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Experts collaborate to save a Rottnest landmark from collapse

Aug 10, 2018

Image of Rottnest Sea Wall

 

The limestone sea wall at Rottnest Island is a welcoming heritage feature that greets everyone as they get off the jetty to start their holiday.

Visitors will now notice a subtle difference to the landmark sea wall, but it’s odds on they won’t know the story behind the change.

Over the past few months, the sea wall has been the subject of a $950,000 collaborative engineering and conservation effort.

Several specialists—including Rottnest staff, a heritage architect, archaeologist, engineers, arborist and a team of specialist stone layers—have been working to protect the wall from potential collapse and restore its distinctive face.

The three-metre high limestone wall is virtually the ‘front face’ of Rottnest and was built about 150 years ago. It stretches some 300 metres in all, along and just below Vincent Way.

Annabel Wills, the Conservation Officer of Built Heritage at Rottnest, says the wall was under pressure from the soil behind the wall, as well as tree roots from nearby Moreton Bay fig trees. The wall was bulging outwards in places and cracked.

“Tree roots bound in the soil behind it were stopping it falling down but at same time they were pushing the wall over because they were growing through the wall,” Wills explained.

Fixing the wall and restoring its stonework was a complex task.

The wall is a single block limestone wall that was had been the subject of various repairs over the decades, and its height had been increased by a metre or so.

As well as that, acrylic paints had been used on the face of the wall and these didn’t allow water to escape through the limestone and allow it to ‘breathe’. The wall would swell in winter when moist, and contract in summer.

In repairing the heritage wall, the Rottnest Island Authority wanted to maintain the health of the heritage trees, even though some large roots had to be cut back to protect the wall.

“We didn’t want to sacrifice one for the other,” says Wills.

The seawall is an important element within the State heritage precinct of Thomson Bay Settlement.  During the planning stages of the work, the Rottnest Island Authority worked closely with the heritage department and Heritage Council to keep them informed of the issues and proposed solutions and to gain their advice and support for the project.

The solution involved an engineering approach—which meant injecting a cement grout into the ground to buttress the wall near it base, plus installing a root barrier to stop further encroachment.

An integral part of the work involved Jason Royal from Arbor Logic to supervise trimming of the tree roots prior to installing the root barrier, as well as an archaeologist Fiona Hook from Archae-aus.

“There were a lot of things found behind the wall,” Annabel says, such as coins, dishes, bottles and bones which were photographed and catalogued.

ANALYSED

“The bones we found were all analysed and were all animal bones, I think because there used to be a lot of livestock around when there was agriculture on the island, pigs and sheep and so forth.”

Working to a plan drawn up by heritage architect Philip Griffiths, structural engineer Rowan Stokes of Wood & Grieve Engineers was responsible for supervising the project.

“We knew without doing any calculations we had some real issues at play here and we needed to stabilise the wall as a matter of public safety,” Stokes says.

The engineering team couldn’t do anything to disturb the wall in any way as vibrations and excavations were likely to cause even more problems.

“The solution we came up with was to grout inject the soil behind the wall,” says Stokes.

“It’s a low-viscous grout that we pumped into the ground and it binds the sand to, in effect, create a big (in situ) block.”

Hundreds of cubic metres of grout were pumped into the ground to mix with the soil, thus becoming a new retaining wall immediately behind the heritage sea wall and out of view.

“In effect, the new cement block is countering those lateral forces and the sea wall now becomes a façade,” explains Stokes.

The second part of the repair job involved making sure tree roots didn’t cause any more problems.

Working with arborist Jason Royal, who trimmed the roots, Colgan Industries dug a two-metre deep trench behind the sea wall and installed a root barrier.

Like the injection of the grout, all of that work is now hidden in the ground.

The third part of the project—restoring the face of the sea wall—is the most visible and was in the hands of John Kelly, who you could say now knows every square inch of the wall.

John is a stone layer with Colgan Industries. He and four other members of his team not only cleaned away the old cement and paints off the length of the wall, they restored it almost to its original state.

“The biggest challenge was to clean the face and get it ready for re-pointing,” he says.

That meant painstakingly chipping of the cement render that had been slapped on in previous years.

“Once we had the cement render off, there was also the acrylic paint on the stone that had to be chipped off,” he says.

To re-point the wall, Kelly and his team used what’s called a hot lime mortar mix technique.

“Using a hot lime mortar mix is as old as it gets. It’s what was done back hundreds of years ago,” he says.

“It involves slaking the limestone from the kiln (to be used in the mix) for three days until the moisture is completely taken out of it.”

OPPORTUNITY

When water is added to it, it sizzles as it reaches a temperature up to 300 degrees, and the chemical reaction turns it into lime powder, and sand and water are added to the mix.

“It was a great opportunity to use a hot lime mortar mix, because I’m pretty sure this wall was built using hot lime,” says Kelly.

The job was then finished with Kelly and his team brushing on a home-made lime wash mixed with ochres to get the required colour.

But that colour will be just a tad different, says Annabel Wills.

“It’s a softer, lighter colour, between a sandstone and ochre colour,” she says, adding that thanks to the work of the specialists involved, the sea wall is going to be in good condition for a long time to come.

“The most pleasing thing now is knowing that the wall is likely to remain for another 150 years.”

Rowan Stokes agrees: “The final result means the wall has come back to life.

“It will be able to breathe, it will look good, it will have a consistent finish with no cement mortars left and more importantly from a structural perspective, the retaining pressures have been relieved and the tree roots have been pruned back and are under control.”

View an image gallery of the restoration of Rottnest Sea Wall.

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