Physico-mechanical properties of thick paleosol in Q1 strata of the Chinese Loess Plateau and their variations during tunnel excavation

To cope with the increasing demand for traffic capacity in the Chinese Loess Plateau, building additional tunnels are required in the thick paleosol strata of Q₁ strata. The thick paleosol strata consist of multiple paleoclimatic stratigraphic units, and the behavior and properties of the paleosol in each unit, which are believed to be quite different from those of common loess, remain scarcely investigated. In this work, we focused on the thick paleosol strata in Eastern Gansu of the Chinese Loess Plateau, the physico-mechanical properties of the paleosol in different paleoclimatic stratigraphic units were analyzed, and the relationship between the chemical properties, microstructures, and physico-mechanical properties of the paleosol was discussed. Besides, the water content of the paleosol was measured on site during the tunnel excavation in the thick paleosol strata, and the trends of the physico-mechanical properties of the paleosol during the excavation were also explored. We found that in the thick paleosol strata, the dry density, water content, liquid plastic limit, swelling potential, and mechanical strengths of the paleosol samples from different paleoclimatic stratigraphic units did not show large discrepancies, but obvious differences could be found in the grain size, maximum swelling pressure, and maximum swelling strain. The subtle variation of the water content had modest effects on the contact mode and cementation degree of the particles, but it could considerably influence the hydration film thickness of the clay mineral particles, which explains the distribution of mechanical properties of the paleosol samples. During the tunnel excavation in the thick paleosol strata, the paleosol around the tunnel sustained high and stable mechanical strength, while its swelling pressure and water content showed a consistent trend. The results can provide basic parameters for the stability analysis of the tunnel built in the thick paleosol strata.