ABSTRACT

Six caverns have been constructed in the world''s longest double tube road tunnel, the Qinling Zhongnanshan tunnel, for 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 1800 m. In-situ rock stress was measured by overcoring in two boreholes close to the cavern sites, where overburden is 400 and 1600 m, respectively. The measurement result indicates very high rock stresses. The caverns 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 only 8 m. Typical Norwegian concept of tunnel supporting was used in the rock support design, which means to use shotcrete and rock bolts as the permanent support. The design was based on empirical approach from rock mass classification and then verified by the numerical analyses. The paper presents the program and outcome of the in-situ stress measurement and the concept and details of the rock support design with special consideration of the high in-situ rock stress.

1. INTRODUCTION

Located in the Shaanxi province, China, the Qinling Zhongnanshan tunnel consists of four tunnels: two railway tunnels and two road tunnels. With a length of 18.02 km the road tunnel, which was opened to traffic in January 2007, is the longest double tube road tunnel in the world. The gross cross section of the road tunnels is 12.8 m x10.5 m, accommodating three driving lanes. For the purpose of special lighting and driving safety six caverns have been constructed. The idea was when a person drives in such a long tunnel he might feel monotony and anxiety and lose concentration, consequently increasing the potential of traffic accidents. Special and bright lighting was designed in the caverns such that the driver would wake-up and have a perception of being outside the tunnel. In this way, the driving safety would be improved.

The tunnels penetrate the Qinling Mountain Range, where the maximum rock overburden along the tunnel route is about 1800 m. According to the experiences gained from the Lærdal tunnel, which is the longest road tunnel in the world and has the similar overburden, it was strongly anticipated that the high in-situ rock stress would have significant impact on the cavern stability. Then a field stress measurement program by overcoring technique was carried out by SINTEF in two boreholes close to the cavern sites. The overburden for the boreholes is 400 m and 1600 m, respectively. The measurement at the low overburden hole was successful, whilst great difficulties were encountered in the high overburden hole. Severe core disking was observed. Finally, the 3D overcoring was replaced by doorstopper (2D overcoring). The measurement result indicates the major principal stress may be as high as 45 MPa.

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