Sand & Gravel Pits
Both sand and gravel are basically composed of rock ground down by the action of water, the former rather more finely than the latter. They are generally recovered from open sand pits located in river valleys or coastal areas using drag cranes to recover the material. The quarry would recover a mix of sand and gravel, the proportion of stones and sand determine whether it is called a sand pit or gravel quarry. In both the basic process is the same and consists of a series of distinct stages as follows:
Winning This is the recovery of the material, in most cases this involved a drag crane working an open pit but some sand was recovered from mines (usually of the 'drift' type).
Picking Mixed in with the desired sand and gravel were larger rocks and stones which had little commercial value and could damage the equipment in later stages of the process. These were removed in various ways, at smaller sites up to the 1970s the most common method was to put the material on a three foot wide conveyor belt (called the picking belt) where people could pull off the big rocks as it went past. The alternative is to use an angled screen, either a set of rods or a heavy metal grid, the desired material would fall through, the big stuff would roll down and off the end. At some sites they used a trommel (a rotating tubular screen), the example shown below is a powered machine of this type suitable for the post 1970s era.
Fig ___ Trommel for a sand or gravel quarry
Washing There would be a quantity of dirt mixed in with the sand and gravel, this was removed by 'washing' with water, the basic idea was to add water so the lighter dirt would float and be washed away to a settling pit but the heavy sand and gravel would sink. The various methods used are discussed below.
Grading This stage separates the sand from the gravel and further subdivides the latter into different sizes (typically small medium and large). Sand was not graded as it tends to be fairly consistent from a single site, some sites produced fine sand, others coarse sands. The grading was done with a screen or trommel as above but with a finer mesh to separate out the fine sand from the stones.
Drying & Bagging Only done with high purity silica sands (as used for glass making). The older method (still in use into the early 1970s I believe) was to spread the sand on a 'drying floor' in a heated building, an alternative is to use a 'rotary kiln'. The kiln is a rotating metal tube set at an angle, the sand is poured in at the top and a fire is fed in at the base (oil or gas were used). By the time the sand reached the lower end of the tube it was dry. This dried sand was valuable stuff and was either bagged in sacks or placed in sheeted open wagons or covered hopper wagons for despatch.
Sand
Most sand is recovered from open cast quarries (known as sand pits) but in some locations it was mined, usually by driving a 'drift' into the side of a hill. A sand mine, although less common, requires a lot less space on a model railway layout.
There are many types of sand, each with a particular application. Some is used as an abrasive, such as on sand paper, for which sand with a high proportion of garnet is employed, but this stuff is also used in some cleaning agents. 'Bath bricks' are blocks (slightly smaller than a house brick) of compacted fine siliceous sand for scouring metal, often used for knife sharpening. They are made from the mixed mud and sand of the river Parrett near Bridgwater. The deposit is dug out, moulded into bricks, and dried. Bath bricks are still made today, up to the 1950s they were mainly sold by hawkers from a barrow (who also sold 'hearth stones', used for cleaning not the stones used for making a hearth). Silt from rivers was also dried and sieved to be made into an abrasive block for cleaning silver, this material was at one time exported all over the world (the last such works in the UK, on the River Trent, closed in 1982 due to a lack of demand for their product). The illustration shows an advert for a sand based metal polish 'Polish De Luxe, Komo Metal Paste, entirely free from grease and a perfect polish, also for mirrors'. This ad wouldn't be seen in the quarry but might indicate a suitable industry for receiving regular shipments of fine sand.
Fig ___ Sand based metal polish advert
Sand recovered from pits or mines varies a lot in its chemical composition, these can be roughly divided into 'construction sand' and industrial or 'silica sand'. Silica sand includes sand used for glassmaking but it also embraces grades of sand used for a variety of other industrial uses. The main deposits of high purity silica sands (as required by several industrial processes) are in Bedfordshire, Surrey and Cheshire although there are odd deposits of good quality sands in various parts of the country. At the end of the 20th Century annual silica-sand usage in the UK was around 4 million tonnes in total, of which sand for glassmaking accounts for around half. Annual usage of construction sand is typically around 40 to 45 million tonnes. Silica sands can cost from twice to ten times as much as the construction grade sands depending on their composition and purity.
