Transcription of quarry operations document
DRA acquired some copies of this overview of Meldon Quarry operations amongst a suitcase of material found in Okehampton signal box. As is often the case with old documents, this one is frustratingly unattributed and undated. It's possible it was written by Frederick Edward Linden Weaver (1906-1997), who was the quarry manager from 1941 to 1970. It is type-written on foolscap paper; this and its language and geological terminology seem to suggest a date no later than the 1960s.
Nor do we know its purpose. As an overview without too much detail, perhaps it was the script of a talk given to a non-technical visiting group.
A couple of hand written sections had been inserted into one of the copies. They break up the flow of the type-written main body of the document, so we have appended them. They contained the odd word we couldn’t read, and a few diagrams which didn't add a lot.
Our comments are in square brackets. Round brackets were in the original.
The whole of the work at Meldon Quarry is concerned with the reduction of the solid mass of Dartmoor stone to the right size and classification to comply with the Specification for Standard Ballast; so it will be an advantage if we first of all give some consideration to the terms of the specification. The first sentence of the Specification is perhaps the most important. 'Permanent Way ballast shall be of granite, flint or other approved hard stone and shall be clean and free from chalk, loam and clay'. As you will have noticed the two operative words are hard and clean.
So as to get the Specification in its proper perspective we will for a moment consider the functions of the stone under the track, although you may have given consideration to this in earlier lectures, it will be an advantage to consider it in relation to Meldon stone. The functions of the stone are briefly:-
1 To spread the loads transmitted through the rails, chairs and sleeper over as wide an area as possible. For this purpose the stone must be hard, otherwise it would quickly disintegrate under the hammering which it receives as the trains pass over the tracks. And it must be of uniform quality so as to give a homogeneous bed under the sleepers. To illustrate this point bear in mind that if the sleepers and rails were laid down on the virgin surface of the ground the varying types of soil, clay, loam, sand, rock, etc. found under a long stretch of line would all react differently to the passing of the trains, and in the softer spots the track would soon sink and the line would become like a switchback, and be unsafe, except for the slowest speeds. So the ballast is laid like a carpet to give uniformity and stability to the track.
2 The ballast must also provide effective drainage, so it must be clean to permit surface water to pass through it. If it has even a small proportion of chalk, loam or clay in it at the outset, it would prevent the free passage of water and the growth of weeds would be encouraged, thereby hampering still further its function of providing effective drainage.
The Specification goes on to deal with the size of the ballast and states:-
'In general, the bulk of the material shall be capable of passing through a 2” ring and be retained on a ¾” ring. The amount retained on a 2” ring shall not exceed 20% by weight and none shall exceed 2½” in its greatest dimensions. Not more than 10% by weight shall pass through a ¾” ring. The ballast in every consignment shall be evenly graded'.
It will be obvious to you that stone of varying sizes within the range of the Specification will knit together better than stones of all the same size.
Now a word as to shape; although the Specification makes no exact reference to it, it is important that the stone should be as cubical as possible as it maintains this shape in service and consolidates well. Flaky stones tend to bridge and break up, and thereby set up disintegration at an early stage. Meldon Quarry has the advantage of possessing a large area of stone very well suited to the production of standard ballast. In general it consists of hardened shale and dolerite (igneous rocks, from the latin Ignis, meaning fire). There are other types of stone, including some weathered bands, which are unfit for quarrying, and have to be avoided and left as the Quarry face moves back.
The first process in quarrying consists of removing what is technically called ‘overburden’. The earth has a solid rock framework beneath the soil, but in a quarry this soil has to be removed for two important reasons:-
1 The statutory regulations governing the working of quarries clearly lay down that the overburden shall be removed so as to prevent dangerous falls [at this point the writer wrote himself a note: ‘illustrate’, but didn’t]
2 It is of the utmost importance to remove as much of the earth as possible at an early stage because none of it must be permitted to become mixed with the stone before crushing and screening, otherwise it would spoil the finished product (refer back to the Specification).
In practice it does not prove possible to remove 100% of the earth by means of overburden clearance, but further attention is given to waste separation during the loading at the Quarry face.
Overburden is cleared as far as possible by mechanical means, a Ruston or Smith excavator loading it into narrow-gauge wagons, or dumpers, and it is then tipped over a bank and a waste tip is formed. The amount of overburden to be cleared varies, but the thickness at Meldon ranges between one and eight feet and in a year up to about 5% of the total weight of material handled must of necessity be tipped in this fashion. This constitutes a considerable item of expense (about 1½d for every ton of finished stone produced). £2 for every train of ballast despatched.
