The physical and mechanical properties of the reservoir bank slope are affected by the water–rock interaction. However, few studies considered the impact of long-term water–rock interaction on the evolution law of mesostructure. Therefore, in this study, the water–rock interaction test was conducted on a slightly weathered red-bed soft rock from the Three Gorges Reservoir area, considering the fluctuation in the reservoir water level. The corresponding pore structure parameters were measured and analyzed based on a scanning electron microscope (SEM) and digital image processing technology. The study showed that: (1) The pore size has been gradually increased, while the number of pores was increased initially and then decreased. Within 12 cycles, the maximum and average pore radius of the rock specimens was increased by 101.02% and 43.32%, respectively, and the porosity has been increased by 26.59%, whereas the number of pores decreased by 22.65%. This indicates the effect of water–rock interaction on the propagation of pores. (2) The pores were changed from oblate to slender by the water–rock interaction. The shape factor was decreased by about 15.79% within 12 cycles. In the meantime, the fractal dimension was increased from 1.20 to 1.28, and more complex structures of pores were observed. (3) The porosity evolution model for the red-bed soft rock was established based on the curve fitting technique. The results can be used as a reference to conceptualize the mesostructure damage of rocks under water–rock interaction.

After the commencement of the Three Gorges hydropower project, the reservoir water level has been fluctuating by 30 m (145-175 m) IntroductionBank slopes have become common as a result of rapid hydropower constructions. For instance, the Three Gorges reservoir has been fluctuating by 30 m (145-175 m). Hence, the rock mass is highly vulnerable to a cyclic water-rock interaction. Meanwhile, this repeated action will reach a state of fatigue, and its effect on both the rock mass and the soil mass will gradually reduce. However, the cumulative effect will result a catastrophic damage [1][2][3][4]. After subsequent progressive failures, the stability of the bank slope has been dramatically affected by the cyclic fluctuations in the reservoir water level.In practice, creep deformation is the predominant type of failure in rock masses. Creep is one of the most important mechanical characteristics of the rock mass. It describes the time-related deformation, slip and failure characteristics of rock materials. Creep and the long-term stability of the rock mass are closely related to each other. In the design and analysis, one should properly address these problems to avoid a catastrophic damage of structures. Due to the rapid development of large scale constructions (hydro powers) the effect of cyclic water rock interaction on the stability of the bank slope will be an inevitable and challenging practical problem. The hydro-fluctuation belt is a governing and a critical zone which can determine the stability of the bank slope [5]. An experimental study was conducted by [6], to investigate the varieties in the uniaxial compressive strength of sandstone rock at dry and saturated conditions. The results revealed that, a significant amount of strength loss (78%) was recorded for the Cretaceous GreenSand when the siliceous sandstone exhibited relatively small amount of strength loss (8%). The extent of sensitivity to water content is predominantly governed by the mineralogical properties of the rocks (proportions of quartz and percentage of clay minerals). Moreover, weak sandstones were less sensitive to moisture. According to [6], the influence of pore water pressure on the mechanical properties of the sandstone rock was negligible. Similarly, a detailed literature review [7] has been done to investigate the effect of moisture content on the strength and modulus of different types of rocks. It was concluded that [7], considerable discrepancies were noticed among the previously

The unloading effect of rock mass excavation is an inevitable practice, and it’s often characterized by a relatively large-scale engineering hazard with a noticeable time lag.A set of unloading triaxial tests were conducted on a sandstone rock to establish the deformation law and the threshold time. Based on the renormalization group theory, the unloading sandstone model was developed by considering the interaction between particles. Similarly, a logistic model was used to predict the unloading damage of sandstone. The unloading time lag damage of sandstone rock was predicted by using the damage threshold. The research shows that: (1) The higher the degree of unloading, the shorter the time-lag failure. (2) The damage range of critical values was optimized. (3) The error between the predicted value and the experimental value of the time threshold was almost less than 5 %, the prediction result was found to be good, and the employed logistic evolution model was reasonable. The findings of this research provide a prediction method and precise information about the mechanism of unloading time lag deformation. Therefore, it can be used as a reference for excavation-support design of underground structures.

