Effects of gas pressure and confining pressure on gas flow behavior in saturated cohesive soils with low permeability

Abstract

Gas flow behavior in saturated cohesive soils having low permeability is a major concern in most engineering practices. The effect of stress, including that caused by gas pressure and confining pressure, on gas permeability must be investigated in detail. In the present study, gas permeability tests with cyclic gas pressure, as well as different gas and confining pressures, were conducted. Results showed that a sudden increase in the gas flow rate appeared when the gas pressure approached the confining pressure. This increase in gas flow was possibly caused by a gas leakage along the lateral boundary, which was not the true gas breakthrough. The real gas breakthrough linearly increased with the increase of intrinsic permeability at a log-log scale. In gas permeability tests, the cyclic gas pressure induced a significant hysteresis: the gas flow rate during decreasing stages of gas pressure was higher than that during increasing stages. In addition, increasing confining pressure weakened this phenomenon. Under a constant confining pressure, gas permeability increased significantly with the gas pressure increased. However, little variation was aroused by increasing the confining pressure under a constant gas pressure. In conjunction with the pore size distribution determined in mercury intrusion porosimetry test, it can be concluded that, gas flow behavior in saturated cohesive soils with low permeability was mainly controlled by the capillary effect, even though the mechanical effect was presented during the gas migration process.