Experimental investigation on gas migration in saturated Shanghai soft clay

摘要

Understanding of gas migration behavior in saturated soft soil is of great importance for designing and construction activities involving applications of pneumatic caisson method and compressed-air tunneling technique in soft ground. For further investigation of mechanical effect on gas migration behavior in such applications, including gas flow rate, gas permeability, as well as the gas breakthrough pressure, step-wisely gas injection tests were conducted on Shanghai soft clay under different vertical loads in this paper. Results show that gas flow rate will increase as the injection pressure increases. The conventional two-phase flow theory, which is mainly governed by capillary pressure, can be used to model gas flow under lower gas injection pressures, but fails to describe gas flow under higher gas injection pressures, due to the significant influence of mechanical stress. As effective stress increases, gas permeability decreases sharply at the initial stage followed by a gradually stabilization stage due to the dissipation of mechanical effect. The mechanical stress has significant influence on gas permeability for specimens with higher compressibilities. Formation of gas flow pathways, either caused by capillary pressure or mechanical stress, can result in gas breakthrough, the corresponding pressure of which is determined according to the evolution of gas flow rate and can be used as the working gas pressure for avoiding significant gas loss in the gas-tight working structures in pneumatic caisson method and compressed air tunneling technique. Gas breakthrough pressure increases with the increase of vertical load. (C) 2017 Elsevier B.V. All rights reserved.