Apart from the expense, waste tips can become very unsightly, and the Town and Country Planning Act gives powers to local authorities to bring pressure on quarrying concerns to carry out their work in an orderly fashion. For instance it is usually contended that waste tips should be grassed down when completed, [at this point in the original was a hand written insert about geology, mostly reproduced at the end of this document] and as far as possible worked out quarry sections should be filled by tipping waste from new workings.
After the removal of overburden the real quarrying begins. The solid face of stone must be tackled to reduce it to more manageable proportions.
There are several variations and it will be quite impossible in the course of this paper to deal with them in detail, so I will briefly describe the principles involved.
Well-holes. There are on the market wagon-mounted drills capable of sinking holes 6 to 8 inches in diameter, and 100 feet deep, and this type of drilling enables charges to be placed in the holes thus made and to be detonated together and burst out the rock on to the Quarry floor.
In all blasting a lot depends on the detailed physical characteristics of the stone, its bedding, joint planes, etc. The best and most economical results can be achieve when the quality of the stone is uniform and there are regular joints or bedding planes. So that a long straight face of stone will yield the best results, if it has reasonably good jointing.
Most of the primary blasting at Meldon is done by means of tunnelling and benching. [there was another handwritten insert here, about explosives, also reproduced at the end of the document]
Perhaps a word or two on the general elementary ideas governing blasting may be appropriate at this stage.
Explosives may be briefly described as substances which occupy a very small volume in the solid or powder form as compared with the volume of gases produced when they explode. For quarry use they are made in cartridge form as small as 1½oz and up to 1lb for insertion into drilled holes. They are also supplied in 10, 25 and 50lbs tins for primary blasting.
The position of the charges is a very important aspect of the use of explosives, and this positioning is generally referred to in terms of the burden, depth and spacing between charges.
As a general rule charges should be placed so that the line of least resistance to the thrust which they exert will be towards an open or free face, and the distance from the charge to the free face, measured opposite the charge, is termed the burden.
Benching starts at the top of the Quarry, and tunnelling starts at the bottom. With benching, rows of ten feet holes are put down and they are charged with about five pounds of explosive each. Tunnelling is carried out by driving a tunnel about 4ft by 3ft into the base of the quarry face, and locating large charges in chambers, which when exploded kick out the toe of the face, thus forming a large heap and allowing the stone from above to slide down the joint planes and add to the size of the heap.
In both benching and tunnelling, local features have to be carefully studied and detailed surveys are made in the case of the tunnel blasts. The absence or presence of jointing, the nature of the stone, the availability of free faces must all receive consideration. Good stemming is essential to ensure that the force exerted by the explosion does its work properly and does not escape through the tunnel or the drilled hole.
In general the yield of broken stone from both benching and tunnelling gives between five and eight tons per pound of explosive used.
Having obtained heaps of stone from the solid the next stage of working it [is] to load and transport the stone to the crushing plants.
The heaps of stone produced by primary blasting are used to provide working places or bays where teams of men are set to work to load the stone to feed to the crushing plants. The stone after primary blasting is of random sizes, some of it far too big to be tackled by hand, so secondary blasting is carried out to break down the stones to manageable proportions.
Secondary blasting consists of placing small charges of explosives in or on the stones which are too big to be handled. It is far more economical to place the charge in the centre of the stone, as a quarter of a pound of explosives so placed will break the stone more effectively than one pound place on the outside, but other considerations have to be borne in mind. The drilling of holes is an expensive process, and the position of the larger stones may be such that it is a physical impossibility to drill them. Cartridges are therefore placed on the stone, and covered with a cap of heavy clay, and when detonated the weight of the clay causes the power of the charge to be exerted downwards, thus breaking the stone.
The stones tipped into the Crushing Plant must not exceed about 14” x 10” otherwise the plant cannot cope with them, so the task of the quarrymen loading stone at the quarry face is to reduce to the correct size the random sized stones met with in the heaps produced by the primary blasting. Teams of rippers and blastmen are in attendance to give attention to the stone heaps, and to deal with awkward situations when large stones jam together in the heaps, and have to be blasted to get them down to the quarry floor.
The quarrymen (known as Fillers) have to break and load stone as quickly as possible, for the production of the quarry depends entirely upon the feeding of stone continuously to the crushing plants.
For this reason P.B.R. [payment by results] is employed to encourage the workers to load the maximum amount each day, so as to keep the crushing plants busy all the time.
In passing we must consider two important points.
1) The stones fed to the crushing plant must be:-
a) Clean, to comply with the Standard Specification, and to prevent fouling the moving parts of the plant, and
b) Not too large, otherwise they would jam in the crushing plant, and overload the prime mover, and cause delay.
