The Influence About Coupled Initial Water Content and Local Expansion on Mechanical Properties and Deformation of the Tunnel in Mudstone

The current research primarily focuses on the impact of full-ring expansion on tunnel safety, but the relationship between local expansion of the surrounding rock and initial water content is rarely studied yet. Therefore, this paper analyzes the mechanics and deformation characteristics of a tunnel passing through an expansive mudstone area in a tunnel project in China. The analysis considers different local expansion areas and the initial water content of the surrounding rock. The research findings indicate that the local expansion of the tunnel's vault and sidewall has the most significant impact on the deformation range of the surrounding rock. Compared to other expansion conditions, the deformation difference can reach a maximum of 156 mm. The order of expansion conditions leading to the development of plastic zone volume in the surrounding rock is as follows: full-ring expansion > vault expansion > local vault expansion > sidewall expansion > invert expansion. When the initial water content reaches a certain level, the expansion effect becomes less significant due to the upper limit of the surrounding rock's expansion capacity. However, an increase in water content will cause the surrounding rock to soften and intensify the expansion of the deformation range. The local expansion of the surrounding rock not only increases the bending moments on the preliminary lining at the section directly affected by the expansive force but also significantly impacts other expansion areas. The local expansion of the vault and sidewall greatly influences the stability of the secondary lining, resulting in some areas having a safety factor lower than the allowed value. However, from the perspective of overall tunnel safety, the secondary lining still provides a certain safety margin.