Abstract
Water inrush of tunnel face is one of the most common geological disasters during tunnel construction in China. Aiming at the rock mass with multi fractures in water-resistant strata ahead of karst tunnel, the compressive-shear cracking property is analyzed by fracture mechanics theory and the change law of rock bridge shear strength with branch crack propagated length under karst water pressure and geo-stress is studied according to Mohr-Coulomb strength criterion. Moreover, the critical water pressure of water-resistant strata with multi fractures under tension-shear failure is deduced. The safe thickness of water-resistant strata with multi fractures ahead of karst tunnel is established based on two band theory and critical water pressure, and the influence of karst water pressure, initial crack length, crack spacing, array pitch of cracks, lateral pressure coefficient and the angle between the crack and the maximum principal stress on the minimum safe thickness of water-resistant strata are discussed. A 3 Dimension Distinct Element Code (3DEC) considering the fluid-solid coupling effect and structural characteristics of rock mass is adopted to study the catastrophe process and the influence of karst cavity scale on displacement and seepage field in water-resistant rock mass ahead of tunnel in the process of sequential excavation. The numerical simulation results show that: The transition from the single effect of unloading on the extrusion displacement of karst tunnel face to combined action of unloading and karst water pressure occurs with the tunnel face advance; The displacement at each measuring point in water-resistant strata continues to increase in the process of tunnel excavation; The extrusion displacement and water flow velocity in tunnel face suddenly increase when the water inrush pathway is about to form; With the increase of karst cavity size, the minimum thickness of water-resistant strata, the displacement of measuring point and pore pressure of crack increase. The study results provide a reference for early warning and prevention of water inrush in karst tunnel face.
Highlights
With the “Belt and Road Strategy” moving forward constantly, more and more tunnel engineering with the characteristics of deeply buried depth, long distance, strong karst, high geo-stress, high water pressure, and complexes logical structure will be built in the western karst region of China.Unfavorably, many kinds of geological disasters often occur in the course of tunnel construction due to the influence of karst development degree, the position relationship between the karst structureProcesses 2019, 7, 686; doi:10.3390/pr7100686 www.mdpi.com/journal/processesProcesses 2019, 7, 686 and the tunnel, karstic fillings and the disturbance of tunnel excavation
This study aims to provide theoretical support for early warning and prevention for water inrush of a karst tunnel face
In the above theoretical analysis, we find that water inrush from the karst tunnel face is usually resulted from the tunnel excavation causing the local rock mass unloading and further reducing the critical water pressure caused by the hydraulic fracturing
Summary
With the “Belt and Road Strategy” moving forward constantly, more and more tunnel engineering with the characteristics of deeply buried depth, long distance, strong karst, high geo-stress, high water pressure, and complexes logical structure will be built in the western karst region of China.Unfavorably, many kinds of geological disasters often occur in the course of tunnel construction due to the influence of karst development degree, the position relationship between the karst structureProcesses 2019, 7, 686; doi:10.3390/pr7100686 www.mdpi.com/journal/processesProcesses 2019, 7, 686 and the tunnel, karstic fillings and the disturbance of tunnel excavation. With the “Belt and Road Strategy” moving forward constantly, more and more tunnel engineering with the characteristics of deeply buried depth, long distance, strong karst, high geo-stress, high water pressure, and complexes logical structure will be built in the western karst region of China. Many kinds of geological disasters often occur in the course of tunnel construction due to the influence of karst development degree, the position relationship between the karst structure. Water and mud inrush are the most common and dangerous problems that tunnel constructors encountered. Water inrush affects the construction safety of tunnel engineering, and endangers human lives and property. It is necessary to study the minimum safe thickness and water inrush catastrophe process of water-resistant strata for the design and construction of karst tunnels
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