Silica sand that contains some clay is used for making moulds in foundries (hence it is called 'moulding sand'), for many years this was an important application but as the iron and steel industries wound down the demand for this purpose fell. In the UK in 2004 glass sand accounted for 53 percent of total sales of silica sand and foundry sand only 11 percent, the remainder being sand for other industrial uses (19 percent) and sand for horticultural and leisure uses (17 percent). Silica sand is used to make calcium silicate bricks, about 90 percent of the brick is sand and it is essential that this sand is of the right type and grade, being rich in silica, clean and granular (see also Lineside Industries - Potteries and Brickworks').
Fig ___ Sand wagon from quarry specialising in moulding sand
Sand constitutes from 33 to 50 percent of the volume of concrete, the sand for making for making mortar and cement is a common quartzose sand but it must be free from salts (it is washed to remove impurities) and it is sifted to obtain consistent grain sizes. Brick makers add construction grade sand to their clay to stop the bricks cracking during 'firing' and they put it in the brick moulds to provide a decorative finish to the outer surface. Sand is used for etching glass and since the later 19th century, when compressed air became available, 'sand blasting' to clean stonework or metals has used quite a lot of sand. Construction grade sand is also used to filter liquids (to remove solids), notably in water works.
Glass works had to be careful about their source of sand, glass sand is low in iron and high in alumina. The three main types of glass for which such sand is used are clear-container glass, float or flat glass (for making windows) and coloured-container glass (these days glass sand is also used in the production of other glass products such as TV tubes, crystal glass and ovenware). Up to the early 20th century the glass makers got sand from Alum Bay in the Isle of Wight and at Lynn in Norfolk and some firms used sand from Reigate, but getting sand that was sufficiently pure was a challenge at the time (some works used crushed quartz or flint instead). In the 20th century other sources were found and exploited, there are suitable sands in Fife (Scotland now produces about 10 percent of the UK's silica sands) and near St Helens in Lancashire (where Pilkingtons set up their glass works).
The sand works at Leighton Buzzard formed the basis of one of Dave Rowe's superb working diorama's and his book on Mechanical Models (see Bibliography) should be read by anyone considering a sand works for their layout. The sand drying works at Leighton was connected to the pit by a 15" narrow gauge railway which used armoured locomotives left over from the First World War to haul trains of side tipping wagons.
Fig ___ Armoured locomotives
These were actually rather large machines, most locos on these narrow gauge quarry lines were little 'Simplex' petrol engined machines by the 1930s.
Sand from the pit is washed to remove clay and other detritus, the sand in Cheshire often has coal mixed in with it. The old method used what was essentially a miners 'biddle' consisting of an inclined trough about fifteen feet long by about eight feet wide, the sand was put in at the top end and water (from a hose) played over it. The sand water mix runs down to the lower end of the trough, the heavy sand collects at the bottom but the water (with the lighter dirt and coal suspended in it) washes out over the end.
By the early 20th century they had developed the 'water ejector' which used a conical hopper in which the sand was mixed with water, this drained into a second hopper where the sand settled to the bottom (and was drawn off) whilst the dirty water overflowed from the hopper into a gutter which lead to a settling pit. In practice, for really dirty sands, there could be up to six hoppers in a row. The rough estimate for the water required was about fifteen times the volume of sand to be cleaned. There were also powered washers, one being a vertical cylinder about nine feet in diameter by seven feet tall, this had powered paddles to stir the mix as the water passed through, emerging at the top and removing the dirt. Another type used an inclined tube containing a screw, the output being collected by a bucket chain. These powered systems were the norm by the later 1930s.
On Mr Rowe's model, depicting a 1950s scene, a rather more modern system is in use, the sand is mixed with water and run through a tall and rather thin looking 'cyclone separator', the sand and clean water comes out at the bottom, the water with the dirt in it comes out at the top.
