Bifurcated tunnels have two major engineering problems of large span and small interval simultaneously, and they pose higher demands on design and construction technology. The Liantang tunnel has a maximum span of 30.1 m, a minimum interval of only 1.05 m and a single excavation face; the existing construction method cannot meet the demand. Based on these features, this paper proposes a reverse expanding excavation method and a stepwise controlled blasting excavation method. By numerical calculation and field testing, the stability and mechanical characteristics of the surrounding rock and primary support using the two methods were researched. The results show that the reverse expanding excavation method, by excavating the pilot tunnel first and then reversely excavating the remaining part, achieves stepwise excavation of the super-large-span tunnel, provides mechanised working conditions and avoids multiple conversions of the excavation method. The stepwise controlled blasting excavation method, by grouting and reinforcing the shared rock first, and then controlled blasting of the later tunnel in steps, ensures the stability of the shared rock, reduces the construction influence of the later tunnel on the existing tunnel and omits the construction of a mid-partition. This study can provide a reference for future similar projects.

中文翻译：

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超大跨度分岔隧道单开挖工作面法

分岔隧道同时存在大跨度、小间距两大工程问题，对设计和施工技术提出了更高的要求。莲塘隧道最大跨度30.1 m，最小间距仅1.05 m，单开挖面；现有的施工方法不能满足需求。基于这些特点，本文提出了逆扩开挖法和逐步控制爆破开挖法。通过数值计算和现场试验，研究了两种方法对围岩和初期支护的稳定性和力学特性。结果表明，逆扩开挖法，先开挖先导洞，再逆开挖剩余部分，实现了超大跨度隧道的阶梯式开挖，提供了机械化工况，避免了开挖方式的多次转换。分步控制爆破开挖法，先对合岩进行注浆加固，后段隧道分步控制爆破，保证了合岩的稳定性，减少了后期隧道对既有隧道的施工影响，省去了对既有隧道的施工影响。构建一个中间分区。本研究可为今后类似项目提供参考。减少后期隧道施工对现有隧道的影响，省去中间隔墙的施工。本研究可为今后类似项目提供参考。减少后期隧道施工对既有隧道的影响，省去中间隔墙的施工。本研究可为今后类似项目提供参考。