In this article we will discuss about:- 1. Tools for Blasting 2. Materials for Blasting 3. Process of Blasting 4. Precautions.
Tools for Blasting:
Following tools are required in the process of blasting:
This is shown in fig. 2-5 and it is used to drill a hole to the required depth.
This is shown in fig. 2-6 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 fig. 2-7 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 explosive, the hole is filled with tamped earth and this needle is kept in 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 fig. 2-8 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 fig. 2-9 and it is used to ram or tamp the material while refilling 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 ends.
Following materials are required in the process of blasting:
A detonator is a contrivance whose explosion initiates that of another. It is in the form of a copper cylinder having diameter and length as 6 mm and 25 mm respectively. It is closed at one end with projecting fuse at other end. It is partly filled with 6 to 9 grains of fulminate of mercury. It is used when dynamite is adopted as explosive. The detonators are fired either by fuse or electric spark.
The blasting powder and dynamite are commonly used as the explosives. The blasting powder is also known as the gun powder and it is a mechanical mixture of charcoal, saltpetre (KNO3) and sulphur. The proportions of charcoal, saltpetre and sulphur by weight are 15, 75 and 10 respectively. Sometimes the saltpetre is substituted by chile saltpetre (NaNO3) in the composition of blasting powder. But as chile saltpetre absorbs moisture, it is difficult to keep such powder for a long time. As a rough guide, the quantity of blasting powder can be obtained by the following rule-of-thumb –
Blasting powder in N = (Line of Least Resistance in m)2 x 1.50.
Thus the quantity of blasting powder required for rocks having L.L.R. = 1 m would be about 1.50 N.
The dynamite consists of 25 per cent of sandy earth saturated with 75 per cent of nitro-glycerine and this percentage composition varies with the nature of work. It is in the form of thick paste and it is ordinarily sold in cartridges. It is very poisonous in nature and develops violent headaches through touch with the skin.
Other explosives which are used in blasting are given in table 2.3 –
It may be mentioned here that Alfred Nobel (1833-96), Sweden’s bearded genius who in the late nineteenth century blasted his way into world fame and fortune by developing dynamite, regretted for its destructive potential and left his vast fortune from it to finance the Nobel prizes which are annual international prizes given by the Nobel Foundation for distinction in physics, chemistry, medicine and literature, and for the promotion of peace.
Nobel was a bachelor and he had more than 350 patents on everything from artificial silk to cinema film. He also dabbed in medicine and had also a passion for the cause of peace. He also wrote poems and essays in spare moments.
These are required to ignite the explosives. They are in the form of a small rope of cotton coated with tar and with a core of continuous thread of fine gun powder. The rate of burning of a good fuse is about 10 mm per second and it is also sometimes referred to as slow-match fuse because the rate of its burning enables the person firing it to move away to a safe place before the explosion takes place. For electric firing, the patented electrical fuses are used.
The blasting is carried out by hand drilling as follows:
(i) The blast holes of required depths and diameters are made with jumpers, dippers and scraping spoons. The 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.
(ii) The blast holes are cleaned. They are made dry by rotating a small iron rod with a rag or a piece of dry cloth tied at its end. After a few rotations, the rod is taken out.
(iii) The charge of gun powder or dynamite is placed at the bottom of hole. A priming needle which is a thin copper rod is placed in position. It is to be coated with grease so as to make its withdrawal easy.
(iv) The remaining portion of blast hole is filled in layers with dry sandy clay, moorum or ant-hill earth. Each layer is to be rammed or tamped hard. The ramming is done by a brass tamping bar. When tamping is being done, the priming needle is frequently turned so that it can be easily removed when the hole is completely filled up.
(v) When the tamping operation is finished, the priming needle is taken out slowly by frequent turning, leaving a long narrow hole and it is filled with gun powder or dynamite as shown in fig. 2-10.
(vi) 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 top and charge of explosive Process of blasting at the bottom.
The detonators are used when explosive is dynamite. For this purpose, the required length of the safety fuse is cut straight from the safety fuse coils with the help of a sharp knife or fuse cutter. Then the freshly cut end of the fuse is gently inserted into the detonator till it touches the charge in the detonator tube.
