On the mechanism of inrush hazards when Denghuozhai Tunnel passing through granite contact zone

Abstract

Triggered by excavation or blasting, the underground water and debris may rush into tunnel like a mudflow, which is the essence of inrush hazards in tunnels. Three conditions are indispensible for the occurrence of inrush hazards, namely, material, space and triggering conditions. This paper introduces the inrush hazards of the Denghuozhai Tunnel, and analyses the mechanism of inrush hazards from aspects of material and space conditions. Studies on the lithofacies, minerals and alteration characteristics reveal that the inrush hazards occurred in the contact zone between geodetic granite and tuff. The types and evolution of the altered rocks are identified which indicates an intrusion of volatile-rich magma into the wall rocks at shallow depth under tensile tectonic environment. Albitized, sericitized and clayinitized alteration thus occurred near the contact zone, leading to the formation of tens of meters wide, loose and deeply-buried soft mineral belts, such as sericite, kaolinite and montorillonite, etc. These altered weak zones provide the materials and space for inrush hazards. It is found from the physical, mechanical and hydrological experiments that a high degree of alteration causes swelling, disintegration and argillization of rock and soil, and thus weakens physical and mechanical properties of rocks. This weakening further lowers the triggering threshold of inrush hazards. Therefore, when tunnel excavation approached the altered weak zone, as the weak zone was not identified and no measures were taken to avoid the accidents, inrush hazards were triggered due to excavation disturbance, stress redistribution, blasting vibrations and groundwater drainage. The experiences from these successful cases will be a valuable references and experiences in the design and construction of similar engineering in the future.