Pushing the envelope at Hinze Dam

In May, the $395 million Hinze Dam Stage 3 redevelopment project will be complete. The project was first commissioned in 2004 and has been co-ordinated by the Hinze Dam Alliance, comprising Seqwater (the asset owner) and three private sector partners: Sinclair Knight Merz (approvals and community management), URS (the designers) and construction project manager Thiess.

The Stage 3 project has raised the Hinze Dam embankment wall from 93.5 metres to 108.5m for a total storage capacity of 310 billion litres (up from 42 billion litres in Stage 1 in 1976 and 161 billion litres in Stage 2 in 1989). The key reason to raise the wall was to mitigate potential flooding in the Nerang River catchment. The project also involved installing a 270m long cut off wall on the right abutment that was up to 55 metres deep and less than a metre thick.

Approximately 390,000 cubic metres of clay was used in the extension of the core of the Hinze Dam wall and was sourced from deposits on the site. The majority of the dam foundation, the left abutment and fresh rock in the spillway and the quarry is a meta-greywacke formation. Greenstone, interbedded with greywacke and chert, occurs on the right abutment and along the footprint of the saddle dam. While the permeability of the greenstone is low, the weathered greenstone, chert and weathered greywacke interface is quite porous, hence the need for the cut off wall in Stage 3. The weathered greenstone on the eastern side of the Nerang River has provided a reasonable quality clay for the dam core.

The filter zone material encapsulating the clay core was from rock quarried by Thiess and crushed on-site by Nordev Contractors. The filter zone protects the clay core from being eroded by the dam water and is then covered by different layers of rock to provide final protection against erosion, giving the dam its strength and retention properties.

The dam?s quarry is owned by the Queensland Government through its subsidiary Seqwater but during construction was leased to Thiess. Approximately 2.1 million cubic metres of ?fresh? rock was removed as well as 780,000m3 of overburden for use as general fill in a new recreation area.

The final cut extends back from Pelican Point to be level with the dam wall. The base of the quarry site was excavated to 85m above the height of the dam (AHD), below the full supply line (FSL) of 94.5m AHD. This resulted in the benching of the slopes and exposed rock faces to a height of 90m AHD.

During construction, vegetation was cleared and soils exposed in the quarry areas for access roads and haul roads and for road upgrades. The borrow area had 250,000m3 of material moved for the construction of the dam clay core. After construction, the borrow area will be re-profiled and rehabilitated.

To assist with the crushing process, Thiess hired Townsville-based quarrying and crushing mobile contractors Nordev to crush the greywacke rock to the very fine specifications required for the aggregate filters and to produce other construction materials for the concrete wall for the spillway. Nordev joined the dam project in June 2008 and remained on-site until November 2010.

Col Wilson, the operations manager for the Nordev fixed and mobile crushing plants at Hinze Dam, explained that the task for his six-person crew was to produce a washed manufactured sand at a grading curve of 0-3 per cent on 0.075 microns. The design team at URS had confirmed that the filtration system would require 2F, 2G and 2H filters for the clay core protection.

?With a grading curve, the wider the envelope,the easier it is to try and get down the centre line,? Col said. ?For the 2F filter, we were required to produce a 5mm sand with very little silt or fines. When we produced the material, we were getting a 12 per cent silt, which meant that the sand had to go through a wash plant to arrive at that 0-3 per cent grading curve.?

Similarly, Col says the envelope on the 2G filter, ?a -20mm run down of concrete aggregate?, also had a tight envelope of 0-2 per cent, again at 0.075 microns, while the 2H filter was a -25mm run down material. Nordev also produced a Bauer 10mm material and Bauer dust.

By the end of its time on the project, Nordev was able to supply Thiess with 187,000 tonnes of the 2F filter, 203,000 tonnes of the 2G, 150,000 tonnes of the 2H, and 21,000 tonnes of the Bauer 10mm and Bauer dust.

To produce such fine gradings, Nordev employed a crushing and screening circuit comprising a 42? x 30? Jaques World jaw crusher as its primary plant, a Terex Cedarapids MVP380 cone crusher as its secondary and a 32? Canica 90 vertical shaft impactor (VSI) as its tertiary. Col added they chose the Canica VSI because ?with the 2F material it had to be a rounded shape?.

There were 16 conveyors in the circuit and two Cedarapids triple-deck 20? x 6? flat deck screens, one to complement the secondary plant and the other for the tertiary. Nordev also employed a Finlay TC15 Sandmaster with cyclones for the washing process.

Nordev commissioned Global Crushers and Spares (GCS) to install the MVP380 cone crusher, the Canica 90 VSI and the two triple-deck 20? x 6? flat deck screens. GCS is a supplier of quality crushing and screening systems with in-house technical and drafting expertise to design and construct crushing and screening plant,

GCS managing director Stuart Wieland explained that the company had three months,rather than 16 weeks, to design, build and galvanise a plant of this type. ?We had to come up with a design for the screen to allow them to mix and blend different materials, so they could meet the tight grading. There was a lot of modelling done on agg flow. We installed all the equipment on schedule while Nordev supplied all the conveyors, did the electrical work and provided the primary jaw. We had a few teething problems which we got through. The greywacke rock was particularly hard, so the secondary crusher was working very hard on single stage crushing. It was doing all the work and it was just in an enclosed circuit, making a -20mm material and that was feeding into the back end of the plant, through the VSI and closed circuit and they were then taking their products off there and washing the sand.?

