Research On Waterburst Mechanism And Prevention Measures In Dashan Freeway Tunnels

Proposed and implemented with the country’s western development strategy, the tunnel construction occupy a pivotal position in the transportation system, waterburst is the most prominent geological disasters in the process of tunnel construction, in which frequently encountered waterburst, bay mud and other geological disasters, seriously impact on construction safety, construction and operation of the tunnel is also very unfavorable. The paper takes Dashan Highway Tunnel as examples, collects typical waterburst disasters and use theoretical analysis, numerical simulation and other means to research waterburst types, mechanism and risk assessment. Corresponding prevention measures is formulated for different levels of risk. The paper made the following main results:(1)Based on collecting and analyzing of large waterburst cases, geological structure, lithology, water-rich aquifers, impermeable rock wall thickness and pressure of underground water are the main factors. According to tunnel groundwater types and geological structure, tunnel waterburst can divide into the following types:Waterburst of surface water, Waterburst of the aquifer water, Waterburst of fault zone (Waterburst of water-rich fault, Waterburst of water conducted fault, Waterburst of impermeable fault), Waterburst of karst pipe (Waterburst of cave, Waterburst of underground river), Waterburst of anticline, Waterburst of syncline and Waterburst of layer-to-layer.(2) According to the common tunnel waterburst, waterburst mainly in the following types:Directly welling type, Fracturing type of hydraulic (Pure split type, Split type of wall stress), Activate of fracture, failure of impermeable weak rock (Bending failure, Shear failure), Fracture extension after construction disturbance, instability of the key blocks. (3) The majority of waterburst process essentially is the groundwater transform into Brinkman flow in waterburst channels from Darcy flow in aquifer until N-S flow in waterburst tunnels. For different inclination of fault waterburst, the fault inclination impact on flow velocity in the constant flow boundary condition, the greater the velocity and the smaller the angle of intersection of the fault and tunnel axial, effect on the flow rate decreases is more obvious. The smaller the angle of intersection of the fault and tunnel axial, the waterburst velocity is larger.(4) Based on the multi-angle, multi-level analysis of the traditional evaluation system, evaluation method, calculate model superior or inferior, two fuzzy mathematical models is selected as waterburst risk assessment model. The paper selectes lithology conditions, terrain conditions, geological conditions, groundwater conditions and other conditions as first level indicators, each one form from a number of secondary indicators, develops a "Tunnel Waterburst Risk Assessment System "software using of "E" language.(5)Drain hole drainage are simulated using software, the results show that the drainage rate of the external drain hole are faster than the internal drain hole; Large drainage holes are faster at the drain holes are in the same area; So when layout the drain hole, the drainage holes are arranged around the tunnel face evenly, and may be appropriate to increase the diameter of the drain holes around the tunnel face.(6) Depending on the different risk levels of waterburst, the corresponding prevention measures are formulated and to develop and the planly comprehensive treatment of post-disaster. Comprehensive treatment of post-disaster programs are: construction drainage plan, drainage holes+full-face ahead of grouting program, the drainage holes+radial grouting program, draining holes+circuitous heading programs and only the pulp wall+full-face grouting program.