Mechanism of rockfall coupled with endogenic and exogenic geological processes: A case study in the upper Triassic limestone mines in the Qamdo area, eastern Tibet

Rockfall in limestone mines is a common geohazard in the Changdu area of eastern Tibet and one of the leading geo-safety issues that mining enterprises and railway projects are faced with. We carried out a detailed geohazard survey using the methods of general geology, structural geology, and geohazard geology. We found the rockfall development pattern, characterized rock mass structural planes, discussed the collapse's mechanism, and established its failure mode. The results show that rockfall sites in the study area show a linear spreading along the fold and thrust zone. Five groups of steep-dip structural planes have developed in the rock body, including the longitudinal joint (S1), the transverse joint (S2), the X-type conjugate shear joints (S3 and S4), and the interlayer shear joint (S5). Paired with regional folds and hedging faults, these structural planes cut the rock mass into broken blocks. The collapses are the product of coupled internal and external dynamic geological processes. The sedimentary foundation of the rockfalls in the Qamdo area is the limestone from the upper Triassic Bolila Formation (T3b) formed in the intracontinental rift basin. The strongly folded orogeny triggered by the Cenozoic (Cz) India-Eurasia collision laid down the tectonic framework in the region, which is the essential condition for rockfall development. The strong Neotectonic movement since the Quaternary (Q), frequent hot and humid climate alternations with abundant rainfall since the Late Pleistocene (Q3), everyday human activities, and other internal and external dynamic coupling effects are the main triggering factors of the rockfall disaster. Three failure modes of rockfall are identified, namely toppling, falling, and sliding. The research results have specific guiding significance for rockfall prevention and control in the karst area and the railway construction in the Sichuan-Tibet area.