Abstract: In order to avoid the damage of deepburied loess tunnel caused by partial failure of primary support, field test on primary support force of deepburied large crosssection loess tunnel is carried out based on the construction of Zaosheng No.3 Tunnel in Gansu Province. The vibrating wire sensor is embedded in primary support to monitor the contact pressure between surrounding rock and primary support, the strain of shotcrete and steel arch and the active force between neighboring steel arches; the finite element calculation model is established to simulate tunnel construction; and the distribution characteristics of primary support internal force are obtained. The results show that: (1) The primary support stress experiences 3 stages with the variation of time, i.e. rapid growth, slow growth, and gradual stability; and the curve turns basically stable after the secondary lining has been closed for 12 days. (2) For space distribution, the upper part of stress is larger than that of the lower part, which shows that the characteristic of biasing is obvious, and the calculated value of load is too small. (3) The crown and sidewall area are the weak parts of primary support, thus measures should be taken and monitoring measurement should be strengthened. (4) According to numerical calculations that the axial force at the end of the bolt is small, the length of the bolt can be appropriately shortened in the design.

Key words: primary support, large crosssection loess tunnel, onsite monitoring, force characteristics, YinchuanXi′an Railway