The Deformation Mechanics And Stability Study Of Bedding Bank Rock Slopes In The Head Area Of The Three Gorges Reservoir

In the head area of three gorges reservoir, bedding bank slopes with variable obliquitydevelop widely, especially large and medium type of rock slope. Commonly, they are stable so thattheir distortion characteristics and stability were rarely studied. But once it failure, the landslide ismassive, abrupt and dangerous. The head area of three gorges reservoir with most special geologicenvironment is one of sector mostly affected by impoundment. At same time, it is the sector withdense population and dense engineering works. So studying the distortion mechanics and stabilityon bedding bank slopes is very important.Based on the data collection, field geologic investigation, deep drilling and laboratory tests,multiple methods were used in this paper, such as statistic analysis, geomechanical analysis, limitequilibrium analysis, numerical modeling and so on. Firstly, on the basis of analyzing thedistribution of regional bedding bank slope, the characteristics of geological structures, developedcharacteristics of typical bedding bank rock slopes, every kind of influencing factor and thedistortion and failure mode of bedding bank rock slopes were studied. Xirangpo slope was used toas a typical example and its primary and secondary influencing factors were studiedquantificationally. Then this paper studied the distortion mechanics and stability under thecondition of the reservoir impoundment. The conclusions are summarized as following:(1) In the head area of three gorges reservoir, developed widely bedding bank slope have thelargest developing probability within large and medium slopes. Their failure strengths are largerthan obsequent slope and crossfall.(2) The bedding bank slopes in the study area can be classified slope with lank layer and slopewith variable obliquity. The latter type is main and various. This paper built five kinds of typicalgeologic structure models of bedding bank slope with variable obliquity, such as the bedding bankslope of upside with large obliquity and downside with low obliquity, the bedding bank slope ofupside with large obliquity, medium with low obliquity and downside with large obliquity, thestairstepping bedding bank slope. Their failure strengths are obviously larger than the beddingslope with lank layer and the linear deformation and failure modulus are all higher than 4.5 million m³/km.(3) In the head area of three gorges reservoir, bedding bank rock slopes main develop at the foldtransition position, the transition position to anticline, Interbedded Strata of Soft and Hard Rock,thin strata with weak interbed. These slopes often have large and medium scale. Gravity resultsprogressive and slow extrusion deformation. Outside dynamic geologic processes are theimportant inducement of bedding bank rock slope failure, such as rainfall, reservoir impounding,earthquake and so on.(4) This paper analyzed the main intrinsic factor and outside dynamic processes affecting slopestability and their process mechanics, then summarized that the failure mode of bedding bankslope with variable obliquity is sliding-crook-bedding slipping.(5) Analysis of the deeper structure of Xirangpo bank slope showed that there are three layers ofinterlayer fractured zone which is bedding, continuous and variable thickness. These interlayerfractured zones and the contact layer between T₂b³ and T₂b² constitute the bottom deformationboundary of Xirangpo slope.(6) Limit equilibrium method was used to calculate the stability of four layers of potential slidingbody of Xirangpo slope. Under the each operating condition and load combination, the slope isstable. From the surface to deeper of bank slope, the stability coefficient is decreasing. The lowersliding body (the third and forth layers) have the dominant influence on the stability of the entireslope. The stability degree of the third and forth layers of sliding body is not high and it is underthe critical state of stability. So Xirangpo bank rock slope has the potential of deeper sliding.(7) This paper combined orthogonal design and analysis module of variance of the software SPSSand did the multifactor sensitivity analysis.The influencing factors include the volume weightγ,cohesion c of potential sliding zone, earthquake peak accelerationα_h and the water level ofreservoir and they have the different influence on the stability of the four layers of sliding body.(8) Finite element modeling software GEO-SLOPE was used to model the characteristic ofsteady-state seepage field and transient-state seepage field of Xirangpo slope during the onehydrological year fluctuating. The modeling results were following:â‘ under the naturalsteady-state, the groundwater hydraulic gradient is small and the water table is very deep.â‘¡when the water level of three gorges reservoir rises quickly, the water table of slope will not risequickly and there is some lag time. The distribution of pore water pressure will be influenced onlyat the variable zone of water level and the slope deep within 100 m.â‘¢the water table has aconvex of shape during the water level decreasing and the distribution of pore water pressure willbe concave. The decrease of water level has important influence below water level and deep placeof the slope. It has not obvious influence on the seepage field above 175m.(9) Based on the modeling result of transient-state seepage filed of the slope, the change of slopestability was analyzed during the fluctuating of water level of three gorges reservoir. The results showed that the stability coefficient will decrease with the increase of water level and it willincrease with the decrease of water level, but the stability coefficient will not come back to thevalue under the initial state of the water level of three gorges reservoir.