Quarrying of stones is an art of extracting stones from the rock beds. The place from which the stones are obtained (by digging or blasting) is known as ‘Quarry’.
Quarrying differs from mining in which various operations are carried out for exploring minerals, such as coal, quartzite, etc. from a mine under the ground.
A good location of a quarry should fulfil the following requirements:
a. A large quantity of good quality stone must be available about the earth’s surface.
b. It should be located near roads and railway lines.
c. Ample space for the installation of crushers, storage of stones and other materials should be available.
d. In case the quarrying is to be done by ‘blasting’; the site of quarry should be away from any permanent structure.
e. There should be proper provision for drainage of rainwater.
Quarrying of Stones-Terminology
Important terms used in quarrying of stones are as follows:
It is the process of quarrying stones (of compact nature) with explosives. The operations in blasting are boring, charging, tamping and firing.
It is a device consisting of a length of slow and even burning composition, which provides adequate time to the person firing a shot or blast to reach a safe place prior to the occurrence of the explosion.
Bickford’s fuse is known as the best safety fuse, comprises an inner core of a fine thread of gun powder entrapped in a cotton rope and burns at the rate of about 60 cm per minute.
It is a device whose explosion initiates that of another. The detonators are fired either by a fuse or electric spark. It is used when usually dynamite is used as explosive.
An explosive is a mechanical mixture or chemical compound of combustible and oxidizing agents which release the energy when fired, almost instantaneously. The blasting powder (like gun powder) and dynamite are commonly used as explosives.
It is an operation of filling the hole containing explosive charge with stiff sandy clay (or other non-inflammable material) with the purpose of enhancing the explosion force.
The degree of fineness or coarseness of texture of a stone or rock is termed as grain.
It is the process of passing the crushed rock material through one or more screens to separate it into a series of products of varying sizes.
It is the moisture (natural) found in the freshly quarried stone.
Line of least resistance:
All the rocks contain lines of bedding fissures or crevices along which these can be split easily. These lines are known as “Lines of least resistance “.
Methods of Quarrying
The different methods of quarrying are:
a. By digging
b. By heating
c. By wedging
d. By blasting
a. Digging method
Digging or excavation of stones is carried out with the help of tools such as crowbars and pickaxes. Only those stones which occur in the form of detached nodules buried in earth can be recovered by this method.
b. Heating method
The use of this method is restored to only in case of those stones which are required in small pieces to be employed for road metal, railway ballast and aggregates.
In this method, fuel is collected on the exposed surface of the portion of the rock to be removed and the fire is burnt for several hours continuously (ordinary bundle wood is employed for burning). The detached portion is then removed with the quarrying tools and then broken into small pieces as per requirements.
This method of quarrying is employed for the rocks which are in the form of layers along which it can be easily splitted (e.g. sedimentary and soft rocks like limestone, marble, slate and laterite, etc.), and the stone is required in blocks for building purposes.
Soft stratified rock can be removed with the help of pick-axes and crowbars but in case of hard rocks the holes are made and grooves are cut at Shorter intervals. Blunt wedges are then inserted in the grooves or conical pins of steel are driven into the holes with a hammer. In place of conical pins sometimes plug and feathers are used. The plug is a conical wedge and feathers are flat wedges with upper ends slightly bent. The plug along with the feathers is applied into the holes and is subjected to hammer blows. If the plugs and feathers are arranged a few centimetres apart and all driven at the same time, the stone will get cracked.
In case of harder stone, the holes are originated by a pneumatic drill.
This method is employed for quarrying hard and compact stones. The various stages involved in the method of quarrying by blasting are as follows:
Boring hole in the rock
The holes are usually made (of desired depth, from 1.25 to 2.5 m deep, and 20 to 40 mm diameter) with a steel bar with knife-edged ends called jumper. When a large quantity of stones is required, holes may be drilled by a drilling machine.
During the drilling operation, water is used to facilitate the operation. The mud and rock powder produced as a result of drilling are removed by a scraper, or a spoon or by a compressed air blast.
Charging with explosive
The drilling of the hole is followed by charging it with an explosive. It should be ensured that the hole is thoroughly dry before being filled with explosives.
Tamping is of paramount importance to prevent the reaction of the explosive along the blasting hole itself. While charging the hole with explosive and prior to tamping a fuse of sufficient length is inserted. Tamping consists in filling the hole with stiff sandy clay by a brass rod called the tamping bar(made of brass 15 mm diameter). Occasionally a priming needle of 1.6 mm diameter is inserted in the tamping materials which is removed after tamping. This is followed by the insertion of fuse to help explosive being fired. A little quantity of gun powder is put in the hole to link the explosive to the bottom of the fuse.
The fuse is kept of a sufficient length as to enable the person firing it enough time to retire to a safe place before the explosion of the charge occurs. The use of electrically firing devices is also made to create the spark needed for the explosion. When the explosion occurs masses of stones around the hole are removed. A good blast produces a dull sound and the mass of the rock should be just displaced without being blown into fragments.
The quantity of blasting powder (or dynamite) necessary for each blast depends mainly on the nature of the rock, the mass of rock to be removed and the position of the hole. As a rough guide, the quantity of blasting powder can be obtained by using the following thumb-rule.
Blasting powder in N (Newton) = (Line of least resistance in meter) ²×1.50
Thus, when the line of least resistance is one meter then the quantity of blasting powder would be about 1.5 N.
The weight of rock loosened is roughly 10,000 times the weight of powder used.
In order to avoid accidents during blasting, special precautions are taken by giving warning just before firing to enable all workmen and supervisory staff to retire to a safer place and all explosives are carefully stored in a magazine specially built for the purpose.
For preventing any misfire, the following precautions need to be taken:
a. The boreholes should be charged with explosives only after these are thoroughly cleaned.
b. As far as practicable, a maximum of ten holes may be loaded and fired at one time successively and not simultaneously.
c. The lighting end of the safety fuse should be cut (with a knife) in an oblique direction.
d. After the insertion of the fuse in the detonator, it should be fixed by nippers.
e. When water is present or hole is damp, the junction of the fuse and detonator must be made watertight by using tough grease, white lead or tar.
The firing of the fuse by electricity entails the following advantages:
a. Ensure safety
b. Saving in labour and time.
c. Efficiency of explosives greatly increased (due to simultaneous firing), eventually making operation economical.
d. Useful for firing fuse underwater or in wet places.
e. No danger of misfire.
f. Proper signalling can be arranged to avoid the occurrence of accidents.