Abstract: To avoid the damage of surrounding rock, pre-stressed anchor cables are installed in high side-wall and cavern intersection area to control the large deformation of surrounding rock. However, research is seldom performed on estimating the pre-load of cables in underground powerhouse with high geo-stress and low strength-stress ratio in the previous literature. Jinping-I hydro-power station is recognised as a typical high geo-stress underground powerhouse. Its cavern exhibits remarkable time-dependent deformation during excavation, which may result in the continuous increase of the inner force of the anchor cable and bars, even the strength failure. To determine an appropriate pre-load, a procedure is proposed for calculating the time-dependent released load of surrounding rock, based on their deformation characteristics. At the same time, a formula is deduced to calculate the pre-load factor of pre-stressed anchor cable. This established formula considers the influences of geo-stress, anchorage force and installation time on the pre-load factor. Comparing the actual values with the theoretical values of pre-load factor of Jinping-I underground powerhouse, the result indicates that the established calculation formula for the pre-load factor is reasonable and can be referenced in practical underground engineering.

Key words: pre-load factor, high geo-stress, underground powerhouse, anchor cable, Jinping-I hydro-power station