| The cyclic fluctuation of reservoir water leads to the damage and deterioration of the Three Gorges Reservoir Bank,causing landslides,collapses and secondary surge disasters.Taking the bedding limestone bank slope in Wuxia section of the Three Gorges Reservoir Area as the research object in this paper,The in-situ fine investigation and test are carried out under the operation state of the reservoir.Through multi-scale three-dimensional laser scanning and image recognition technology,the rock mass deterioration mode and criterion are established,and the rock mass deterioration degree of the hydro-fluctuation belt is classified;Taking Qingshi 6#slope as an example,the geological strength index GSI(t)of bank slope rock mass under long-term cyclic fluctuation with water level is established by indoor experiment,cross hole acoustic wave test and underground television;Combined with physical model test and discrete element simulation,the GSI(t)function of rock layer and elastic-plastic plate theory are introduced,and a bedding deterioration buckling mechanical model and critical deflection section equilibrium equation considering the rock mass deterioration effect are established.The long-term stability of the bank slope is analyzed,and the deterioration disaster characteristics and instability mechanism of bedding limestone bank slope are revealed;The concept of phased adaptive prevention and control based on the deterioration evolution process is discussed.In view of the two major contributing factors of deterioration disaster of Qingshi 6#slope ’leading edge deterioration+trailing edge extrusion’,the pre reinforcement design scheme is proposed,and the scheme optimization and effect simulation analysis are carried out.The main conclusions are as follows:(1)Based on the investigation of rock mass structure of bank slope in Wuxia section of Three Gorges Reservoir Area,the bedding limestone slippery geological structure widely distributed in Wuxia is identified,and three typical deterioration characteristics and main deterioration distribution areas of bedding limestone bank slope are analyzed and summarized;Taking Qingshi 6#bedding limestone bank slope as a typical case,this paper focuses on its engineering geological background,hydrological conditions and deterioration deformation characteristics,and reveals the binary deterioration characteristics of‘rock+structural plane’ of Qingshi 6#slope.(2)①The point cloud response characteristics of rock mass deterioration can be divided into three types:crack manifestation-extension,collapse-block falling,and translation-rotation.The crack manifestation-extension type can be subdivided into:crack extension,crack widening and crack deepening.The point cloud response of the three sub categories all exhibit strip-like erosion-induced extensions,but the deformation forms and locations are different.The rock mass deterioration rate De=n/N is proposed.According to deformation thresholds of the C2C and M3C2 algorithms,the deterioration rates De corresponding to different deformation thresholds can be obtained.With an increase in the deformation threshold,the deterioration rates De decrease.The deterioration deformation and collapse blocks are mainly carried out along the bank slope structural surface and weak fracture zone with the change order of mm～cm,and there is no obvious deformation in other areas.②The comparative analysis of the direct method and the analogy method of the Jiandao Peak case shows that the analogy method is operable and accurate.A qualitative grading standard for general,stronger,and strongest of rock mass deterioration is proposed,and the deterioration indexes Det,Jov,Jv of rock mass are defined,improved and counted,and quantitative indicators corresponding to different deterioration degrees are given.It is recommended that when Det≥0.05,ΔJov≥0.65 orΔJy≥1,the degree of deterioration is strongest.There may be three development modes of limestone rock mass deterioration,two of which will eventually enter a state of accelerated deterioration;According to different deterioration effects,there are mainly two fracture evolution paths of limestone rock mass.(3)①The deterioration experiment of water level cyclic fluctuation shows that the deterioration rate of T1d argillaceous strip limestone is significantly higher than that of T1j dolomitic limestone.The governing equation of nuclear decay deterioration of limestone block is established,and two kinds of lithological deterioration characteristics are described by introducing deterioration constant λ and half-life T1/2.Due to the existence of the structural plane of on-site rock mass,its deterioration rate is much higher than that of the indoor rock.②Combined with the in-situ cross hole acoustic velocity measurement under water level circulating drawdown,the GSI quantitative value method is constructed.Based on the quantitative relationship between Jv and GSI,the GSI andΔGSI of 10 typical cases are calculated using the analogy method.When the degree of rock mass deterioration is strongest,ΔGSI≤-4.0.ΔGSI and Det satisfy an exponential function relationship,and ΔGSI and ΔJv satisfy a linear negative correlation.