Geological occurrence characteristics of shallow gas in Yangtze River Delta and their effect on cross-river tunnel
To study the unfavorable geology of the occurrence of shallow gases and their effects on the tunnel engineering in the plain areas along the middle and lower reaches of the Yangtze River,the geological occurrence characteristics of shallow gas in the region were studied by means of special geological survey and geochemical analysis.In the study,the shallow gas geology occurred in the Chongtai tunnel of Yangtze River was taken as an engineering background and the analysis was carried out based on the geographi-cal,sedimentary history,sedimentary facies characteristics of the Yangtze River Delta.The adverse effects of the geological occur-rence characteristics of shallow gas on engineering were pointed out,and targeted prevention and control measures were proposed.The results show that the regional strata are overlap with each other,and the sequence is complete and obvious in the Yangtze River Delta.The silty clay of shallow marine facies are both gas-producing and capping layers,and the thick sand layer covered below is a good reservoir of gas.In the tunnel site,there are typical sedimentary characteristics of the Yangtze River Delta,and the shallow gas in the stratum belongs to the primary methane biogenic gas.The eruptive gas store in the silt soil and silty sand micro-thin interlayers inside the muddy clay layer of shallow marine facies,and it belongs to the self-generating,self-reservoir and self-sealing biogas with-out long distance migration.However,the cross-river section of the tunnel in the primarily passes through thick,fine and medium sand layers of estuarine facies have good gas storage condition,but there is no evidence of large gas accumulation.Based on the occurrence characteristics of shallow gas in cross-river tunnel section,the impact of this occurrence feature of the shallow gas on tun-nel engineering is limited,but it still cannot be ignored in the engineering construction,and targeted prevention measures should be taken to avoid the hazard caused by shallow gas geology.
Yangtze River Deltashallow gasgeological occurrenceunfavorable geologycross-river tunnel
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