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
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.
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.
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