Failure Mechanism And Evolution Of The Anaclinal High Carbonatite Rock Slopes With Soft Toes In The Three Gorges Region

The vast region of southwestern China is characterized by deep river valleys,steep gradient and complex geological structures,where a great number of high and steep slopes are located.Therefore,many large-scale geological disasters and instabilities of engineering slope are occurred in this area.Especially in the region where layered rocks are widespread.Usually,the disaster formation conditions and factors are more complicated since these rocks may fail in toppling.The aim of this study,which is relied on the National Natural Science Foundation: Experimental study on bending fracture mechanism and strength of thinly layered composite soft-hard rock masses(41672300),is to investigate the toppling failure mechanism of anaclinal rock slopes and the water–rock interactions in reservoir region due to the fluctuation of water level.Based on the detailed investigation of the more than ten anaclinal rock slopes in the Gongjiafang–Dulong section of the Wu Gorge in Three Gorges region,together with rock mechanical tests,the rock mass structures are mapped in detailed and rock mass quality are determined.Besides,by the analysis of data from deformation monitoring and local failure phenomenon at the slope toe,the correlation between slope displacement and reservoir level are also investigated.In the end,the time–dependent collapse process of the anaclinal reservoir rock slope are simulated and reconstructed by using the TSD–DEM method,considering the interactions of water erosion,softening and seepage between rock mass and reservoir water.The main work and conclusions of this study are as follows:(1)The geological structure,lithology and hydrological conditions of the study area are analyzed by collecting and summarizing the geologic background information of the Wu Gorge.The lithology,geologic structure and hydrological conditions of the Gongjiafang–Dulong section are determined.It is revealed that the rocks are a suit of shore to shallow sea facies carbonaceous carbonate and clasolite sedimentary formations.Controlled by the Hengshixi anticline and influence by the severe river incision,the anaclinal rock slopes are finally formed.(2)The rock mass structure and rock mass quality are investigated by uav photography,drilling and artificial mapping.Based on the data collected,three widespread dominant sets of discontinuities were identified.They include high continuity bedding planes(referred to as J0),and two sets of low to medium continuity cross joints(referred to as J1 and J2,respectively),and cut rock into blocks.Since the developments of the three set of discontinuities are different,the structural types of rock mass are also different in different formations: rock mass in the upper slope belongs to thick to medium layered type while rock mass at the slope toe belongs to thin layered one.(3)The slope structure is determined by in situ and laboratory tests.These tests reveal that the strength of rock at the slope toe is lower than that of rock in the upper slope.Considering that rock mass in the upper slope belongs to thick to medium layered type while rock mass at the slope toe belongs to thin layered one,it is concluded that the slopes in the study area belongs to anaclinal rock slope with a soft base.(4)The investigation reveal the slope toes in the Gongjiafang–Dulong section are eroded by the fluctuation reservoir level.And the typical erosion signs include argillization of the weak intercalated layers and erosion cracks/erosion caves.It is revealed that rock masses at the slope toe are mainly characterized by flexural toppling,which also induce a blocky toppling failure at the middle section of the slope.(5)Long-term monitoring data reveal deformation differences at the slope: larger deformations occurs at the slope toe with the direction pointing down to the river valley while rock masses at the upper slope deform less.Besides,the monitoring sites record a growing displacements of the slope,which suggests that the decrease of the sloep stability.Analysis of data from deformation monitoring suggests that the temporal movement of the slopes shows seasonal fluctuations that correlate with reservoir levels and drawdown conditions,with induced slope acceleration peaking when reservoir levels are lowest.(6)Based on the varying behaviors of different components of carbonatite under the action of water(including short and long term)and its influences on the macroscopic mechanics behaviors,a time-dependent strength degradation DEM method(TSD-DEM)has been built by developing customized code using the Fish language,which considers the interaction between rock and water.The ability of the model to predict the rock behaviors under the short and long term action of water.(7)Taking the G2 rockslide as an example and considering the influence of fluctuation reservoir level,the evolution of the collapse process of the anaclinal slopes with soft toes and reservoir erosion mechanism has been studied by using the TSD-DEM method.The result reveal that the collapse process of the slope can be divided into three stages,i.e.,the erosion and softening of the slope toe(taking the longest time),the collapse of the toe(fail in flexural toppling)and the collapse of the upper slope(fracture of the middle rock beam and slide of rock mass at the upper section).