Research On The Bedding Sandstone Unloading Mechanical Properties Of The Three Gorges Reservoir Under Water-rock Interaction

In the latter part of the construction of the reservoir area, we will inevitably encounter the blasting, and excavation of the reservoir slope, which will cause a lasting damage deterioration. At the same time, during the operation time while the dam has been completed, the water level of the reservoir will form a hydro-fluctuation belt, the rock and soil of this part stays in a dynamic situation changing from alternating dry to saturated under the condition of sharp fluctuation. This kind of saturated-dry alternation is a kind of “fatigue effect ”for the rock and soil, which will eventually lead to the deterioration of rock properties. Therefore, it is necessary to study the degradation effects of the bank slope rock under water-rock effects.This paper selects the slope rock from Three Gorges hydro-fluctuation belt as the research object, based on the theoretical analysis, the research for the physical and mechanical properties of bedding sandstone considering periodical water-rock cyclic effects has been conducted, aiming at detailed research for the unloading mechanic properties of bedding sandstone under the soaking-drying cyclic effects. The main conclusion are as follows:（1） The results from sonic test and saturated test of bedding sandstone show that longitudinal wave velocity varies greatly for different bedding plane, the longitudinal wave velocity of parallel bedding samples are larger than the perpendicular ones, and with the process of water-rock, the longitudinal wave velocity of two kinds of bedding sandstone presented decline tendency. At the same time the saturated density increases with the process of the tests.（2） The tests results for secondary porosity of different bedding samples show that, the secondary porosity of perpendicular bedding samples are a little bit larger than the parallel ones, both of them increase with the process of water-rock tests, and the increment of perpendicular bedding samples is more obvious.（3） By analyzing the iron concentration changing law of soaking solution from each period water-rock test, we can know that the concentration of Ca²⁺ 、Na⁺ 、K⁺and SiO₂ changed a lot. The concentration of Ca²⁺ decreased first, and then increased until it reached a stable state finally; Na⁺ and K⁺ are increasing all the time but the increase rate are becoming slower and slower; the concentration of SiO₂ keep decreasing all the time. The reason for the decrease of the iron concentration isbecause the reaction rate of samples’ inner elements with water chemistry and the mineral that produced.（4） The axial compression test results show that, with water-rock interaction proceeds, the axial compressive strength, elastic modulus, and poisson’s ratio of two kinds of bedding samples showed a different degree of weakening, and the weaken degree of perpendicular bedding sample is larger than parallel bedding samples; at the same period of water-rock test, the axial compressive strength and elastic modulus of perpendicular bedding samples are larger than parallel ones, which showed the bedding direction has an obvious effect on the mechanic properties of sandstone samples（5） Triaxial loading test results showed that with the increase of confining pressure, triaxial compressive strength of two kinds of bedding samples have obvious increase; with the process of water-rock tests, the compressive strength of samples gradually weakened at the same confining pressure. Generally, the week amplitude under low confining pressure is greater than at high confining pressure; by contrast with two different bedding sandstone samples, the compressive strength of perpendicular bedding samples is larger than parallel ones.（6） In the uniaxial and triaxial loading tests, the trend of the stress-strain curve is substantially the same, and with the increase of soaking-drying numbers, the failure strain showed a gradual increasing trend, indicting that the samples gradually softened with water-rock interaction, the compaction section of perpendicular bedding samples are long which showed they are relatively soft.（7） With the cyclic water-rock interaction, the compressive strength, elastic modulus, cohesion, internal friction angle and Poisson’s ratio gradually decreased, the compressive strain related to peak strength in different confining strength gradually increased, and the change law are basically the same, the deterioration rate is relatively large in the former two periods and then gradually became stable, the deterioration situation can be well matching with the function.（8） According to the triaxial unloading tests results with two different kinds of bedding samples, combined with the consideration of water-rock interaction,with the help of continuous damage mechanics and Weibull statistical distribution theory, the sandstone’s deterioration process under the effects of soaking- drying has beendescribed, a damage evolution equation has been established, and the parameters m and f0 in the weibull statistical distribution has been calculated, and then compare the stress-strain curve came from the tests with the stress-train curve calculated from damage constitutive model...