在连拱隧道施工时，往往采用三导洞法施工，即首先开挖中导洞，然后施做中隔墙，再分别开挖左右导洞。开挖中导洞主要是为了首先施做中隔墙。地铁的修建必然涉及到岩土体的开挖， 施工过程中产生的过大地表沉降和变形会对地面建筑物以及地下管线的安全造成巨大威胁。地铁暗挖车站几乎都是大断面的开挖，施工难度大，稍有失误将会造成不可估量的损失。
本文的研究结论可供确定导洞的开挖顺序及开挖方案提供参考。
The research conclusion of this thesis can be taken as a reference for determining excavation order and excavation scheme of pilot tunnels.
在许多隧道工程中，经如辛普龙大隧道，都要在隧道全面开挖前打一个导洞。
In many projects, like the Simplon, for example, a small pilot tunnel is driven before the full diameter of the tunnel is excavated.
实测数据表明：施工引起的既有地铁隧道结构变形以沉降为主，沉降主要发生在导洞施工阶段；
The monitoring data show that the structural settlement is the prominent response style of the existing subways, and the settlement mainly happens during the pilot-drift construction.
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