin at southern part of Qin-ba Mountainous Region were selected as the object of this research,and unsaturated-saturated hydrological response and surface deformation monitoring system was established on a test-site by selecting typical shallow landslides,and long-term field monitoring experiments under complex environmental conditions were conducted.Based on site investigation,field landslide monitoring test,unsaturated hydro-mechanical tests,the methods of theoretical analysis and numerical analysis,comprehensive research on the typical rainfall-induced shallow landslide triggering mechanism was conducted,considering different rainfall characteristics,antecedent hydrological conditions and local geological condition,unsaturated soil-water coupling effects,for the aim of putting forward a kind of suitable assessment criteria and early warning system for a small basin in the Qin-ba Mountains Region.The main results achieved are as follows:(1)By means of remote sensing image,field geological environment and geological disaster investigation,the geological conditions of landform,geological structure,stratigraphic lithology and slope structure in Renhe Basin are summarized,and the correlation between spatial distribution of the landslides and geological conditions is statistically calculated.In addition,three main failure modes of shallow landslides in the basin are summarized,and the corresponding mechanisms of rainfall infiltration process are analyzed.(2)In this paper,a convient field test method for measuring unsaturated soil water retention curve(SWRC),was obtained by the collection of test data under both desorption and absorption paths,and confirmed that significant hysteresis characteristics existed,and the bigger the overlying soil stress(reduced soil density)is,the more obvious lag behaves,reflecting that the soil porosity decreased when the water channel connectivity is reduced after absorbing process,weakening the soil adsorption ability;Two groups of undisturbed silted clay SWRC tests with different initial porosity were carried out by using the stress-related pressure plate instrument,and SWRC considering the volume change under the desorption and absorption paths.The desorption path test results show that the saturation degree of samples considering the volume change was higher and more accurate than that without considering the volume change.In addition,different initial porosity affects the characteristic value of SWRC,including the air entry value(AEV),the higher the initial porosity,the stronger the drainage capacity,that is,the higher the drainage rate and the lower the AEV value.With continuous increase of suction,SWRC with different initial porosity tend to coincide with each other,and thus the influence of void ratio on SWRC gradually decreases.Therefore,soil structure is an important factor influencing soil water characteristic curve before a certain matric suction threshold is reached.By comparing the characteristics of each SWRC model,the van Genuchten model is the most suitable to describe the characteristics of silty clay in each stage,and its fitting correlation is the highest.(3)A landslide monitoring system of unsaturated-saturated hydrological response and slope surface displacement was established by selecting a typical shallow landslide in the basin.By using different types of sensors and an automatic data acquisition system,the relationships among physical quantities such as rainfall,temperature and humidity under complicated environmental condition and the water content,matric suction,groundwater level and the slope surface displacement data were analyzed.The hydrological response under different meteorological conditions were analyzed,among which the response time of volume water content and matric suction at the near-surface layer was the shortest and the fluctuation range was the largest.The response time gradually increased at depths and the change rapidly decreased.Generally,the saturation degree of soil layer below 1.0 m depth remained relatively high for a long time(about 70%,with suction close to 0 k Pa)and the hydrological parameters were relatively constant,it is suggested that the influence of transient unsaturated seepage flow under the effect of rainfall was mainly concentrated within superficial layers.In the late rainy season,when the saturation degree of shallow layer remained relatively higher,the frequent rainfall lead to the formation of unstable local positive pressure head at the interface over stratums with lower permeability,which resulted into the decrease of effective stress and the loss of soil strength,which can largely induce landslide.On the other hand,the characteristics of wetting front movement with different initial water content distribution was analyzed.Overall,wetting front movement rate and direction rely on antecedent spatial distribution of moisture content and matric suction,and vertical wetting front movement rate sharply decreased at depths,and near-ground infiltration rate was much higher than that of the lower soil layers,verifying that transient rainfall infiltration mainly influence the most shallow layers.Finally,a piecewise calculation method was proposed,and a modified Green-Ampt wetting front model considering the effect of slope inclination was applied to describe the wetting front,and the simulation results were in good agreement with the actual situation.(4)Based on understanding of the periodically hydrological response of unsaturated seepage,finite difference algorithm and simplified unsaturated strength theory and the infinite slope model were applied to reconstructed the triggering event occurred on July 18,2010 in the Ren River basin of three typical landslides,mainly considering the interior geology conditions,including the slope structure influence on the mechanism of rainfall induced landslides.The results show that concave slope with a thick layer,unsaturated-saturated wetting front moving to the lower part,leads to the decrease of suction and unstability of the slope.For the convex slope with thin layer or decreasing layer thickness due to the presence of an abrupt scrap,the weak permeability of soil layer or bedrock under it is easy to form hydrostatic pressure head near the interface over lower permeability layer after rainfall infiltration,and the positive pore water pressure was considered the intrinsic mechanism of such landslides.(5)On the basis of fully understanding unsaturated-saturated hydrological response process of rainfall landslides,the early warning model of landslide with multi-scales can be thus proposed.For a single landslides,this paper proposed a simplified model considering the hydro-mechanical process of a landslide,and the warning threshold was determined according to the slope stability by using the rainfall intensity and duration and defined as in the form of critical Intensity and Duration curve(I-D),and analyzes antecendent water content effect on this critical curve,the greater the rainfall intensity,a relatively shorter time was needed for inducing the instability of landslide,and vice versa.However,when the average water content is lower,the time required for the formation of critical pore water pressure near the sliding surface is relatively long;on the contrary,when the initial average water content is high,the time required for the formation of critical pore water pressure is relatively short due to averaged higher permeability of soil.In addition,the TRIGRS model,by using the DEM data,rainfall sequence,the regional distribution of geotechnical parameters and the thickness of soil,regional slope stability was elaborately back-analyzed for the rainfall event on July 18,2010.The results was fully compared with the actual landslide area,and show that the model can well predict regional landslide stability. |
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