A Numerical Investigation on the Hydraulic Fracturing Effect of Water Inrush during Tunnel Excavation

When a high-pressure water source is located near a tunnel under excavation, water inrush is commonly associated with a hydraulic fracturing effect. To study the hydraulic fracturing effect of water inrush (HFEWI), flow-rock failure process analysis (F-RFPA2D) was adopted to simulate the water inrush process. The simulated results indicated that a stress disturbance area formed in front of the excavation face and that a hydraulic fracture zone formed in front of the karst cavity. Similarly, stress concentrations formed in front of the excavation face and the karst cavity. The hydraulic fracturing effect was characterized by stress concentration, and the local hydraulic crack propagation was the result of stress concentration. In addition, a pore pressure gradient formed in the crack-free area of the surrounding rock, and the occurrence of hydraulic cracking was the root cause of the significant change in water flow. When the hydraulic cracks initially formed and expanded, the zone of crack activity was large. As the cracks continued to expand, the range of activity decreased and finally concentrated directly in front of the excavation face. Additionally, the shapes of the water inrush channel obtained by the experimentation and numerical simulation were basically the same: semielliptical. During the evolution of hydraulic crack initiation, expansion, and penetration, the bottom of the excavated borehole was initially dry and then experienced seepage and water inrush. Finally, the minimum safe thickness of the rock wall was calculated to provide a safety guideline for this type of water inrush.

The Minimum Safe Thickness and Catastrophe Process for Water Inrush of a Karst Tunnel Face with Multi Fractures

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.

COVERED KARST LANDFORMS: COMPLEXIFICATION OF METHODS TO ESTIMATION OF MORPHOMETRIC PARAMETERS IN ENGINEERING PURPOSES

The research is aimed to search of optimal solution in integration of various methods for predicting the sizes of karst-suffosion deformations on the ground surface or at the base of shallow foundations. Necessity of integration is dictated by engineering-geological conditionsof the karst areas and the complexity of their reliable study with using modern approaches (drilling, geophysics etc.). From this point of view increasing of authenticity of the final forecast is in direct dependency from completeness of investigation of sizes of observed surface karstforms and features of behavior soils massif above the karst cavity. The mechanism of the karst process in the soils thickness is defined, first of all, by geological structure (thickness and interleaving of different soil layers), hydrogeological conditions and the physical-mechanicalproperties of soils. At the present day are known three basic mechanisms of soil deformation over karst cavity: karst-collapse, karst-suffosion and karst-suffosion-collapse mechanism. Even in the conditions of one site these mechanisms can be interleave during the year, which makesthe karst formation process hard-to-predict in part of diameters of sinkholes and subsidence zones. Today are known and applicable next four methods to estimations of possible sizes of karst-suffosion deformations: 1) method of analogy, 2) probabilistic method, 3) deterministic(calculated) method, 4) laboratory physical modeling method. Each of four methods for predicting the sizes of karst-suffosion deformations has its own advantages and disadvantages, which limit the possibility of using any one of them in different natural environments of karstdevelopment. To improve objectivity and reach necessity accuracy of estimation of diameters and depths of surface karst deformations in engineering-geological purposes a block-scheme of integrations of different forecasting methods is recommended. This scheme is based on the data about surface karst area, the depth of occurrence of karst rocks and level of responsibility of the projecting object.

Mapping deeply buried karst cavities using controlled-source audio magnetotellurics: A case history of a tunnel investigation in southwest China

Karst cavity mapping is attracting great interest from engineering geologists because of its relation to the dangerous geohazards faced during engineering construction. Ground-based geophysical methods still face challenges in karst mapping, and concealed karst cavities potentially pose threats to tunnel construction in southwest China. Given the significant contrast in electrical resistivity between karst cavities and their host rocks, geoelectrical methods are widely used for mapping these cavities. We have developed a successful case history of mapping karst cavities on a planned railway route using controlled-source audio magnetotellurics (CSAMT). Scalar CSAMT, with frequencies ranging from 0.5 to 8192 Hz, was used for field data acquisition. A full-frequency domain apparent resistivity correction method was used for near-field corrections. Electromagnetic array profiling (EMAP) filtering was used for topographic and static shift corrections, and the Bostick conversion was used for data interpretation. Our study indicated that the results of the Bostick conversion with EMAP filtering were more acceptable than the results of rapid relaxation inversion and nonlinear conjugate gradient inversion in this case. The G Tunnel is a key tunnel along the Gui-Guang high-speed railway in southwest China. Initial geophysical and engineering geologic results suggest that the bedrock of the survey section of the G Tunnel route is sandstone. A CSAMT survey with three inline sections and three crossline sections over the tunnel route was conducted in two phases to verify the rock conditions of the tunnel route. A concealed karst cavity with a low-resistivity anomaly was found on the tunnel route and was verified by the borehole. Data from the CSAMT survey significantly refined our understanding of the subsurface engineering geologic conditions along the tunnel route.