Abstract：Six caverns are designed in the world¢s longest twin-tube road tunnel，the Qinling Zhongnanshan tunnel，for the special lighting and driving safety purpose. The tunnel is excavated through the Qinling Mountain Range of Shaanxi Province，China，where the maximum rock overburden is about 1 800 m. In-situ rock stress was measured by overcoring method in two boreholes close to the cavern sites，where overburden is 400 and 1 600 m，respectively. The measurement result indicates very high rock geostresses. To meet the lighting requirements，the spindle-shaped caverns are designed to have a length of 200 m and a maximum span of 22 m. However，restricted by the distance between the two existing tunnels，the minimum width of the pillar between the caverns is left to be only 8 m. The main features of the caverns include：(1) high in-situ stress；(2) generally good rock；and (3) small pillar in comparison to the cavern size，which imposes great challenges to the rock support design. The support design is based on empirical approach from rock mass classification Q-system；and it is then verified by the numerical analysis. The supporting system，including both temporary support and permanent support，consists of bolting and shotcreting. In order to ensure the cavern stability during construction and in operation，a complete construction procedure，including sequence of excavation，bolting and shotcreting，is specially defined. Monitoring measures are also proposed. For the caverns located in the extremely high stress region，a flexible support system is designed，allowing the rock mass to partially deform before the permanent support is installed，thus ensuring cavern stability and the well-functioning of the rock bolts and shotcrete as permanent support. Commercial codes FLAC3D and Phase 2 are used in 3D and 2D analyses，respectively. The purpose of the three-dimensional analysis is to demonstrate the 3D effect and variations of stress and deformation along the tunnel axis；while the two-dimensional analysis is to study the functioning of support elements in each excavation stage. Numerical analysis verified the support design with controlled deformation and well functioning of the permanent support elements of both rock bolt and shotcrete.
