Study On The Characteristics Of Shallow Gas Occurrence In The Loess Beam Tunnel Site Of Mianjiu Expressway And Its Harm To The Tunnel

The West Sichuan Plateau is located in the convergent belt of Yangtze Landmass,the Songpan—Garze and Western Qinling Orogenic,there are kinds of geological structural forms and a large number of metamorphic rocks,Multiple tunnels have been exposed to high concentration of CH4,SO2 and H2S shallow gas when traversing the metamorphic rock subterranean formation,The Huangtuliang Tunnel that situated on the north of the West Sichuan Plateau is 13 km long and the maximum depth is 1092m.It has important meaning to study the occurrence features and migration patterns of shallow gas in tunnel construction.This paper undertaked many works to the Huangtuliang Tunnel on the basis of research large sum of articles related gas tunnel at home and abroad,such as collecting relevant engineering geological data,field geological surveys,wellhead gas measurement,laboratory data analysis,COMSOL Multiphysics software to numerical simulation of tunnel,etc.The main results of this paper show as follows:(1)The stratum that the Huangtuliang Tunnel traversing is Palaeozoic middle Devonian Sanhekou Zu formation,the mainly lithology is politic slate and sandstone,local carbonaceous slate and granite porphyry.The shallow gas types in the tunneling area are methane gas and the main genetic types of methane are natural gas and inorganic gas.(2)The analysis of the wellhead gas measurement data shows:the methane concentration is positively related to depth-drilling,relatively high in fault fracture zone,the concentration in hanging wall is higher than in foot wall,and different in defferent lithology rock.(3)The main reservoirs are sandstone and carbonaceous slate in the location of the tunnel,and there are high gas outburst risk in local anticline,syncline,fault fracture zone and magmatic intrusion structure.(4)Using the large software COMSOL Multiphysics to proceed approximately numerical simulation on surrounding rockmass of tunnel excavation under ground stress and gas flowing conditions.The results shows that there are prominent stress concentration phenomenon on the middle positions of two sides,top and bottom of tunnel,where easily occur secondary cracks.After the tunnel excavation,the gas pressure in the surrounding rock decreases gradually with the increase of the exposure time of the surrounding rock under gas-solid coupling,and the gas pressure tends to stabilize after 120 days of excavation.(5)According to the methane concentration of wellhead gas measurement,the location of the tunnel has shallow gas resources with total reserves of 112.02×104m3 approximately,and forecasting the gas emission quantity of the tunnel excavation face is 0.78m3/min,Thereby,it can be judged that the Huangtuliang Tunnel is low gas tunnel;At last,the 4 high risk locations of tunnel construction are forecasted based on the engineering geological condition and occurrence features of shallow gas.