| The land terrain of the large basin in China has contained rich natural gas resources,which has been an important energy fortune for the national economic growth.However,due to the gas with the features of the toxicity and explosive characteristics observed from the aspect of the physical and chemical properties,it has been harmful for the railway engineering.When making the excavation of a tunnel,harmful gas will be likely to be escaped from the tunnel,causing a serious safety risks to the construction and operation of the project.Since the development of the new century,the number of the high-speed railway tunnel construction has been increased dramatically.The harmful gas in the tunnel construction has been a common phenomenon,there have also been many accidents taken place such as poisoning suffocation,gas combustion,explosion or outburst,causing serious casualties and property losses.In view of the research on the railway tunnel engineering of harmful gas geological started relatively late,in recent years in the study of coal related gas tunnel has been popular gradually.In addition,the study on the oil-type gas and other harmful gases,at present,in the aspect of the harmful gas,has seldom been researched.Especially,the disaster reduction and harmful gas tunnel assessment,disaster prevention measures shall be considered into the alignment selection for the railway construction,in which the systematic study has been emphasized.At the same time,due to the restrictions of the elevator,high plane linear standard and low flexibility of interval alignment and other factors,the constructions of the high-speed railway and expressway engineering,the areas of the harmful gas occurrence cannot be avoided by the design for the route selection.Therefore,how to recognize and evaluate the risks for the tunnel engineering has been focused on in many occasions.Therefore,it has been of a great significance to study the principles and methods of disaster risk reduction and selection in the hazardous gas area along the high-speed railway,establishing a reliable risk assessment system on the hazardous gas tunnel,scientifically to select and evaluate the lines for the disaster reduction and risk assessment,and specifically to formulate risk prevention,control and treatment technical measures to achieve disaster reduction and prevention objectives.Based on the collections from the existing regional geological data and construction experience of the Chengdu-Gui Yang high-speed railway,this paper has expounded the geological environment backgrounds of the south of China,setting Sichuan Basin as an example,the types and characteristics of the underground harmful gases have been presented in that areas.Therefore,the distributions of the harmful gases along the Chendu-Gui Yang high-speed railway have been illustrated in this paper,combining with its cause mechanism and concluding a qualitative evaluation system on the high-speed railway line selection in hazardous gas area,putting forward the guiding principles for the line selection with convinced examples.In view of the alignment selection for the high-speed railway through the hazardous gas areas shall be determined in the design and cannot be changed,correspondingly,the risk assessment systems for the tunnel with the hazardous gas shall be established up.And the measures of preventing and controlling harmful gas along the tunnel construction alignment shall be put forward.The following main results and conclusions have been achieved below:(1)Based on the analysis on the stratigraphic lithology and geological structure combining with the characteristics of the south of China like Sichuan Basin,it has been shown that the regional geological situations have been complicated,in which the geological problems for the engineering have been prominent with the occurrence of the underground harmful gas.The occurrence times for the main rocks for the oil-type natural gas in the area have been traced back to the Asinian,Cambrian,Permian and Triassic.The rocks have been verified with a high maturity,in which the gas hydrocarbon has been in a good condition.The harmful gas of oil-type has been presented with the features of the shallow strata by means of faults,cracks and micro-cracks to form air bags,which has threatened the safety of the tunnel construction along the alignment.(2)Based on the landform of the topographic,geomorphological and geological features in the southern areas,the Cheng-Gui Yang high-speed railway in sichuan basin(Road section from Leshan to Xingwen),the alignment has been divided into three engineering geological areas: alluvial plain,southern Sichuan hilly area and northern Gui Yang low and middle mountainous area.Considering the impacts on gas occurrences in a tunnel and oil and gas structure,oil and gas reservoir,rock and oil and gas display,oil and gas and weathering rocks,it has been concluded for the distributions of the natural gas along the alignment,which has been summarized as the two laws.At first,the closer to the core of the region trap has been,the higher the gas density has been formed.The deeper the burial of the soil layer has been,the higher the gas density has been presented,on the contrary,the lower the gas density has been,the situation has been changed correspondingly.(3)On the basis of the characteristics on the gas disaster,the selection for the alignment of the high speed railway has been studied with some reasonable requirements,incorporating the following considerations in terms of the harmful gas disaster formation mechanism,research on the disaster,gas,railway engineering geology and engineering geology and other basic theory to put forward the principles on the effective alignment selection for the high-speed railway construction and finally formulate to practical systems to be obeyed.The systems of the harmful gas in high speed railway line selection should be formulated in compliance with the guidelines called "High risk avoidance of the harmful gas;Choose the low risk safe passage or position;Adopting reasonable engineering forms or measures".Meanwhile,a summarized and proposed slogans which has been marked as follows "First round to avoid short,short pass,small buried depth,tunnel group,elevation,considerable decision",in which the guidelines have been paid more attention by local governments.(4)According to the guideline for the risk decision-making construction,a large number of problems may be encountered during the construction.This paper has combined with the author's own experience and put forward the four principles for the tunnel risk evaluation on the tunnel construction with the hazardous gas.At the same time,the four principles for the alignment selection have been suggested in a feasible way by the author.According to the analysis on the harmful gas in tunnel engineering,geological condition and human influence,the evaluation index from three aspects concerning the factors in terms of the regional gas content,reservoir elevation difference line distance,crack rate,porosity,coefficient of fault sealing,capping thickness,folding wing Angle,hydraulic migration,the strength of hydraulic seal,hydraulic dissipation block types and prospecting quality,has been put forward with a total of 11 indexes.(5)Based on a large number of the surveyed sample data,especially in the Chengdu-Gui Yang high-speed railway project,the laws of occurrence,migration and escape of harmful gas in trap structure,the calculation formula of the escape of harmful gas have been derived.By the scoring method and numerical analysis method of evaluation index values range have been building-up,in which the corresponding risks have been divided into four levels: level?for low risks,level?for medium risk grade,level ? for higher risk,level ? for highest risk.(6)In view of AHP subjective weighting method,coefficient of variation objective weighting method and game theory aggregation model,the index weight has been calculated and distributed,and then the membership value of each level has been calculated by combining with the associated cloud model.Finally,the risk probability value of harmful gas tunnel has been drawn.Shizhushan tunnel,nanchanggou tunnel and xinglong ping tunnel in the southern margin of Chengdu-Gui Yang high-speed railway in sichuan basin have been selected as the typical examples for the risk evaluation verification.The final evaluation has been showed that Shizhu mountain tunnel and Xinglong ping tunnel have presented the high risk,however,Nanchanggou tunnel have had a medium risk,and the evaluation results have been consistent with the actual situation.(7)According the requirements of the disposal measures and engineering structure prevention measures,they have been divided into two categories.Combined with the constructions of Chen-gui high-speed rail and the geography conditions of the Xinglong ping tunnel,the related data for the harmful gas development and engineering geological conditions of the tunnel has been calculated,using numerical analysis method for the tunnel under the condition of pressure into the ventilation tunnel endogenous wind field,simulation analysis on the gas concentration distribution and motion has been employed as well.Therefore,the results have been shown that under the condition of pressure into the ventilation,tunnel there have been parts of the gas density on the high side.In order to keep the gas density of the whole tunnel in the allowable range,ventilation shafts and local fans should be considered in the design to ensure construction in a safe manner. |
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