Wallsend Reservoir Showcases Steel Versatility

With a diameter of 88 metres and a footprint of 1.5 acres, a recently constructed water reservoir at Wallsend in the Lake Maquarie area of New South Wales challenged the notion that only small water tanks could enjoy the benefits afforded to steel construction of low maintenance, quality construction and durability.

Steel tanks have long had a reputation for low maintenance, quality construction and durability. Now a project is also showcasing steel as a competitive option for much larger water supply projects than previously thought practical.

Journey to the Lake Macquarie area of NSW and you'll find one of Australia's largest all-steel water reservoirs. Completed in mid-April 2,000 the 900 tonnes of predominantly pattern plate steel contains 55 megalitres of water to guarantee supply to the boom region well into the new millennium.

Hunter Water commissioned the South Wallsend No 2 Reservoir to complement an existing 45ML steel tank on the site. The utility estimates the additional capacity will be required to guarantee supply to an estimated future regional population of up to 150,000 people.

The new reservoir will become a landmark in itself, with a diameter of 88 metres, four strakes standing just over 10m high and a footprint of around 1.5 acres. According to contractors Pacific Steel, the reservoir is one of the largest diameter steel water tanks in the country, with only a handful of tanks with diameters around 100m surpassing it.

"It's by far the largest tank we've ever built," says director Peter Way. It presents an opportunity for the Sydney based steel fabricators to showcase their tank building talents. Although formed only in 1995, all of the company's directors have a strong industry background and extensive experience in steel fabrication and tank construction over many years.

Tank building, especially for such large reservoirs, is a highly specialised field.

Correct Sequence

"Following the correct sequence of steps is vital. And there's no manual that I know of which tells you how to go about it. It comes down to experience, you have to know what you're doing to get a successful result," says Way.

The sheer size of the Wallsend Reservoir project created additional engineering challenges; chief among them the very real threat posed to the tank's structural rigidity by the forces of thermal expansion.

The risk is greatest during the construction process. Once full of water, the liquid's cooling and temperature dissipation properties reduce temperature extremes as the sun heats different parts of the structure during the day.

"With 1.5 acres of exposed floor you've got a fair bit of expansion across that area on a hot day. If the sun's beating down and it's 35 degrees, you can end up with the floor bubbling up in the middle if it's welded together too early," Way says. This is why the correct construction sequence is so critical.

The successful construction of larger tanks also requires intimate knowledge of the effects of steel plate interaction with structural geometry.

"With a smaller diameter tank the tighter radius gives the walls strength. On this project most of the plate is only 12mm thick and is not rolled at all. In comparison with the width of the tank it's like paper," Way explains.

Again, the greatest risk to be managed occurs during construction, when the relatively thin plate is unable to support its own weight. "If there's any sort of wind the incomplete tank could buckle, which is a real possibility.

Generally smaller tanks with a tighter radius present less of a problem in this regard."

Once complete, sifting rings around the circumference and an internal spider roof will provide sufficient stability for the Wallsend Reservoir, but any mistakes during the construction process are costly for both builder and client. It's not possible to repair any steel plate damaged or buckled by wind during building work, the plates have to be cut out of the tank and replaced with new material.

To minimise the risk of buckling, Pacific Steel specified extra supports and restraints during the construction process, including 40 two tonne concrete blocks distributed around the wall of the tank. Additional guy ropes hold the 13 metre long plates of steel firmly in place while awaiting permanent welding. Initially these are tack-welded to allow for the steelwork to expand and contract without causing damage.

The bulk of the floor plates require no fabrication or trimming and are delivered direct to site. The remainder accompany the wall plates to Pacific Steel's Warragamba workshop south west of Sydney for trimming and weld preparation.

Twenty millimetre thick BlueScope Steel 350 L15 XLERPLATE®* destined for the bottom strake of the tank are then rolled to the correct radius. This critical process determines the correct curvature for the rest of the structure. The remaining three strakes are constructed from 12mm XLERPLATE®*. On such a large radius tank the 13 metre lengths can be pulled into shape on site during the final weld process.

Firm Base

On site construction begins with the 8mm BlueScope Steel 250 grade XLERPLATE®* floor being laid in place but not fully completed until later in the process, again guarding against the risk of thermal expansion buckling. An annular ring runs around the circumference of the tank and is of higher grade 16mm 250 L15 XLERPLATE®*. It provides the firm base necessary for wall construction.

The first strake is raised into position sheet by sheet and restrained to allow vertical joints to be welded before work commences on higher strakes. Welding the floor to the wall is a final stage process. According to Way, welding is a critical skill in tank construction.

"The trick is to control the joint so it remains curved. It's very easy to end up with a peak as the joint shrinks under heat," he says.

Once complete the inside surface of the reservoir will receive a two stage epoxy coating and the exterior an inorganic zinc treatment. Finally the interior surface is fully spark tested to ensure the coating is consistent.

Way believes there are a number of reasons behind the increasing popularity of steel water tanks with customers, including water utilities.

"The biggest benefit with steel is its improved ability to accommodate expansion more readily. It's also non-porous, so in the longer term, and particularly with the new technology coatings available now, it's a low maintenance option."

Quality control during construction is another area where steel offers strong benefits. "You can't live with any weakness at all in a concrete tank, whereas in a steel tank you won't get any weakness if it's been welded correctly - and that's very easy to determine through visual inspection at any stage of the process," he says.

Although the Wallsend Reservoir, with its circumference of almost one-third of a kilometre, represents a major engineering feat, Way believes there are even bigger steel tanks on the horizon. "This tank has a wall thickness at the bottom of only 20mm, which is fairly thin plate. So if the plate thickness is taken as the limiting factor of your construction rather than the diameter of the tank itself, we've still got a long way to go in size before we reach anywhere near the limit."

* From 1 August 2002, BlueScope Steel Plate Products are known as XLERPLATE®