Abstract: The artificial control of pre-exhausting shallow gas is a necessary measure for preventing geological disasters before the construction of Hangzhou Metro. By analyzing the gas-water migration process during the formation of shallow gas reservoirs, it can be found that original pore water pressure and matric suction in aeration zone of reservoirs present linear distribution along the thickness; and the pore water pressure is equal to hydrostatic pressure of the same position. The original saturation distribution of reservoirs can be obtained through experimental research on soil-water characteristic curve of reservoir sample and in-situ survey data. Based on analysis of the gas-water migration inside of reservoirs during gas release, deformation studies of gassy sand in wetting process were carried out by GDS unsaturated stress path triaxial system; and a method to predict settlements of the aerated stratum induced by gas release was proposed. Results show that the shallow gassy sand going through natural dewetting during the formation of reservoirs is further compacted; the wetting deformation of sand caused by decrease of saturation degree or matric suction is very small during water immersion or re-wetting induced by the engineering measure of gas release; and the effect can be ignored; settlements of the shallow gassy soil caused by controlled gas release decrease gradually from top to bottom along the thickness of reservoirs; and the total settlement is about 1‰-5‰ of the total thickness of gas reservoirs.

Key words: shallow gas, gassy sand, gas release, wetting deformation, settlement