Mechanism of excavation-induced cracking of the protective layer of a rock bench in a large underground powerhouse under high tectonic stress

Abstract

Excavation-induced unloading fractures in the rock mass can easily cause rock engineering structural failure under high stress. This research work investigated the unloading fracture characteristics of the protective layer of the rock bench in Shuangjiangkou underground powerhouse in detail, thereby revealing the unloading fracture mechanisms via numerical simulation and true triaxial unloading tests, and proposed an optimal method for protective layer excavation. The study found that unloading cracking failure in the rock mass is asymmetric in Shuangjiangkou underground powerhouse, with the fracturing degree of rock mass on the downstream protective layer being greater than that on the upstream side. The dip angle of the unloading fracture plane in the downstream protective layer increased with the aspect ratio (k) of the protective layer and gradually coincided with the diagonal of the protective layer. The stress concentration in the downstream protective layer was caused by the tectonic stress, the stress changed in the form of multistep loading σ1 and unloading σ3 during layered excavation. Tensile-shear unloading cracks were formed in the rock under the excavation stress path, becoming steeper and denser, and gradually parallel to σ1 as the unloading amount of σ3 increased. As k increases, the σ1 in the protective layer becomes steeper, causing the unloading fracture plane to steepen and eventually coincided with the diagonal of the protective layer. A formula for designing the protective layer width was proposed based on the relationship of the dip angle of the fracture plane with k. The findings lend themselves to implications to engineering geology in that engineers should consider the asymmetric failure characteristics of the tunnel caused by high tectonic stress, and formulate asymmetric excavation and support schemes during the construction process of rock benches or other similar structures.