Stability evaluation of the sequential excavation method of large-section double-arch tunnel in fly ash accumulation stratum through physical model

There is no precedent for the construction of large-section double-arch tunnels in fly ash accumulation stratum in China. The fly ash stratum is characterized by a loose structure, poor cementation, and low bearing capacity, which present significant challenges in tunnel design and construction. Based on the Yanpingba tunnel project, this paper conducts a model test on a large-section double-arch tunnel crossing a fly ash accumulation stratum, analyzing the tunnel’s mechanical behavior under various sequential excavation methods. The results show a large settlement of the vault and upper stratum of the double-arch tunnel excavated in the fly ash accumulation stratum. In addition, the stratum can easily become unstable, so it is necessary to reinforce the stratum by grouting. Compared with the subsequent tunnel, the advance tunnel is more affected by excavation, and the risk of structural damage is higher. Compared with the central diaphragm (CD) method and the bench method, the single side drift method is more conducive to safe construction. A mid-partition wall can effectively reduce the settlement of the stratum, but it is easily deflected to the side by the bias pressure. Adding support on the side of the advance tunnel of the mid-partition wall is recommended.