Abstract: The Yujingshan tunnel of ChengduGuiyang highspeed railway passes through a giant karst cave. There is a largediameter unstable wedgeshaped body at the roof of the karst cave. To prevent the tunnel roof from falling off and collapsing under this working condition, a joint reinforcement of the roof with an expanded shell hollow grouting anchor rod and a low prestressed fully bonded anchor cable is proposed. Furthermore, threedimensional numerical models of the surrounding rock with no roof support, only bolt support, and combined support with bolts and anchor cables are established using the finite element software. Meanwhile, the reinforcement effect of the roof surrounding rock when the support is lagging is considered. A comparative analysis is conducted on the variation rules of the anchor rod, the axial force of the anchor cable, the displacement of the surrounding rock and the plastic zone. Finally, an effective roof protection measure is found. The results show the following: (1) The timely bolt and cable support can effectively control the displacement of unstable surrounding rock and the development of plastic zone, achieving good reinforcement effect. (2) The axial force of anchor rod can be significantly reduced by applying anchor cable structure in time, so as to avoid the failure of anchor rod due to overlarge axial force. (3) The application of expanded shell hollow grouting anchor and low prestressed fully bonded anchor cable structure can guarantee the safety of the Yujingshan tunnel.

Key words: giant karst cave, highspeed tunnel construction, anchor cable, rock bolt, finite element software, roof protection