QUARRYING AND BLASTING OF ROCKS

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 thus used for various engineering purposes.

Quarrying by blasting: In this method, the explosives are used to convert rocks into small pieces of stones. 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, aggregates for concrete, etc.

Tools for Blasting:

Following tools are required in the process of blasting:

1. Dipper: This is shown is figure and it is used to drill a hole to the required depth.
2. Jumper: This is shown in figure and it is used to make blast holes. It is more effective in boring a nearly vertical hole.
3. Priming needle: This is shown in figure and it is used to maintain the hole when tamping is being done. It is in the form of a thin copper rod pointed at one end and provided with a loop at the other end for handling. After filling the hole with explosives, the hole is filled with tamped earth and this needle is kept at the centre so that its removal or withdrawal will develop a passage for the insertion of fuse to cause explosion. 
4. Scraping spoon: This is shown in figure and it is used to scrap or remove dust of crushed stone from blast holes. It is in the form of an iron rod with a circular plate attached to one end and provided with a loop at the other end so as to facilitate its handling.
5. Tamping bar: This is shown in figure and is used to tamp or ram the material while filling blast holes. It is in the form of a heavy brass rod of 10 mm to 15 mm in diameter and it tapers a little at the end.

Figure 1. Tools for Blasting

 Process of blasting:

1. The blast holes of required depth and diameters are made with the help of dippers and jumpers. A small quantity of water is added at intervals to make the rock soft and to convert dust into paste. Such paste is easily removed by scraping spoons.
2. The blast holes are cleaned and dried by rotating a small iron rod with a piece of dry cloth tied at its end.
3. The charge of gunpowder or dynamite is placed at the bottom of the hole. A priming needle is placed in position. It is to be coated with grease so as to make its withdrawal easy.
4. The remaining portion of the blast hole is filled in layers with dry sandy clay, moorum and ant hill earth. Each layer is to be rammed or tamped hard. The ramming is done by a tamping bar.
5. When the tamping operation is finished, the priming needle is taken out slowly by frequent turning leaving a narrow hole and it is filled with gunpowder or dynamite.
6. A fuse is inserted in the hole and it is kept projecting outside the hole to a length of about 600 mm to 900 mm. Thus a link is formed between the fuse at the top and charge of explosive at the bottom.
7. The free end of the fuse is fired and the explosion takes place and the rock is disintegrated into small blocks.

Line of least resistance: The rocks contain fissures, cracks, faults or bedding plane. When explosion occurs, gases are formed. If blast hole is tamped sufficiently hard, it will not be possible for the gases to come out through the blast hole. In such a case, the gases will follow the line of path which offers the least resistance. Such a line is known as line of least resistance or LLR. In practice, LLR is taken as the shortest distance between the centre of the blast hole and the nearest rock surface.

Precautions in blasting:

Following precautions are to be taken in the process of blasting to avoid the occurrence of serious accidents:

1. Failure of explosion: Sometimes a charge fails to explode due to any reason. In such a case, a fresh blast hole is made near the hole that has failed and the process of blasting is repeated. The fresh blast hole should not be too near the failed hole. In many cases, the explosion of fresh blast hole will also explode the charge of failed blast hole and in such a case, it may result into serious accidents.
2. Needle and tamper: These should be made of copper or bronze and not of steel. A spark is formed when steel strikes the rock. Hence, if they are of steel premature explosion will take place and it may result into serious accidents.
3. Notice of blasting: Nobody should be allowed to enter the area where blasting is being done. The notices and visible signs as red flags should be placed at suitable places along the periphery of such area. 
4. Retreat to a distance: The fuse adopted should be such that a worker can retreat to a safe distance after firing it. For larger work, the whistles or sirens may be used to warn the workers to go to a safe place before explosion takes place.
5. Seepage of water: If water is entering the blast hole, the charge of explosive should be placed in thin iron plate.
6. Skilled supervision: The work of blasting should be entertained only to the trained and experienced persons. 
7. Storing: The explosives should be stored very carefully. They should be placed in specially constructed buildings known as magazines or store houses.