Whatever the washing process the result was waterlogged sand, which had to be dried off somewhat before it could be sold. To date I have not confirmed the older methods of doing this although I gather some pits simply spread the sand out on a bank, sometimes onto wooden boards to drain. The resulting damp sand could be shipped out for construction work, some sites used their tipping narrow gauge wagons, usually hauled by small petrol engined locos in the 1930s, running to an elevated gantry (usually supported on sand lime brick pillars) where it was tipped into railway wagons or road lorries.
At smaller works men with shovels would load the sand, or a small steam or petrol crane could be used. From the later 1930s a tractor with a front loader bucket would be increasingly seen on this duty and after World War Two these would increasingly use hydraulic rams in place of cable and pulleys. The example shown dates from 1952.
Fig ___ Ferguson TE tractor with 'front loader' bucket (1952)
By the 1940s (quite possibly earlier) some sites were using a jigging table to get the bulk of the water out of the sand. This was a table set at an angle, the wet sand was tipped onto the lower end and the table vibrated in an oscillating circular manner, on the up-swing the sand was thrown up and the table moved back a bit before it landed. In this way the sand migrated up the table and by the time it fell off at the top end it was damp rather than wet. The scene on Mr Rowe's layout has one of these machines. The technical term for this machine is 'Circle-throw vibrating equipment'
Properly drying the sand after washing resulted in a more valuable product for some applications, and saved a lot of trouble at the glass works or other customer. To get really dry sand the most common approach by the 1930s was to use drying sheds, these were quite tall two storey buildings with the 'drying floor' on the upper level, heated by multiple coke furnaces. The buildings at the larger works (some catering to several pits) were generally well built brick structures, I understand that most had corrugated iron or asbestos roofing. The sketch below is based on a very poor photo, the prototype was a large building in the far distance.
Fig ___ Sketch of a drying shed at a larger sand pit
The sand could be delivered to the sheds either using a narrow gauge railway with tipper wagons running up an incline, or by belt conveyor (which did not need to be covered as the sand was damp). Once spread on the drying floor the sand was turned until dry (I am not sure if this was ever done mechanically, it was certainly done by men with shovels at some works).
The drier on Mr Rowe's post-war model was an oil fired rotary kiln, roughly six feet in diameter and about 20 feet long, much more compact than the shed. The damp sand was supplied by a conveyor belt and fed in to a hopper incorporating a screen for gravel at the higher end and the dried sand was taken away via a covered conveyor extending from a pit beside the dryer to a set of raised silos (it has to be covered as the dry sand would blow off an open belt).
Returning to the drying shed system, once dried on the upper floor of the building the sand was shovelled into hoppers which fed a set of screens mounted in the upper part of the lower floor, I believe these were rotating 'trommel' type screens which separated out the sand from the gravel or shingle (small stones) usually found in it but some works just had a mesh screen. At some works the sand emerging from the screens was directed into another hopper and bagged. At one works in Leighton Buzzard in the 1930s they then used their internal 2 foot gauge railway to transport the bags to a wharf where it was transshipped onto standard gauge wagons for despatch.
Instead of being bagged the sand might be fed via a covered conveyor to a set of elevated silos from where it could be poured into road lorries or railway wagons (later hoppers) for shipping out to the customer. This seems to have been the norm by the 1950s.
The covered conveyors could be suggested using square section wood strips supported at an angle on metal pylons and clad in corrugated iron sheet. To transport the damp sand on Mr Rowe's model there were open and working sand conveyors (he used typewriter ribbon with sand glued to it) but an easier alternative for the damp sand would be to use side-tipping skips on an elevated track (you could go for an overhead rope-way but I do not know of a prototype that used one). In both cases the main works could be off stage, perhaps screened by trees to the rear of the layout. Do remember that the angle of the conveyor belt must be shallower than the 'angle of repose' of the sand (the angle of repose is the steepness of the sides of a pile of the material in question), that means nothing steeper than about 40 degrees maximum for dry sand.
As noted above the gravel or shingles, separated out at the washing plants and drying sheds, were used for road work and other purposes.