The tight specifications on the grading envelope did mean some trials, even as the crushing process went full steam ahead. ?When we first took the contract on,? Col Wilson explained, ?we looked at the specs and considered how we were going to go. It was either a sandscrew or this cyclone wash plant because of the tight specifications. There was a lot of trial and error and we had to change liners because on crushers you have different profile liners to make sure you can get the choke feed to get the material into specification. It would have taken us about two months to iron out most of the bugs.

?We were also limited to operating only 10 hours a day, five days a week, due to the ERA (Environmentally Relevant Activities) in place. We didn?t have the opportunity of running a night shift, so everything had to be produced in 10 hours a day, five days a week.?

To assist with the main plant?s daily output, Col added that Nordev utilised a mobile plant to produce the 2H material. The mobile plant did not operate all the time, he said, ?but it would have produced about 80,000 tonnes of material. Based on our numbers, we were processing up to 1600 tonnes a day on the main plant. However, because we had to reduce our tonnes to ensure the material was in spec, we averaged 1250 tonnes a day with the main plant?.

Stuart Wieland added that the screens? unique design was also significant in the processing of the sand. ?The design of the front chutes on the 20? x 6? screens were as such that on any given level, we had four flaps, which if you opened one, would allow you to take the material to the top deck and blend it with the material on the next deck. You could do that four times, in increments of 25 per cent. That happened all the way down, so you could blend to meet the grading.?

In addition to the grading, Nordev also had to contend with the properties of the meta-greywacke rock, which Col Wilson described as ?one of the hardest and most abrasive materials? he had come across in his 20 year career in the quarrying industry.

?The rock was pre-wet in the quarry to minimise dust which presented more challenges with meeting the specs. As soon as you get moisture on the rock, that will stick the fines together and influence the testing results. It was difficult to crush the blue fresh rock,? he added, ?because there wasn?t a lot of ?free winnings?, the smaller stuff like 2.36mm down, the crushers had to actually produce that material. With the specifications, it?s been a big learning curve for me over the last three years. The rock changed daily too. If they were blasting the rock at a certain level, we?d have a really good run, but if they blasted just 10 metres lower and the material didn?t stand up to tests and we were out by one or two per cent then we?d have to chop and change. You had to have your finger on the pulse all the time. It was very hands on.?

Stuart Wieland agreed that the grading curve in the 0.075 was particularly challenging for GCS, ?especially with only one compression crusher and then one VSI. If you?d had another crusher in the circuit, it would have been easier but that would have pushed the costs up, and Nordev had decided to do it with two crushers. The modelling showed it was possible but the proof is always in the eating of the pudding! It was an interesting job because six months down the track we could compare all the results with what we came up with on agg flow and they were almost identical. Greg Rains (Nordev?s managing director) spent a good eight weeks at night modifying the agg flow circuit to best optimise the process. The correlation between the theory and actual results was remarkably close.?

The completion date for the supply of aggregate for the dam?s clay core was in September 2010. However, between September and November 2010, Nordev continued to provide meta-greywacke for road base and concrete in the quarry pit with a mobile crushing and screening circuit comprising a 42? x 30? Parker jaw crusher (primary plant), a Finlay 683 screen, a Fintec 1080 cone crusher (secondary) and a Sandvik UH440 tracked cone crusher (tertiary). In a 10 hour shift, Col stated that the mobile plant was capable of producing between 1800 and 2000 tonnes of road base.

?We have a three stage plant for the roadbase because we?re still using blue fresh rock,?Col added. ?Normally, you can probably do a road base in a two-stage plant but because of the degree of difficulty and the nature of the rock, it needed a three-stage plant. All the other contracts we?ve done, such as in northwest Queensland, Cloncurry and Mount Isa, it?s always been primary and secondary plants that have produced a road base.?

In all, combining the outputs of both the fixed and mobile plants, Nordev was able to provide Thiess over a two and a half year period with between 700,000 and 720,000 tonnes of crushed aggregate for the filter protection for the dam?s clay core, road base for the car parks and internal roads on-site and 10mm material and a concrete dust for the concrete batching plant.

Peter Kinsella, the Alliance Manager for Thiess on the Hinze Dam Stage 3 project, paid tribute to Nordev, saying that the contractor had been presented with a challenging project brief and acquitted itself extremely well. ?The 390,000m3 of clay in the core was sourced on-site. And that?s a big credit to Nordev and its team. They were uncertain at first how they would produce the filters, but they brought in the gear and took the time to produce the 2F filters on-site out of the local material. This saved Thiess in terms of both time and costs. It means we didn?t have to cart in a significant amount of material from elsewhere. That would have meant a whole lot of trucks on the road, particularly on Nerang-Murwillambah and Gold Coast-Springbrook Roads, and would have impacted on the local community.?

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