Woodsford Quarry Comes On Stream
Hills Group open their first mineral extraction operation in Dorset
Hills Group are an independent, family owned, regional business based in Swindon, Wiltshire, but with operations extending from Gloucestershire, across Oxfordshire to Berkshire and down to Dorset. Originally founded as a brick-making company in 1900 by Edward Hill, today, four generations later, the multifaceted, multi-million pound business employs around 385 people and encompasses the production of quarry products (Hills Quarry Products Ltd), waste-management and recycling activities (Hills Waste Solutions Ltd) and the building of quality new homes (Hills Property Ltd).
The Hills Quarry Products (HQP) division operates a network of five quarries across south-west England, producing a full range of sand, gravel and limestone aggregate materials, as well as three plants producing ready-mixed concrete and floor screeds. Several of the division’s quarry sites are also licensed for the tipping of inert materials which, in turn, provide a source of recycled products and topsoil. This year, as the company celebrates its 110th anniversary, Hills have marked a southward expansion of the business with the opening of Woodsford Quarry, their first mineral extraction operation in the county of Dorset.
The site
Located on agricultural land in the Frome valley about 5 miles east of Dorchester, Woodsford Quarry lies between the villages of Crossways and Woodsford and comprises a low-level, state-of-the-art mineral processing plant and associated ready-mixed concrete facility. The site had been earmarked as a potential development area in the county’s mineral plan for a number of years before the landowner was finally granted planning approval from Dorset County Council, albeit with several conditions relating to noise levels, hours of work, water discharge, road improvements, footpaths, landscaping etc. With permission secured, some two years ago competitive tenders were sought for operation of the quarry and the bid from Hills Group proved successful.
The total area of the site is 163.3ha and the current consented reserve of sand and gravel amounts to 4.2 million tonnes. Based on a target output of around 175,000 tonnes per annum, the operational life of the site is estimated at 24 years. Progressive low-level restoration of the site, incorporating an improved drainage scheme, will ultimately return the land back to full agricultural use.
Access to the quarry is from the south-west corner of the site via a new entrance on to the Crossways to Dorchester road. This was constructed early last year by Bath-based Ikon Construction, who went on to build the site’s main and internal asphalt access roads and processing plant foundations during spring 2009. Although the plant and operations in the north-western part of the site are well screened by landforms and hedgerows, as part of the site preparation works, the ground level in the vicinity of the plant was lowered by 2m and environmental bunds were constructed along the north-eastern and western boundaries of the site to further reduce the visual and noise impacts of the operation.
Geological setting
The extraction site is situated close to the western extent of the Tertiary Reading Beds (Bagshot Beds sequence) on the western margin of the geological structure known as the Hampshire Basin. The valley gravel forming the economic mineral at the site occurs at low elevations predominantly within the valley of the river Frome, which flows from west to east across the region. The mineral itself (a flint sand and gravel) is of Pleistocene Age and composed of materials derived from plateau gravels that occur to the south of the quarry. The deposit, which varies in thickness from 3m to 5m, overlies beds of the Lower Greensand formation and is considered as an upper aquifer. As such, the water table is affected seasonally with maximum elevations occurring between January and March, followed by a steady decline during the summer months. However, groundwater levels at the site will be controlled by pumping during the winter months to allow dry working of the quarry all year round.
Extraction and feeding
The depth of overburden varies across the site but generally comprises around 400mm of topsoil/subsoil with occasional silt/clay lenses measuring 100–300mm in thickness. Removal of these materials is carried out in house by HQP between April and August using a Volvo EC290 excavator, two Volvo A25D articulated dumptrucks and a Komatsu D61PX dozer. As the site is still in its early stages of operation, the topsoil and subsoil are currently being stripped separately and stored in amenity bunds until required for restoration purposes. As the site develops, however, the soils will be stripped and directly placed on previously worked-out areas as part of the ongoing restoration programme.
