Abstract: The unloading and relaxation of rock mass caused by excavation of dam foundation in water conservancy and hydropower project is an essential problem affecting engineering quality and safety.Identifying the temporal and spatial characteristics of unloading and relaxation of rock mass, and providing data basis for reinforcement engineering design are of substantial significance.In this paper, the unloading effect and development of rock mass caused by excavation of a high arch dam foundation are summarized in temporal and spatial senses based on long-term observations via acoustic logging during excavation.Results unveil that(1) unloading and relaxation of rock mass mainly occurs in the first eight months after excavation and basically ends after 10-12 months. (2) Unloading and relaxation develops rapidly at first and then slows down.(3) The more complete the structure of the original rock mass, the lower the unloading effect, and vise versa. (4) An optimal slope ratio leads to a smallest thickness of relaxation zone. (5) Unloading relaxation of rock mass develops gradually from the shallow to the deep. The degree of unloading relaxation of shallow rock mass is evidently higher than that of deep rock mass, and so does the duration of unloading. (6) Structural plane and weak interlayer are important parts of unloading relaxation which affect the stability of large rock blocks.

Key words: dam foundation excavation, unloading relaxation, acoustic logging, temporal and spatial effects, development characteristics