Study on the mechanism of displacement mutation for jointed rock slopes during blasting excavation

During blasting excavation of jointed rock slopes, the rock masses of the slopes often suffer an abrupt change in displacement, which is called displacement mutation. This kind of displacement mutation seriously threatens the stability of jointed rock slopes. Taking the left-bank spandrel groove slope in the Baihetan hydropower station as an example, the displacement behaviour of the jointed rock slope under excavation was surveyed based on the field monitoring. It is found that the displacement mutation of the jointed rock slope is closely related to the dynamic disturbances of blasting excavation. Then the numerical simulations based on the FLAC3D program were conducted to study the mechanism and influencing factors of the displacement mutation caused by blasting excavation. The results show that the displacement mutation is attributed to the combined actions of blast loading and transient in-situ stress release occurring on blast-created free surfaces if high in-situ stress exists. Under the compression of blasting pressure and in-situ stress, strain energy is accumulated and stored in the rock masses of slopes. As the blasting pressure decays, the accumulated rock strain energy due to blast loading is rapidly released. Meanwhile, the stored rock strain energy under in-situ stress is also quickly released as blasts create a free surface and the in-situ stress initially exerted on this face is suddenly released over a very short period. It is the rapid release of the rock strain energy that causes joint opening and displacement mutations in jointed rock slopes. Magnitude of the joint opening displacement is positively related to the strain energy release rate. The blasting load at higher peak pressure, longer rise time and shorter fall time and the transient in-situ stress release with a higher initial stress and a shorter period correspond to greater strain energy release rates, and thus will result in larger joint opening displacement. •Displacement mutations are observed during blasting excavation of a jointed rock slope.•Rock strain energy is accumulated and stored under blasting pressure and in-situ stress.•Rapid release of accumulated and stored rock strain energy causes displacement mutation.•Magnitude of displacement mutation is positively related to strain energy release rate.•Mutation displacement contains joint opening displacement and strain displacement.