Experimental investigation on gas migration behaviour in unsaturated sand-clay mixture

Understanding gas migration behaviours in unsaturated sand–clay mixtures is of great importance to design and construction involving applications of shield-cutting-tool replacement using pressure. A gas permeability test apparatus for unsaturated soil was upgraded to investigate gas migration behaviours further. A series of gas, liquid and solid three-phase coupling gas injection tests were performed on unsaturated clay under different moisture contents and sand–clay ratios. There are three stages of gas migration in unsaturated sand–clay: steep drop stage, transition stage and stability stage. In unsaturated sand–clay with low moisture contents and gas pressures, gas migration is affected primarily by the slippage effect. When the gas pressure is greater than 300 kPa, the permeability remains nearly unchanged. In this case, the gas permeability of the soil sample is affected primarily by the internal structural characteristics of the unsaturated soil sample and those of the shrinkage film. The ideal gas permeability of the soil used in actual engineering and the empirical formula for calculating the pressure of the shield pressure-holding system are summarised.