Rock Fragmentation by Blasting

B. Mohanty

• Publisher: CRC Press
• Year: 2020
• Print ISBN: 9789054108245
• eBook ISBN: 9781000150766
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

This collection of symposium papers covers a wide range of topics on rock fragmentation, from carefully documented case studies to attempts, for example, at fractal representation of the fracture process itself.

Modelling of blasting process

‘Blastability’ and blast design

Andrew Scott

Julius Kruttschnitt Mineral Research Centre, The University of Queensland, Qld, Australia

ABSTRACT: Rock mass properties largely control blasting performance. However, despite many years of fundamental work aimed at describing the role of rock mass properties in blasting, these properties are not routinely included in blast design procedures. This paper reviews the conventional methods used to account for rock mass properties in the design of blasts and discusses the concept of ‘blastability’. In the absence of suitable methods for the incorporation of rock mass properties in blast design, an approach is suggested where blasting engineers can develop a ‘local blastability scheme’ in order to derive basic designs for different rock mass conditions. These basic designs can then be optimised through operating experience assisted by monitoring, analysis and modelling.

1. INTRODUCTION

There are four critical aspects to a rock blasting system: the rock mass, the explosive, its distribution and the detailed detonation timing. The latter two aspects have received quite rigorous analysis in recent years with the development of a number of useful computer based blast design editors and models. Explosive performance can be described by complex detonic codes but these have largely remained the domain of a few experts and have yet to be linked to blasting performance in a manner suited to routine blast design. Rock mass properties have a controlling influence on blast performance but remain too complex to be explicitly incorporated into current blast design procedures.

Geotechnical engineers have approached this problem from a different perspective. The design of most rock excavations is guided by rock mass classification schemes. These schemes rate the suitability of a rock mass for a particular duty based on simple measures of rock mass properties. The properties used are those that can be readily measured and that affect the critical engineering requirements for that duty. Each property is weighted in the overall analysis to reflect its influence. A similar approach is being used by the JKMRC to describe the behaviour of a rock mass when blasted. The challenge is to define a ‘blastability’ index which incorporates the most relevant parameters according to their influence over blasting performance. The resulting description of ‘blastability’ can then be used to derive basic blast designs for new blasting situations.

2. BLASTING MECHANISMS

The properties of the rock mass are of fundamental importance to the design of blasts. The use of standard designs without regard to any variation in rock mass properties will lead to either over blasting in some areas and under blasting in others or, more typically, consistent over-blasting. Fines generation and damage to adjacent structural rock are also common results. The real cost of these departures from targeted performance can be several times the cost of the initial drilling and blasting operation (Scott 1992). If the desired blasting results are to be achieved over a range of rock mass conditions, it is necessary to incorporate the most relevant rock mass properties into the blast design process.

Blasting performance is determined by the interaction of the detonation products of an explosive and the confining rock mass. Rock mass properties dominate this process. The blasting engineer is therefore faced with the challenge of determining which rock mass properties most influence blasting performance in each situation and of deciding how designs should be changed to suit different geological conditions.

Sarma (1994) provides a useful description of the blasting process as a background to modelling explosive-rock interaction. According to Sarma’s model, when an explosive detonates, the ingredients of the explosive are rapidly converted into gaseous products at very high temperature and pressure. The high pressure gases impact the blast hole wall and transmit a shock wave into the surrounding rock as shown on Figure 1. The stresses resulting from the shock wave compress and crush the rock in the vicinity of the blast hole in response to the blast hole pressure and the strength and stiffness of the rock. As the rock is compressed and crushed, the vol...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Organization
• Sponsors
• Fracture dynamics
• Modelling of blasting process
• Blast induced fractures and rock damage
• Fracture control blasts
• Fragmentation assessment and image analysis
• Explosive energy and fragmentation
• Specialized blast design
• Blasting and productivity
• Posters
• Author index