Modelling a Sand Quarry or 'pit'
Modelling the complete works of pit, drag line crane, washing, de-watering, drying, bagging and despatch point would require rather a lot of space, however for model railway purposes we can simply have a loading bank for damp sand and some silos for the dry sand, both served by conveyor (or with the damp sand brought over in side tipping narrow gauge wagons). An alternative would be to have a sand mine with the mine trucks passing into a series of buildings representing the washing, de-watering and drying stages of the process but this would still require more space than a simple loading point.
A couple of 'worked examples' may be of help and the most basic operation would recover a mix of sand and gravel for use in the construction industry.
I had a chat with Ian Franz who worked at a site run by Cooper Ready Mix (a concrete supplier) in the later 1960s, the first example is based on the information he supplied.
The pit was waterlogged, the two cranes used were both Ruston-Bucyrus, an RB30 and an RB38 drag lines. These emptied the sand onto the bank beside the pit where it was left for the night to drain. The following day the same cranes transferred the pile into a hopper which fed onto a three foot wide picking belt where two men removed the larger stones (known as 'duck stones'). At the end of this belt it dropped into a second hopper and onto a conveyor which took it to the top of the 'plant'. This was a large structure, roughly four stories tall and about twenty feet by thirty feet in floor area. On the example in question there was a roof but very little by way of sides. The sand and gravel mix was poured into a tall hopper/washer, about eight feet (2.6m) in diameter. On the inside, a few inches down from the top was a four inch (100mm) wide gutter with a raised lip, the water was fed in at the sides near the base of the hopper and 'bubbled up' through the sand, bringing the detritus with it. The water then ran over the sides of the gutter and was lead away down a pipe to a settling pond (after the dirt settled out the water could be reused). The base of this washing hopper tapered down into a pipe about 4 inches (100mm) in diameter which directed the washed sand and gravel onto a series of inclined vibrating screens, the sand falling through the first screen was collected in a hopper beneath. The second screen separated out the small 'pea gravel' and a third screen separated the remainder into 'large' and 'small' stones, all collected in hoppers beneath the screens. To decouple the vibration from the structure the vibrating plant sat on giant vertical springs, about eighteen inches (45cm) in diameter and about four feet (1.3m) long (these can be seen in the first floor bay just right of centre on the illustration below). At intervals the base of one of the hoppers would be opened and the contents fell into a screw conveyor leading to the end of the plant, lying at the base of the structure in the right hand end are a couple of twelve inch diameter, four foot long, screws used to move the material coming out of the hoppers. A screw elevator carried the sand up a pipe (close to a foot in diameter) known as a 'giraffe neck'. The pipe could swing through an arc and deposited the sand and three grades of stones onto separate piles.
Fig ___ Typical sand plant - 1960s to date
For modelling purposes the 'plant' building could be clad in corrugated metal sheeting (there was a lot in there, including the washer hopper, screens on their heavy springs, pumps, motors and conveyors). A simple front loader tractor could be used to load railway wagons, or there could be a series of loading bunkers beside the track, fed by separate inclined conveyors from the plant (the bunkers could be positioned at one end of the plant to save space). The area where the piles were formed, and the plant itself, was all concreted over. From the piles at the Warrington works described above a brand new Caterpillar 944 bucket loader was used to transfer the materials to lorries.
Fig ___ Typical bucket front loader - 1960s to date
Construction sand was shipped in low-sided (typically three plank) open wagons, BR built a number of half-height tippler wagons which were used for sand traffic (amongst other things). Gravel could be moved in mineral wagons, probably 5 plank opens or BR standard steel wagons or tipplers, it could also be transported in hopper wagons.
Some jobs, such as glass making, require sand which has been processed to remove impurities, for example flint glass uses dried sand which has been washed and bleached with acid to remove any traces of iron. Sand which has been dried is shipped in sheeted open wagons or in covered hopper wagons (Graham Farish offer such a wagon in the range of modern air braked wagons). See also 'Lineside Industries - Glass Works'. In the BR era a lot of silica sand was moved in covered hopper wagons, the example shown is a vacuum braked Covhop from 1962 reserved for glass-sand traffic.
Fig ___ Glass sand Covhop wagon
Not all hopper wagons reserved for sand traffic were branded with a company name, several Prestwin wagons reserved for sand traffic simply had the word sand in three inch high lettering at the top of the usual markings.
