The end stability of a dam, whether it be a concrete arch gravity dam or a rockfill dam, depends on the quality of materials used in construction. Not only must the materials be clean of deleterious particles; they must be of excellent shape and contain tough, clean particles. Obtaining sand for dam construction has, historically, been difficult and at many sites the coarse and fine aggregate sources have both not been available. When the aggregate source is from hard rock the creation of quality fines has been difficult.

In New Zealand in the 1960’s, Jim Mcdonald had the idea for the rock-on-rock Barmac vertical shaft reduction concept to help resolve these prodlems. In partnership with Brian Bartley, Mcdonald developed the early versions of the machine and were awarded numerous patents for their innovations. Since those early days there have been significant hardware improvements and market strides around the world.

The latest Barmac incarnation, the B-Series VSI, is an ideal third or fourth stage crusher designed to crush the complete range of ores, rocks and minerals. Barmacs combine high velocity impact crushing with attrition crushing to produce cubical product & quality sand. Due to the pure rock-on-rock principle, impeller shoes or impact anvils are not needed to achieve reduction on Barmac crushers.

Crushing action

The overall crushing action in a Barmac B-Series VSI is a complex process because several different crushing mechanisms are involved. It is, in essence, a high-energy centrifugal rock pump. Several fracture mechanisms – impact, shatter, shear, attrition and abrasion – achieve size reduction in the Barmac. Reduction results from the high intensity inter-particle and particle-to-rock-bed collisions occurring within the crushing chamber and rotor.

This continual process replenishes the rock lining while at the same time maintaining a rock-on-rock chain reaction of crushing and grinding. The particles in the crushing chamber collide until they have lost sufficient energy to enable them to drop out of the particle cloud and leave the crushing chamber. Typically the residence time in the crushing chamber is between 5 to 20 seconds.

In response to the ever-demanding requirements from the comminution industry to further reduce the cost of reduction, the cascade phenomenon (introducing a second stream of material, in a controlled quantity, into the crushing chamber) was investigated. The result was the discovery of increased turbulence in the crushing chamber.

The introduction of the second stream of material causes a supercharging of the particle population within the crushing chamber, improving the energy transfer between particles and providing ‘crushing for free’.

The second stream of material does not come into contact with the moving rotor. Therefore the extra ‘work’ done by introducing the second stream of material does not increase wear costs nor does it consume any more energy.

Several design features such as rotor size, cascade ratio, rotor speed and crushing chamber liner profile, can be adjusted to enable a wide range of control over the final product grading.

Material for dams

Particle shape is perhaps the most important factor for producing a high quality concrete aggregate. Cubical shaped aggregate performs better than aggregate of a poorer shape. The advantages of using cubical shaped aggregate in concrete are:

• Improved workability in placing the mix with less time and effort required.

• Mixes are more pumpable.

• Concrete is easier to finish and gives a better surface with less time and effort required.

• Less cement is required per cubic metre of concrete, for a given strength.

• Higher strength for a given cement content.

• Higher strength aggregate in the mix.

• Better mixing of ingredients, fewer voids, less bleeding and better consistency.

• Better shape leads to better packing of the particles. This results in less voids and structurally sound in-situ fill materials.

The Barmac has been developed to bring both a shaping and cleaning mechanism to the production of these types of coarse and fine aggregates. The crushing action breaks down soft material at a faster rate than the harder, tougher particles. This results in a product that is free of the softer materials that would perform poorly when subjected to the loads and conditions involved.

Throughout the world, concrete producers are moving towards the use of crusher fines – traditionally a waste product – as manufactured sand products. Extensive testing is showing that manufactured sand when processed by a rock-on-rock crusher can be used to make concrete with superior strength characteristics and at a lower cost than concrete made with natural sand.

Case studies

Clyde dam (New Zealand)

In the early 1980’s the New Zealand Government commissioned and managed the construction of the Clyde dam in the South Island.

As the project proceeded it became obvious that the sand fractions were in short supply and were poorly graded. The percentage of sand in the designated concrete aggregate for the project had been over estimated. The combined problem of variable gradings and supply prompted investigation into alternatives, such as transporting sand from another source and manufacturing sand on site from the surplus coarse aggregates.

Manufacturing the sand on-site seemed the most practical and after costing the options a Barmac VSI was installed. The contractor then proved to the government department managing the contract that Barmac Sand could stand up to microscopic tests for shape comparison with natural sand. Trial batches carried out showed no increase in water demand and no loss of workability and pumpability. The Barmac produced both coarse and fine sands to specification, drastically reducing costs on the contract.