The open end of the cap of the detonator is then gently compressed with nippers to grip the fuse securely in position. The dynamite cartridge is opened at one end and a hole is made in the charge with the help of a brass needle. The detonator is inserted into the hole in dynamite cartridge.
The paper of the cartridge is closed up and it is secured firmly round the fuse immediately above the detonator with the help of wire or twine i.e. strong string or cord formed from two or more threads of hemp, cotton or the like twisted together. This is known as the primer cartridge and it is gently inserted in the blast hole so that the free end of the fuse is outside the hole.
(vii) The free end of the fuse is fired. This can be done either with a match or with electricity.
The firing by electricity has the following advantages:
(a) It ensures safety because the firing operation can be done from a great distance from the site.
(b) It results in saving of time and labour.
(c) The firing is simultaneous and hence, the efficiency of explosives is greatly increased, thereby making the operation economical.
(d) It is useful for firing fuse under water or in wet places.
(e) It is possible to arrange for proper signaling so as to avoid the occurrence of accidents.
(f) There is no danger of misfire.
(viii) The explosion takes place and rock is disintegrated into small blocks. A good blast produces a dull sound. Such blocks are collected and taken for further treatment.
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 a serious accident.
(2) Line of Least Resistance:
The rocks contain Fissures, cracks, faults or bedding planes. When explosion occurs, the gases are formed. If blast hole is tamped sufficiently hard, it will not be possible for the gases to come out through 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 the Line of Least Resistance or L.L.R. In practice, the L.L.R. is taken as the shortest distance between the centre of charge and nearest rock surface, as shown in fig. 2-10. The length of L.L.R. plays an important part in determining the quantity of explosive required in the process of blasting and hence it should be carefully decided.
(3) Needle and Tamper:
These should be made of copper, brass 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 a serious accident.
(4) Notice of Blasting:
Nobody should be allowed to enter the area where blasting is being done. The notices and visible signs such as red flags should be placed at suitable places along the periphery of such area. It is desirable to avoid the blasting operations late in evenings or early in mornings. The fixed hours of blasting operations should be made known to the public.
(5) Retreat to a Distance:
The fuse adopted should be such that a worker can retreat to a safe distance after firing it. For large scale work, the whistles or sirens may be used to warn the workers to go to a safe place before explosion takes place.
(6) Seepage of Water:
If water is entering the blast hole, the charge of explosive should be placed in thin iron plate or in water-proof paper.
(7) Skilled Supervision:
The work of blasting should be entrusted only to the trained and experienced persons. The responsible person should ascertain the fact that the charges exploded are equal to the charges fired with the number of explosions heard.
The explosives should be stored very carefully. They should be placed in specially constructed building known as the magazine or store-house.
The precautions to be taken for storing of explosives can be enumerated as follows:
(i) In case of failure of electric power, only torches should be used for lighting, if required and in no case, the flame lantern should be used.
(ii) No outsiders should be permitted to enter the magazine campus and the authorized persons must wear magazine shoes or shoes with no nails.
(iii) The detonators should be kept entirely away from other explosives.
(iv) The different explosives should be placed in separate boxes.
(v) The electric wiring must be concealed for lighting purpose and it should be frequently checked by competent persons. An open and loose wiring of any temporary nature should not be allowed.
(vi) The explosives should be handled gently and they should not be; thrown or dropped in any case.
(vii) The Firing or smoking should be strictly prohibited within a radius of 50 metres from the magazine.
(viii) The magazine should be located away from residential buildings, important structures, public places and places of fuel storage.
(ix) The magazine should be protected by a high barbed wire fencing on its boundary with caution boards and watchman should guard it round the clock.
(x) The magazine should be protected from the lightning and the thoroughly efficient lightning conductors should be provided, one at each end of magazine.
(xi) The store-house should be always locked and the key should be kept in the safe custody of the responsible person.
(xii) Under no circumstances, the magazine should be constructed within a distance of 0.50 km from any working kiln or furnace.
(xiii) Under no circumstances, the magazine should be opened during or on the approach of a thunderstorm and no person should remain in the vicinity of the magazine during such storm.