水库运行期岸坡消落带红砂岩抗剪与抗压强度劣化机制

Degradation mechanism of shear strength and compressive strength of red sandstone in drawdown areas during reservoir operation

目前在水库运行期岸坡消落带环境条件模拟方面,对消落带岩石在库水位周期性升降条件下宏观强度劣化规律进行研究时,主要侧重于“湿干”交替作用环境条件的单一模拟,鲜见对水库运行期岸坡消落带岩石所处的岸坡应力、“湿干”交替和库水渗透综合作用的真实环境条件进行模拟。自主研发了能模拟岸坡应力、“湿干”交替和库水渗透综合作用环境的水库岸坡消落带软岩三轴试验系统,以三峡库区马家沟滑坡红砂岩为研究对象,开展红砂岩的单轴、三轴压缩试验,并分析了岩石试样的破坏模式;同时,通过扫描电镜(SEM)和X射线衍射(XRD)等测试方法,获得红砂岩的微细观结构和黏土矿物成分随“湿干”交替作用次数增加的变化规律。研究结果表明:红砂岩的峰值抗压强度随“湿干”交替作用次数增加逐渐劣化,其中前4次“湿干”交替过程中,峰值抗压强度劣化幅度较大;第6~8次“湿干”交替过程中,劣化幅度逐渐减小。随“湿干”交替作用次数的增加,红砂岩的黏聚力也逐渐下降,第1~4次“湿干”交替过程中,黏聚力的下降幅度较大;第6~8次“湿干”交替过程中,黏聚力的下降幅度明显减小。随“湿干”交替作用次数的增加,红砂岩的内摩擦角有所降低,但变化规律不明显。红砂岩中的黏土矿物因水化反应产生不均匀膨胀、收缩,导致碎屑矿物颗粒之间的胶结作用遭到一定程度的弱化,在宏观上表现为抗剪强度、抗拉强度的劣化;抗剪强度、抗拉强度的劣化使得红砂岩在轴向荷载的作用下更容易发生破坏,是导致峰值抗压强度发生劣化的直接原因,而红砂岩中胶结物胶结作用的变弱是红砂岩峰值抗压强度发生劣化的根本原因;同时,渗透压对峰值抗压强度、抗剪强度的劣化具有一定程度的促进作用。研究成果可为水库运行期岸坡稳定性动态分析与评价提供科学的依据。

So far, the researches on the degradation of macroscopic strength of rock under cyclic wetting-drying conditions in the course of reservoir operation mainly focus on simulations of single environmental conditions without considering the combined actions of slope stresses, wetting-drying cycles and seepage water pressures. The triaxial test system of soft rock in the drawdown areas is developed independently to simulate the combined actions of slope stresses, wetting-drying cycles and seepage water pressures. The red sandstone from Majiagou landslide is selected as the study object. The uniaxial and triaxial compression tests on the red sandstone are carried out, and the failure modes are analyzed after each wetting-drying cycle.Meanwhile, the variation of microstructure and clay mineral content of red sandstone undergoing different numbers of wetting-drying cycles is always explored by means of the scanning electron microscopy(SEM) and X-ray diffraction(XRD).The results show that with the increasing number of wetting-drying cycles, the peak compressive strength of red sandstone decreases gradually. In the first four wetting-drying cycles, the peak compressive strength decreases obviously. After the sixth wetting-drying cycle, the decline trend of the peak compressive strength decreases gradually. With the increasing of wetting-drying cycles, the cohesion of red sandstone decreases gradually either, and the cohesion decreases a lot during the first four wetting-drying cycles. The reduction extent of cohesion is obviously reduced in the sixth to eightth wetting-drying cycles. The friction angle of red sandstone decreases with the increasing number of wetting-drying cycles, but the change rate is small. The uneven expansion and shrinkage of clay minerals caused by hydration reaction weaken the cementation between clastic mineral particles to a certain extent, which embodies the deterioration of shear strength and tensile strength on the macro level. The deterioration of shear strength and tensile strength makes red sandstone more vulnerable to failure under the action of axial loads, which is the direct reason for the deterioration of the peak compressive strength. The weak cementation of detrital minerals is the fundamental reason for the deterioration of the peak compressive strength. Meanwhile, the seepage water pressures can promote the deterioration of the peak compressive and shear strength to a certain extent. The research results can provide a scientific basis for the dynamic analysis and evaluation of the slope stability during the operation of reservoirs.