Abstract:With the expressway tunnel tending to be long and wide,its stability is becoming the key problem for the design and construction. The Hejiagou expressway tunnel,which is double-lane tunnel with separate up and down lines,is taken as an example. The mechanical response of excavation and support is studied using the three-dimensional fast Lagrangian analysis of continua(FLAC3D) method. The numerical simulation reveals the distribution characteristics of displacement and stress for the underground structure under the condition of dynamic step excavation and support. The distribution of plastic zone is graphically presented for the different construction stages and its mechanism is also discussed in detail. The obtained conclusions offer a guideline for the design and construction of tunnels in rocks and soils.