Mechanical Analysis Of Dynamic Excavating Tunnel Portal In Feeble Ground

Tunnel portal, at which the burial depth of soil is shallow, the rock is weathered seriously and the geological condition is complicated, is highly vulnerable to the terrain bearing heterogeneous pressure and potential geological hazards such as landslide, collapse and so on. Recently large quantities of tunnels are constructed, but many tunnels at the excavation stage encounter to common difficulties such as excavation difficulty, roof collapse etc. so transferring the plane of construction is necessary, in other words, the tunnel has to be excavated on the other side. So it is very important that the rocks'mechanical properties and displacement characteristics are understood when the tunnel portal is excavated.The research is based on the Yuba tunnel project at the Shuicheng-Panxian highway in Guizhou province. The review of design and construction of tunnel portal at home and abroad is done. The 3D numerical simulation of the construction process is conducted by finite element method analysis software MIDAS/GTS , which includes six numerical models of three excavation methods in Yuba tunnel project. According to the in-situ measured data, the comparison of rocks'mechanical and displacement properties at the construction stages of applying three excavation methods is made. The appropriate construction program at tunnel portal is summarized. The study includes:1.the 3D elastoplastic model of Yuba tunnel is set up, which contains six dynamic excavating process at the tunnel portal using full-section excavation method, half-section excavation method, and half-section combined with temporary inverted arch excavation method. The mechanic properties of surrounding rock and retaining structures at the excavating process applying the six methods are studied.2.The settlement tendency of vault and ground at the construction stages of six excavation methods is analyzed respectively. A comparison between the simulation results and the in-situ monitored data is made, which shows the numerical simulation is reliable. 3.The mechanical properties of surrounding rock and retaining structures and the settlement properties of vault and ground at these construction stages are also studied. Some steps are proposed pertinently to improve the stability of tunnel portal in construction.