③Combined with multi-stage cross hole acoustic wave and generalized H-B criterion,the stress state of triaxial compression can improve the strength of rock mass structural plane and obviously inhibit the deterioration.GSI system has a wide applicability for rock mass deterioration evaluation,and the application scope declines with the decrease of GSI.The aging curves of GSI(t)and Erm(t)considering the deterioration process are constructed by exponential regression,and the two and three-dimensional strength deterioration envelopes at different depth structural planes are obtained.The envelope of strength deterioration shows that the deterioration sensitivity of surface structural plane is higher than that of deep structural plane,and the application scope of H-B criterion decreases most obviously.GSI(t,h)spatiotemporal function shows that the deterioration time t is similar to the impact weight of the rock mass structural plane buried depth h on rock mass deterioration.④The GSI(t)deterioration equation at the structural plane combining with the intact rock block strength damage equation N(t)=N0e-λt can establish the binary deterioration structure of bank slope.The GSI(t)time series function is continuously optimized through long-term cross hole acoustic wave test,so as to provide support for the long-term stability analysis of follow-up limestone bedding bank slope.(4)①The model test show that the whole bank slope is in a stable state for a long time before impoundment.After impoundment,the bank slope deformation intensifies until ’buckling failure’ with deterioration,rock mass deterioration shortens the instability process of ’deterioration-buckling’.The trailing edge of the bank slope slides down along the rock formation and the front edge is uplifted.The peak velocity of the same rock stratum is similar during buckling failure.The movement characteristics of the rear part of the ’buckling point’ are relatively consistent,the front part is relatively discrete.The’buckling failure’ is the turning point and apex of bank slope energy release.②Displacement and stress both show signs of early response,which gradually increase with deterioration evolution.The stress produces ’concentration-release’ during buckling failure.The stress change is significantly ahead of displacement,indicating that stress monitoring is more effective.The core of stress monitoring is to determine the ’key section’.For the ’deterioration-buckling’ bank slope,the sharp increase of stress at the’deflection section’ at the front edge can be an important characterization of the critical instability of the bank slope.③Combined with the geological strength index and elasticplastic plate theory,the analytical solution of deterioration buckling critical plate length is derived.As the deterioration progresses,the sliding thrust F gradually increases and the bending stiffness K decreases.The critical deflection section lcr shows a rapid initial decrease and gradually tends to a certain value,which is the "key section" of the bank slope treatment.The influence of rock strata angle α on lcr and relative plate length ratio RPL is significantly greater than that of rock layer thickness hi,and the rapid decline angle α of bank slope lcr is between 40° and 60°.④Numerical simulation shows that the deformation of the trailing edge is larger than that of the leading edge when only considering the trailing edge pushing.With the deterioration of the rock mass in the hydro-fluctuation belt,the deformation increment of the front edge is greater than that of the trailing edge.The stress is concentrated at the bank slope deterioration zone,a plastic yield zone is formed first at leading edge.The trailing edge opens,slides along the plane to squeeze the rock mass of the leading edge,prompting the leading edge to further bend and uplift until the potential ’shear outlet’ penetrates failure.’Reservoir water fluctuation+rock mass deterioration’is extremely unfavorable to the stability of bank slope,which is much greater than simply considering static/dynamic hydraulic pressure of reservoir water wave.⑤The trailing edge pushing always exists in the evolution process of deterioration buckling,which is the premise of bank slope catastrophe failure.However,the dominant factor of bank slope instability is the continuous deterioration of rock mass in the hydro-fluctuation belt.The Qingshi 6#slope is currently in the process of evolution toward ’strong bending uplift’.Due to the continuous deterioration of the rock mass in the hydro-fluctuation belt,it may gradually evolve from a stable/basically stable state to an under-stable state after more than 10 deterioration cycles.(5)Due to many factors affecting bank landslides,a combination of multiple treatment methods is often used.Considering the phased adaptive control of the deterioration evolution process,it is found that the bank slope pre-reinforcement in the middle or before the deterioration stage is the best window for bank slope treatment.The more timely and early the pre-reinforcement of the bank slope drawdown zone is more conducive to the self stability of the bank slope and achieve the best treatment performance.Two design methods are proposed for the two main contributing factors of the evolution process of ’deterioration-buckling’ of Qingshi 6#slope,’rock mass deterioration of drawdown zone+continuous extrusion of the trailing edge’.Scheme 2 is recommended from the perspective of ecological environment protection and construction safety:Lattice anchoring and shotcrete reinforcement at the leading edge and prestressed anchor cables at the trailing edge.The numerical simulation further shows that the plastic zone of the front edge drawdown zone and the overall displacement of the bank slope are greatly reduced after the treatment of scheme 2.The overall slope protection engineering structural unit is in normal working state.The slope stability coefficient increases and remains at 2.06 after treatment,indicating that scheme 2 is effective in early prevention and control by cutting off the disaster factors of deterioration. |
