Damming for dairying

South Island farmers’ love affair with irrigation is proving a boon to a South Canterbury company.   HUGH DE LACY REPORTS.

Tony Moir and Bruce Tinnelly were too busy with other earthmoving projects to get much involved in the South Island dairy boom, but with the farming focus switching to irrigation they’ve found a new niche in building water storage dams.

Moir and Tinnelly are joint company managers of Paul Smith Earthmoving of Timaru, a company founded by Paul Smith, who is now largely retired though still operating a Tekapo quarry, about 25 years ago. The company offers a range of services from roading, forestry and stone-work through to site-works, excavation, demolition, carparks and driveways.

Last year, however, it took on a new kind of project, a large lined irrigation dam on Dermott O’Sullivan’s Cricklewood farm southeast of Fairlie. Holding 385,000 cubic metres on a 5.5 hectare footprint, the dam’s water is gravity-fed from the Tengawai River, part of the Opihi River system serviced by the Opua dam, under two resource consents allowing a total take of 103 litres a second.

The dam was to allow the conversion of the 495 hectare farm from mixed cropping to a 1000-cow dairy farm, with 380 hectares spray-irrigated by two centre-pivots, one of 700 metres and the other 600 metres. Apart from installing lanes for a few earlier projects, Paul Smith Earthmoving had largely skirted the decade-long dairying boom, but the Cricklewood dam took Tony Moir back to his contracting roots on big dams in Australia.

In 1986 Moir formed the patriotically-named Kiwi Construction company in Canberra, and worked with one of Australia’s biggest construction companies, Leighton’s, on the Tuggeranong Dam, a 10 hectare recreational lake serving the Australian capital. It took two years to build, and as well as hundreds of thousands of cubic metres of earthmoving, it involved construction of concrete spillways, walls and structures.

Kiwi Construction also had a hand in a similar project in the Blue Mountains, a water clarification plant involving extensive earthworks and concrete construction that cost $6 million.

Returning to New Zealand in 1996, Moir formed the South Canterbury Concrete Construction Company which did a series of siteworks in the Timaru and Temuka central business districts, before teaming up with Tinnelly, previously operations manager for Sicon, to buy out Paul Smith Earthmoving. They retained the company name with the addition of the year of purchase, 2002.

The Cricklewood dam was originally designed on a 13-hectare footprint and three metres deep, but it was estimated that the combination of evaporation, seapage and nor’wester-driven waves of up to 1.5 metres slopping over the batters would result in too much wastage.

Accordingly the footprint was reduced to 5.5 hectares and the depth more than trebled to 9.5 metres.

Early on O’Sullivan decided to further reduce seapage by having the dam lined with low-density polyethylene sheeting. While construction of the dam was generally straight-forward in terms of earthmoving, the polyethylene made it crucial that the floor of the dam be free of stones or anything else that might puncture it under the weight of the water. Given the rockiness of the Fairlie sub-soil, where boulders up to 500mm in diameter are common, this proved a problem.

The company addressed it by moving a crusher onto the site to produce enough sand to cover the footprint to a depth of    between 10 and 20 centimetres using a standard agricultural lime-spreader. The availability of the crusher was the result of the company in 2005 launching a subsidiary, Paul Smith Aggregates, operating a Nordberg SW348 mobile screening plant, a Kobe crusher, and a Nordberg LT80J jaw-crusher.

The polyethylene sheeting was laid over the sand by the Christchurch branch of Skellerup Industries, using an excavator adapted to run the material out like carpet from rolls that were 140 metres long by seven metres wide and weighed 1.4 tonnes.

The whole job took 20 weeks, with the earthmoving performed by a 50-tonne and a 20-tonne excavator, a 20-tonne grader, two 40-tonne dump-trucks, two heavy rollers, a couple of water carts and a D5 bulldozer.

“The batters were to be re-soiled and sown out in grass, so we had to finish them off with hand-rakes, which was a pretty time-consuming job,” Tinnelly told Contractor.

Demott O’Sullivan said that without the dam the farm, which is about half river-flats and the rest rolling clay downs, could never have been converted to dairying.

The pumps have the capacity at full stretch to shift up to 160 litres a second out of the dam. It was touch-and-go at the start though, because just as the dam was completed in November the regional council, Environment Canterbury, imposed water restrictions that meant it couldn’t start to be filled until the rains came in December.

“We’ve actually got the capacity to pump out more water from the dam than we can meter in, so we’ve been mining it over the summer,” O’Sullivan says.

The project was designed by O’Sullivan’s son Tim, an irrigation consultant. The O’Sullivans have since applied for a high-flow consent allowing them to take up to 350 litres a second when there’s excess water in the Tengawai.

The Cricklewood dam has opened up a new market for Paul Smith Earthmoving, with more and more Canterbury farmers looking at ever-larger on-farm water storage projects separate from the proliferation of community schemes either already completed or being organised. The company is about to start work on a dam of similar size to Cricklewood, albeit it only three metres deep, near Oxford in North Canterbury.

More significant though is another private farm project for which the company has submitted a price. It’s a North Otago dam with batters 20 metres high that will hold 1.2 million cubic litres.

Nor is that likely to be the last that Paul Smith Earthmoving will bid for. After a century-and-a-half of predominantly dry-land farming, landowners the length of the South Island’s east coast are scrambling for water to make their farms summer-safe, so demand for earth dams can only escalate.