Therefore the specification of P.B.R. Terms lays it down that ‘stone must be separated from waste’, and ‘oversize stones must not be loaded’. If a worker is found to be at fault in either instance he must re-sort the stone, and will be credited once only for the stone which he handles twice.
Now it must be known to all of you that a primary consideration in all production centres is to keep the material moving from process to process without delay, and avoid bottlenecks. Therefore the track layout is designed to supply each loader with an empty wagon as soon as he has completed a full one.
Apart from the method of loading into wagons, a new method of loading into steel pans is gradually being introduced; this method gives the worker a slightly easier task for the stone can be more easily slid into or lifted over the low sides of a pan than it can be lifted up over the side of a wagon. It is a partly mechanical operation, for the lifting is done by a mobile crane, and the stone tipped into tipping lorries which run the stone straight to the crushing plants.
In quarrying we are, of course, dealing with a heavy commodity, and wherever possible gravity is used to lessen the cost of moving the stone, and to reduce the cost of production. For instance:-
The rail tracks from the quarry faces are set with a down gradient of 1 in 90, the main tracks have a down gradient of 1 in 300 to the crushing plants so that locomotives are assisted in pushing the loads to the crushing plants.
Similarly the stone fed to the jaws of the crushing plants are tipped down a sharply inclined shute which carries it down to the working part of the plant.
At Meldon two main gyratory crushing plants are employed each capable of dealing with from 60 to 80 tons of stone an hour. As mentioned previously stones sized up to about fourteen inches to ten inches are fed to the plants, and I will give you a brief illustration of what happens then.
The stones are nipped between a manganese steel mantle mounted on a shaft which rotates with an eccentric motion, and a double ring of tapered manganese [steel, presumably] curved plates called concaves. This breaks down the stones until they are small enough to pass through the aperture at the base of the mantle, set at 2¼”, which you will recall is the upper limit allowed by the Specification.
This crushing plant is set in a massive steel shell below ground level and it discharges the broken stone into a chute which feeds it into the buckets of an elevator conveying the stone to the Screen House about 100 feet above ground level.
Here the stone is discharged on to a conveyor and carried to the actual screen. Here, of course, the stone arrives in varied sizes from 2¼” down to dust and the function of the screens is to sort it out into the required sizes for distribution to the storage hoppers.
The screens are of the Niagara Vibratory counterflow type and the random sized stones flow down the screens, and are constantly being thrown into the air by reason of the vibrations of the deck which is agitated by an eccentric shaft rotating at 1000 r.p.m.
The rapid movement of the screens separates the stones and allows them to pass through the apertures, the steep inclination of the screens passing them quickly forward to the discharge chutes when they reach the right deck.
So that if the top deck of a screen has 2” mesh and the next deck has ¾” mesh the product flowing out from between them will obviously be correctly graded Standard Ballast. This is fed by gravity to a long conveyor which can be shuttled to spread the ballast evenly into the long storage bins situated beneath the screen house. There is in fact storage room for nearly 2000 [cubic?] yards of standard ballast from the two Crushing Plants.
I should like to draw your attention to the part which gravity plays in the handling of the product, that is why the crushed stone is carried up the high elevator to the screen house to permit gravity to play its part in returning the finished product straight into the hopper wagons.
From the storage bins the stone is discharged direct into the trains of hopper wagons passing slowly underneath them, the track beneath the bins is on a slight down gradient, so that the wagons can be let down under the bins without a shunting engine being continuously employed.
Now a word or two about the construction of the hopper wagons. You will be aware of their external shape, but the inside is designed to discharge the whole of the contents with a bare minimum of manual work, the inclined steel plates assisting the flow of the stone to the centre of the vehicle where the discharge doors are opened to allow the ballast to be lowered on to the track exactly where it is required for use.
So it is interesting to reflect that after the stone has been manhandled at the quarry face is passes through all the succeeding operations of being crushed, screen and loaded at the quarry, and then being unloaded on the site where it is required for use without being touched by hand again.
Now there are many other processes upon which there is only just time to touch, but which are essential to the output of stone. They include:-
1 The production of compressed air, by means of oil engined, two stage air compressors. The air is raised to a pressure of 100lbs per square inch, and this is conveyed through pipelines to all parts of the quarry, where the supply of air is fed to the drilling machines, the air is discharged against the piston of the machine and drives it forward to strike a series of rapid blows against the end of the drill steel and drive it forward into the rock.
2 The Drill Sharpening Shop
Here the carbon steel drills are forged into the correct shape and hardened so that they will cut holes in the rock for the purpose of placing the charges of explosives for blasting.
3 The Washing Plant
Chippings used as concrete aggregate must be absolutely clean, and to ensure this cleanliness they are taken to the Washing Plant, and sprayed with water as they pass through a long barrel formed of wire cloth, which by being rotated sorts the stone into the sizes required for the different kinds of concrete.
