| Hydropower development is an important part in implementation of the strategy of "West China Development". Large-size hydropower stations constructed in the west regions are mostly located in high mountains and deep canyon areas with complex topographic and geological conditions; as a result, there are plenty of high and steep reservoir bank slopes. During operation of a reservoir, water is impounded and drained periodically as planned; the substantial rise of fall of reservoir water level has an effect of "fatigue damage" and repeated weakening on rock mass in the hydro-fluctuation belt. Under fluctuation of reservoir water level, the deterioration effect of water saturation-dehydration circulation of rock mass in the hydro-fluctuation belt will intensify the rheological behavior of rock mass in the hydro-fluctuation belt of reservoir bank slope, or even lead to rheological deformation failure. Moreover, since the slope in dam site area is relatively close to the dam, under the effect of fluctuation of reservoir water level during operation of the hydropower station, the long-term stability of the slope will directly affect the long-term safety of the dam.In combination with the program of the National Natural Science Foundation of China titled "Study on the Rheological Mechanism and Long-Term Stability of Soft Jointed Rock Mass of High Reservoir Bank Slope under Periodic Substantial Fluctuation of Water Level"(41172281), this paper takes sandstone and argillite in the "hydro-fluctuation belt" of high reservoir bank slope in the dam site area of Longtan Hydropower Station, a national key project, as the research object. With consideration to the fluctuation of reservoir water level with which the reservoir bank slope can frequently encounter during operation of the Hydropower Station, a study is conducted on the deterioration mechanism and effect of rock samples from the hydro-fluctuation belt under water saturation-dehydration circulation; uniaxial and triaxial compression tests, tensile test and triaxial rheological test are performed after different times of "water saturation-dehydration" circulation. The rule of damage and deterioration of sandstone and argillite in the "hydro-fluctuation belt" after water saturation-dehydration circulation is investigated and analyzed. Based on the Hoek-Brown strength criterion, the cumulative damage rate of rocks after water saturation-dehydration circulation is introduced; the damage effect of water saturation-dehydration circulation on rocks is considered; the Hoek-Brown strength criterion is improved; and the "quantitative GSI table" corresponding to the improved Hoek-Brown strength criterion is established. On the basis of the rheological test, a nonlinear visco-elasto-plastic rheological model (DNBVP model) considering the damage after n time(s) of water saturation-dehydration circulation of rocks is proposed; VC++programming is used for secondary development of this model based FLAC3D software. A typical section of the zone where rheological body B is located in the dam site area of Longtan Hydropower Station is chosen for numerical simulation calculation and analysis of the stability of the high reservoir bank slope in the dam site area under the deterioration effect of water saturation-dehydration circulation, the "method of slope displacement back analysis based on BP-PSO algorithm'" is adopted to obtain through inversion calculation the optimum rheological parameters of the DNBVP model corresponding to the "rock mass" in the numerical calculation model considering the rheological property. The validity of the rheological damage constitutive model proposed is demonstrated in combination with the findings from field monitoring of the rock mass of high slope in the dam area within the fluctuation cycle of reservoir water level; then, this model is used to calculate and predict the long-term stability of the high slope in the dam area under fluctuation of reservoir water level, and relevant prevention and control measures are put forward for reference in engineering practice. In the context that a number of water conservancy and hydropower projects have been completed and put into operation in succession in Midwest China, the research findings are of great theoretical and application value for revealing the rule that the mechanical parameters of rock masses of numerous reservoir bank slopes change with the fluctuation of reservoir water level, as well as for applying this rule in evaluation and analysis of the long-term stability of rock and soil masses of reservoir bank slopes.The following research findings are achieved:(1) From the indoor uniaxial compression, tensile, triaxial compression and triaxial rheological tests, and the analysis of the macro-and micro-failure patterns of the rock samples after the triaxial rheological test, it is found that "water saturation-dehydration" circulation has an obvious damage and deterioration effect on the physical and mechanical properties of sandstone and argillite.