Failure Mechanism of Unbonded Prestressed Thru-Anchor Cables: In Situ Investigation in Large Underground Caverns

Prestressed rock anchor cables are commonly utilized in geotechnical and mining engineering (Peliua et al. 2000; Tezuka and Seoka 2003; Koca et al. 2011) because their installation increases the effective strength and stability of the reinforced rock (Maejima et al. 2003; Li et al. 2012). Nevertheless, in some cases, prestressed cables have failed in large underground caverns or tunnels because of excessive deformation of the anchored rock mass or inadequate assumptions used when designing the cables (Li 2004; Lu et al. 2011; Gong et al. 2011). During the failure process of anchor cable, the stress redistribution and progressive deterioration of the reinforced mass rock can adversely affect the overall stability of the free surface, manifested as large deformation or indeed collapse of rock mass (Galvez et al. 2006; Zhu et al. 2010). Therefore, a deeper understanding of the mode and mechanism of prestressed anchor cable failure will better inform the design process of anchor cables to mitigate future failures. Prestressed anchor cables can be categorized into three general groups based on the anchoring method (Jarred and Haberfield 1997; Chen and Yang 2004): (1) tip-grouted anchor cables that have grout-bonding segments and free segments; (2) fully grouted anchor cables with anchor wires that are fully bonded with the rock; (3) prestressed thru-anchor cables that have two anchor bases without grout-bonding segments. The tip-grouted and fully grouted prestressed cables have been the subject of research more than the thru-anchor cables because they have more extensive applications (Spang and Egger 1990; Hyett et al. 1995; Serrano and Olalla 1999; Huang et al. 2002; Cai et al. 2004; Ugur et al. 2011). Even less attention has been paid to the unbonded prestressed thru-anchor cables (UPTACs) based on the limited information available in the academic and professional literature. The mechanical interactions of UPTACs are different from those of groutbonding cables because the UPTAC does not have a groutbonding section, but has two anchor bases. Under loading, the prestressed cable can restrain the deformations of the anchored rock, and the anchored rock can also transfer the rock stress to the cable via the bridge of anchor bases. This paper focuses on evaluating the failure mechanism of UPTACs based on a case study of underground caverns in Sichuan Province, China. Several external failure modes of the UPTACs observed in this project are first presented and special design techniques for the UPTACs in the large underground caverns are summarized based on in situ investigation: failure depth of disabled UPTACs, break face, measured working load and installation method.