Sand Producers
The major users of bulk sand are the construction industry and the glass industry, firms in both areas operated a number of the sand pits around the country. There were however a lot of smaller firms set up to recover the sand, mainly dealing with construction grade material, only the larger pits could afford the drying plant.
Pilkington Brothers (the glass makers) started extracting sand deposits at Rainford in 1880 and had a siding put in at Mill Lane (on the LNWR branch, the siding also served the factory of Rainford Potteries Ltd to ship their glazed earthenware pipes, tiles and bricks. Other sidings on the opposite side of the line were provided for coal, building materials and agricultural produce although the glass business was so important the yard was known as Pilkington's Yard.
Sand was brought to the Pilkington Brothers sand wash by narrow gauge wagons where it was transshipped to standard gauge wagons for onwards movement to the sheet glass works in St Helens (the Rainford sand only being suitable for sheet glass, the silver sand for the Plate Works coming from abroad).
The wagon illustrated has a rather modern looking livery but this is a Pilkingtons sand wagon (note no doors) from the 1930s. They also had a livery with a single diagonal toward the right hand end and larger lettering on the upper left of the body.
Fig ___ Pilkington's wagon for sand
British Industrial Sand came into being in 1951 as a result of a merger between the Standard Brick and Sand Co. of Redhill and Joseph Boam of King's Lynn. This company became Hepworth Minerals & Chemicals Ltd (I believe in the mid 1990s), in 1998 it became part of Sibelco Minerals & Chemicals Ltd. In 2001 Sibelco Minerals & Chemical Ltd. merged with Watts Blake Bearne & Co plc to form WBB Industrial Sand (WBB is owned by a Belgian company, Sibelco).
Watts Blake Bearne & Co plc produced ball clays (used for pottery making) at their Bovey Basin in South Devon and the Petrockstowe Basin in North Devon for over 300 years. Since 1873 they have also produced kaolin (china clay) from the deposits of south-western Dartmoor.
Gravel
Gravel is basically what most people would refer to as pebbles, river-deposited rounded stones. It may be recovered from quarries inland using a drag crane (in a similar way to sand) or dredged up from water filled gravel pits on the coast (when it is often referred to a shingle). One unusual gravel works in the Orkney Islands has a monorail system on which a skip is lowered into a catchment in the sea and cable hauled up to a hopper on the cliffs above. The monorail was built the island of on Lambholm, it uses a standard Ford engine to operate the cable haulage and the tide re-fills the catchment every day. This equipment, operated by Alexander Sutherland (Contractors), was still in use in the early 21st century.
Principal sources of gravel in the UK are along the South East coast, notably at Dungeness in Kent and Chesil Beach in Dorset. Gravel is also a by-product in many sand quarries. At one time gravel was a major commodity in the building trades, but since the Second World War it has largely been replaced by crushed stone chippings. Gravel pits usually extend down below the water table and needed to be pumped out as they got deeper, old abandoned pits are these days just lakes, often set aside for recreation.
The gravel dug out of the ground has to be separated from the sand and dirt, this required a washing plant, the simple option being an inclined trough and a hose as described for sand above. At larger works a powered system was used, either a variation on the 'jigging table' (described in 'Lineside Industries - Mines - General Introduction') or a trough with a screw driving the gravel along, I believe both types would be enclosed in a simple shed (an easy option to model).
The gravel also has to be separated from any sand (which being as heavy as the gravel is not washed out), some gravel pits also sold the sand recovered. This was done by dropping the gravel onto a 'riddle' (a set of metal bars in a frame and at an angle, small stuff falls through, big stuff rolls down and off the end). By setting the bars to widen out toward the lower end the gravel could also be divided by size in the same action, however I have not yet seen a UK plant that divided the gravel in the open using this method. The riddle could (I think) be combined with the washing stage but I have found no references to this being done. The riddle could be served by a man with a shovel (done at smaller works, he would shift about 20 to 25 cubic yards in a ten hour working day) or by a crane using a drag line or grab. The example shown is sketched from a photograph taken in the 1930s and was served by the quarry crane.
