| Slope stability is one of the main engineering geological problems in the construction of large-scale hydropower projects in western China.The atomized rainfall formed in the flood discharge process of hydropower station will affect the slope stability to different degrees,and may lead to the potential instability of the slope failure,threatening the operation of hydropower station.Shuangjiangkou Hydropower Station is located at the junction of Markang County and Jinchuan County,Sichuan Province.It is the fifth level of the 28-level development scheme of the 3 reservoirs in the main stream of Dadu River.The dam is an earth-core dam with a height of 315m,a maximum discharge head of 253m and a maximum discharge of about 8200m~3/s.The atomized rainfall of flood discharge will have a significant impact on the stability of the slope of the large accumulation body on the left bank.Once the overall stability fails,the operation safety of the power station will be seriously threatened.Therefore,on the basis of the study of the basic characteristics of the slope,the evaluation and prediction of the stability of the slope in the atomized area is of great significance to the treatment of the slope in the atomized area and the safe operation of the power station.Based on the"double estuary dadu river hydropower station alteration rock development characteristics and engineering influence research"project as the backing,through detailed engineering geological survey and prospecting means such as drilling,checked out the deposit slope engineering geological conditions of atomization,material composition and the structure characteristics,the cause of deposit slope is analyzed and its failure mode,evaluation of the stability of natural slope;Based on checking the scope and intensity of atomized rainfall in flood discharge design conditions,the change of seepage field of the slope in atomized area under different rainfall conditions was studied.On this basis,Geostudio and FLAC3D were used to calculate and evaluate the stability of the slope in atomized area,and the following main results were obtained.(1)The slope of the left bank flood discharge atomization area is an ancient accumulation body with complex structure formed by multiple periods of collapse.Its length along the slope is 1200m～1500m,width along the river is 1300m～1500m,thickness is 5m～130m and volume is about 1994×10~4m~3.According to the difference of material composition and structure,it can be divided into three regions.The accumulation body on the right(Ⅰarea)is 30m～130m in thickness and about1818×10~4m~3 in volume.It can be roughly divided into two layers.The lower part is a single piece of gray granite gravel soil with a thickness of 20m～80m.The upper layer is grey white solitary gravel with yellowish brown silty clay containing gravel,with a thickness of7.8m～30m.The structure is relatively dense and formed by multiple collapses.The step-like yellow silt was developed at 2600m～2700m on the surface of the slope,indicating that the main body of the deposit was formed about 104ka B.P.In addition,over the elevation of 2700m on the right,there are 5m～9m thick granite fragments covering the yellow silt,which are formed by recent collapse.On the left side(Ⅱarea),the accumulation body is 5m～30m in thickness and 175×10~4m~3 in volume.It is mainly composed of granite fragments and gravel with local boulders.The structure is relatively dense and formed by multiple collapses.The leading edge of the slope(Ⅲarea)has outcropped bedrock and the lithology is granite.(2)The accumulation body is located on one side of the concave bank of Dadu River.The longitudinal profile shows that a bedrock platform with a width of 110m～150m is developed along the elevation of 2380m in the front of the accumulation body.The side of the platform near the upstream Feishuiyan Gou is in the shape of a trench,which is Feishuiyan ancient trench.The tug-of-war elevation of bedrock platform is 138m,which is equivalent to the elevation of Dadu RiverⅢterrace base,and is formed by the lateral erosion of the current of the concave bank of Dadu River.Combined with the yellow silt overlying the accumulation body,it can be inferred that the main body of the accumulation body was formed inⅢterraces of Dadu River,about 101～104ka ago.(3)The formation of the accumulation body is mainly controlled by slope structure,river erosion and earthquake.This period of granite slope has the structure of the large-scale slip cracking failure conditions,the dadu riverⅢlevel terrace formation period,the lateral erosion in dadu river concave shore side of the river forms the open platform for the foundation,and induce granite slope sliding cracking failure happens,and then induce granite slope under seismic action such as high large-scale instability destruction,At the foot of the slope,a single piece of gravel soil with dense structure was formed,which caused the diversion of Feishuiyan Gou by Dadu River in Yongsay.Subsequently,several high level collapses of different scales occurred,forming the accumulation body of colluvium with slope deposit.Under the erosion of the Dadu River in the later period,the front of the accumulation body was disintegrated and destroyed,and with the rapid uplift of the crust and the rapid cutting of the Dadu River valley,bedrock slope was formed in the front of the accumulation body.(4)At present,there is no obvious deformation and failure of the accumulation body,and only partial collapse occurs in the excavation section of the construction path,which has a small scope and good overall stability of the slope.The calculation results of limit equilibrium method show that the slope is in a stable state as a whole in the natural state,and the leading edge elevation is in a basically stable state in the area of 2435m～2620m.(5)The empirical formula proposed by Liu Xuanlies was used to calculate the influence range of flood discharge atomization.Taking the dam site as the starting point,the distribution length of the dense fog and rainstorm area along the river is 693m～1071m,the width of the vertical river is 472.5m～630m,and the elevation of the tug-of-war is252m～441m.The length of thin fog and light fog area along the river is 1575m～2362.5m,the width of vertical river is 787.8m～1260m,and the elevation of tug-of-war is472.5m～787.5m.(6)According to the spray range of flood discharge,the seepage field characteristics of the slope under different working conditions were studied by numerical simulation.The results show that there is no transient saturated area on the surface of slope under natural condition,and the water pressure accumulation is not obvious near the interface of foundation and overburden.Under the condition of rainstorm,the saturated area is formed in the middle and back part of the slope with small slope and thickness,and the hydrodynamic pressure is generated near the interface of foundation and overburden.Under the flood discharge atomization rainfall condition,the affected area of flood discharge atomization rainfall,that is,below the elevation of 2750m,changes from unsaturated area to saturated area on the slope surface,and hydrodynamic pressure is generated near the interface of foundation and overburden.Under the conditions of flood discharge atomization and rainstorm,the slope surface changes from unsaturated area to saturated area,and high hydrodynamic pressure is generated near the interface of foundation and overburden.Therefore,the flood discharge atomization rainfall is easy to produce saturated area on the slope surface,reduce the matric suction of rock and soil mass,and at the same time,form hydrodynamic pressure on the interface of foundation and overburden.(7)The numerical simulation results show that the slope is in a basically stable state on the whole under the condition of flood discharge atomization,and only a small-scale circular sliding failure occurs in the area of the excavation section at the front edge of the accumulation body at 2435m～2750m. |
