Deformation Analysis of Shallow Gas-Bearing Ground from Controlled Gas Release in Hangzhou Bay of China

Ground disturbance and deformation caused by a controlled gas release prior to engineering construction are commonly encountered geotechnical and geoenvironmental problems in shallow gas geology areas. In this work, typical characteristics of shallow gas geology and gas-charged sand in the Hangzhou Bay of China are analyzed. It is found that the occurrence configuration feature of sand in the shallow gas enrichment zone of reservoirs is similar to that of the common unsaturated soil, but its pore gas pressure is several times larger than the atmospheric pressure. The effect of the suction history on the deformation characteristics of sand is investigated by triaxial simulation tests using the Global Digital System (GDS). The deformation of reservoir sand caused solely by the suction reduction in the process of gas release is negligible, and the main deformation is attributed to the increasing net mean stress arising from the gas pressure reduction in the reservoir. Based on the findings from the triaxial tests, a simplified constitutive model for the gas-charged sand under the special stress path of gas release is developed to describe its deformation behaviors. Then, combined with the two-phase flow equation of incompatible fluid seepage, the proposed model is applied to the three-phase soil-water-gas-coupled analysis of a typical gas-bearing ground with a gas exhaust shaft. The numerical results match the field monitoring data well. Thus, the proposed method can be used to quantitatively evaluate the geoenvironmental effect of an engineered gas release on existing adjacent structures in shallow gas regions.