Fig ___ 1930s Gravel sorting and cleaning riddle
A powered option is the 'trommel', a rotating drum with holes in it, set at an angle. The holes get bigger along the drum, allowing progressively larger pebbles to fall though into bins underneath.
After washing and grading the gravel is stockpiled in the open, subsequently being loaded into railway wagons from over track loading hoppers, or in earlier times by men with wheelbarrows and shovels. The hoppers were filled by (usually) the same crane that recovered the gravel from the pit. Typically this would be a rail-mounted drag crane (often running on two parallel narrow gauge tracks), caterpillar tracks came in during the 1930s (tracked cranes are easier to model). The silo itself was often pretty rustic, the example shown below is from a photograph taken in the 1930s, note that the diagonal braces and corner posts are not cut to length. The hopper is wood, lined with metal plates. At the base of the hopper would be a sliding 'gate' to control the flow into the wagons.
Fig ___ 1930s gravel loading hopper
For more modern gravel works, particularly since World War Two, the hopper would most likely be metal and a shallow hopper is available in the Heljan range which would serve (intended for use in a Continental loco depot). This is quite a large structure which would require a conveyor feed from a silo or stock pile for this application.
For larger works with a long term supply available a larger silo type building might be constructed, either an over-track bunker or a building beside the tracks feeding the wagons through a chute or chutes on the lower sides. On the few maps I have found showing older gravel pits there was no indication of a weighbridge, although these were all small and used lorries to transport the product, so they could have used a nearby public weighbridge. For larger rail connected establishments I suspect a trail weighbridge would be provided.
The illustration below is based on a tracing of a photograph of a British gravel quarry in the 1930s. A diesel engine is housed in the shed A, this drives all the equipment. The washed gravel is hoisted in a skip up the inclined track B. At the top it is tipped into a hopper C which feeds the trommel, the trommel itself is covered in a sheet metal cowling. The first section of the trommel D has small holes in it to extract as much sand as possible, the gravel then enters a concentric set of rings E, the centre one having the largest holes. This separates out the gravel, apparently into two grades which are fed down chutes F into the bunker. The outermost ring collects the last of the sand, all of which is directed down the side chute G. The gravel bunker has a series of loading chutes H for filling road vehicles or railway wagons.
Fig ___ 1930s gravel depot
This is a genuine prototype, but if space is tight we can alter the arrangement slightly to reduce the depth from front to back considerably. Firstly the trommel is rotated to run parallel with the gravel bunker, and instead of the concentric rings (which are tricky to model at the end) we could have a standard punched tube to separate out first the sand, then the small gravel and then the larger gravel. Any really big rocks could be allowed to fall out of the far end and down a chute to be passed to a stone crusher. The trommel itself would be enclosed in a sheet metal cowling as shown above to keep the dust down a bit. The incline and skip for loading the trommel is easy to model, some similar installations had a pit with a bucket chain elevator instead. The incline is now placed at one end of the bunker to be loaded by horse and cart, lorry or tipping narrow gauge railway trucks (these would preferably be end tippers). The gravel bunker is made long enough for two or four wagons (in British N that means either 3,5 inches or 7 inches), the sand chute extends out from the end with the incline and skip beside it. The engine is moved directly under the sand chute and the whole assembly could be mounted on a raised bank (to make loading the recovered sand by hand easier). To add movement the 'flywheel' on the trommel drive could rotate and the skip could be made to rise and drop. The resulting structure would end up in the region of 6-10 inches long and about two inches deep in British N, which would fit in a corner even if very tight 9 inch radius curves were in use on the main line, if this looks too complicated to make consider the simple crushed rock loader shown below.
Fig ___ Compressed 1930s gravel depot
In the post war era gravel pits were operated using crawler-cranes (often rather large), tractors with 'front loader' buckets (again these got larger over time) and big tipper lorries (which required access ramps down into the pit). Where a rail loading point was required the norm was a row of quite shallow hoppers, perhaps four (one per wagon), filled by a bucket chain elevator or just a tractor with a front-loader bucket.
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