The dirty water from the process is fed to large storage tanks and allowed to settle, and the clay thus produced is used in connection with secondary blasting.
4 The Organisation of the Quarry Staff
An organisation chart showing:-
1) Quarry Manager
2) Technical Assistant, for survey, development, mechanical design, etc.
3) General Foreman, for all day-by-day routine working throughout the quarry, with sub-foremen taking charge of minute-by-minute operations in each separate bay, controlling drillers, blastmen, rippers, and all other quarrymen.
4) Office, comprising Chief Clerk, controlling sections for :-
(b) Accounts, Paybills etc
(d) Statistical, train running etc
5 Maintenance Staff
Maintenance foreman, responsible for the continuous overhaul of plant and machinery, cranes, lorries, dumpers, locomotives, electric motors, etc; with a team of fitters, electricians, welders, and a further section maintaining buildings, wearing parts of chutes etc, and comprising carpenters, mason and a painter.
The working of quarries is governed by Statute, mainly in the form of the Special Rules of the Quarries Act of 1894, and the Statutory Rules and Orders of 1938 as supplied to quarries. [DRA has copies of both acts]. The whole object of these Regulations is to set down a code of practice which must be followed, with the object of carrying out the work in the safest possible manner.
Rule 1 of the 1894 Act lays it down that 'the working of the sides and of the overburden or tops of the quarry shall be carried on so as to prevent dangerous falls'.
The Rules also give instructions which must be observed with regard to the use of explosives, insisting that proper provision must be made for the storage and handling of the material, and laying down the procedure which must be followed when explosives are used.
Rule 24 states that every shot must be fired by a competent person properly authorised.
Rule 31 provides that due warning must be given by an efficient system of signals before blasting is commenced and when it has finished.
A very important group of Rules is numbers 52 to 55. These rule that a daily inspection must be made by a competent person appointed by the owner of every working place from which danger might arise, and the external parts of all plant, machinery, etc; including ropes, chains and tools, and that steps shall be taken at once to remove any danger revealed by the inspection.
A true report must be made and signed forthwith by the person making the examination in an approved book, and the record must be made available for the examination of H.M. Inspector of Mines and Quarries for at least a year after the date of each entry.
Rule 60 states that every person employed at the Quarry who notices anything that appears unsafe, or likely to cause danger, shall forthwith report it to the Owner or Agent.
The 1938 Regulations cover most of the points already dealt with in the 1894 Act, but gives rather more attention to Health and Welfare of the workers, and lays down instructions restricting the hours to be worked by women and young persons. It also deals at length with the construction and regular inspection of fencing, chains, ropes, and lifting tackle, also steam boilers, air receivers etc
Appendix 1 – Geology
Meldon’s stone consists mainly of shale (this is as rock which falls within the group known geologically as sedimentary) – which as the word denotes owes its derivation to having been laid down in layers underwater or on land (by wind etc) and the separate layers are separated by fine cracks.
[Here there was the writer’s diagram of an igneous intrusion metamorphosing shales, and an explanation of dip and strike].
Dip - Rock beds seen in a quarry or railway cutting are often found to incline in one direction or another – and at varying angles – and the rock is said to dip or incline to the north etc according to the compass direction of the line, and
Strike - the line at right angles to the dip is called the strike
Although the rocks were all laid down horizontally originally they have been tilted by volcanic disturbances at a later date.
This has been caused by :-
a) Bedding, the fine [something] planes between the layers of sediment marking a period of change
b) Shrinkage cracks, caused by the heating or drying out of the rocks (in the same way that mud cracks in the sunshine)
c) Shear Joints, caused by pressure at the ends or beneath
The joints are of the utmost importance in quarrying as they represent planes of weakness which can be of great assistance in blasting or winning the rock. But too many joints can indicate too much weakness and too many laminations can make the rock quite useless.
Appendix 2 – Explosives:-
1½ lbs of T.N.T. occupies about 1 pint. When exploded it for a moment occupies a pint – but a very high pressure of some 200 tons per square inch (remember that the compressive strength of the toughest shale is more than about 20 tons) so with the explosion a devastatingly powerful compression wave of gas pushes out in all directions.
The chief merits of an explosive are power and safety. For blasting work a rather slow acting explosive which rends the rock is preferred to a very rapid one which shatters it. Safety is achieved by having an explosive which will burn without exploding and which can be struck without exploding. The milder explosives used commercially are generally ammonal (ammonium nitrate and aluminium powder).
To explode the charge a detonator is used (this gives a violent shock by exploding about a gram [?] of a sensitive shattering explosive like mercury fulminate or lead azide).
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