Hard Rock Extraction
Basics Of Rock Blasting
Hard rock is extracted in an inverted "tier" method, these tiers or benches are cut by means of explosives.
Boreholes are drilled behind the quarry face at a calculated distance and interval, to release a given amount of burden.
The length of the borehole controls the height of the working bench, the inclination of the borehole from the vertical controls the repose of the bench face, at most quarries this inclination can vary from vertical to 8o, the holes are charged with explosives and detonators and then packed or stemmed with inert material, to control the force of the blast.
In this manner the amount of explosives used in the blast is controlled, and varying the amount of explosives used in each blast cannot be carried out without affecting other factors.
The Environmental Impact Of Hard Rock Blasting
Department of Environment guidelines in the form of Mineral Planning Guidance notes, have recommended a ground vibration limit for hard rock blasting of between 6.0 and 12.0 mm/s at the nearest residential property as being acceptable.
The ground vibration can be affected by certain blast design parameters:-
- The maximum instantaneous charge or MIC is the amount of explosives fired at the same moment in time.
- The number and frequency of delays. The introduction of a delay sequence can reduce the size of the maximum wave produced.
- The height of the working bench and therefore the length of borehole.
- The number of "decks" or layers of explosives and detonators in each hole.
- The spacing, burden and number of holes, in the blast ratio.
- The diameter of the shot hole, which will effect the amount of explosives used.
Vibration can be generated within the ground by any dynamic source of sufficient energy. It is composed of various waves, known collectively as seismic waves which spread radially from the vibration source, decaying rapidly as distance increases.
Four interrelated parameters are used to define ground vibration magnitude : Displacement - Velocity - Acceleration - Frequency.
Much investigation has been undertaken into the damage potential of blast induced ground vibration, resulting in an adopted method of monitoring. This allows for results to be obtained in terms of the Peak Particle Velocity (P.P.V.), which is measured in mm/s.
Research work has been undertaken by various Authorities around the world into vibration levels that are likely to induce damage in properties, both cosmetic and structural.
Values in excess of 50mm/s are necessary to produce appreciable structural damage.
The onset of cosmetic damage can be associated with lower levels and a limit of 12.7mm/s is therefore recommended.
The question is often asked
“What is the long term affect of vibration on my property ?”
The vibration levels from normal every day occurrences can be seen below.
As shown the level of vibration on a property from rock blasting are comparable with every day household events, the major psychological difference is in that rock blasting is an alien operation most people never experience and therefore are unfamiliar with. There are no proven incidents of damage to property from rock blasting in Leicestershire where quarrying has been carried out since the last century.
Human perception levels are difficult to precisely define since they will vary from one person to another.
The human body is very sensitive to vibration, which can result in concern being expressed at energy levels well below the threshold of damage.
A person will generally become aware of blast induced vibration at levels of around 1.5mm/s and under some circumstances this can be as low as 0.5mm/s, even though such vibration is routinely generated within any property and is entirely safe.
The British Standards Institution have produced a relevant document, BS 6472: 1992, which specifically mentions blasting vibration. A satisfactory magnitude of 8.5mm/s at a 90% confidence level with an absolute limit of 12.7mm/s is recommended for such impulsive vibration at residential property.
Leicestershire County Council have generally adopted a maximum vibration limit of 6.0mm/s, with a confidence level of 95%, as part of the conditions covering blasting within modern Planning Permissions. This lies at the lower end of the vibration range recommended, as part of a planning consent, by the Department of the Environment.
Air Over Pressure
Whenever an explosive is detonated transient airborne pressure waves are generated.
This can be controlled by the amount of stemming used in each borehole and by employing a good borehole drilling technique to control the amount of burden throughout the length of the borehole.
The one piece of rock can only be worked once, and within the same quarry the homogeneous qualities of the material can differ, either by natural impurities found within the structure of the rock or by the introduction of fracturing and shattering of the rock from adjacent blasts, these are known as overbreaks.
Air Over Pressure can also be affected by these overbreaks or cracks and fissures in the rock mass permitting the explosive gases generated to vent to atmosphere.
Although it is possible that air over pressures could cause structural damage those produced by routine blasting operations under normal atmospheric conditions are not likely to do so. Many hundreds of air Over Pressure measurements undertaken over a wide range of conditions indicate that relatively rarely do air over pressures exceed 125 dB. Within a range of values of + or - 15 dB some 7% of values are found to be in excess of this figure.
The weakest parts of a structure that will be exposed to air Over Pressure are its windows, and so these are the most likely to suffer damage. Poorly mounted panes might be forced out of their frames while improperly mounted panes that are pre-stressed will be cracked and broken more easily.
Air Over Pressure values of 150 dB could be enough to damage a badly mounted pane with most windows breaking at 170 dB. Structural damage can be expected at 180 dB. Although structural damage is unlikely, air Over Pressure does play a most important role in the annoyance aspect of blasting . Relatively low levels can be sufficient to cause the rattling of loose ornaments or windows and hence give the impression of a significant ground vibration shaking the property.
Impulsive vibrations as low as 0.5mm/s can cause complaints when accompanied by such secondary noise effects. This is because the average person forms a judgement based largely on his or her psychoacoustic responses, and is usually unaware of the important distinction between the characteristics of the motion alone and the sound effects that accompany it.
Leicestershire County Council have generally adopted a maximum Air Over Pressure limit of 120 dB peak linear, as part of the conditions covering blasting within modern Planning Permissions.
Vibrock Blast Vibration Document
United States Bureau of Mines
Page Last Updated: 11 March 2009