Influence of acid rain on slope instability mechanism—a case study in Sichuan provincial highway, China

Abstract

Acid rain has important influences on the physical and mechanical properties of rock as a result of mineral conversion and dissolution of cement. There are clear spatial correlations between the occurrence of slope failure and rainfall pH value in acid-rain areas. Although some fatal landslides have been ascribed to acid rain, those landslides all consisted of sedimentary rocks that generally contained carbonate minerals, which are particularly susceptible to dissolution in acidic conditions. This paper describes the mechanical deterioration of the structural plane based on acid-rain simulation experiments on a slope cut in hard rock along Sichuan provincial highway S211, China. In the experiments, the cohesion and internal friction angle of the structural plane fell to 35.4% and 6.8% of their original values, respectively, and the strength deterioration exhibited a power-law relationship with the exposed area of the rock bridges. Mineralogical and hydrochemical analyses including polarising microscopy, X-ray diffraction, scanning electron microscopy, and inductively coupled plasma optical emission spectrometry were employed to describe the micromechanisms of weakening. Mechanical weakening of the structural plane is attributed to deterioration of both the composition and the structure. Transformation from hard essential minerals to clay minerals through pyroxene chloritisation, plagioclase illitisation, illite smectitisation, and chlorite dissolution reduced the strength properties of rock bridges. In addition, abundant secondary micro-cracks were formed by propagation of worm-like dissolution pits, thus disintegrating rock bridges and further reducing the strength of the structural plane. This paper provides a better understanding of slope failure mechanisms in areas affected by acid rain.