In this article we will discuss about:- 1. Definition of Quarrying 2. Sites for Stone Quarrying 3. Important Considerations 4. Methods.
Definition of Quarrying:
The process of taking 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 stones, thus obtained, are used for various engineering purposes.
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 a quarry, the operations are carried out at ground level in an exposed condition.
Sites for Stone Quarrying:
The selection of site for a quarry of stones should be done after studying carefully the following aspects:
(i) Availability of tools, power, materials and labour for the easy and efficient working of quarry;
(ii) Availability of site for the dumping of refuse and avoiding health hazards, if any;
(iii) Distance of quarry from roads, railways, sea coast, etc. and proximity to the transportation facilities;
(iv) Drainage of quarry pit;
(v) Easy availability of clean water in sufficient quantity all the year round;
(vi) Economy in quarrying;
(vii) Facility of carrying and conveying stones from quarry;
(viii) For quarrying by blasting, absence of permanent structures in the nearby area;
(ix) Geological data regarding rock formations at the site;
(x) Quality of stone available from quarry;
(xi) Quantity of stone likely to be obtained from quarry;
(xii) Results of trial pits; etc.
Important Considerations for Starting the Quarry:
Following are the important considerations which are to be carefully paid attention to before actually starting the quarry:
(i) Examination of Rock Surface:
The exposed surface of rock bed should be carefully examined. The presence of cracks and fissures are to be noted. The planes, along which stones will easily split, should be found out to make quarrying operations quick and economical.
(ii) Lay Out:
It is necessary to prepare a complete lay out of various stages involved in quarrying operation. The faulty planning leads to the failure of quarry.
(iii) Men and Machines:
There should be proper co-ordination between men and machines employed on the quarry so as to obtain maximum advantage from them.
(iv) Removal of Top Surface:
The loose soil and soft rock present at the top surface of quarry should be removed. The material obtained from top surface is unsuitable for construction work and hence it should be rejected. The dense rocks are available at a depth which depends on the weathering qualities of a rock.
(v) Structural Stability:
The stones should be removed from the quarry without affecting the structural stability of its sides. If proper precautions are not taken, there may be serious slips or landslides with disastrous results.
Methods of Stone Quarrying:
Following are the three methods of quarrying:
(i) Quarrying with hand tools
(ii) Quarrying with channelling machine
(iii) Quarrying by blasting.
Each of these methods of quarrying will now be briefly described.
(i) Quarrying with Hand Tools:
Following are the three different ways of doing quarrying by the use of hand tools:
(a) Digging or excavating
(a) Digging or Excavating:
In this method, the stones are merely excavated with the help of suitable instruments such as pick-axes, hammers, shower, chisels, etc. This method is useful when soft stones occur in the form of large or small blocks.
In this method, the top surface of rock is heated. This is usually done by placing pieces of wood or by piling a heap of fuel over the surface and setting a steady fire to them for some hours. Due to unequal expansion, the upper layer of rock separates out. It is indicated by a dull bursting noise. The detached portion of rock is then removed by suitable instruments such as pick-axes, crowbars, etc.
This method is useful when small blocks of more or less regular shape are to be taken out from quarry. It is suitable when the rock formation consists of horizontal layers of shallow depth. It is possible to obtain by this method fairly rectangular block required for coursed rubble masonry.
In this method, if rock surface contains cracks or fissures, the steel wedges or points, as shown in fig. 2-2 and fig. 2-3 respectively, are driven through such cracks by means of hammers. The blocks of stone are then shifted and they are removed with the help of suitable instruments.
If natural cracks are absent, the artificial cracks are to be formed. A line of holes is drilled along the rock surface. The diameter of hole is about 12 mm. The distance between successive holes is about 100 mm to 150 mm. The depth of hole is about 200 mm to 250 mm. The plug and feathers are placed into these holes as shown in fig. 2-4.
A plug is a conical steel wedge. A feather is a flat steel wedge with its upper end slightly curved. A plug is placed between the feathers and all plugs are then simultaneously driven by hammer. A great force is exerted due to enormous pressure and a crack is developed along the line of holes. If stone is comparatively hard, the pneumatic drill may be employed to prepare holes for plug and feathers.
If rock is comparatively soft, only wood plugs may be used. They are placed in the holes and are kept soaked in water. When wood plugs swell or expand, a great force is exerted and the rock splits along the line of holes.
The wedging is adopted for costly stratified rocks which are comparatively soft such as laterite, marble, limestone, sandstone, etc. The wedging is preferred to the blasting, wherever possible.
(ii) Quarrying with Channelling Machine:
In this method, the channelling 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 m to 3.70 m.
The process consists of the following steps:
(a) The channels are cut around the stone block which is to be removed from the rock mass.
(b) The horizontal holes are drilled beneath the block.
(c) The wedges are driven into the holes and the block is then broken loose from its bed.
This process of separation of stone from the rock mas is almost invariably employed in case of marbles, lime stones and other soft sandstones. It is possible to separate very large blocks of stones from the rocks by the application of this method.
(iii) Quarrying by Blasting:
In this method, the explosives are used to convert rocks into small pieces of stones. The main purpose of quarrying stones by blasting is to loosen large masses of rocks and not to violently blow up the whole mass so as to convert it into very small pieces of practically no use.
This method is adopted for quarrying hard stones, having no fissures or cracks. The stones obtained by blasting are usually of small size and they are used as ballast in railways, aggregate, for concrete, road metal, etc. The process of blasting is important with respect to the stone quarrying.