Study On The Stability Of Loess Bank Slope Under The Condition Of Reservoir Water Level Change

Nangoumen Reservoir area is located in Yan ’an City,Shaanxi Province,3.2km upstream of Hulu River,a tributary of the Luohe River.Currently,the reservoir has been put into use,but many engineering geological problems have occurred in the process of operation,such as multistage slide of the loess bank slope in the reservoir area,which has a certain impact on the geological environment around the bank,part of the cultivated land and the siltation and water quality of the reservoir.At the same time,the rainy season precipitation and the change of water level in the reservoir area lead to the instability of the bank slope,especially the loess bank slope,bank slope slide often occurs.Therefore,based on the investigation and analysis of the engineering geological conditions of the reservoir area,this paper evaluates the stability of the bank slope during the operation of the reservoir through field sampling,laboratory test and theoretical analysis.The main research contents and achievements are as follows:(1)The grain size gradation,basic physical indexes and capillary water rise height of the loess on the bank of the reservoir were tested.The grain size composition of loess in the reservoir area was determined,and the maximum height of capillary water rise was determined to be 1.11 m.(2)Direct shear tests were conducted on the undisturbed loess,saturated loess and the loess after five dry-wet cycles on the bank of the reservoir,respectively,to study the changes in the shear strength of the loess under different states,and analyze the reasons for the changes in the shear strength from the microscopic point of view by means of microstructure test(SEM).(3)The saturation permeability coefficient of undisturbed loess with different depths in reservoir area was obtained through the indoor variable head test,and the research showed that the permeability coefficient decreased with the increase of depth.The reasons for the difference of saturation permeability coefficient were analyzed combined with the pore characteristics of loess on the bank slope.(4)Based on the filter paper method,the soil-water characteristic curves of the undamaged loess with the representative bank depth of 3m and 6m were measured.The two curves have obvious double "S" shape,which is a typical double-peak soil-water characteristic curve.The V-G model was used to fit the two soil-water characteristic curves in sections,and according to the unsaturated permeability theory and the CCG model,the relationship between the permeability coefficient and the volume water content of the undamaged loess was obtained indirectly.(5)The seepage analysis and stability evaluation of two typical bank slopes were carried out by using GeoStudio software,and the variation rule of bank slope stability coefficient was obtained when the reservoir water level was rising and falling.When the water level of the reservoir is 844.23 m,the bank slope of P52 point is in an unstable state,and the bank slope of P95 point is in a basically stable state.When the water level rises,the bank slope stability coefficients of P52 points and P95 points decrease.When the water level drops,the bank slope stability coefficient decreases more obviously.The bank slope at P52 points is in an unstable state,and the bank slope at P95 points is in an understable state when the water level falls back to the initial water level.The decrease of water level obviously increases the possibility of local instability of the bank slope.