As workers finished repairs this month on four cracked pontoons at the Aberdeen 520 pontoon construction site, new cracks have turned up in one of the massive cement structures, prompting the state Department of Transportation to call in a panel of experts to help figure out what is causing the cracks to occur, and how to fix the problem.
“These pontoons must be safe and functional for 75 or more years,” said state Transportation Secretary Paula Hammond in a press release. “On any big project like this, there are issues that arise and refinements that are made. We welcome the expertise of our panel as we address the things we’ve encountered and look forward to successfully building the new State Route 520 floating bridge.”
While the new cracks present another problem for the project, the state still believes work is on track to begin the first float-out of the pontoons in July or August.
The original cracks occurred during the “post-tensioning” process in May, when high-tension steel tendons are stretched through the pontoon to strengthen it. Cracks were discovered after that initial process.
To repair the problem, workers removed concrete and then inserted rebar at both ends of the three mammoth 360-foot-long longitudinal pontoons. The same process was also applied to the smaller 240-foot-long cross pontoon. The fix took about three weeks, which included the time to let the concrete cure. The pontoons were tested length-wise again and everything turned out fine, according to Mike Cotten, the design build manager for the state Department of Transportation.
But when the cross pontoon was tensioned vertically, new cracks were discovered on that pontoon, Cotten said.
The cracks have stalled construction on the next wave of pontoons as engineers try to figure out what’s going on.
At the peak of construction in February, there were about 375 workers on site, including 177 from Grays Harbor. As of June 1, there were 349 workers with 96 from Grays Harbor. Current employment numbers are not available.
However, gone are the two shifts of workers who had worked 20 hours at a time. Instead, as a tour of the facility on Tuesday showed, the site was nearly silent in the late afternoon with most everyone gone by 4 p.m.
Late Tuesday afternoon, there were just a handful of workers working to “de-tension” the cross pontoon that showed the newest cracks. They worked to cut away the steel tendons, while using extreme caution to ensure the tendons didn’t whip about. The crew was kept small and proper safety procedures were followed, according to Kiewit-General, which holds the contract to build the pontoons.
With less work, there’s less of a need for a big crew, Cotten said.
There are workers combing the pontoons for any hint of a crack. They actually map where the cracks are that exceed six-1,000ths of an inch. Crews treat the hairline cracks with an epoxy injection of resin and follow up with a waterproof membrane, he said.
Cotten said that besides the repair work, there’s still some workers on site in preparation for the second wave of pontoons.
The peer review panel of industry experts convened this week will take a look at the repairs done so far as well as the process that could be done to repair the new cracks.
The review panel started Monday, with interim findings expected during the week of July 9. The panel has experts with backgrounds in structural analysis, pre-stressed concrete and steel design, risk assessment and long-span bridges under seismic conditions, according to an agency press release. Some have backgrounds in work on the Interstate 90 and Hood Canal floating bridges and the Tacoma Narrows Bridge.
Cotten said that engineers were not expecting the cracks. However, there was no way to do testing before hand to ensure it would not happen during the post-tensioning process. Preliminary tests on the concrete mix were done at the Satsop Business Park. Cotten said those tests made sure that the concrete mix was working. He says he doesn’t believe that’s a problem, although the panel will look at it, just in case.
Engineers also oversaw construction of a smaller-scale test pontoon at the Aberdeen site. However, the model wouldn’t allow the post-tension tests to occur.
“This is the first opportunity we had to do the testing on the real pontoons and, now, we need to see if there’s anything we could do better,” Cotten said. “Is the issue with the engineering or construction or something else?”
The construction site will build 77 pontoons with the goal of completing all of them by 2014.
Before the initial cracks were found, the contractor had hoped to float out the pontoons this month. The construction site has a mammoth gate that will be opened and then water will fill the casting basin so that the pontoons can be floated out. Only when the existing pontoons are floated out into the Harbor will work be able to ramp up on the next cycle of pontoons and more workers be brought to the site.
Pontoon assembly is slated to begin on Lake Washington this summer, the state agency said. Other floating bridge construction work continues on pontoons in Tacoma, anchors in Kenmore and bridge supports on the east side of Lake Washington in Medina, according to the agency press release.
Check out video of the pontoons, including a look inside of them: http://youtu.be/Mze6Wb7KQhY