One of safety problems caused by blasts is the dynamic tensile fracture in underground mines.Theoretically,a tensile stress wave from a free surface is the major reason for such dynamic fracture.To confirm this,the paper presents a single shot and production blast tests in LKAB Malmberget mine.The result from the single shot indicates that when a free surface is 8.9 m far from the nearest charge position,the surface is still seriously destroyed,even though it is a decoupled charge.In full charged blastholes,as the free surface is 20 m away from the nearest charge,the spalling is also marked.However,as the delay time was increased from 10 to 30 ms,spalling in the roof disappears,indicating the rock fracture in the roof is controllable.The field investigation indicated that (1) rock fall from the roof of a drift after blasting happened not only close to a blasthole but also far from it; (2) eyebrow break caused by tensile fracture is a common phenomenon in sublevel caving.Finally,the paper introduces some measures to reduce the tensile rock fracture caused by blasts in underground mining.
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