Research On Bank Collapse Prediction Method Of Reservoir In Loess Plateau Area Based On Limit Equilibrium Theory

After the reservoir is impounded,the surrounding environmental geological conditions will change,and the bank slope within the scope of reservoir backwater will recede under the action of reservoir water,resulting in soil and water loss on the bank and reservoir siltation,which may lead to secondary disasters such as surge wave in serious cases.Based on this,this paper proposes the prediction method of bank slope collapse width based on the limit equilibrium method through the field investigation and adjustment of bank slope collapse of valley-type reservoir on Weibei loess Plateau,as well as the indoor physical mechanics and saturated permeability tests.The reasonable accumulation coefficient is determined by a large number of numerical calculations,which provides a basis for the prediction of bank collapse in the loess region.The main achievements of the paper are as follows:(1)Based on field survey and mapping,the stratigraphic structure and bank collapse characteristics of the riverbank slope in the Loess tableland of Weibei were analyzed.The reservoir lithology consists of aeolian loess,slope-deposited soil and silty clay from top to bottom.The water depth of the reservoir from upstream to downstream increases,and the bank slope collapse on both sides becomes stronger.The loess in the upper part of the bank slope is upright,while the middle and lower part of the bank slope is gentle.At the same time,the statistical results show that the average slope Angle of stable deposit of collapse material above the reservoir water level is 35°,and the average slope Angle of underwater deposit is 18°.The morphology of the unflooded part of the bank slope was the same as that before the reservoir was impounded.(2)Based on laboratory experiments of physical mechanics and hydraulic properties,the basic physical indexes,strength characteristics,saturation permeability and anti-disintegration performance of loess bank slope soil are analyzed,which can provide experimental basis for predicting the collapse width of loess bank slope.The results show that the direct shear results can be used as the prediction parameter of bank collapse width,and the influence of reservoir water on bank slope is mainly reflected in the strength of deteriorated soil without considering the influence of osmotic pressure in the prediction of bank collapse in loess area.(3)The prediction method of bank collapse width of loess bank slope based on limit equilibrium and the selection principle of relevant parameters are proposed,which can not only predict the final width of bank collapse,but also better reflect the process characteristics of bank collapse,such as the stage characteristics of bank collapse width growth and the nonlinear characteristics of bank collapse width growth with time.(4)Taking the Kachugin method as a reference,the key parameters of the collapse width prediction model based on the limit equilibrium method were studied.The results show that under the same conditions,the underwater accumulation Angle has little influence on the predicted width of collapse bank.Meanwhile,when the ratio of water depth to bank slope height is the same,the predicted width increases with the increase of bank height,and fluctuates with the increase of slope.When the bank slope height is the same,the predicted width increases with the increase of water depth-bank slope height ratio.When the ratio of water depth to bank slope height is the same as that of slope height,the accumulation coefficient(a)increases with the increase of slope height(H)and water depth to bank slope height(b).The relationship between underwater accumulation coefficient a,water depth-bank slope height ratio b and bank slope height H is as follows: a=0.02H-0.5b+0.3.