Effect of loess and paleosol distribution on tunnel stability in thick paleosol strata of Chinese Loess Plateau

During the construction of tunnels in the Chinese Loess Plateau, the primary focus has traditionally been on the loess surrounding the tunnel. However, the swelling of paleosol and the low mechanical strength of loess in thick paleosol strata elevate the risks associated with tunnel construction. The distribution of loess and paleosol significantly influences tunnel stability. In this paper, we monitored the surrounding rock displacement and support structure pressure of a tunnel built within the thick paleosol strata. The thermal-mechanical coupling model was employed to simulate and calculate the swelling of paleosol, and the impact of loess and paleosol distribution on tunnel stability was thoroughly discussed. The results confirm the reliability of the thermal-mechanical coupling numerical model. At the vault and spandrel, where the interface lies between the thick paleosol strata and thick loess strata around the tunnel, the surrounding rock displacement initially increases and then decreases with the decrease in tunnel depth. However, the surrounding rock pressure exhibits an inverse variation, reaching a minimum when the interface is at the bottom of the lower bench. In areas with a thin loess layer around the tunnel, the surrounding rock displacement decreases, while the surrounding rock pressure increases with the reduction in the height of the thin loess layer. Furthermore, in comparison with surrounding rock with a full paleosol section, the displacement safety factor is 0.39-0.40 smaller, but the pressure safety factor is 1.21-1.24 larger when the interface is at the bottom of the lower bench. Simultaneously, the pressure safety factor is 0.17-0.41 larger, but the displacement safety factor is only 0.08-0.09 smaller with the thin loess layer in the upper bench. These findings provide a valuable theoretical reference for the construction of tunnels in thick paleosol strata and multi-strata environments with swelling soil and weak mechanical strength soil.