Blast vibration has long been associated with quarries, but separating fact from fiction has often been a problem. Blasting is necessary to break rock from the ground, prior to it being taken for processing. In the early days of aggregate quarrying, explosives were not used particularly scientifically although some attempts were made to optimise use to minimise cost and maximise output. In most cases, high explosives would be detonated instantaneously — either packed into near vertical drill holes or occasionally into horizontal tunnels running into the face from quarry floor level. The latter could take six months to excavate and the blasting events were such big occasions that the local populace would turn out to watch. They were also the subject of reports in the national trade press. These methods would generate both considerable ground vibration and air blast ('overpressure') often perceived over a wide area. Indeed the residents on the higher ground on the outskirts of Bath, often reported hearing blasts from Emborough and Vobster, seven or eight miles away!
For safety, environmental and economic reasons blasting techniques have changed out of all recognition over the last 30 years. Instead of instantaneous detonation, explosives in each drill hole are initiated in a predetermined sequence so that there are delays of a few milliseconds between each activation. The shock waves generated not only direct much more energy into breaking up the stone effectively (and less into sound and air vibration), they create much lower levels of ground vibration in the surrounding area. The explosives used are now far safer to handle. Blocks which were initially too large to go through the crusher used to be broken up by secondary blasting which was particularly noisy and potentially very dangerous. This is now done by 'drop balling' or using jib-mounted breakers.
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