Blasting
Blasting and Energy
The basis of good blasting design is to achieve the desired degree of fragmentation in the rock, safely and economically1 2. The need for blasting varies significantly between the types of mineral being worked. For sand and gravel workings it is rarely needed. In a hard rock quarry it is necessary not only to loosen the rock but to fragment and move it away from the face of the quarry to produce a muck pile.
A schematic diagram of a multi-row blast with vertical holes is given in Figure 2, illustrating the major features which will be referred to throughout this section. The decking shown in the boreholes is the separation of the explosive column into two sections, which is often not required.
Figure 2. View of multi-row quarry blast with vertical holes.
A description of the mechanics of rock blasting, and the ground borne waveforms it produces, is given in the Themed Review L0089 (pp11-15).
In general terms, the levels of vibration will get higher as the amount of explosive increases, and will decrease as the distance between the blast and the monitoring location increases. The amount of explosive is taken as the largest amount which is detonated at one particular instant in time, known as the Maximum Instantaneous Charge (MIC) measured in kilograms. Most quarry blasts are multi-hole blasts with small delays between each hole being detonated to improve fragmentation and reduce vibrations. The "8 millisecond rule" has been applied for many years, on the assumption that adjacent holes which are detonated less than 8ms apart could be considered as going off simultaneously, thus increasing the actual MIC and therefore raising vibration levels. However, this is no longer thought to be a major factor441, and the increasing utilisation of highly accurate electronic detonators (assessed in two ALSF projects L0032 L0036) means that delays of less than 8ms are now regularly used where appropriate427.
For many years the effects of charge weight and distance have been combined to produce a parameter known as Scaled Distance. For ground vibration the square root of the MIC is used, but for air overpressure the cube root is used. So, for ground vibration, Scaled Distance = distance /√MIC (or distance x MIC-0.5 ), where distance = distance in metres from blast to monitoring location, and MIC = maximum amount of explosive in kg detonated at any given moment.
The levels of vibration will also depend on the blast design, the nature of the rock between blast and receptor, and the ground at the monitoring location. Therefore, the relationship between vibration and Scaled Distance uses "site factors" to take account of these in the following way:
Peak Vibration = a.(SD)b,
where SD = Scaled Distance, and a and b are site factors which are determined by
monitoring. The units of vibration and methods of calculation will be outlined in later sections.
Continued with Units of Measurement TOP Return to Introduction