| The “anti-dip stratified rock slope with embedded rock beam” in the Three Gorges Reservoir Region refers to a stratified reservoir bank slope which has a strike almost consistent with and a dip opposite to that of rock strata, at the lower-middle section of which exists thick-layered rock beam. The Gongjiafang landslide in the Three Gorges Reservoir Region was chosen for a case study. After a field survey into the natural and engineering geological conditions of this bank slope and similar slopes around as well as compilation of data collected, the deformation phenomena and the characteristics of this type of bank slope were classified, and the evolutionary relationship among different deformation characteristics was analyzed. On this basis, a geologic model for staged deformation and failure of the anti-dip stratified rock slope with embedded rock beam was established to determine the main control factor and development trend at each deformation stage. The rock mass strength weakening coefficient was introduced to simulate the weakening effect of long-term scouring by reservoir water on rock mass in the hydro-fluctuation belt of bank slope, and analyze the deformation development trend of this type of bank slope when the strength of rock mass in the hydro-fluctuation belt was weakened. The finite element method was used to analyze the response relationship of the stability of this type of bank slope with the fluctuation rate of reservoir water level and the unsaturated permeability. Last, based on the research findings, guidance was offered to failure prevention and control design for this type of bank slope.The following conclusions have been drawn from the above research work:(1) Based on the classic G-B model for toppling failure, an improved G-B model for slope deformation zone division was proposed according to the toppling deformation characteristics of the anti-dip stratified rock slope with embedded rock beam; the zones after division included sliding zone, rock beam zone, toppling zone and stable zone. The existence of the rock beam zone has a locking effect on the toppling deformation at the upper section; this zone becomes the boundary between the sliding and toppling zones, and bears more gravity component of the overlying rock mass. As a result, the stress on rock mass in the lower sliding zone is significantly reduced, which is greatly helpful for maintaining the slope stability.(2) On the basis of the improved G-B model, the failure process of the anti-dip stratified rock slope with embedded rock beam was analyzed so as to divide the process into four stages: ① stage of small deformation of bank slope in the valley formation period; ② stage of stratum toppling deformation; ③ stage of slope toe weakening and slipping; ④ stage of rock beam fracture, toppling, collapsing and sliding. Among them, ① and ② are long-term deformation stages; ③ and ④ are failure and instability stages.(3) With the weakening of rock mass in the hydro-fluctuation belt, the critical weakening coefficient for rock mass in the hydro-fluctuation belt is 63% when there is no rock beam at the middle section of the bank slope; the critical weakening coefficient for rock mass in the hydro-fluctuation belt reduces to 50% when rock beam exists at the middle section of the bank slope. Therefore, it can be seen that the rock beam at the middle section has a certain anti-sliding effect. In addition, under the same weakening coefficient for rock mass in the hydro-fluctuation belt, the maximum displacement and the maximum shear strain increment at the slope toe reduce significantly due to the anti-sliding effect of the rock beam.(4) Under the weakening effect of scouring by reservoir water, the strength of rock mass in the hydro-fluctuation belt reduces, resulting in decline of the slope stability; the area of shear failure and tension failure in the bank slope increases continuously, extending upward gradually from the slope toe till the rear edge of the sliding mass. The superficial rock mass is mainly subjected to tension failure and the deep rock mass to shear failure. When rock beam exists at the middle section of the bank slope, with the weakening of rock mass in the hydro-fluctuation belt at the slope toe, the plastic zone on the lower sliding surface of the rock beam first becomes connected; then, the plastic zone of the rock beam gradually expands untill the rock beam is fractured. At this moment, the upper and lower sliding surfaces are connected, and the entire bank slope is subjected to unstable failure.(5) The change trend of slope stability is closely related to the fluctuation rate of reservoir water level. When the reservoir water level rises, the slope stability coefficient first increases, then decreases and last tends to be stable; when the reservoir water level falls, the slope stability coefficient first decreases, then increases and last tends to be stable. At the same penetrability coefficient, the higher the rising rate of reservoir water level is, the faster the slope stability coefficient will increase; the higher the falling rate of reservoir water level is, the faster the slope stability coefficient will decrease; the higher the fluctuation rate of reservoir water level is, the faster the bank slope will reach the peak of stability coefficient.(6) The change trend of slope stability is also closely related to the permeability coefficient. No matter whether the reservoir water level is at the rising or falling stage, under the same fluctuation rate of reservoir water level, the slope stability declines as the permeability coefficient of sliding mass decreases.(7) For the anti-dip stratified rock slope with embedded rock beam in the Three Gorges Reservoir Region, as the permeability coefficient of sliding mass decreases and the falling rate of reservoir water level increases, the bank slope is more likely to be subjected to unstable failure.(8) The residual thrust method assumes that connected potential sliding surfaces exist in the bank slope; based on this assumption, the fourth failure stage(stage of rock beam fracture, toppling collapsing and sliding) will be defined as the current stage of the Maocaopo bank slope; the main factor controlling slope deformation at this stage is whether the rock beam is fractured or not. However, the Maocaopo bank slope is actually at the third failure stage(stage of slope toe weakening and slipping); the main factor controlling slope deformation at this stage is the weakening and slipping of rock mass in the hydro-fluctuation belt at the slope toe.(9) After comparison of the prevention and control effects under different conditions by numerical simulation, the following findings have been obtained: at the third failure stage, the scheme of slope protection with lattice needs a low construction cost and produces a better prevention and control effect, while the scheme of bolt anchoring needs a higher construction cost but has a poorer effect; at the fourth failure stage, the scheme of bolt anchoring has a better prevention and control effect, while the scheme of slope protection with lattice hardly has any effect. Therefore, it can be concluded that the same prevention and control method may have quite different effects at different stages.(10) By comparison of the prevention and control effects of the two schemes applied at different failure stages, a staged discussion idea has been proposed for failure prevention and control design of bank slope: it is suggested that a qualitative analysis should be first conducted for the bank slope; after the slope failure stage is confirmed, the instability mode and main control factor can be determined. On this basis, guidance can be offered to engineering prevention and control design for the bank slope, so as to choose the prevention and control method in the most reasonable manner. |
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