Quarrying of stones

Articles > Quarrying of stones

The process of raking out stones from natural rock beds is known as the quarrying. The term quarry is used to indicate the exposed surface of natural rocks. The difference between a mine and a quarry should be noted. In case of a mine, the operations are carried out under the ground at great depth. In case of quarry, the operations are carried out at ground level in an exposed condition.

Methods of Quarrying

The methods of quarrying the stone may be five:

Digging or excavating
Wedging
Heating
Quarrying with channeling machine
Blasting

Digging or Excavating: Stones buried in earth or under loose overburden are excavated with pick axes, crow bars, chisels, hammers, etc.

Wedging:

This method is mainly used for the rock of sedimentary type, which are comparatively soft, such as sandstone, limestone, marble, slate, laterite.

In this method, first of all naturally occurring cracks or fissures are located in the rocks, to be excavated. The steel wedges or points are then driven with the help of hammer, in hammer fissures or cracks and stones are detached. The split out blocks of stone can be converted into marketable forms and supplied to users.

If natural cracks or fissures do not appear, there artificial cracks will have to be formed. For this, a lime of holes is drilled along the rock surface. The holes are normally 12 mm in diameter and 20 to 25 cm deep depending upon the type of rock and also desired size or thickness of the excavated stone. The holes are generally drilled with the help of pneumatic drill. After drilling, each hole is provided with two pieced of feathers and a conical wedge or plug inserted in each hole. Sometimes, if rock to be excavated is soft, dry wooden plugs are inserted between the feathers put in the holes. After lightly fitting all the wooden plugs, water is sprinkled on them. Wet wooden plugs swell and exert a large force to split the stone along the line of the holes. After splitting, the stone is taken out and converted into desired forms.

Heating:

Heating is most suitable for quarring small, thin and regular blocks of stones from rocks, such as granite and gneiss. A heap of fuel is piled and Fred on the surface of rock in small area. The two consecutive layers of the rocks separate because of uneven expansion of the two layers. The loosened rock portions arc broken into pieces of desired size and are removed with the help of pick-axes and crowbars. Stone blocks so obtained are very suitable for coarse rubble masonry. Sometimes, intermediate layers are to be separated from the top and bottom layers. In such a case, the intermediate layer is healed electrically and the expansion separates it from the other two.

Quarrying with channeling machine:

In this method, the channeling machines driven by steam, compressed air or electricity are used to make vertical or oblique grooves or channels on the rock mass. These machines make rapidly the grooves having length of about 24 m, width of about 50 mm to 75 mm and depth of about 2.40 cm to 3.70 cm.

The process consists of the following steps:

The channels are cut around the stone block which is to be removed from the rock mass.
The horizontal holes are drilled beneath the block,
The wedges are driven into holes and the block is then broken loose from its bed.

This process of separation of stone from the rock mass is almost invariably employed in case of limestones, marbles and other soft sandstones. It is possible to separate very large blocks of stones from the rocks by the application of this method.

Blasting:

In this method, the explosives are used to convert rocks into small pieces of stones. This method is used when stone to be excavated is of very hard variety and it has no cracks or fissures. Moreover, if stone is to be excavated on very large scale, blasting method will have to be adopted. No definite size blocks can be excavated by this method. After blasting, the excavated stone is sorted out in different sizes and categories. Explosives such as blasting powder, blasting cotton, dynamite and cordite are used. The operations involved are boring, charging, tamping and fining.

Boring:

Holes are drilled or bored in the rock lo be dislodged. For bard rocks, machines drilling is used instead of hand drilling.

Charging:

The holes arc dried completely and the required amount of' charge is placed in the holes.

Tamping:

After placing the charge in the hole, a greased priming needle, projecting a little outside the hole, is placed in the hole which is then filled up with damp clay or stone dust in layers tamped sufficiently with a braced tamping rod. The priming needle should be kept on rotating while tamping is going on. This is done so that the needle remains loose in the hole. The priming needle is then taken out and 60 to 75 per cent of space created by withdrawal or needle is filled with gun powder. A bick ford fuse, a small rope of cotton coated with far, is placed just touching the needle. The other end of the fuse is kept at sufficient length so that the person igniting it can move away to a safe place.

Firing:

Blasting powder and cordite are ignited by means of a fuse, whereas gun cotton and dynamite are exploded by detonation.

Storage of Explosives:

The explosives should be stored in a magazine (a special type of building) which should be away from residential areas, petrol depots, etc. the magazine should have ventilators at high levels and should have concealed wiring. It should be protected from lighting. Smoke or fire should not be allowed in the nearby area. Explosives should be protected from extreme heat or cold and also from moisture. These should be handled carefully and gently. The magazine should be surrounded by barbed wire and the entry should he restricted.

Quantity of Explosives required:

The quantity of explosive required depends upon several factors such as strength of explosive, method of blasting; number of bore holes their sizes, position, etc and the type and mass of rock to be dislodged. It is very difficult to incorporate all the factors in an expression and obtain the exact amount of explosive required.

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