The sand and gravel deposit is extracted from the dry working using a Volvo L150F wheel loader and fed into a Finlay 595 mobile screener. This unit pre-screens the ‘as dug’ material at 50mm and feeds the –50mm fraction on to a 750mm wide field conveyor for delivery, at a rate of 180 tonnes/h, to a surge pile with a live capacity of 800 tonnes. Meanwhile, the rejected +50mm material (predominantly large flints) is discharged to a stockpile to await subsequent campaign crushing using a mobile crusher.
Processing operations
The main processing plant was designed, installed and commissioned to Hills Quarry Products’ specific requirements by Leicestershire-based main contractors Wileman Engineers Ltd. Erection of the low-level plant (all elements are less than 8m in height) was completed during summer 2009, with final commissioning taking place during October and November. It is designed to wash and size the primary screened ‘as dug’ sand at a maximum throughput rate of 180 tonnes/h to produce, in conjunction with a Linatex sand plant, a full range of washed sands, single-size gravels and ballast materials for supply throughout Dorset and neighbouring Somerset. The plant is equipped with a PLC system, supplied by Bridges Electrical, which controls all aspects of its operation, such as feed rate and start/stop sequencing. In addition, all conveyors are fitted with rotation sensors to facilitate unmanned running. The quarry’s weighbridge and ticketing operations are conducted through a computerized system supplied by Avery Weigh-Tronix.
Recovery of material from the surge pile is achieved via a 30m long x 3m diameter Armco-type tunnel, supplied by Asset International. A rubber-lined chute in the roof of the tunnel feeds material, via a Rowecon motorized cut-off gate, on to a variable-speed belt feeder which, in turn, discharges on to a 160m long x 800mm wide troughed and part-inclined tunnel reclaim/field conveyor for delivery to the plant. The belt feeder is PLC controlled via a Siemens Acumass belt weigher to maintain a consistent feed from the surge pile to the processing plant at the desired rate.
On arrival at the processing plant, the –50mm material is fed in to a Hewitt Robins 4.2m x 1.8m double-deck washing screen equipped with SCS polyurethane deck modules. The top deck cuts the material at 40mm while the bottom deck cuts at 5mm. Any +40mm rejects are sent, via conveyor, to a ground bay for subsequent reprocessing by a mobile crusher, while the –40mm +5mm fraction is conveyed to a Wileman 2M3 Pnu-Drive 2m diameter x 3m long contraflow scrubber barrel for attrition to release the sands and clays ahead of the final grading screen. Fresh water feed requirements to the washing section amount to approximately 1,100 gal/min.
The second and final screen is a Hewitt Robins 4.2m x 1.25m double-deck horizontal screen. Once again, both decks are fitted with SCS polyurethane modules, the top deck cutting at 20mm and the lower deck cutting at 6mm and 10mm. Any 5–6mm product is sent to a ground storage bay while the 6–10mm and 20–40mm fractions are conveyed to their respective ground stockpiles by a pair of static inclined conveyors. The predominant 10–20mm fraction is conveyed to its ground stockpile by a radial conveyor.
All the stockpiling and transfer conveyors used within the plant are powered by Renold drives and gearboxes of varying sizes and equipped with either 800 or 600mm wide x 500/3-ply all-nylon belts with rubber covers, supplied by RAP Conveyors. The conveyor drive shafts run in INA FAG self-aligning ball bearing plummer blocks, while the belts themselves travel on 102mm diameter three-roll, equi-length, drop-in idler sets with sealed-for-life bearings supplied by Rulmeca UK Ltd. Scraper sets from RH Conveyor Services are fitted under all head drums for belt-cleaning purposes.
