INTRODUCTION

In today's major surface mining operations, the firing of large multi-row blasts, commonly fragmenting >100,000m3 of rock, generates forces which, unless carefully controlled, produce considerable fracturing beyond the design blast perimeter. Understanding and reducing this fracturing is of great importance. Minimal fracturing at and beyond final or semi-permanent faces is a prerequisite of

  1. minimal slope degradation and instability and, hence,

  2. minimal hazards and optimum productivity.

On the other hand, good fragmentation and looseness of the blasted rock is necessary as close as possible to the final face, so as to minimize

  1. the use of specialised equipment and, thus

  2. operating costs.

Because the design and effects of blasting can be controlled within wide limits, mine operators have the opportunity to promote pitwall stability by minimising last-induced damage. Before this opportunity becomes ability, however, it is first necessary to determine

  1. the principal mechanisms, intensity and extent of overbreak, and

  2. how each of the several blast parameters affects overbreak.

Hamersley Iron Pty. Limited operates two large iron ore open pits at Torn Price and Paraburdoo in the North-west of Western Australia. At Torn Price, conventional mining methods are used in that both ore and waste are drilled and blasted in 15m high benches and then loaded by shovels. multiple rows of vertical 381mm diameter blastholes are fired, an average blast fragmenting about 300,000t of rock. At r Present, about 40 × 106t are blasted annually.

The Torn Price orebody is a bedded deposit consisting of several bands of high-grade hematite separated by thin (0.09–3.5m) shale bands. The shale underlying the orebody (Mt. McRae Shale) has an average thickness of 60m but, in places, is thinned due to folding. En echelon folding on two major axes, faulting and erosion have resulted in a complex surface representation. However, the orebody is basically formed of synclinal structures, and most of the pit slopes are, therefore, formed in the footwall shale. This shale is weak, non-brittle and heavily-jointed, and exhibits well-defined and variably-spaced bedding plane partings. Compressive strengths vary from 3 to 35MPa, the mean being 12MPa. Values of Poissons Ratio are high (approximately 0.40), and specific gravity is in the range 2.2–2.6.

OVERBREAK MECHANISMS

There are at least five mechanisms by which significant amounts of blast-induced overbreak can occur, viz.

  1. radial fracturing,

  2. internal spalling,

  3. fracturing along boundaries of modulus contrast,

  4. gas extension of discontinuities and strain wave-induced cracks, and

  5. release-of-load fracturing (Hagan, 1977).

The above mechanisms are listed approximately in their chronological order of occurrence but by no means in their order of decreasing importance. In most weak rocks, mechanisms 3,4 and 5 (see listed above) have the greatest ability to create excessive overbreak.

(Figure in full paper)

Radial fracturing

When the front of an explosion-generated strain wave passes, a cylindrical shell of rock immediately around each blasthole is subjected to intense radial compression, and tangential tensile strains then develop. If these tangential strains exceed the dynamic tensile breaking strain of the rock,

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