Abstract: The setting mode and construction time of the second layer steel arch support in double primary support are of great importance. As a result, the layout mode of inner and outer layers of steel arch and the construction time of the second layer of primary support for the double primary support of high geostress soft rock tunnel with large deformation are analyzed and studied by taking Yuntunbao Tunnel on Chengdu-Lanzhou Railway for example. The study results show that: (1) The staggered arrangement of the steel arch in the double primary support is more beneficial to the control of the primary support structure stress and surrounding rock deformation. (2) The second layer of primary support should be carried out before damage of the first layer of primary support; as for Yuntunbao Tunnel, the distance between the second layer of primary support and the tunnel face should be within 7 m.

Key words: high geostress soft rock tunnel, double primary support, layout of steel arch, construction time