粉煤灰堆积体地层连拱隧道开挖错距影响分析

粉煤灰堆积体自稳能力差，受力即破碎成散颗粒，隧道穿越粉煤灰场区时失稳围岩对施工安全构成极大威胁。以盐坪坝连拱隧道为依托，根据粉煤灰力学性质对比选用了塑性-硬化本构，通过有限差分软件FLAC3D对不同开挖错距施工进行数值模拟，研究5、10、15 m不同开挖错距对粉煤灰堆积地层中双连拱隧道的围岩和结构的影响。结果表明：中导洞台阶开挖错距不宜超过10 m, 15 m错距工况相比10 m错距工况应力最大值增长了36.82%,左右洞采用小开挖错距可以有效减少中导洞拱顶位移；中导洞开挖完成后左右洞拱顶竖向位移达到总沉降量约50%,左洞拱顶沉降位移与开挖错距近似线性增长，后行洞开挖过程中对先行洞造成的影响较大；主洞采用不同开挖错距时应力集中区域分布在不同位置，开挖错距大于10 m后结构应力增幅快速增长，15 m错距相比10 m错距应力最大值增长了17.28%,主洞开挖错距不宜超过10 m。

Analysis on the Influence of Excavation Offset of Multi-arch Tunnel in Fly Ash Accumulation Stratum

The self-stability of fly ash accumulation body is poor, and it is broken into scattered particles after stress. When the tunnel passes through the fly ash field, the unstable surrounding rock poses a great threat to the construction safety. Based on the double-arch tunnel of Yanpingba, the plastic-hardening constitutive model is selected according to the comparison of mechanical properties of fly ash, and the finite difference software FLAC3D is used to simulate different excavation offsets. The working conditions of the upper and lower steps of the middle pilot tunnel and the left and right tunnel excavation offsets are set as 5 m, 10 m and 15 m, respectively. The influence of different excavation offsets on the surrounding rock and structure of the double-arch tunnel in the fly ash accumulation stratum is studied. The results show that the step excavation offset of the pilot tunnel should not exceed 10 m, and the maximum stress of 15 m offset increases by 36.82 % compared with that of 10 m offset. The small excavation offset of the left and right tunnels can effectively reduce the displacement of the arch top of the pilot tunnel. After the completion of the excavation of the middle pilot tunnel, the vertical displacement of the left and right vaults has reached about 50 % of the total settlement. The settlement displacement of the left tunnel vault increases approximately linearly with the excavation offset, and the excavation of the subsequent tunnel has a greater impact on the first tunnel. The stress accumulated and distributed in different positions when the main hole was excavated with different offsets. The stress increment of the structure increased rapidly after 10 m offset, and the maximum stress of 15 m offset increased by 17.28 % compared with 10 m offset. The excavation distance of the main tunnel should not exceed 10 m.