Transient response analysis of saturated-unsaturated subgrade considering pot cover effect

The presence of impermeable cover layers, such as those found in airport runways, high-speed railway ballastless tracks, and highways, can impede the migration of water vapor within the subgrade. Under the influence of seasonal freeze-thaw cycles, the accumulation of moisture beneath these cover layers is known as the "pot cover effect." This phenomenon is highly likely to induce subgrade frost heave and uneven settlement, posing significant hazards. Current research on this effect primarily focuses on its formation mechanisms and the patterns of water vapor migration, with limited attention given to the potential dynamic disaster issues associated with the pot cover effect. This paper establishes a one-dimensional saturated-unsaturated subgrade model, simplifying the cover layer as an impermeable boundary condition. The Biot theory for saturated soil and the wave propagation theory for unsaturated media are employed to simulate the affected and unaffected regions by the pot cover effect, respectively. Based on the eigenfunction method, transfer matrix method, and precise time integration method, a semi-analytical solution for the transient response problem is directly provided in the time domain. The correctness of the solution presented in this paper is verified through two degenerate case study. The results of the case analysis indicate that the pot cover effect significantly influences the excess pore water pressure. The peak pore water pressure in the pot cover layer is approximately 18.66 times higher than that without the pot cover effect. As the initial water content of the subgrade increases, both the pore water pressure and the solid phase displacement show an increasing trend. In engineering practice, the dynamic disaster issues caused by the pot cover effect require special attention.