(2) Based on the tests, the rule of damage and deterioration of sandstone and argillite under different times of "water saturation-dehydration" circulation is revealed in terms of compression strength, elasticity modulus, tensile strength, cohesion and internal friction angle.(3) On the basis of the Hoek-Brown strength criterion, the cumulative damage rate of rocks after water saturation-dehydration circulation is introduced; the damage effect of water saturation-dehydration circulation on rocks is considered; and the Hoek-Brown strength criterion is improved, which provides a theoretical basis for obtaining physical and mechanical parameters of field rock mass under water saturation-dehydration circulation, and also builds a "bridge" for parameter transformation.(4) In combination with the research findings at home and abroad, and based on the GS1scoring table in the generalized Hoek-Brown criterion put forward by E. Hoek, a "new quantitative GSI table", comprising structure type of rock mass, volumetric joint count of rock mass Jv, integrity coefficient of rock mass Kv, structure rating of rock mass SR, joint characteristic coefficient Jc, and surface condition rating SCR, is established. The new quantitative GSI table solves the problem that the volumetric joint count of rock mass JV usually cannot be reasonably determined; meanwhile, according relevant norms of China, the integrity coefficient of rock mass KV is used to replace the volume of rock mass Vb to solve the problem of reasonable quantitative determination of rock mass integrity, and four qualitative indices including surface characteristic, surface condition rating SCR, joint characteristic coefficient Jc and rock quality designation RQD are used to selectively verify the quantitatively determined value of surface characteristic of rock mass through comparison. The new quantitative GSI table, which utilizes the interval number theory to represent the uncertainty of GSI, is more aligned with the actual situation of value determination of mechanical parameters of field rock mass; it gives consideration to the operability of selective acquisition of field test data, and introduces multiple indices to jointly determine the final intersection of GSI, ensuring the accuracy of GSI quantitative determination. In the meantime, a new basis is provided for quantitative transformation of mechanical parameters of rocks to mechanical parameters of rock mass.(5) The rheological test curves of water-saturated (0time of water saturation-dehydration circulation) sandstone and argillite are taken as examples to investigate and analyze the rheological property of sandstone and argillite in the "hydro-fluctuation belt" of high reservoir bank slope in the dam site area. The findings are as follows:(a) The rheological curves of sandstone and argillite both undergo the instantaneous elastic deformation stage, the decelerated rheological stage, the stable rheological stage, and the accelerated rheological stage. Under low axial stress, sandstone and argillite only have the first three stages; the foresaid four rheological stages take place successively only when the axial stress is close to or reaches the critical failure value,(b) Under the first four levels of axial load, the rheological rate of the sandstone samples first decreases, and then becomes close to zero or reaches a stable value. Under the fifth level of axial load, the rheological rate of the sandstone samples first decreases, then becomes close to zero or reaches a stable value, and finally increases till failure of the samples. Under the first four levels of axial load, the rheological rate of the argillite samples first fluctuates irregularly, and then becomes close to zero or reaches a stable value on the whole; but local fluctuation still exists, because argillite is more sensitive to water than sandstone as it has more microfissures, microdefects or nonuniformity, and nonuniform fluctuation of axial strain is induced under the long-term effect of the rheological test. Under the fifth level of high axial load, the rheological rate of the argillite samples first decreases, then becomes close to zero or reaches a stable value, and finally increases till failure of the samples,(c) The axial strain and axial strain rate curves of sandstone and argillite are not quite smooth; the local strain curve segment and the strain rate curve segment are both subject to slight fluctuation and mutation, and this phenomenon is more violent in argillite. The cause of such phenomenon is as follows:the nonhomogeneity of the internal structures of sandstone and argillite leads to micro-weakening and fracture of the rock samples, upsetting the original stress equilibrium;under the long-term sustained effect of constant axial stress, the microdefect parts of internal materials of the rock samples gradually get damaged;after long-time accumulation, the positions in rock materials where the strength is relatively low exhibit inhomogeneous deformation failure at micro-parts due to incapability of bearing the rheological damage induced by the long-term accumulation effect of microdefects, further resulting in rock deformation which gives rise to irregular fluctuation and mutation. In water-saturated state, the mutation in argillite is more frequent than in sandstone, which indicates that in water-saturated state, the axial stress has more obvious damage effect on the microdefects of argillite, resulting in frequent mutation of the axial strain curve of argillite.