Effects of delay time on crack coalescence between two boreholes

Abstract

Delayed detonation widely occurs in rock blasting, which has a significant influence on the propagation and interaction of explosion-induced stress waves. This affects the rock fracture interconnection between adjacent boreholes in controlled blasting. In this study, the effects of delay time on crack coalescence between two blastholes in contour blasting are experimentally, theoretically and numerically investigated. The rock cracking in two-hole blasting on 200 × 200 × 50 mm granite plate is first experimentally examined, in which the delay time is determined by high-speed photography and the rock fracturing is identified by digital image correlation. Then, two-hole blasting with delayed initiation is analytically modelled, and the fracture propagation behaviours in blast tests are explained based on the evolution of explosion-generated stress waves and peak stress distribution between two holes. Subsequently, 3D finite element models are built in LS-DYNA to simulate the evolution of explosion-produced pressure, rock fracturing and energy dissipation in two-hole controlled blasting with delayed initiation. The results indicate that in two-hole contour blasting, the stress superposition zone, which produces local stress enhancement in the vicinity of the stress wave collision point, moves closer to the later detonated hole. The superposed stress facilitates the preferential extension of blast-created cracks along the connection line of boreholes, and the crack from the later detonated hole extends farther than that from the first detonation hole. The optimal crack coalescence in contour blasting archives when the delay time allows stress waves to approximately propagate through the hole spacing.