Water Sensitivity Of Loess And Prediction Of Rainfall Induced Shallow Loess Landslides

The Loess Plateau in China is the region of loess with the largest distribution area,the most complete layer and the thickest accumulation.Taking advantage of this,stratigraphy and paleoclimate based on loess have been extensively studied by Chinese researchers in the subject of quaternary and environmental geology,and keep a leading position in loess research over the world.In terms of engineering geology,loess is regarded as a kind of special soil for its collapsibility.Seas of studies have also been done on this,and plays an important role in guiding the engineering practice in the constructions of cities and major projects in the loess area.In addition to the narrow sense of collapsibility,loess also has properties of broad water sensitivity.Loess is a special type of soil with strong water sensitivity,contributing to the complexity of the mechanism,and difficulties of prevention and mitigation of loess landslides.Which also makes the Loess Plateau becoming one of the three regions with the most serious geohazards in China.Taking loess plateau as the research area,based on the properties of water sensitivity of loess,to reveal the mechanical mechanism of water sensitivity and its spacial and temporal distribution at the Loess Plateau,and to explore the technique of prediction of rainfall induced loess landslides.The main research contents and innovative achievements are as follows:1.The mechanical mechanism of water sensitivity and its spatial and temporal variation is revealed.On the basis of systematically summarizing the engineering properties of loess,the tests on unsaturated characteristic curves for loess locating at the different geomorphological unites and different deposited layers were conducted,and the databases of soil-water characteristic curves,permeability curves,and the matric suction curves for loess at the Loess Plateau were established.Based on summarizing the mechanical behavior of water sensitivity of loess,its mechanical mechanism is revealed,and gives the definition of loess' s water sensitivity.The test results show that the water sensitivity of loess changes with deposited time and locations regularly.In space,from west to east and from north to south,the water sensitivity of loess is weakened.In terms of time,the water sensitivity of loess increased from old to new sedimentary age.2.The hydrological process and mechanism for the uniform and preferential flow in the loess slope is revealed.Based on the technique of traditional electrical resistivity tomography,the measuring instruments and electric-supply mode are improved,which can be used for long-term 3D monitoring in the field,achieving equipment unattended,automaticmeasurement and data remote transmission.By establishing the relationship between water content and resistivity,the hydrological process of the infiltration and the spatial and temporal distribution for water content are captured dynamically,and the law of rainfall infiltration in the loess slope with different slope types is revealed.With the high precision geophysical detection technology and the 3D tomographic scanning techniques,the in-situ uniformed and preferential infiltration tests were carried out.According to the in-situ test,the stochastic numerical model was set up,and the uniform and preferential infiltration processes of the test loess slope were reproduced,and revealed its infiltration pattern.The method can be used to predict the hydrological process of uniform infiltration in loess slope under different rainfall conditions.3.Based on the water-stress coupled model and the theory of progressive failure calculation,the failure mechanism of loess slope is revealed.Guided by the theory of unsaturated soil considering matric suction,taken soil water characteristic curve as a link,water-stress coupled model for loess slope was set up,and the mechanism of water and stress interaction was revealed.Guided by the theory of progressive failure calculation,taken the local factor of safety(LFS)as a link,the probability analysis model was established.The failure and deformation mechanism of loess slope was revealed from three aspects: rainfall infiltration mechanism,water-stress coupling mechanism and progressive failure mechanism,and the failure modes based on the law of rainfall infiltration in the loess slope was analyzed.4.The regional rainfall-induced loess landslides was predicted by extending the probability analysis model for single slope.Based on the monitoring data from the observation base of loess landslides in Yan'an city,the water content distribution for loess slope with different geometries,under different rainfall conditions.According to failure probability related to water content distribution in loess slopes,the prediction criterion for rainfall-induced loess landslides was established,for the different combinations of three slope types of convex,concave and linear,and the three slope gradients of 30,40 and 50.Then,extending the calculating results for the above combinations to the main district of Yan'an city,and analyze the failure probability for each slope,according to its belonging to or similar combinations.Finally,the landslides occurred in the “7.3” rainstorm were used to verified the prediction criterion,and gave a good fitting.