Meanwhile, the sand and fines released by the washing screen and the scrubber barrel are pumped to the holding sump ahead of a Linatex compact sand plant and dewatering screen system. Some 1,150 gal/min of water and up to 100 tonnes/h of –5mm solids are pumped from the sump to the centre of a 1.22m diameter Linatex S-Type classifier. Here, the coarse fraction reports to the base of the classifier vessel and is extracted by a Linatex 200/150mm bore pump and fed to a Linatex 660 G4 hydrocyclone. This apparatus washes and thickens the product, removing –75 micron material via the overflow while the underflow discharges ‘coarse’ sand on to a VD15 dewatering screen, which removes the free moisture prior to stockpiling via conveyor.
Concurrently, the overflows from the S-Type classifier and the coarse hydrocyclone are directed to a collecting sump and then pumped by a 150/125mm bore pump to a Linatex 375 G4 hydrocyclone. As before, this unit washes and thickens the product and removes, via the overflow, any unwanted –75 micron silts and excess water. This time, however, the underflow discharges ‘fine’ sand on to the VD15 for dewatering prior to stockpiling. The single dewatering screen is fitted with a dividing plate down its length, to maintain separation between the coarse and fine sands.
Silt and excess water from the sand plant is discharged to a ground sump where a Warman 150SP vertical pump delivers the liquor through 300m of 280mm diameter HDPE fused pipework to the quarry’s silt lagoons. Connected by a series of channels and with a potential silt storage capacity of 98,000m3, these allow hindered settlement of the solids and provide clean water for re-use in the washing process. The clean water pump, a 37kW Flygt electric submersible, delivers 350m3/h back to the plant via 360m of 250mm diameter HDPE fused pipework.
The loading out and rehandling of the final sand and gravel products is carried out by a Volvo L120F wheel loader equipped with an on-board weighing system, with deliveries to customers being made by the company’s franchised vehicle fleet as well as additional contract hauliers, as required. The Volvo L120F loader is also used to feed aggregate materials to Woodsford Quarry’s on-site ready-mixed concrete plant.
Concrete plant
The fully enclosed Econocrete C100 combined dry batching and wet-mix plant, supplied by Basford Plant Ltd and owned and operated by Hills Quarry Products, has the capacity to process 60–80m3 per hour and features a 4m3 dry batch capability and a 2m3 Teka THZ 3000 single-discharge pan mixer.
The plant is fed via a 10-tonne capacity loading hopper and 650mm wide x 30° inclined radial chevron belt arrangement. Aggregate materials are stored in six overhead bins, each with a capacity of 50 tonnes, while cement products are stored in three 60-tonne silos which incorporate a silo safety system. The plant also includes a 32,000-litre recycled water storage and pumping system, a 4,500-litre clean water header tank, a four-admixture pumping and weighing system and a 4m3 aggregate and cement weighing machine.
An elevated and air-conditioned control room houses the plant’s automated control system, which comprises a Windows-based BPL computerized batching system with multi-recipe and batch data-recording facilities. Full independent manual control is also provided.
Open day
The opening of Woodsford Quarry received a positive response from guests who attended an open day at the site on 12 May 2010. Visitors included local residents, members of the site liaison committee, representatives of Dorset County Council, suppliers, environmental groups and customers. Those present were taken on tours of the site and shown the processes involved in extracting minerals and producing ready-mixed concrete. Thames Valley Archaeological Services were also on hand to provide a display of interesting finds from the quarry.
Commenting on the open day, Hills Quarry Products’ group director, Alan Mackenzie, said: ‘This type of event gives us the opportunity to meet with stakeholders and to work together, via future liaison meetings, to provide the minerals that society needs to build our schools and hospitals and improve the environment we live in. We have a reputation for providing quality and service to our customers, and I believe the new operation at Woodsford will allow us to continue to do this.’
Acknowledgement
The editor wishes to thank Hills Group for permission to visit the site and, in particular, Alan Mackenzie, group director - Quarry Products; Steve Bowman, quarry manager; Andrew Liddle, operations manager; Peter Andrew, business development manager; and Matthew Joyce of Wileman Engineers Ltd, for their help in the preparation of this report.
