Study On Gas Disaster Risk Assessment System Of Tunnel Engineering

Gas disaster is one of the major disasters during the tunnel construction. Gas disasters are in the forms of poisoning, asphyxia, combustion, explosion, and outburst of coal and gas, among which gas explosion is most likely to happen with serious consequences. Since more and more tunnels passing through coal-bearing strata, gas accidents are on the rise accompanying the construction and development of basic communication in our country. Therefore, it is so urgent to realize the significance of preventing and mitigating gas disaster that a effective risk assessment method of gas disaster should be established.Based on the research of gas tunnel in our country, a relatively perfect gas disaster risk assessment medhod and system of tunnel has been set up through the following measures. Above all, geo-environmental conditions and gas situation in the construction have been analyzed; combining qualitative analysis and quantitative evaluation, the influencing factors of gas disaster also have been studied, including geological factors, gas factors and human factors. The results not only reach the goal of risk assessment, also make a progress in investigative techniques and assessment method in tunnel engineering. The main achievements of this work are shown below:(1) Analyzing the geo-environmental conditions of gas tunnel in our country, based on the gas formation, distribution and occurrence, the conclusion that gas situation has closely related to China's regional gas distribution can be made: that is high gas tunnels always locate in regional high gas zone or high gas belt, vice verse.(2) According to the principles and methods of geological engineering theory, a concise gas tunnel classification evaluation method has been set up, mainly based on geological macroscopical estimation. The method takes distributed gas zone, buried depth of tunnel, and geological structure, seam thickness, the degree of exposed groundwater in tunnel site areas as evaluation indexes, and adheres to general evaluation method factors to carry out the classification.(3) Considering relatively numerous materials in design and relatively more demanding of projects, meanwhile, due to classification evaluation method, and referring to gas tunnel railway technical specifications of 2002 version, a suitable gas disaster engineering risk assessment method should be established. Gas tunnel engineering risk assessment is composed of two parts: division evaluation and risk assessment, and amend by superposition construction factors. Division evaluation is a risk assessment of each division under natural geological conditions, meanwhile, the background value and basis of construction risk assessment, which includes six evaluation indexes: gas emission, pressure, geological structure, the type of coal structure, groundwater conditions, and depth between working zone and the coal. Different prevention and control measures, gas management, construction experience, and technical level may bring out different level of risk. Those four factors can be taken as amending factor; methods of assessment can be divided into comprehensive evaluation method of general factors, fuzzy comprehensive evaluation method, and extension evaluation method.(4) Tunnel face risk assessment method of gas tunnel engineering has been set up for the potential risk of gas outburst in dangerous area of outbursting (dangerous zone). Taking the maximum buried depth, geological structure, dynamic phenomenon in advance borehole drilling, maximum gas pressure, maximum initial velocity of gas emission in borehole, and coal structure as the outburst risk evaluation indexed, by discriminating each of them to conduct the predictive assessment.(5)The complete gas disaster risk assessment system, applicable to tunnel engineering, is composed of three parts described above: gas tunnel classification evaluation, gas tunnel engineering risk assessment, and tunnel face risk assessment. Therefore, it can make a prompt and accurate assessment of gas disaster in any phase, such as route selection, designation, construction, and take appropriate engineering control measures. At the process of establishing the system, both subjective (analytic hierarchy process) and objective (interactive matrix method, grey correlation method) methods are resorted to calculate the of index in each phase. The identical significance of each index demonstrates the justifiability of calculating weight using subjective and objective methods. In order to reflect the subjective and objective information simultaneously, this paper adopts mestasynthesis weight approach to assign the indexes.(6) This paper has applied the Zipingpu tunnel to perform the gas disaster risk assessment of tunnel engineering, and achieved a relatively good results. By means of geo-environmental conditions of Zipingpu tunnel, research has been done on the actual status of evaluation indexes, the values of which been assigned according to quantitative criteria. On this basis, adopting gas disaster risk assessment system, combining qualitative analysis and quantitative evaluation, to assess the risk of Zipingpu tunnel. The results are accordance with the actual situation of tunnel, correspondingly, prevention and control measures has been put forward in light of risk level integrated with geological conditions.