Deposits sources of inrush hazards for the Liangshan Tunnel passing through deeply buried granite

摘要

The Liangshan Tunnel, located in Zhangzhou, Fujian Province, in Southeast China, is constructed in deeply-buried granite. A large-scale mud and sand inrush occurred during the excavation of the main tunnel and the left bypass pilot tunnel at a depth of 270 m. This paper analyses the sources of the inrush deposits from the aspects of mineralogical composition and isotopic composition. An investigation of the regional geology showed that the inrush events occurred near diabase veins intruding into geodic granite. Petrographic and mineralogical analyses revealed that the diabase veins and the surrounding geodic granite were altered into AI-clay (in which altered illite is dominant) and AM-clay (in which altered montmorillonite is dominant), respectively. The mineralogical compositions and microstructures of the AI-clay and AM-clay were apparently different from those of the WK-clay (in which weathering kaolinite is dominant) near the ground surface. The AI-clay was formed due to the alteration of geodic granite surrounding the veins and was the product of the albitized alteration of geodic granite during the rock consolidation period and the illitized and kaolinitized alteration after the magma phase. The AM-clay was formed due to the alteration of diabase and was produced by the chloritized and montmorillonitized alteration of diabase subjected to hydrothermal circulation. The minerals of WK-clay in the completely weathered granite outside the clay zone were weathering kaolinite and illite. Meanwhile, the hydrogen and oxygen isotope compositions of montmorillonite and kaolinite further prove that the inrush deposits were the product of hydrothermal alteration, instead of weathering. This study not only clarifies the sources of the great amount of mud and sand in the deeply buried granite tunnel but also resolves the confusion regarding the weathering depth that cannot be explained by weathering theory.