Transportation engineers suspect the timber pilings supporting the Simpson Avenue Bridge in Hoquiam had been deteriorating for decades, but the weakening substructure remained unnoticed despite several inspections.
Bridge engineer Dave Bruce with the Department of Transportation said just one underwater inspection in 2002 ever hinted at a problem, but the changing level of the riverbed, low visibility and debris combined to hide the severely damaged pilings until unexplained bridge movements begged a closer look in August of this year.
"Now we have a better picture of what was happening, probably for decades," he said.
Transportation officials ordered an emergency closure of the 83-year-old downtown Hoquiam drawbridge Aug. 5 after determining about 70 of the 176 timber pilings supporting the eastern drawspan were either completely missing or badly damaged by marine life.
As the pilings deteriorated and weight shifted to other aging piles, the bridge began to shift earlier this summer, leaning slightly toward the river. Bruce said the remaining pilings are significantly overloaded.
Project engineer Scott Ireland said crews have carefully worked to stabilize the overworked piles while engineers design a permanent steel-reinforced replacement foundation.
"We understand the importance of this bridge to the community," he said. "We're working hard. We're doing in weeks what usually takes years."
History of inspections
The first inspection records for the bridge date back to 1976. Bruce said those inspections used above water checks and underwater sonar to check the riverbed level.
Diving inspections, by a combination of commercial and Department of Transportation teams, started in 1992. Bruce said the span was also inspected in 1997, 2002 and 2007.
A Daily World review of the inspection reports from the 2002, 2007 and 2010 dives found officials did not suspect any pilings had been damaged until this summer. The 2002 report said divers suspected the pilings might be exposed beneath the footings, but the 2007 report found the mud floor had again risen to cover up the footings.
"Overall, the submerged substructure components are in satisfactory condition," the 2007 report stated, adding, "There are some corner spalls at the bascule (draw span) piers, but nothing structurally significant. ... Although the visibility was limited, the dive plan afforded an excellent tactile inspection of the submerged substructure components."
The concrete and metal bridge has been supported by dozens of untreated timber pilings. Bruce said the concrete and sediment around the pilings usually protect the piles from marine worms and other borers that feed off the wood.
The bridge was designed to protect the pilings with a seal between the concrete footings and the timber piles. Records show the depth of the riverbed can change dramatically from year to year, but always appeared to cover the pilings.
"Those have probably been uncovered for decades and we never knew it," Bruce said, adding, "That mud and that silt in the Hoquiam River is very dynamic."
Engineers now believe the changing current washed away, or scoured, mud near the eastern Pier 2 and then siphoned sediment from behind a wooden coffer dam surrounding the pilings.
Once the sediment was sucked out, it could not fill back in, creating a hollow of water around the pilings, he said. But the mud could still build up around the cofferdam to hide the exposed area.
In 2002, sonar readings put the riverbed level at below the footings, but Bruce said divers never suspected any damage to the pilings. A full inspection was also impeded by the wooden coffer dam and debris around the pier.
"Results of the fathometric survey indicate the channel bottom to be considerably deeper than that shown on the construction drawings around Pier 2, resulting in approximately 2 to 4 feet of undermining on the channel side of the bascule," the 2002 report stated.
The team suggested placing rocks along the pier to reduce scouring, which never happened. Bruce said such a repair would not have protected the exposed pilings.
The next underwater inspection in 2007 shows the channel floor had returned to several feet above the footing.
"For the 2007 dive it was obviously filled back in," he said.
Emergency Response
Maintenance crews first noticed a problem when lowering the two drawspans after a marine vessel passed through in early July.
"The two tips actually touched, which is not normal," he said, but crews chalked it up to warm weather and filed the tips down.
But a couple weeks later the tips still were not aligning properly and a new gap had opened on the eastern span, he said. Engineers also found concrete stops designed to keep the bridge from moving during an earthquake had engaged to hold up the bridge.
"The pier had obviously moved," he said.
Transportation officials and a commercial dive team launched an emergency inspection that found the riverbed had dropped to expose the pilings and worn a large hole in the wooden coffer dam. For the first time ever, a diver poked his head behind the dam to inspect the pilings.
"We had a difficult time even getting to the footing," Bruce said. "There's just a forest of remnant piles. ... (but) he was able to find a hole in the coffer dam big enough to get his head and shoulders through."
Bruce said the "clear water" space around the pilings had created a "smorgasbord" for marine worms, which completely devoured some pilings and reduced other 16-inch timbers to wrist-sized twigs.
The 2010 report from multiple dives on Aug. 3 and Aug. 6 paint a startling picture of the damage cause by marine borers on the pilings.
"Many of the piles are completely destroyed – only a small stub 2- to 3-inches high projects above the mudline; most of the remaining piles were found to have been reduced to a 2- to 4-inch-diameter core," the report stated, adding, "only a few of the piles were found to have approximately 50 percent of the original cross section remaining."
"Most of the remaining piles indicated on the original drawings could not be seen or felt by the diver; even when the diver employed a long boat hook in a side-to-side sweep of the exposed area."
Engineers reported the remaining pilings are now supporting more weight than they were ever designed to hold. Bruce said the ground may be supporting more of the weight, but the remaining pilings may be crushing or leaning under the additional strain.
"At what point does the bridge say, 'I'm done standing up?'" he asked with no definite answer. "All bets are off. ... They were way overloaded. There's no way of knowing how much (weight) the ground is supporting."
Repairs in works
Ireland said Rognlin's Inc. construction crews have finished a temporary "tie-back" fix, using guy wires to anchor the weakened pier to the shore.
"It's very challenging circumstances," he said. "There's a lot of work that goes into this."
Crews tensioned the cables two weeks ago and finished clearing sediment out of a chamber in the pier last week. He said the $1.6 million project was completed on a 30-day emergency contract.
Bruce said the cables pulled the weight back to center it over the remaining piles, so the load is better distributed.
"We've physically moved the bridge backward," he said. "We've done some good."
Crews will now demobilize the equipment as engineers work to complete designs for a permanent solution. Ireland said the permanent fix will likely involve four, three-foot shafts at each corner of the pier to replace the foundation.
"You can't do it from underneath, so you have to do it from the sides," he said, noting disturbing the ground too much could also undermine the weak piles still holding the bridge. "We have to be very careful about how we approach this."
Ireland said officials will post another emergency contract for the permanent fix in the coming months, seeking at least three bids from companies "well-versed" in heavy marine construction.
No cost estimate is available for the project. Ireland said the department has applied for emergency federal transportation funds to pay for the repairs.
Officials still believe the bridge can be re-opened by November.
"We're trying to go fast," he said. "We're going to do our best to open this bridge as soon as we can."