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Perfect Partnering

Washing and screening on the M6 Toll

In early 2000, in conjunction with CAMBBA, Powerscreen Washing Systems began to consider the supply of equipment to process site-won materials for the construction of the M6 Toll. This included site investigations along the entire 44km route, as well as the design plant package to suit CAMBBA's construction programme. It was identified early on that the Weeford cut would realize in the region 1.5 million m3 of material suitable for processing into quality aggregates.

Programme and materials

The processing programme had to be linked to the construction requirements and needed to have the flexibility to meet programme variations which could arise as the detailed design was being finalized. Although at the tender stage the programme and requirements had agreed, the contract had to allow for change and flexibility had to be accommodated within the processing plant layout.

The initial programme showed a processing feed requirement of 8-9,000 tonnes a week through the main washing plant, which has since increased to a current production level of 14,000 tonnes a week.

The washed requirements shown in table 1 will be produced over a 104-week period, while the CBM aggregate requirement of 596,500 tonnes will require screening and crushing within a 54-week period.

It was calculated that demand vs supply would lead to two critical path programmes and therefore the necessary stocks and stocking area would have to be provided.

Washed stocks would have to comprise in the region of 300,000-400,000 tonnes of six different products in order to meet drainage, blended material, PQ concrete and structural concrete requirements on a parallel demand programme.

Raw material needed to meet these requirements will be excavated and delivered to a stockpile adjacent to the plant by CAMBBA. At any one time, this will amount to up to 100,000 tonnes.

Site investigation

Working in conjunction with CAMBBA, Powerscreen Washing Systems excavated trial pits to verify the volumes and gradings of the site-won materials. The materials investigated comprise gravel deposits which form part of the Kidderminster formation of the Triassic Sherwood sandstone (Bunter pebble beds). Discussions were held with other quarry operations in the area and gradings were taken for comparison. This gave an accurate cross-section of the materials available which helped with process plant design.

Plant design

To meet CAMBBA's requirements, Powerscreen Washing Systems had to design the plant to include provision for: crushing oversize +37.5mm in a closed circuit; producing an oversize 6C; attrition scrubbing of material, with by-passing of this process when required; graded aggregates to BS 882; two washed sands to BS 882 and BS 1200; and a guaranteed minimal loss to the silt lagoons.

Consideration was also given to silt- and water-management systems to ensure the availability of a fresh water supply during the summer months.

It was decided to locate the main washing and screening plant within Hanson Aggregates' Weeford Quarry, which is situated adjacent to the contract.

Plant specification

The Powerscreen plant comprises: a Commander 25-tonne primary feed hopper; an M 100 feed conveyor to the primary screen (with full walkway access); a static 16 x 5 twin-deck full rinsing screen capable of handling up to 320 tonnes/h (with full walkway access and rolling chutes for ease of maintenance); a T5032 radial oversize conveyor feeding either a Pegson 1000 Maxtrak mobile crusher or the stockpile, as required; a static log-washer capable of handling up to 175 tonnes/h of coarse aggregates; a log-washer by-pass conveyor for use when scrubbing is not required; a T6532 transfer conveyor to a static secondary 16 x 5 part-rinser twin-deck screen (with full walkway access and rolling chutes for ease of maintenance); and five T5032 conveyors for stockpiling finished aggregates.

For flexibility of gradings, all screens are fitted with polyurethane modules, and all chutes, catch boxes, wash boxes, impact areas and wear areas have been lined with heavy-duty wear-resistant rubber supplied by Screening Consultancy and Supplies Ltd.

The sand-classification plant was supplied by Floatex and includes: a self-regulating sump feeding a sand-classification tank with underflow on t o a 1.5m x 3.0m dewatering screen; and a self-regulating sump tank with two Floatex C500 cyclones mounted above a 1.5m x 3.0m dewatering screen and automatic re-blend chute.

Fines recovered from the secondary screen and log-washer are fed to a sump and pumped to the Floatex plant using a Warman 100 RVSP pump, while process water is pumped to the silt lagoons via a Warman 200 SVSP pump and 200m of 10in piping.

Fresh water is supplied via a Grindex Maxi N 8in submersible pump through 150m of 10in piping, which supplies the required volumes of water to the screens and sand plant at 30 1b/in2.

A Milltronics MS1 belt scale with GSM connection provides both on-plant and remote monitoring of hourly throughput, while a weighbridge supplied by Mettler-Toledo is used to monitor load-out weights.

The entire plant is powered by a Caterpillar genset supplied by Finning.

Satellite operations

While site preparation and installation of the main processing plant was taking place, a mobile satellite plant was installed at Weeford within two days to provide drainage aggregates, 6N, 6P, sand, 6C and starter layer material at the rate of 7,000 tonnes a week.

This plant comprised a Chieftain l400 which removed -5mm on the bottom deck and +37.5mm on the top deck, with the middle product being fed directly into a mobile SW70 log-washer for scrubbing and then into a second Chieftain l400 which split the aggregates into BS 882 drainage products.

Other processing operations on the M6 Toll included the Kingswood Lakes embankment toe, which required the blending of site-won sands with imported recycled stone through a Powerscreen three-bin blender and Ml00 stockpile conveyor, producing a total quantity of 90,000 tonnes.

In various areas of the construction site it was found that the topsoil was rich in gravel, and last summer much of this material was recovered by processing it through a Powerscreen 615 Trommel, for starter layer drainage, or feeding through the main wash plant, as required.

CBM production takes place at three different locations along the length of the construction site. This material is processed through several Powerscreen Chieftain 1400s, while the +40mm is removed and crushed using a Pegson Metrotrak. Products are stockpiled separately to allow a measured re-blend of crushed aggregates with natural aggregates in order to achieve CBM strengths with controlled levels of cement.

These processing plants operate at feed rates to 300 tonnes/h, and total site production has reached up to 55,000 tonnes a week.

Material handling

Raw material is fed to the main processing plant's feed hopper from the adjacent stockpile using a Volvo L l20C wheel loader. Processed materials are loaded directly on to road vehicles for site delivery, taken to the stocking area by two loading shovels, or loaded into dumptrucks for delivery to the concrete batching plant.

Each of the CBM screening plants is fed directly from the face by a 360? excavator and the processed material is stocked adjacent to the operation by loading shovels.

Maintenance and support

As production is critical, the selection process for outside plant suppliers included a requirement that they should be located within one hour of the construction site and able to offer rapid response times with spares immediately available from stock.

To support the Powerscreen plant, carefully monitored consignment stocks of spares are kept on site for programmed maintenance schedules, and general maintenance is conducted outside normal production hours to minimize downtime.

Health and safety

The combination of an aggregates processing plant within a construction site requires stringent levels of control regarding health safety and working permits. A programme operator training with CAMMBA prior commencement of site work was undertaken and monthly inspections and certification operators and prime movers is rigorously enforced and audited. Regular toolbox involving the construction and processing teams also take place to ensure that working practices are continually employed.

In addition, prior to use, all Powerscreen equipment had to pass CAMBBA's plant inspection, which included checks on levels, guarding and access for maintenance.

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