In many projects, like the Simplon, for example, a small pilot tunnel is driven before the full diameter of the tunnel is excavated.
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.
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。
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