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.

1 Introduction

Blasting plays an extremely important role in mining and rock engineering. On the other hand, blasting causes some negative effects on mining safety and environment. For example, blasting usually induces rock break or fracture in and near the surface of a tunnel or a drift in underground mines. In sublevel caving mining, such rock break usually appears in the roof and wall of a production drift or other rock structures (Zhang, 2005; 2011). Blasting can cause rock fall from the roof of a drift or tunnel, even though the place of rock fall may be far from the blastholes. Blasting may also initiate a seismic event (Eremenko et al., 2009). All the above phenomena are dynamic rock fracture problems caused by blasts. On this background we will first analyze dynamic fracture near a free surface, often called spalling. Then we will show different types of dynamic rock fracture in sublevel caving and discuss the measures to handle the problems.

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