Abstract: The excavation damage zone (EDZ) in hard brittle country rock exhibits noticeable time effect after excavation, as rock strength varies with time caused by the high geostress, which leads to the development of surrounding rock excavation damage zone and shows time-dependent characteristics. The LSSVM-PSO intelligent inversion analysis method considering time effect was established based on the time effect evolution model of rock strength, and measured EDZ data collected in the test tunnel of Jinping Ⅱ Hydropower Station, such as borehole monitoring, ultrasonic test and deformation monitoring were used as target functions. The orthogonal design method, least square support vector machine (LSSVM) model and principle of particle swarm optimization algorithm were used to simulate the EDZ’s time effect evolution process between the time excavation ended and 25 days after in Jinping Ⅱ Hydropower Station. The study result shows: 1) Under high geostress, the principal geostress direction dominates the EDZ expanding, furthermore, the minimum principal stress direction is the main direction for EDZ extending and the failure zone (failure approach index, FAI≥2) also concentrates in this area. 2) The area of plastic zone after excavation develops to be “S” shaped curve. The area change of the plastic zone is relatively slow at beginning, then shows a linear increasing tendency and gradually becomes stabilized. 3) The plastic zone growth is rapidly increased from 3rd to 10th day after excavation, during which period rock burst is most likely to occur. The research results can provide guidance to the time effect characteristics of EDZ expanding in hard brittle surrounding rock mass under high geostress.

Key words: excavation damage zone, failure approach index, time effect, intelligent inversion