Conducting dredging works with an excavator on a barge is just one of many initiatives WA Limestone Contracting is employing to construct a breakwater in Bunbury, Western Australia
Multi-million-dollar construction contracts will often present a host of challenges, but when it involves building a structure within the sea, it can become a lot more complicated.
WA Limestone Contracting is all too familiar with this, having been awarded a $69.2 million joint-venture contract with Italia Stone Group for the construction of a breakwater in Bunbury, Western Australia.
This job is part of the broader Transforming Bunbury’s Waterfront project, undertaken by the Department of Transport in partnership with the South West Development Commission, which aims to enhance the waterfront area, promoting tourism and economic growth in the region.
Some of the main tasks of the contract include supplying and placing large quantities of rock to build the breakwater at Bunbury’s Casuarina Boat Harbour – providing sheltered water – as well as significant seafloor dredging to improve navigation and provide a solid foundation for the breakwater.
With 35 quarries across the state, WA Limestone Contracting (in joint venture partnership with Italia Stone Group, WAL ISG JV) was the ideal candidate for the job – not to mention its 50 years of experience.
In addition to protecting the harbour, it’s envisaged the breakwater will also provide a safe haven for up to 300 future boat pens during stormy weather.
There’s an exposed section to the north of the harbour and, during winter months, it can be quite choppy and can be dangerous for boats. The construction of the breakwater is just one part of the broader long-term vision for the Bunbury area.
Deep dive
Winning the contract for the job in July 2024, WAL ISG JV wasted no time getting started, arriving on site only a month later.
Before any construction of the breakwater could commence, unsuitable silt and clay material on the seabed needed to be removed.
WA Limestone Contracting project manager James Della Bona says WAL ISG JV engaged Rhode Nielsen, a world-leading dredging contractor from Denmark to take on this work.
“They used a trailing suction hopper dredge (TSHD) – it was an impressive effort,” Della Bona says.
“It broke up the material on the seabed and sucked the silty material up a tube, filling a hopper within the ship. The ship would then sail 10km offshore and dispose of it.”
In areas that were too shallow for the TSHD, a back-hoe dredge was used. A long-reach excavator from WAL ISG JV was deployed on a barge to dig out and remove the material. What came up in the excavator bucket was placed onto a split hopper barge and disposed offshore.
Dredging was completed in November 2024, and WAL ISG JV has been working on the site since.
“We make the joint-venture work by splitting the rock requirements down the middle: 50 per cent WA Limestone Contracting and 50 per cent Italia Stone Group,” Della Bona says.
“The quarries are key to this job, because it’s a matter of how much rock we can produce and how quickly we can transport it to the site.”
Heavy armour
While Italia Stone Group operates a quarry close by in Roelands, WA Limestone Contracting was not so lucky with the distance of its main quarry in Byford, Perth, which Della Bona says is “a good hour and a half drive to Bunbury”.
However, a partnership with another quarrying business enabled WA Limestone Contracting to also extract resources from the B&J Catalano Shenton Ridge quarry, not far from the site.
When construction of the breakwater commenced, ‘core rock’ – which is used for the stability and structural integrity of the breakwater – needed to be placed. Della Bona says these rocks varied in size, with a maximum weight of around one tonne.
Much larger ‘armour’ rocks were then placed on the outer layer of the breakwater to protect it from waves.
“The armour can be of varying sizes based on where the design models predict the biggest waves are going to hit the structure,” Della Bona says.
“For the main structure, rocks of 60–300kg were used as lighter armour, and on top of that we now need to put either 300kg to one-tonne rock sizes, or one to three-tonne variations.
“It all depends on where the most wave impact is going to be, which is decided by the consultants engaged by our client, the Department of Transport. If they think there’s going to be larger waves on a certain part of the structure, they’ll specify a heavier rock to be placed.”
An important milestone was recently reached with over 300,000 tonnes of rock placed at low level to complete the 460-metre full-length northern breakwater.
Della Bona adds most of this rock was for the core, with WAL ISG JV now focusing on the armour. Rocks used to construct the armour need to be formed with the design specifications requested and placed with precise accuracy. To ensure this, WA Limestone Contracting’s and Italia Stone Group’s fleet of excavators working on the breakwater are all GPS-integrated.
“You can’t just dump the rocks in the water and walk away, you need to make sure each one is well placed and interlocked with the surrounding rocks,” Della Bona says.
Safe practices
A variety of earthmoving machinery is being used for the project, including wheel loaders to load the rock, dump trucks delivering and unloading it, as well as several different sized excavators used to place the rocks.
“We’ve got Cat excavators in the 30 to 50-tonne class that are well-suited for placing rock close in the structure,” Della Bona says.
“We also have a 125-tonne long reach excavator, which is very useful for placing rock right down to the toe of the breakwater.”
Although most of the works have taken place from land – apart from dredging – Della Bona says there are various safety precautions that must be considered due to the proximity of the water.
“It’s key we have safe work distance from the water, and we have to make sure the machines stay away from any dangerous areas,” he says.
In addition to the construction of the breakwater, WA Limestone Contracting and Italia Stone Group has also been contracted to repair a seawall and a revetment nearby.
Similar principles to the breakwater will apply for these repairs, as Della Bona says a sturdy core will need to be placed in the right position.
“At this stage, all three jobs are scheduled to be completed simultaneously in the October to December period later this year,” he says.
“The whole job will require 400,000 tonnes of rock – it’s a pretty big number.”