Study On The Risk Of Water Busting Hazard In Balang Mountain Tunnel

With the construction and development of domestic transportation infrastructure, people pay more attention to water inflow forecasting and risk assessment of the tunnel in the fissured bedrock in complex geological environment conditions. The proposed Balang Mountain tunnel will be built in Balang Mountain which is at the junction of Xiaojin, Wenchuan and Baoxing counties. The tunnel is a section of Provincial highway line S303 and it is the only way to connect nine ring lines, Giant Panda Nature Reserve of the Wolong, and the East Sacred mountain --Siguniang Mountain. For the steep terrain, bad weather and high altitude, the proposed tunnel project is to improve the conditions of road crossing the mountain, improve the traffic safety and enhance Giant Panda Nature Reserve of Wolong and the travel resources of East Sacred mountain --Siguniang Mountain.This paper first introduces the rock structure in the tunnel site areas, identifies the recharge, runoff and discharge conditions, the rule of migration and hydrodynamic characteristics of the groundwater system, analyses water-bearing media types of the underground water, evaluates the rock permeability, on this basis, the risk indicators of water inflow into the Tunnel is selected. Then analyses the probable water inflow sections and points during the tunnel construction, calculates and analyses water inflow in the tunnel for checking on rationality of the risk indicators, the influence of water inflow into the Tunnel on the project and the environment is further evaluated.The main research contents and results are as follows:(1) Characteristics of lithological association in the aquifer. The groundwater in this area can be divided into pore water in the loose rocks of Quaternary and fissured water in the bedrock. Due to the large topographic relief and deep cut which make recharge and runoff areas not large, Pore water in the loose rocks is mainly in the form of phreatic or perched water, whose ability of containing water is weak - medium, and effected by seasons and climate significantly; fissure water in the bedrock is mainly recharged by rain, then supplied by water from ice and snow melting in the mountain areas, whose dynamic seasonal change is remarkable and exposure is generally in the ditches or low-lying areas.(2) Study the permeability of rock mass. Through the packer permeability test in the deep holes and the indoor core permeability test, the results of two tests show that the permeability coefficient in sandstone is larger than in slate, and the permeability coefficient is decreased with the depth gradually increased, when it reaches a certain depth, the permeability coefficient will be stable.(3)Study on dynamic field of the groundwater and field of water chemistry. Through survey and tracing spring points and analysis the information of underground water, chemical compositions and Hydrogen and Oxygen stable isotopes revealed by deep holes, we study the recharge, runoff and discharge conditions and the cyclic-alternating rule of the groundwater. The groundwater circulation in the southern slope of Balang Mountain is mainly shallow. The effect of the rock medium on hydrochemical type of the groundwater is not very clear, and the mineralization is generally not high; the groundwater circulation in the northern slope of Balang Mountain is mainly deep, mainly due to "U" shaped valleys in the northern slope are more developed, and the slope is gentle.(4) Analysis of water system. According to the recharge, runoff and discharge characteristics of groundwater in tunnel site areas, class 1 systems of the groundwater are divided: groundwater system in the northern slope of Balang Mountain and groundwater system in the southern slope of Balang Mountain. Then sub-groundwater systems of the groundwater are further divided, due to the spatial distribution of the confining layer groups. The groundwater systems of Haizi ditch and Gaodianzi ditch are risk for the tunnel by the initial judgement.(5) Risk indicators select of the water inflow. This article initially establishes the risk evaluation indexes of the fissured bedrock tunnel mainly by collecting and tidying related data on the risk classification of the water inflow and former research results, combined with the geological structure, hydrogeological conditions and other relevant factors of the tunnel, meanwhile, presume the risk segment of the tunnel site areas.(6) Calculation of the water inflow. The water inflow of typical sections of the Balang Mountain tunnel which has been segmented is calculated by traditional, classic formulas of the tunnel water inflow (such as rainfall infiltration method, hydrodynamics method, etc.), and combined with Visual ModFlow numerical simulation, the ranges of the water inflow in typical sections of the tunnel site areas are initially got.(7) Comprehensive evaluation of Tunnel water-busting risk. High-risk sections of Balang Mountain tunnel are obtained though risk indicators, then verified by water inflow calculations. It proves that the risk sections of the tunnel are matched with the values of water inflow. The water inflow of the tunnel is small, and the risk is small; only in the transitions of the foldings and in the broken belts, is the risk relatively large, which needs special attention during the construction and operation phases. Tunnel excavation will affect runoff and discharge conditions of the pore water in loose layers .But the leakage of the water inflow in the tunnel is main from the water in bedrock fissures, which affects little in the ability of soil containing and storing water and in the damage of surface vegetation.