Study On Supporting Structure Mechanical Properties Of Tunnel In Loess Portal Sector

In-situ test work was carried, in order to analyze the mechanical properties of the supporting structure in the Loess Tunnel Portal, the project located in Liujiaping NO. 2 loess tunnel, which belongs to NO. 14 of National Trunk Qingdao-Yinchuan Shaanxi Habitat Wubao to Zizhou paragraph. On account of the geological character and the engineering conditions of the tunnel, one arch section and two entrance section have been selected to measure. The monitor contents include time-dependent regulation and the distribution characteristics of arch foundation pressure, steel arch stress, concrete stress of first support, surrounding rock pressure, shotcrete stress, rock bolt stress, link bar stress, contact pressure between first support and second support, concrete and steel stress of second support. Meanwhile, simulation analysis is carried by FEM software MIDAS/GTS comparing the numerical simulation results with the in-situ test data , some valuable conclusions are gained as follows:1. The effect of the arch and the pipe shed support structure is significant, and their stress stage is more complex. The arch stress has mainly initiated by the pipe shed, and the steel arch stress and shotcrete stress are impacted by the pipe shed stress. The stress role was impacted by spatial effects.2. The arch dome subsidence should be paid attention mostly in course of tunneling portal constructing.3. The shotcrete, steel arch and the link bar of first support play an important role in tunnel portal, so these components need to pay enough attention in designing and construction stage. And when the steel arch support is employed, the bolt arch will be in compression, which is contrary to the design aims. As a suggestion, system arch bolt should be replaced by synchronously foot lock bolt, and the arch foot anchor bolt should be strengthened.4. According to the monitoring results of tunnel portal, tunnel entrance in the loess section and the shorter tunnel should be reinforced in the second lining.5. The stress distribution regularity of FEM simulation results has good consistency with monitor date. However, the stress value has great divergence, which indicates the complexity of geotechnical engineering.The research has a promotive effect on the design and construction of the loess highway tunnel portal, and the calculations can be used for reference for other engineering.