(6) The constitutive model related to the rheological behavior of rocks established according to the element combination model of linear rheological body will deviate from the reality to some degree:a nonlinear viscoplastic body consisting of nonlinear viscous elements and plastic elements connected in parallel is connected with the Burgers model in series to further propose a nonlinear rheological model that can describe the viscous, elastic and plastic properties of rocks-NBVP model; the3D rheological constitutive equation of the NBVP model is derived. The rheological test curves of water-saturated sandstone and argillite are taken as examples, and the three-parameter model, the Burgers model and the NBVP model are respectively adopted for fitting of measured data. After comparative analysis, it is found that the established nonlinear visco-elasto-plastic rheological model-NBVP model is relatively consistent with the results of the triaxial rheological test of sandstone and argillite, and that the NBVP model can ideally describe the accelerated rheological stage of rocks.(7) The damage variable is introduced into the NBVP model to construct a nonlinear visco-elasto-plastic rheological model considering the damage after n time(s) of water saturation-dehydration circulation of rocks-DNBVP model:meanwhile, the corresponding damage variable expression is obtained based on the results of the rheological test, and the3D rheological constitutive equation of the DNBVP model is derived; from analysis of the test data, it can be seen that the DNBVP model can well describe the curve of the whole rheological process of rocks after water saturation-dehydration circulation.(8) The equation of the nonlinear visco-elasto-plastic rheological model (DNBVP model) is deduced in detail to obtain the3D central difference scheme of stress increment required for secondary development of the model; referring to the difference expression of stress increment of "the nonlinear visco-elasto-plastic rheological model (DNBVP model) considering the damage after n time(s) of water saturation-dehydration circulation of rocks", and using VC++programming, a callable dynamic linking library (.dll) is generated, which can realize the development of the DNBVP rheological model in FLAC3D software. After analysis of the calculation results of the triaxial numerical model test of the rock samples, it is found that the DNBVP model after secondary development is rational and correct. After determining the number of times n of water saturation-dehydration circulation, the axial displacement at the top of the rock samples is calculated; from analysis of the axial displacement, it can be seen that as the number of times n of water saturation-dehydration circulation increases, the axial displacement also increases, which also means the damage is aggravated; this result is relatively consistent with the test data, proving the correctness of the proposed "nonlinear visco-elasto-plastic rheological model (DNBVP model) considering the damage after n time(s) of water saturation-dehydration circulation of rocks".(9) A typical section of the zone where rheological body B is located in the dam site area of Longtan Hydropower Station is chosen for calculation of the rheological property under the deterioration effect of water saturation-dehydration circulation. Combining the indoor rheological test of rocks with the field monitoring data, using the proposed "nonlinear visco-elasto-plastic rheological model (DNBVP model)", and adopting the "method of slope displacement back analysis based on BP-PSO algorithm", the optimum rheological parameters of the DNBVP model of the "rock mass" in the hydro-fluctuation belt of high slope are obtained through inversion calculation and are used in calculation of the long-term rheological property of high slope. Then, the displacement values of the monitoring points obtained through inversion calculation are compared with the field measured displacement values, and it is found that the change trends of the field measured displacement values and the rheological calculated values are basically the same, and that the magnitudes are also close to each other, indicating the correctness of the adopted "method of slope displacement back analysis based on BP-PSO algorithm" and the proposed "nonlinear visco-elasto-plastic rheological model (DNBVP model)" Last, the model is used for calculation and predictive analysis of the long-term rheological property of the high slope of the typical section of the zone where rheological body B is located at the left bank in the dam site area of Longtan Hydropower Station, and relevant prevention and control measures are put forward for reference in engineering practice. |
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