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Innovation in Demolition

A crane operator lifting a concrete block.
Cab view from the derrick barge WOTAN at the Concrete Sill Removal project in Jacksonville, Florida

As the first 400-ton concrete block emerged from the water, hoisted by the derrick barge WOTAN with a special lifting frame, crew members clapped and cheered. This was the culmination of nearly eight months of preparation and planning.

“Everyone was relieved because before this point all work was essentially unproven based on the environment,” said Ryan Gielow, Manson project engineer on the Concrete Sill Removal project at the Blount Island Marine Corps Support Facility in Jacksonville, FL.

From day one of the bidding process, it was clear that this was a unique project that would require an unconventional approach and an experienced team ready to think on their feet.

The submerged concrete sill was constructed in the late 1970s as a critical component to a graving dock intended to build floating offshore nuclear power plants. Due to the lack of economic demand, the project went under and further construction was halted. “The inlet was already there, and they were essentially trying to make it into a massive dry dock. The construction of the concrete sill was one of the few things that got done before the project stopped,” says David French, Manson project manager.

The facility was later taken over by the U.S. Marine Corps to be used as a support facility; however, the sill – measuring 32.6 ft. wide, 426.5 ft. long and 14.5 ft. tall – created problems for deeper draft vessels. They would often have to light load or wait for high tide in order to be able to make it past the sill, which sat at -37.6 ft. MLLW. To create better access logistics, the Corps wanted 9.4 ft. of the sill height removed so the depth would be -47 ft., ensuring no logistical issues at any time of day. Manson decided to remove 10 ft. to be completely sure the objective was met.

Manson’s clamshell dredging crew joined the team to excavate approximately 10,000 CY of material around the sill, which then allowed access for the diving team to remove any protruding rebar and to set the vertical and horizontal saws. All cutting operations were performed by subcontractor Bluegrass Bit Company, using a patent-pending continuous diamond wire saw cutting frame that was hydraulically operated from the surface.

Even with all this preparation and a special cutting frame, there were still challenges to attend to along the way. The sill was built on top of rock.

“With the rock adjacent to the sill, that limited the saw frame placement required to make full depth cuts. The team utilized jack hammers, heavy digging rock dredge buckets, and a vibratory hammer connected to a 60-ft. probe to remove the rock,” Ryan says.

The job worked 24 hours a day, six days a week – sometimes seven – racking up a lot of man-hours. Logan Diving and Salvage provided a large chunk of that manpower, with up to 24 divers on the job in a single day.

“There were very few activities where the divers were not involved,” Ryan says. “In total, Logan performed 1,770 dives without incident or injury.” Of course, communication with the dive teams was key to ensuring the work would go off without a hitch.

There were many unknowns in this project, given that it had never been done before, but one unknown that worked to the team’s advantage was that both the vertical and horizontal cuts were performed at a faster rate than anticipated. In each block, divers core-holed 8-in. diameter, 36-in. deep cores and then grouted rods in place which would later be bolted to the lifting frame.

This project was the first for Mohammad “Mo” Kanaan, a field engineer who joined Manson straight out of university in 2019. Having helped with the pre-construction phase after award and the initial project mobilization, he had a complete understanding of the critical lift plans and was excited for the hands-on experience of putting everything into action.

“I worked with the divers – they were our main workforce – to make sure when we set the saw or were using the core drills to core holes, all the steps lined up,” Mo says. He oversaw the verification process of each step, seeing to it that the saw was installed correctly, the core holes drilled at the correct dimensions, etc. “There would really be a point of no return if we didn’t get the cuts right, so we had to make sure it was done correctly the first time.”

Once the rods were in place, the team had two rigs going at a time, working their way from each end of the sill. Bluegrass would complete the horizontal cut for a given block, then the divers would mount the lifting frame to the rods so Manson’s derrick barge WOTAN could remove it.

This was the moment of truth.

“No one really had an image in their minds of what that was going to look like coming out of the water,” Mo explains. “We knew the dimensions of it, but visualizing it is another thing. Seeing it coming out of the water with the WOTAN hoisting it and this lifting frame on it – that was one of the most exciting parts of the project.”

It was also a major moment of relief. As soon as the first block came out of the water, the learning curve was out of the way, and the team was fully confident that the plan was streamlined. The first block was lifted Sept. 11, 2020 and the last was Oct. 21, 2020.

The WOTAN offloaded the blocks at Manson’s Jacksonville Yard, where United Brothers Development Corporation broke them up into small pieces to be taken to a concrete recycling plant to be made into “crush-crete.”

“When you put it in hindsight, we worked on the project for a little under eight months, and out of those eight months, it was just 40 days where we were actually lifting. The rest was all prep,” Mo says.

David puts it all down to great teamwork and communication, not only among Manson staff, but all the subcontractors, and with the Corps.

“We knew from the beginning it was going to be a very labor intensive job. The people who worked this job were very dedicated,” David says. “We had good people. Everybody hung in there for the long haul to get the job done.”

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