Changes in the micromorphology of joint surface under dry-wet cycling have a direct effect on the mechanical properties of the jointed rock masses, which in turn affects the deformation stability of the bank slope of a reservoir. In this study, we design and carry out a test that aims to quantity the effects of repeated rise and fall of a reservoir on the properties of a jointed rock masses. The results are as follows: first, the roughness, local gradient, and undulation of the joint surface gradually decreased under repeated dry-wet cycling. In addition, the height parameters and texture parameters showed a steep decrease followed by a slow decline. The deterioration was particularly obvious over the first 5 dry-wet cycles. Second, the roughness coefficient of the joint surface, the compressive strength of the face wall, and the basic friction angle were gradually reduced under dry-wet cycling. The shear strength of the jointed rock masses (obtained both quantitatively and experimentally) showed a deteriorating trend controlled by the deterioration of the micromorphology, the strength of the face wall, and the frictional properties of the joint surface. Finally, the dry-wet cycling process determined trends of changes in the microstructure parameters and mechanical properties of the joint surface. Our research results provide a good basis for the analysis of the deterioration and failure of rock masses within the hydrofluctuation belt of a bank slope.

Long-term interaction between stress and water pressure leads to creep damage of reservoir bank slope. As a result there will be instability of the bank slopes in many water conservancy projects. The rock mass creeping effect of coupled stress-water pressure was studied by using a typical sandstone rock from the Three Gorges reservoir area. The experiment was conducted by using the rock immersion-air-drying cyclic load rheometer device (designed and manufactured by our research team). Based on the experimental results, the following key points were observed: 1) the creep strain and the steady-state creep rate was increasing when the water pressure increased (at the same stress level). Under the same water pressure, the increase in the axial pressure resulted in the increase in the creep strain and steady creep rate of the sandstone specimens. 2) the increase in the axial pressure increased the creep strain and steady-state creep rate of the sandstone specimens while the water pressure increased. The mechanical properties of the sandstone specimens were affected by the water pressure. 3) the water infiltrates through the pore surfaces. As a result, the rate of deformation will increase while the bearing capacity and long-term strength of the rock decrease. This paper provides a solid theoretical foundation for the evaluation and prediction of reservoir geological hazards.

During the long-term reservoir operation, the seismic capability and dynamic response characteristics of the bank slope are of great importance to its safety evaluation content. Aimed at typical bank slopes, considering reservoir water level fluctuation and soaking-air drying cyclic interaction, an experiment has been designed and conducted. In addition, the cyclic loading test with different stress amplitudes was carried out in different water-rock cycles. The laboratory results indicate that (1) during the immersion-air dry circulation process, the damping ratio and damping coefficient of sandstone gradually increased while the dynamic elastic modulus decreased. It is obvious that the dynamic elastic modulus of sandstone decreases dramatically during the immersion-air dry circulation process, especially in the first six periods. Also, its variation curve fits with the logarithmic curve. (2) When the cyclic load stress amplitude increases from 10 MPa to 35 MPa, the damping ratio and coefficient of the rock sample gradually decreased while dynamic elastic modulus increased. each dynamic parameter shows a more obvious variation trend when the stress amplitude is lower than 25 MPa. (3) During the water-rock interaction process, the closely knit microstructure of rock gradually becomes loose and porous, which resulted in the degradation of macroscopic physical and mechanical properties of sandstone. (4) In the analysis of the seismic response of the bank slope, the actual water-rock interaction process and the seismic level of the bank slope should be more considered. To find out further accurate reflection in the earthquake resistance and dynamic response of the bank slope, it is necessary to select the reasonable dynamic parameter to carry out seismic research.

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During the reservoir operation process, the long-term security and stability of the bank slope is affected by dynamic response characteristics of its seismic action directly. Aimed at the typical bank slope existing in the actual reservoir environment, an experiment considering reservoir water level fluctuation and soaking-air-drying cyclic water-rock interaction has been designed and conducted while the cyclic loading test was performed in different water-rock cycles. Research results indicate the following: Firstly, in the process of water-rock interaction, the dynamic characteristics of sandstone show evident degradation trend, with the increase of the damping ratio and Poisson's ratio and decrease of dynamic elastic modulus, and the former six water-rock cycle degradation effects are particularly obvious. Secondly, the numerical analog computation analysis of dynamic response in typical bank slope shows that as the water-rock interaction period is increased, the dynamic response of the slope hydro-fluctuation belt zone increases gradually, while the other parts weaken. irdly, under the long-term water-rock interaction process, the hydrofluctuation belt zone gradually becomes a "soft layer" which is sensitive to the earthquake effect and dynamic response, resulting in a direct influence on long-term seismic performance of the bank slope. erefore, it is necessary to make better protection for the bank slope hydro-fluctuation belt zone.