Field observations and interpretation of extensional fracture in hard rock surrounding deep underground openings
Charlie C. Li*
Deep Resources Engineering. 2024, 1(1): 100006. 
doi.org10.1016j.deepre.2024.100006.pdf
Abstract: Extensional fracturing often occurs in hard rock masses during excavation at depths, for example, >1000 m below the ground surface. Surface-parallel fractures are created in the surrounding rock mass, which is typically subjected to stresses parallel to the free rock surfaces after excavation. These are called extensional fractures because the strains perpendicular to the fracture planes are extensional and the opposite surfaces of each fracture tend to separate from each other as soon as the fracture is created. These fractures predominantly propagate parallel to the maximum principal stress σ1 in the surrounding rock mass. This study analyses extensional fractures observed during excavations in cut-and-fill mining stopes in a deep metal mine. This analysis explores the process of extensional fracturing during excavation in an undisturbed rock mass. In general, intensive spalling occurred on the roof surfaces immediately after the excavation of the undisturbed rock mass. This spalling terminated after a certain depth of rock failure, while burst sounds intermediately emitted from the surrounding rock mass, indicating that rock fracturing was ongoing at depth. In the subsequent cutting slices, the spacing between the extensional fractures decreased with increasing mine-out space in the stope. An extensional fracturing criterion was proposed based on microscopic observations of microcrack development in the rock in response to applied stress. The crack initiation and extensional fracturing processes are associated with two critical extensional strains which are related to the secondary stress state in the position. In areas close to the free rock surface where σ3 = 0, the stress for crack initiation is (σ1+σ2) = 0.4σc, whereas the stress for extensional fracturing is (σ1+σ2) = 0.8σc.
Highlights:
• Field observations of spalling / slabbing fracture in the surrounding rock mass of cut-and fill stopes are presented.
• The primary failure mode of hard rock at depth is extensional parallel to the rock surfaces of the underground opening.
• A criterion for new initiation and extensional fracture is proposed.
Keywords: Extensional fracture; Rock fracture; Failure criterion; Rock burst; Hard rock
Cite: Li, C. C., Field observations and interpretation of extensional fracture in hard rock surrounding deep underground openings. Deep Resources Engineering 2024, 1 (1): 100006. https://doi.org/10.1016/j.deepre.2024.100006
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