The Polkemmet reclamation scheme is situated on very old land dating back to the Dinantian era (about 350 million years old) and beyond and much of this cannot be excavated in the normal fashion. This means it is often necessary to blast the rock with explosives to fracture or fragment the rock mass to enable processing. The process of blasting can give rise to vibrations, noise, dust and fumes, which can all be controlled by good design and operation. It is important to understand that not all the energy produced when the explosive is detonated goes into breaking the rock. Some of it is "lost" in the form of heat, sound (causing noise), displacement (when this is excessive it causes fly rock) and ground shaking seismic waves. Seismic waves are vibration which travel through the ground and which can be divided into to two types, those which travel through the rock (body waves) and those which travel along a boundary such as the earth's surface (surface waves). The distance travelled will depend on the size of the source of vibration (e.g. blast) and the nature of the rock through which the waves travel. Seismic waves will spread radically from the vibration source decaying rapidly as distance increases.

Here comes the science bit!
There are four interrelated parameters that may be used in order to define ground vibration magnitude at any location. These are:

• Displacement: the distance that a particle moves before returning to its original position, measured in millimetres (mm);
• Velocity: the rate at which particle displacement changes, measured in millimetres per second (mms-1);
• Acceleration: the rate at which the particle velocity changes, measured in millimetres per second squared (mms-2) or in terms of the acceleration due to the earth's gravity (g);
• Frequency: the number of oscillations per second that a particle undergoes measured in Hertz (Hz).

After years of detailed research around the world, a complex arithmetical formula has been agreed using the above and other parameters such as pi and amplitude. The formula gives a value called PV (particle velocity). It is the maximum value of particle velocity in a vibration event called the peak particle velocity, that is of most significance and this will usually be measured in three independent, mutually perpendicular directions at any one location in order to ensure that the true peak value is captured. These directions are longitudinal (or radial), vertical and transverse.
The maximum of any one plane measurement is the accepted standard world-wide and is recommended by the British Standards Institution and the International Standards Institute, amongst others, as the basis for all the recognised investigations into satisfactory vibration levels with respect to damage of structures and human perception.
When defining damage to residential type structures the following classifications are used:

• Cosmetic or threshold: the formation of hairline cracks or the growth of existing cracks in plaster, drywall surfaces or mortar joints.
• Minor: the formation of large cracks or loosening and falling of plaster on drywall surfaces, or cracks through bricks / concrete blocks.
• Major or structural: damage to structural elements of a building.

Comprehensive research programme undertaken by various bodies around the world during the 70s & 80s determined that vibration values well in excess of 50 mms-1 (the PV) are necessary to produce structural damage to residential type structures. The onset of cosmetic damage can be associated with lower vibration levels, especially at very low vibration frequencies, and a limit of 12.7 mms-1 is therefore recommended for such relatively unusual vibration. For the type of vibration associated with the type of blasting carried out at the Polkemmet reclamation scheme, the safe vibration levels are seen to be from 19 - 50 mms-1.

Domestic properties will exhibit cracks that may be wrongly attributed to blasting activities. There are many additional reasons why properties will develop cracks, for example:

• Fatigue and ageing of wall coverings;
• Drying out of plaster finishes;
• Shrinkage and swelling of wood;
• Chemical changes in mortar, bricks, plaster and stucco;
• Structural overloading;
• Differential foundation settlement - particularly after times of prolonged dry spells.

It has been found that daily changes in humidity and temperature can readily induce strain of the order that is equivalent to blast-induced vibration of from 30-75 mms-1. Typical domestic activities will produce strain levels corresponding to vibration of up to 20 mms-1 and greater (boy racer with his loud exhaust and massive sound system racing along the street!!!)

The fact that we are very sensitive to vibration can result in subjective concern being expressed at energy levels well below the threshold of damage. We will generally become aware of blast-induced vibration at levels of around 1.5 mms-1, although under some circumstances this can be as low as 0.5 mms-1. Even though such vibration is routinely generated within any property and is also entirely safe, when it is induced by blasting activities it is not unusual for such a level to give rise to subjective concern. Such concern is also frequently the result of the recent discovery of cracked plaster or brickwork that in fact has either been present for some time or has occurred due to natural processes as outlined above. Virtually all complaints regarding blasting arise because of the concern over the possibility of damage to owner-occupied properties. Such complaints are largely independent of the vibration level. In fact, once an individual's perception threshold is attained, complaints can result from 3% to 4% of the total number of blasts, irrespective of their magnitude i.e. we tend to exaggerate (it's in our nature).
An explanation of the necessity to use explosives and the likely effects as perceived by a site's neighbours can allay the concern of a significant proportion of those inhabitants of neighbouring property. The Polkemmet reclamation scheme operator has considered the perception threshold level prior to the design of each and every blast.

Prior to operations starting, like the other environmental issues such as dust, noise and odour, receptors in and out with the site boundary were recognised to be potentially sensitive to the effects of blasting and vibration with most of these properties located at a distance of more than 215m from any working face. These receptors are the same ones as those identified as dust receptors etc but with some additions. As with any operation, standards have to be applied and by using the recommended blast charge weight of 40kg, at this distance it was found that vibration levels are lower than the recommended criterion contained in British Standards. However, 5 of the locations are potentially located inside this distance i.e. are located within 215 m of a proposed excavation area but this only occurs for short periods of site operations. When this occurs, blasting techniques change to ensure that the vibration criterion is not exceeded. For example, at distances between 215 and 150 m, the use of a 19 kg charge would satisfy the suggested criterion. This could be achieved by decking the explosives in the borehole such that the normal charge was, generally, split in two. So problems arising from blasting can be mitigated by changing the blast pattern, using smaller diameter boreholes or splitting the charges or a combination of each, this is especially the case should basting be required at a distance of closer than 150m.

Before the Polkemmet reclamation scheme started, Vibrock Limited, an independent firm of environmental consultants, specialising in blasting and vibration assessment, carried out a study and the mining contractor at Polkemmet reclamation scheme has designed blasting operations in accordance with Vibrock's recommendations. A programme of blast monitoring has been established and the results of such monitoring indicate compliance with the vibration criterion and the data used provides valuable input to future blast design.

It is also in the contractor's best interest to always reduce vibration, both within the ground and airborne, to the minimum possible as it increases the efficiency and economy of blasting operations.