Analysis of macroscopic, microcosmic, and microscopic characteristics of red-bed soft rock in Central Yunnan and its mechanical response in construction

The red-bed soft rock mass in central Yunnan, characterized by terrestrial clastic sedimentary strata with a red color, is mainly found in the Chuxiong area of Yunnan Province. Understanding its properties is crucial for managing large deformations in soft rock tunnels. This study focuses on the Fenghuangshan Tunnel in the Chuxiong Section of the Central Yunnan Water Diversion Project. Various laboratory tests were conducted to investigate the macroscopic mechanical properties, microcosmic hydrophysical properties, microscopic composition, and pore structure characteristics of the red-bed soft rock at different burial depths. The finite difference simulation software was utilized to model the excavation process of these tunnels, exploring the deformation and mechanical response laws of the surrounding rock, which were then verified through on-site monitoring. Results indicate that with increased burial depth, rock mechanical properties are enhanced; water's weakening effect on soft rock decreases by 28%; primary and secondary disintegration resistance indices increase by 13.3% and 8.5%, respectively; the content of hard hydrophobic minerals like quartz rises by 21%; pore structure is reduced; pore size distribution becomes more uniform; and particle arrangement becomes more compact. Stress concentration at the tunnel foot and steps junction is evident. As burial depth increases, vertical and horizontal displacements of the surrounding rock rise, but the vertical displacement growth rate slows while the horizontal displacement convergence rate accelerates. This study provides valuable references for similar projects.