Abstract: Tertiary semi-diagenetic rocks were sampled from the water-rich section of the Wangjiazhai tunnel for particle screening, X-ray diffraction, and scanning electron microscopy tests. Based on the dry density of the field samples, remolded samples were prepared for triaxial compression tests to examine their mineral composition, microstructure, and mechanical properties. Finally, a construction disaster analysis of the tunnel was conducted using Mod-Flow simulation of groundwater levels. The research findings reveal the following: (1) The grading of tertiary semi-diagenetic rocks from the Wangjiazhai tunnel is uneven, with weak cementation characteristics leading to notable water sensitivity and water softening under low moisture content. As moisture content increases, the shear strength of the samples first rises and then declines, peaking at 459.03 kPa when the moisture content is 11%. When the moisture content is below 12%, clay minerals form cements that fill particle gaps, enhancing sample cohesion. However, with further moisture increase, a large volume of free water accumulates in the particle gaps, disrupting the cementation structure and significantly reducing shear strength. (2) The water inflow simulation results for the Wangjiazhai tunnel indicate that water inflows in two typical water-rich sections are 2 805 and 5 025 m³/d, respectively, suggesting a high likelihood of water inflow disasters during tunnel excavation, with substantial water volumes involved. (3) The physico-mechanical properties of tertiary semi-diagenetic rocks show strong water sensitivity and a tendency to soften in water. The high-pressure, water-rich environment, coupled with engineering disturbances, are the primary factors inducing geological disasters during the Wangjiazhai tunnel excavation.

Key words: highway tunnel, tertiary semi-diagenetic rock, engineering properties, scanning electron microscopy, triaxial test, softening effect, Mod-Flow simulation