72 / 2019-07-29 00:08:52

Weakening mechanism of structural plane in gabbro slope caused by acid rain

acid rain,structural plane,weakening mechanism,gabbro,dissolution

水库地质灾害防治与生态环境保护 > 表生地质作用与地质灾害演化过程

Three Gorges Project lies in China’s hinterland, which gains remarkable achievement since it has been constructed in 2006. However, as the water level in the reservoir rises, gradually exposed potential geohazards, e.g., landslides, along Three Gorges Reservoir makes the reservoir area become a susceptible zone of natural disasters. The current research on the triggering factors of landslides are mainly focus on the impact of rainfall and the variation of reservoir capacity. Only a few present studies considered the influences of environmental factors. Three Gorges Reservoir Area is located in a typical acid rain area of China (Fig. 1). Zhang and McSaveney (2018) and Zhao et al. (2009, 2011) have investigated some landslides in and around Three Gorges Reservoir Area, and believed that acid rain plays a subtle role in slope instability.
The present study took a failure gabbro slope from Dadu River valley which belongs to the Upper Yangtze River Basin as an example to reveal the mechanical properties of rockmass structure varying with acid rain influence. Here, the fetched gabbro samples were processed into cylinders of 5.5 cm diameter and 2 cm height, and their top surface were processed into paired two (P1 group), three (P2 group), and four (P3 group) ridges or grooves, respectively, to simulate different average waviness and unevenness of structural plane. The width and height of each ridge and groove are both 3 mm (Fig.2). Then, we established the corresponding relationship between the volume of hydrochloric acid (pH=3) used in the 7 and 14 days’ acid rain simulation experiments and local precipitation of 20 and 40 years by the equal amount of Hydrogen. Finally, an integrated approach based on portable direct shear apparatus, polarizing microscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM) were used to analyze the degradation of the mechanical properties of structural plane after the simulation experiments.
The direct shear testing illustrates that the lower the discontinuity persistence (k), the higher the cohesion (c) and internal friction angle (φ) will be, but all samples have varying degrees of decline regarding c and φ under the acidic condition. After two weeks’ acidic corrosion, the c of the samples tumbles more than 33%, while φ is reduced by 6.8% at most. Those negative correlation trends are shown in Fig. 3. The microscopic observations of gabbro samples show that the corrosion induced by acidic solution has two effects on gabbros: the erosion along some high energy sites and the dissolution on the surface of primary minerals. As the acidic corrosion continues, the former propagates the micro-cracks from 1 to 5 μm width (Fig. 4a) and the latter results in the dissolution pits from 0.5 to 35 μm diameter and they are not well-distributed. SEM observations show that these pits are not simply voids but filled the clay particles, indicating that the dissolution of primary minerals and the precipitation of new components occur nearly at the same location (Fig. 4b). This proposition is demonstrated by the adhesion of white newborn filmy clays along the micro-crack (Fig. 4c).
The XRD results show that (Table 1 and Fig. 5) the 7 days’ acidic corrosion by acid rain simulation experiments lead to a decrease in the relative percentages of rock-forming minerals, especially plagioclase and pyroxene, but an increase in clay minerals, including chlorite, kaolinite, illite, and sericite increase. When the acidic corrosion lasts for two weeks, the relative percentage of pyroxene keeps decreasing with an increase in kaolinite. Quartz is one of the relatively stable and less soluble minerals in acidic solutions (Wray and Sauro, 2017), the increase in the relative percentage of quartz implies that the acidic corrosion process of gabbro is accompanied by the considerable dissolution of minerals.
The acid rain simulation experiments denote that weathering rates accelerated by acid rain significantly decrease the shear strength parameters of the structural plane. Four decades of acid precipitation infiltrates into the gabbro mass through the structural plane, changing the compositions and structures of the gabbro and influencing the mechanical properties of the discontinuities. Corrosion process of rock-forming minerals in the junction of joint segments and rock bridge causes the generation of secondary micro-cracks, leading an increase in discontinuity persistence and a considerable reduction in cohesion. Meanwhile, as the dissolution of the gabbro, transformation process from the tectosilicates calcic plagioclase and inosilicates pyroxene to phyllosilicates kaolinite, illite, and chlorite also weaken the intergranular bonds and shear strength of rock bridge. The above acidic corrosion processes synthetically give rise to a significant reduction in shear strength of the structural plane, and consequently, lead to the gabbro avalanche on the side of S211 provincial highway with about 900 m3 volume in September 2015.