The clean coal storage silos at Rio Tinto’s Mount Thorley Wark worth mine, located near Single ton, are a crucial to the productivity of the whole operation.
The sizable structures, each with circumferences of 48 m, are designed to store up to 3000 tonnes of coal before it is transported by rail to the Port of Newcastle.
The structures were constructed in 1984 with a single 250 mm layer of reinforced concrete.
However, less than 12 months after construction was completed, cracks became visible on the external faces of the silos.
The mine decided to apply 154 post- tensioned steel cables horizontally around the silos in order to restrain the walls against the coal loading and prevent further cracking.
While the cables proved effective for some time, they eventually began to corrode and fail.
This caused the silos’ capacity to decrease steadily every year, so the mine sought out Newcastle-based engineer ing firm Izzat Consulting to propose a more long-term solution.
Remedial building contractor Build corp Asset Solutions was commissioned in 2008 to carry out the repairs.
By this stage, the silos could only store 25% of their total capacity.
The company’s construction manager, Brendan Walsh, oversaw the $3.2 million project.
“Izzat developed a plan to remove the cables, repair the concrete and then apply layers of carbon fibre to rein force the structure,” he told Australian Mining.
“The problems arose because the silos were under-reinforced from the start.
“In some places we actually had to apply up to four layers of carbon fibre in continuous 300 mm wide strips.”
According to Walsh, the Izzat engi neers chose carbon fibre for its high tensile strength and resistance to corro sion.
“We could have simply replaced the cables, but they would have inevitably failed again,” he said.
Because the silos were still opera tional while the project was underway, it was difficult to gain access to the structures.
“We were only allowed to work on two metre high horizontal sections of the silo at any one time to ensure we did not weaken the structure in any way,” he said.
“So basically, the work would alter nate between two metre areas at the top and bottom of the silo.”
The original tender documents recommended the use of scaffolding.
However, the repair team quickly realised this would not be satisfactory.
“To install full-height perimeter scaffolding all over both silos would have been a bit of a headache, because in some cases it would actually restrict the working space,” Walsh said.
“It was also quite expensive to put that much on structures so large.
“So we decided to use a specially- designed mast climber system, which reduced the cost by around $300,000 and actually gave us a much better working method.”
The access system consisted of six vertical mast climbing units fixed at equal intervals around each silo’s circum ference, as well as separate decks fitted between each mast.
“This meant the silo was divided into six separate working areas and we could use any one of the decks to travel up and down the structure,” Walsh said.
“The decks could also be locked together to provide access to the complete circumference.
“Each section was basically set up as a climbing workshop.”
The team had to devise a method to safely restrain and remove the cables, which had tensile forces of 20 kN.
“We fitted about 20 steel plates to prevent the cables from flying away from the structure when we cut them,” Walsh said.
After cutting the cables, the workers removed the damaged concrete with hydro-demolition, using low volumes of water at pressures up to 20,000 psi.
The concrete was then replaced with shotcrete to provide a stable surface for the carbon fibre.
In all, the project involved 566 m of external crack repairs, 1633 m of internal crack repairs, 14.42 km of carbon fibre wraps and two kilome tres of carbon fibre laminates.
Withstanding the deluge
The project hit a major stumbling block when heavy rainfall and flooding hit the Hunter region mid-way through work on the first silo, forcing the site to be evacuated.
“The silo actually filled up with water because the roof had been removed and one of the post-tensioned cables actually snapped under the strain of this extra load,” Walsh said.
“This forced us to revise our methods and make sure the cables were secured before we restarted work.”
Since the project was completed, the silos have been running at full capac ity and are expected to last another 15 years before further maintenance will be required.
“Carbon fibre is completely resist ant to temperature variations, rain and hail so it only requires minimal main tenance,” Walsh said.
“Ultraviolet radiation will cause deterioration, but that can be prevented by applying a resistant coating to the surface.
“If this is done regularly, the carbon fibre could have a limitless lifespan.”
• Buildcorp Asset Solutions
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