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Up to the early 1900s, the output from stone quarries was dependent upon the employment of a large labour force and to a great extent on a stable localised market. The working and supply of stone was by this time, becoming a commercial proposition so greater use was being made of railways to haul bulk supplies to outlying areas. Short hauls were still dependent on the use of cumbersome horse- drawn wagons, either direct to the user or to rail heads, where the stone was transferred into privately owned railway trucks.
Where water transportation was available, as at quarries adjoining rivers and canals, stone was tipped over private wharves into barges and shipped into towns for internal distribution or very limited haulage to sites outside.
Stone Getters
At the quarry face, stone getters would work in gangs varying in numbers from two to four men, with each individual quarry man having to provide his own tools, these would include sledge hammers, bars and shovels and each gang was even required to pay for all explosives used by that gang. The gang was also responsible for it's own particular "piece" of the rock face.
The method for working the stone was simple, but arduous. Each gang undertook its own blasting, rock breaking and haulage within the quarry. The removal and excavation of a face was affected by a form of benching system employing relatively short holes drilled using a hand-held jack hammer or wagon type machinery. The fireman responsible for boring and firing would use black grain powder, which was packed into the drill hole and well bedded with the fuse. The fuse was sometimes just wheat straw filled with a similar explosive, which had been damped to give it a slow burn rate. Following the blasting, the loose stone was "barred down" by the fireman, broken or crushed with hand tools by the gang and loaded into skips, drums or drays which were then pushed on narrow gauge light rail tracks or hauled by horses to loading bays within the quarry. Little or no secondary crushing of the stone took place. Where the stone was required for road purposes the material was stacked into lay-bys outside the quarry or at other prearranged locations until required for use. If the stone required additional crushing or reducing into appropriate sizes for road works, then this work was carried out by the roadmen.
Probably the most significant technological development to the quarrying process was the advent of steam power for both the plant operation and the road haulage. During the early part of the 19th century, the steam rail locomotive became the principal means of bulk haulage of materials and stone through out the country. The movement of stone by road was still in its infancy. Although the steam traction engine had been in existence for many years and its static counterpart widely used, the Prohibitive Act of 1865, which required a road vehicle to be preceded by a man with a "red flag" on turnpike roads, placed limitations on the use of the steam driven traction engines for road vehicles.
After 1896, when the "Red Flag" Act was repealed, the heyday of the steam traction vehicle started and for nearly 30 years it was the mainstay of heavy transportation by road. The repeal of the Act made commercial haulage by mechanised power a practical and economic proposition and it became feasible to haul 6 ton as far as Salisbury and back in a single day from the Mendips.
Steam Power
The passing of the Motor Car Act of 1903 was a further important advance and led to the development of the petrol lorry and more sophisticated road steamers.
The passing of the Motor Car Act of 1903 was a further important advance and led to the development of the petrol lorry and more sophisticated road steamers.
Steam development within the quarry was of major importance, providing easier methods of drilling and the installation of the small single rotary crushing mill driven by static engines or chain driven from traction vehicles allowed greater volumes of aggregate to be processed. Other early developments included the steam crane for bulk lifting.
While the small family investments in local quarrying remained, progress in the installation of plant was comparatively slow. Blasting was assisted by the use of power driven machines, with boring done by pneumatic drills, supplied with air from steam compressors. Narrow or standard gauge railways were developed for working inside quarrying areas. Loading techniques showed some advances with the development of rail mounted steam cranes with grabs and later, well into the 1920's, the steam driven shovel.
Mechanical methods of operation resulted in the gradual reduction of the large labour force. The late 1920's saw the installation of a new, power driven processing plant, which was the precursor of the type of plant now commonly used within modern quarries, with primary and secondary crushers and screen decks for aggregate sorting and sizing. Screening showed considerable advancement. In earlier days there was little demand for chippings. Small quantities could be readily produced, cheaply, by small rotary screens installed adjacent to each mill and capable of sieving out aggregate of the desired size. Generally chippings of under 1 inch were regarded as waste. By the 1930's the pattern had changed and chippings, hitherto incidental to the aims of production, were required in quantity. With the introduction of new crushing machinery, the rotary screen gave way gradually to the horizontal screens which separated crushed stone into about a dozen sizes varying from 2 inch (50 mm) down to dust.
Steam operated plant was followed by electrically operated machinery, the power, pre-war, being provided in the main by generators. Meanwhile other forms of power were used, first the gas engine and then in the late 1930's the diesel engine.
Two main types of crushing machinery were used, the jaw and the gyratory crusher. Over the next twenty years, apart from increased production and the demise of the smaller family-owned quarrying companies, there was little change in production method.
However, during the last fifty years or so the industry has undergone its greatest change, the developments being closely associated with larger units, increased production and large-scale capital investments.
The Present
Explosives are now principally used for primary blasting. Modern blasting techniques enable operators to detach large sections of the quarry face in a single drop. The blast is now designed using laser mapping and bore-hole logs. Once all the available data has been analyzed, the boring pattern is agreed and drilling commences. The bore holes are placed in a precise pattern to ensure the maximum amount of stone is dropped in the most efficient manner, using as little charge (explosive) as possible and that the resultant aggregate is of a sizes to pass through the crushing plant without further processing.
After blasting, the broken stone is normally loaded into Dump Trucks by Face Loading Shovels and delivered to the crushing plant. The crushing plant in use today varies considerably in design and layout. However, typically the stone is discharged into a ground level hopper. Stone is then drawn from the intake hopper by an apron feeder and chain curtains control the flow of rock. From the apron feeder the stone falls onto a vibrating "Grizzly" which on some plants has a plate of 100mm square perforations. The under-sized stone falls onto a conveyor belt and the over-sized passes into the primary crusher. These crushers vary in size and design but generally they are "Jaw" or "Gyratory" type. The crushed stone falls from the crusher onto a conveyor belt which is parallel to a belt carrying the under-sized from the grizzly, running from the base of the primary crusher house to a scalping screen.
The crushed rock of approximately 150mm is discharged into a surge bin, prior to secondary crushing. Stone from the surge bin is conveyed for further processing. It passes through two secondary crushers in line, giving an overall reduction in size of minus 40mm. This crushed material is discharged into the tertiary crushers, set to give 20mm product. At some hardstone units where a higher-grade product is required for road surfacing purposes, the shape of the aggregate is the most important factor during manufacture. In order to achieve a cubical material, four stages of crushing may be employed.
This material is conveyed to the main screen house where it is passed over three linear motion screens fitted with polyurethane decks. These screen decks are set up to produce aggregates in the following sizes: Dust; 5mm, 6mm, 10mm; 14mm, 20mm and rejects.
The layout of plant and actual processing procedures can vary considerably from quarry to quarry, dependant upon the characteristics of the rock that the quarry works and the end use for which the aggregate is to be used.
Certain applications such as Surface Treatment Aggregates require the aggregates to be free from surface dust. All aggregates will have a certain amount of dust adhered to the surface as the crushing process inevitably produces dust. In the case of Surface Treatment Aggregates, only a small proportion of the aggregate will be encapsulated in a bituminous coating, so it is vital that this bitumen adheres to the aggregate and not to surface dust. In normal road asphalts or concrete, individual stones are surrounded by other stones of varying size, bitumen or concrete and this entrapment means that any surface dust is less critical to the bond of the bitumen or concrete to the aggregate. To ensure that Surface Treatment Aggregates are clean and free from dust, the screened and sized aggregate Is passed through a washing process.
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