Blasting in soft rocks: technical considerations and case studies in rock salt mines.


Jonathan Aubertin, Morton Salt - Weeks Island mine; D. Jean Hutchinson, Geological Sciences and Geological Engineering, Queen's University; Mark Diederichs, Geological Sciences and Geological Engineering, Queen's University

Soft rocks such as evaporites and aggregates present unique challenges around underground exploitation and blasting processes. Soft rock blasting requires special considerations around blast geometry, charge selection, and fragmentation control. Soft rocks often exhibit moderate stiffness and low inherent strength. These features effectively prevent the rock mass from building strain energy during a blast event, and prohibits the traditional violent brittle release and associated projection observed with hard rock blasting. To palliate, artificial relief and free surfaces are incorporated in soft rock blast designs as large slots created with undercutters or auger drills. Moreover, high VOD explosives with rapid pulses tend to lose much of their breakage capability near the borehole, and effectively choke development blasts. Non-ideal detonation by design through line initiation, smaller boreholes and decoupling, are examples of counterintuitive solutions for better breakage in soft rocks. The following presents conceptual reasoning behind soft rock responses to blasting from a series of technical studies performed at Morton Salt and K+S Windsor Salt. It is shown that a longer blast pulse reflected at a free surface plays a key role in burden removal through the generation of localized failures. Operational conditions from underground salt mines are referenced to showcase practices around blast patterns and explosives selection. Case studies for blasting optimization and fragmentation control are presented to demonstrate the applications of discussed concepts. A series of blast tests surveyed via LiDAR are reviewed to highlight rock salt unique response during blasting events.  Special considerations are highlighted for explosives selection to match excavation geometry and pattern design.
Keywords: Soft rock, drilling and blasting, underground mining, rock salt, case studies, best practices