Steady-state analysis of heat–moisture–salt coupling in unsaturated layered soil

Based on the theory of porous media, the multifield coupling problem of heat–moisture–salt in unsaturated layered soil under the steady condition is studied considering that the pore of unsaturated soil is filled with liquid water, dissolved salt, water vapour and dry gas. Based on the conservation of mass and energy equations, the paper discusses the mass conservation equations and energy conservation equations for water, gas and salt in unsaturated soils. Considering the one-dimensional steady-state problem, the state equations are obtained by selecting temperature, pore air pressure, pore water pressure and salt solution concentration and their derivatives as the state variables. Under the condition of the boundary values of adjacent soil layers in a given unsaturated soil, the non-linear variable coefficient differential equations are solved by using the shooting method and the validity of the model is verified by comparing with existing steady-state experimental results. Based on numerical examples, the differences in temperature field, pressure field and salinity field between unsaturated layered soil and homogeneous soil are analysed.