Rio Jordão river diversion dam (Brazil)

A consortium in Brazil designed a crushing plant to produce manufactured sand for one of the largest dam operations in Brazil – the Rio Jordão river diversion dam in the southern state of Paraná. 100% manufactured sand was used in the construction so the products had to be of the highest quality. There were two major factors that influenced the choice of crushing technology; the material and the product specifications.

The material was basalt, which, due to its cleavage planes, is easily crushed in the initial stages. However, in the later stages, it tends to be very resistant and difficult to fragment. In addition to this, specifications dictated that fines below 80 microns should not exceed 15%.

Initially, the installation involved two primary jaw crushers, three secondary cones and a Barmac VSI alongside a cone crusher in the tertiary section. At this stage, the Barmac generated 60% of the plant’s sand production of 180 tph. Following pilot tests, it was decided to increase the plant sand capacity to 200 tph by installing a quaternary Barmac VSI. The installation of the second Barmac increased the capacity of the plant by 20tph with minimal changes to the existing plant.

The Barmacs were fundamental to this plant’s ability to produce manufactured sand with the specified gradation and shape characteristics. In addition to this, the Barmacs allowed the plant to exceed the design capacity, contributing to the completion of the dam forty days ahead of schedule.

Matahina dam (New Zealand)

The Matahina dam in New Zealand required reconstruction after an earthquake caused consolidation of the underlying alluvial sediments resulting in unacceptable dam settlement. To provide aggregate for this project, a specialised plant capable of 170tph was built on site. The plant was required to be in operation 22 hours a day, six days a week, with minimal unexpected downtime to meet the tight project deadlines. It was also required for the plant to produce high quality aggregates with little to no deleterious materials present in the final product.

Aggregate was sourced for the dam’s reconstruction from a nearby quarry. The shape of the aggregate needed to be compliant with tough particle shape criteria detailed in the contract documents. To achieve the shape required, two Barmac VSI’s were installed into the tertiary phase of the plant.

The Barmac rock-on-rock crushing mechanism produced high quality cubical aggregate that performed well in this type of application.

Three Gorges dam (China)

The Three Gorges hydro power project in China is a massive construction project requiring mammoth resources of sand and aggregate. A shortage of natural sand in the area, and the high costs associated with transporting it from elsewhere, prompted the construction corporation to look at manufacturing sand.

metso-minerals proposed two Barmac VSIs to produce manufactured sand from a minus 40mm cone crusher product.

The two Barmacs operated alongside some rod mills, making manufactured sand from the hard, abrasive granite. Comparison trials showed that each 440kW Barmac matched the production capacity of three 220kW rod mills, while wear costs were reduced to US$0.40 per ton of sand from the more expensive US$1.00 per ton seen in the rod mills. Trials showed that by utilising the Barmac’s unique cascade system, production of minus 5mm sand was increased by 7% for no extra power or wear part costs.

Thus, by installing Barmac VSIs in the quaternary stage, rather than cone crushers, the customer reduced the number of mills operating. The need to increase sand production to 1000tph, together with the success of the existing machines led to the purchase of a further three Barmac VSIs.

Mian Hua Tan dam (China)

The supply of high quality aggregate for concrete, both coarse and fine, was of extreme importance for the construction of this major dam. The contract called for the production of 1Mt of coarse aggregate and 400,000t of fine aggregate within a period of seven months.

An investigation revealed that gravel reserves were not large enough to satisfy coarse aggregate requirements. The reserves of natural sand were also not sufficient for the construction requirements. The sand that was available was located below the pond level of the dam and its continuous availability could not be guaranteed.

Most importantly, however, the sand was predominantly single sized, lacking both the coarse and fine fraction. Without the extensive use of admixtures and mineral fillers, such sand would have created workability problems during placing as well as permeability problems when hardened. In any event, the use of mineral fillers and admixtures would have been too costly.

Sufficient reserves of high quality granite were identified in close proximity to the dam. Due to the unsuitability and insufficient quantity of the naturally occurring gravel and sand, it was decided to mine and crush the granite to produce the total requirements of both the coarse and fine aggregate for this project.

In China, sand has traditionally been manufactured by processing coarse aggregate through rod mills. Since this is a wet process, the sand must either be de-watered or allowed to drain naturally for at least five days before it is dry enough to be transported by the materials handling equipment without the aid of mechanical equipment.

