Study on rock fracturing in smooth blasting under initial stress

Abstract

In practical smooth blasting, rock fracturing is influenced by in-situ stress, which frequently leads to underbreak/overbreak and excavation damaged zone and consequently poses a serious threat to the safety of the excavated structure. This study systematically investigates the behaviour and mechanism of rock cracking under coupled initial stress and smooth blast utilizing experimental, theoretical, and numerical methods. Model experiments involving three-hole smooth blasting on 200 × 200 × 50 mm granite specimens are conducted under various static stresses. The resulting fracturing patterns are identified using the digital image correlation technique. The model testing examines rock cracking behaviours, fractal characteristics of rock fractures, and underbreak/overbreak. Experimental results indicate that the extents of rock fracturing and the fractal dimension in both the excavation and reserved zones show an overall decreasing trend as the static stress increases. Meanwhile, the area and depth of underbreak/overbreak also present a decreasing trend, suggesting that static stress is beneficial to the performance of smooth blasting. Then, the variations of crack patterns in model tests are theoretically analysed based on the transmission and superimposition of detonation stress waves combined with the static stress distribution near perimeter holes. Theoretical results emphasize the crucial role of the second hole and indicate no evidence of the third hole influencing the generation of connection fractures. The initial stress induces hoop tensile stress concentration on borehole-wall and generates the smallest hoop compression stress distributed vertically to the excavation line, which controls the initiation and extension of fractures in smooth blasting along the connection line of boreholes. Subsequently, the fracture evolution under coupled initial stress and smooth blast is numerically modelled using LS-DYNA. The numerical results vividly illustrate the necessity of a small burden to efficiently fragment the rock within the excavation zone when smooth blasting is applied to rock excavation under high in-situ stress.