Oxidation of black shale and its deterioration mechanism in the slip zone of the Xujiaping landslide in Sichuan Province, Southwestern China

This work investigated the chemical water-rock interaction of black shale interbedded with limestone along a bedding slip zone and how its deterioration affects the surrounding rock mass in the Xujiaping (XJP) landslide. Many dissolved pits were found on the limestone, and geochemical phenomena were investigated. Rock and water samples from the site were analysed for mineralogy, chemical composition and hydrochemistry. In slip zones, water-rock chemical processes occur in the bedding fractures of interface between black shale and limestone. Thus, a bedding water-rock cyclic reaction experiment was designed with limestone, black shale and black shale interbedded with limestone. Many major elements and heavy elements (Fe, Mn, Si, Zn, Ni, Al, S, Mg, Ca, Na, K, Co and Sr) dissolved out, demonstrating that strong dissolution occurred because of the acidic water during the black shale water-rock interaction. The limestone neutralized the acidic water through black shale oxidation in the interfacial fractures between black shale and limestone, causing deterioration of the slip zone. The acidic water from the fissure network inside the black shale strata migrated along the bedding to the exposed surfaces of cliffs and rock fractures, then evaporated to form secondary mineral phases, including melanterite, rozenite, szomolnokite, and gypsum. The water-rock chemical interaction in the XJP landslide included dissolution, oxidation, dehydration, and neutralization reactions. The accumulation trends and hydrochemical properties at different reaction stages of ions dissolved from the surrounding rock mass in the bedding direction were revealed. The deterioration mechanism was expanded: (i) rock-forming and carbonate minerals were especially prone to dissolution by sulfuric acid from black shale oxidation in the slip zone, and (ii) volume expansion due to the crystallization force of precipitated minerals caused further fracture expansion and deformation. Therefore, geochemical analyses can effectively elucidate the long-term development and nature of slip zones in landslide investigations.