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Drilling and blasting is the controlled use ofexplosives and other methods, such as gas pressure blasting pyrotechnics, to breakrock forexcavation. It is practiced most often inmining,quarrying andcivil engineering such asdam, tunnel or road construction. The result of rock blasting is often known as arock cut.
Drilling and blasting currently utilizes many different varieties ofexplosives with different compositions and performance properties. Highervelocity explosives are used for relatively hard rock in order to shatter and break the rock, while low velocity explosives are used in soft rocks to generate more gas pressure and a greater heaving effect. For instance, an early 20th-century blasting manual compared the effects of black powder to that of a wedge, and dynamite to that of a hammer.[1] The most commonly used explosives in mining today areANFO based blends due to lower cost thandynamite.
Before the advent oftunnel boring machines (TBMs), drilling and blasting was the only economical way of excavating longtunnels through hard rock, where digging is not possible. Even today, the method is still used in the construction of tunnels, such as in the construction of theLötschberg Base Tunnel. The decision whether to construct a tunnel using a TBM or using a drill and blast method includes a number of factors. Tunnel length is a key issue that needs to be addressed because large TBMs for a rock tunnel have a high capital cost, but because they are usually quicker than a drill and blast tunnel the price per metre of tunnel is lower.[2] This means that shorter tunnels tend to be less economical to construct with a TBM and are therefore usually constructed by drill and blast. Managing ground conditions can also have a significant effect on the choice with different methods suited to different hazards in the ground.
The use of explosives in mining goes back to the year 1627,[3] whengunpowder was first used in place of mechanical tools in theHungarian (nowSlovak) town ofBanská Štiavnica. The innovation spread quickly throughout Europe and the Americas.
The standard method for blasting rocks was to drill a hole to a considerable depth and deposit a charge of gunpowder at the further end of the hole and then fill the remainder of the hole with clay or some other soft mineral substance, well rammed, to make it as tight as possible. A wire laid in the hole during this process was then removed and replaced with a train of gunpowder. This train was ignited by aslow match, often consisting simply of brown paper smeared with grease, intended to burn long enough to allow the person who fires it enough time to reach a place of safety.[4]
The uncertainty of this method led to many accidents and various measures were introduced to improve safety for those involved. One was replacing the iron wire, by which the passage for the gunpowder is formed, with one of copper, to eliminatesparking that could ignite the powder prematurely. Another was the use of asafety fuse. This consisted of small train of gunpowder inserted in a water-proof cord, which burns at a steady and uniform rate. This in turn was later replaced by a long piece of wire that was used to deliver an electric charge to ignite the explosive. The first to use this method for underwater blasting wasCharles Pasley who employed it in 1839 to break up the wreck of the British warshipHMSRoyal George which had become a shipping hazard atSpithead.[4]
An early major use of blasting to remove rock occurred in 1843 when the British civil engineerWilliam Cubitt used 18,000 lbs of gunpowder to remove a 400-foot-high chalk cliff nearDover as part of the construction of theSouth Eastern Railway. About 400,000 cubic yards of chalk was displaced in an exercise that it was estimated saved the company six months time and £7,000 in expense.[4]
While drilling and blasting saw limited use in pre-industrial times usinggunpowder (such as with theBlue Ridge Tunnel in the United States, built in the 1850s), it was not until more powerful (and safer)explosives, such asdynamite (patented 1867), as well as powereddrills were developed, that its potential was fully realised.
Drilling and blasting was successfully used to construct tunnels throughout the world, notably theFréjus Rail Tunnel, theGotthard Rail Tunnel, theSimplon Tunnel, theJungfraubahn and even the longest road tunnel in the world,Lærdalstunnelen, are constructed using this method.
In 1990, 2.1 billion kg of commercial explosives were consumed in the United States (12 m3 per capita), representing an estimated expenditure of 3.5 to 4 billion 1993 dollars on blasting. In this year the Soviet Union was the leader in total volume with 2.7 billion kg of explosives consumed (13 m3 per capita), and Australia had the highest per capita explosives consumption that year with 45 m3 per capita.[5]
As the name suggests, drilling and blasting works as follows:
The positions and depths of the holes (and the amount of explosive each hole receives) are determined by a carefully constructed pattern, which, together with the correct timing of the individual explosions, will guarantee that the tunnel will have an approximately circular cross-section.
During operation,blasting mats may be used to contain the blast, suppress dust and noise, for fly rock prevention and sometimes to direct the blast.[7][8]
As a tunnel or excavation progresses the roof and side walls need to be supported to stop the rock falling into the excavation. The philosophy and methods for rock support vary widely but typical rock support systems can include:
Typically a rock support system would include a number of these support methods, each intended to undertake a specific role in the rock support such as the combination of rock bolting and shotcrete.
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