在软弱地质条件下大断面隧道施工中，采用新奥法施工时，目前国内外采用的施工方法大体上可分为2种，即CRD法和双侧壁法。初期支护过量下沉是当前隧道施工中的一个突出问题，由于过量下沉引起初期支护侵限需要换拱的情况普遍存在，下沉超过限度造成塌孔继而造成重大事故的情况也时有发生。目前使用的施工开挖方法，都不能很好地解决下沉和侧向收敛问题。接连不断的隧道跨塌事故已充分证明了这一点。一种新的施工方法——下部导洞法，不仅可以从根本上解决初期支护下沉和收敛问题，而且施工安全、施工速度快、有利于提高初期支护的施工质量，并且可以明显的降低工程造价。

Under the guiding of New Austrian Tunneling Method, there are two basic construction methods, i.e., CRD construction method and both side drift method, for large crosssection tunnels in weak strata. The excessive settlement of the primary support is a serious problem in the construction of large crosssection tunnels, which causes the projection of the primary support structure into the tunnel clearance and thus the replacement of the primary support structure.  Because of the excessive settlement, serious collapse accidents occur one after another. For the moment, all the traditional construction methods can not effectively solve the problem. A new tunnel excavation method, i.e., bottom heading method, is recommended, which can not only completely solve the problem of primary support settlement and convergence, but also can guarantee the construction safety, improve the construction speed and construction quality and reduce the construction cost.