Although water is freely available, treating the process water is expensive due to the high initial cost of equipment and the on-going cost of chemical additives and sludge disposal. High water usage (4000 litres of water per m3 of sand) also adds to an already high process cost.

After investigating sand production methods used on other dam sites, dry crushing was selected as the most economical and environmentally friendly method of sand production.

By processing all minus 40mm aggregate through a Barmac a constant supply of high quality sand and aggregate with superior particle shape was assured.

The plant contained a primary jaw crusher, gyratory secondary crusher and cone tertiary crusher. Two Barmacs were installed with the express purpose of producing coarse aggregate with low flakiness index and for the production of high quality manufactured sand for the project.

To ensure the removal of clay, the minus 20mm material emerging from the primary crusher was discarded. The plus 20mm material was placed in the primary surge pile before being processed by the secondary gyratory crusher into the secondary surge pile.

A tertiary cone crusher crushed the plus 140mm material. The 40mm to 80mm material was either stockpiled to be used as extra coarse aggregate or returned to tertiary crushing for further reduction.

All of the minus 40mm material was processed through the Barmacs and screened to produce the medium and fine aggregate and sand requirements of the project.

To ensure reliable concrete production, the sand stockpile was completely enclosed to exclude rainwater.

All fractions easily surpassed the flakiness requirements of the contract specifications.

Burnett River dam (Australia)

The 300,000 megalitre Burnett River dam, which is currently being built, is located on the Burnett river, 80km southwest of Bundaberg in Queensland, Australia.

A shortage of water in the Burnett Basin was limiting agricultural production as well as urban and industrial development. The development of the dam was required to provide additional water for existing and new irrigated farms, meet existing and future urban and industrial water supply demands, allow further diversity of agricultural production in the lower Burnett area and support citrus growers and horticulture.

Two mobile crushing lines feed a stationary plant for tertiary crushing through a cone crusher and quaternary crushing for shaped material through a Barmac VSI. In total the plant processes over 300 tph running between 393 to 433 hours per month. Since commencement of operations at the Burnett River dam, the plant has already processed enough aggregate to create a stockpile of over 750,000 tons.

In that time the Barmac was critical in producing the necessary fines for the all-in Roller Compacted Concrete (RCC) final product. RCC has the same ingredients as conventional concrete: cement, water, and aggregates, but is much drier. It can be placed quickly and easily with large volume earth-moving equipment. Sections are built lift-by-lift in successive horizontal layers so the downstream slope resembles a concrete staircase. Once a layer is placed, it can immediately support the earth-moving equipment to place the next layer.

The dam is scheduled for completion near the end of 2005.

Ridges Basin reservoir (US)

The Ridges Basin reservoir is part of the Animas-La Plata Project, to provide water for Southern Ute and Ute Mountain Ute Indian tribes and water districts in the states of Colorado and New Mexico. Based near Durango, Colorado, at an elevation of over 1980m above sea level, the filter material processing plant is scheduled to commence full-scale operation during the spring of 2005.

The Ridges Basin reservoir dam will be over 80m high and will stretch for about 400m from one canyon wall to the other. It is an earthfill embankment structure requiring about 900,000t of washed and dry processed filter materials. It is anticipated that the whole plant will process approximately 2.3Mt of raw feed in two plus years of operation.

The borrow area is a sand and gravel deposit. Due to significant concerns over the presence of ‘soft’ materials in the borrow, considerable attention was given to how the material would degrade during processing, stockpiling, and placement in the dam. To alleviate these concerns, a dry attrition drum scrubber was placed in the Primary stage of this 1150stph primary plant to produce a multiple pass degradation process. All minus 6.3mm material from the primary scalping process is rejected. A closed circuit Nordberg HP cone crusher is used to process an average of 500stph of material to the secondary stockpile.

Two Barmac VSIs, utilising the cascade function, have been selected in the secondary section, which has an average processing capacity of 500 stph, due to their ability to both upgrade finished material quality and produce superior cubical products at high tonnage rates. The Barmac’s ability to handle the application was verified at Metso’s Mineral Research and Test Center located in Milwaukee. This data was then incorporated into the design of the secondary circuit and the tertiary wash circuit.

The Barmacs are fed a 37mm x 6.3mm (1-1/2” x 1/4”) cone crusher product and operate in a closed circuit.

The three (3) washed products are Zone 2 – 100% minus 9.5mm (-3/8”) x 0-5% P#200, Zone 2A – 100% minus 9.5mm (-3/8”) x 0-7% P#200, Zone 3 – 100% minus 37mm (1-1/2”) x 0-5% P#8.