Abstract:

Aiming at solving problems regarding the support to cope with the large and small deformation of surrounding 〖HJ4.5mm〗rock of soft rock tunnels with high ground stress, different applicable conditions are emphasized for two support options, namely support by ordinary prestressed anchor cables and support by largescale prestressed yielding anchor cables. Analysis is made on the risks of adopting ordinary prestressed anchor cables support for soft rock tunnels with certain large deformation, and specific countermeasures are proposed. With the trial section of Haidong water conveyance tunnel of the Central Yunnan water diversion project taken as an example, calculation and discussion are made on the threshold between the relatively "large deformation" the relatively "small deformation" of the surrounding rock. The importance of applying prestress to anchor bolts and cables is verified by means of principle analysis, theoretical calculation, and experimental research. It is considered in this paper that: (1) Ordinary prestressed anchor cables are suitable for tunnel sections with relatively "small deformation", while yielding anchor cables are suitable for tunnel sections with relatively "large deformation", and the two types of anchor cables apply to different occasions and cannot replace each other. (2) When ordinary prestressed anchor cables are used in a tunnel, the applied prestress cannot be too high, otherwise there will be unexpected risk of tendons breaking and bouncing out of the tunnel. It is suggested that tensioning and grouting cannot be started until the overall deformation of the surrounding rock becomes stable. (3) The preliminarilydetermined threshold between the relatively "large deformation" and the relatively "small deformation" in the trial section of Haidong water conveyance tunnel is 30 cm, which needs to be further verified and corrected during the field test. (4) Prestressed anchor bolts and cables can improve the lithological parameters of the surrounding rock in the anchoring area and increase its strength and stiffness. (5) It is expected that the preliminary study on the trial section of Haidong water conveyance tunnel with "small deformation" surrounding rock could attract the attention of experts in the industry to get more comments and further improved and refined suggestions, which will be reflected and implemented in the subsequent trial sections.

Key words: soft rock tunnel, surrounding rock support, large deformation, small deformation, prestressed anchor cable, yielding anchor cable, construction risks and countermeasures