Clear, Deep Water Aids Bridge Inspection, Divers Say
By Karen Gould

Diver engineers (from left) Mark Gipson, project leader Patrick Conroy, Brian Dillworth, and Paul Caterino performed underwater pier inspections on the Mackinac Bridge in July and August. Diver engineers Heath Pope and Clay Brookins (not pictured) also worked on the project.

Engineers who inspected the Mackinac Bridge below the water's surface this summer say the structure is in such good shape that future inspections can be scheduled every seven years, instead of every five years as previously recommended. The final report is expected before the end of the year.

"It's virtually unheard of that a consultant would suggest reducing the amount of inspection work," said Bob Sweeney, executive secretary of the Mackinac Bridge Authority (MBA). "They really are sending us a strong message. It shows how confident they are that the piers are in very, very good condition."

With a digital camera and a video camera, a team of engineers inspected the 31 underwater piers to depths of 134 feet from July 23 through August 9. The edited film and the photographs will be included with the final inspection report and serve as references for future inspections.

At 90 feet below the surface of the Straits and connected to fresh air hoses, Heath Pope inspects the area around the north tower pier of the Mackinac Bridge. Although not needed for the dive, the equipment was being tested by the company during the inspection. (Photograph courtesy of Collins Engineers)

"That bridge is very sturdy," said Patrick Conroy, the inspection project manager from Collins Engineers Inc. of Holland, "and it is not going anywhere."

He has 20 years of experience as a diver engineer.

This is the second time the firm has conducted the underwater inspection, which used to be done by the bridge maintenance crew. It was a popular job with the crew, said Mr. Sweeney, who also is a certified diver. The Mackinac Bridge Authority decided to contract the work with an outside firm five years ago, because inspection technology was changing rapidly and a hyperbaric chamber was needed on site for divers returning from deep water.

The chamber aids divers by equalizing air and water pressure at various depths. One is needed for dives deeper than 100 feet, said Mr. Conroy, although any dive deeper than 80 feet is a valid reason to have the chamber available.

In the clear water, said diver Paul Caterino, he saw big carp, lots of zebra mussels, and some construction debris from when the bridge was built. Visibility ranged from 25 to 50 feet. Crayfish and salmon were also spotted.

Many of the bridge inspection projects the engineers perform, said Mr. Conroy, are in rivers with fast moving, muddy water with limited visibility. Some inspections are done by feel, with murky water obstructing visibility.

"I've done dives where, in a river six feet deep, it is full of silt and there is no visibility," agreed diver Brian Dillworth. "Instead of looking at it, you feel the bridge. In this case, you see everything. There is very little wrong with the structure [of the Mackinac Bridge], so I spent very little time describing defects. We're not finding much."

Diver Mark Gipson said the visibility made the job enjoyable.

"At 60 feet," he said, "you can look up and see the sun coming through the water and the bottom of our boat."

But being enjoyable doesn't make it easy, Mr. Conroy points out.

"It is difficult, deep diving," he said. "Normally, we don't dive this deep. This isn't like an easy job, it is just that the water is so clear, we can see each other."

The water surface offers its own challenges. Thunderstorms, 20-mile-per-hour winds, and high waves can cancel inspections.

On the good days, four diving engineers worked from their 25- foot boat, two of them diving. Before entering the water, the team reads aloud their last report on the area to be inspected. The review offers them a basis for comparison.

"It's a complex project," said Mr. Conroy, "because of the depth of the water and getting all the personnel and equipment together."

The divers go through a series of emotions on the dive.

"The first thing is a fear of the unknown, and then after that, it is kind of a thrill that you are doing this," said Mr. Conroy. "Then it becomes a job of video taping."

The buildup of nitrogen in their system, caused from diving at deep depths, keeps divers alert to their time underwater. They are underwater for no more than an hour and use hand signals to communicate to each other, although they are not in communication with the two engineers on the boat.

During the 90-degree days of the inspection, he said, the water temperature at the surface was 68 degrees and, below 90 feet, 48 degrees. The divers wear dry suits or very thick wet suits.

"There is a definite thermocline where the temperature changes, where the layer above is warmer and then it gets colder," he said.

Inspectors looked at the piers and their base. The pier base is just under the surface of the water and is much larger than the pier.

"All you see are these columns going into the water," said Mr. Conroy, "and then 10 feet down, approximately, is this much bigger pier base."

The top of the base is an area the inspectors examine for ice damaged. The build up of ice at that depth can harm the top surface of the base structure. They also look for breaks in the steel sheeting that surrounds all of the pier bases. The sheeting consists of curved sections that lock into each other. They can come apart, although if this happens, it's usually during construction, said Mr. Conroy.

"We've only found two locations where it has," he said. "It's a very minor thing."

The engineers also look for impact damage and corrosion, although they do not expect to find any scouring under and around the pier base. Scouring is more com- mon with bridge piers in fast-moving rivers, he said.

"Typically you don't see scouring here," said Mr. Conroy of the Straits, "because it is such a wide channel and the flow is not as great."

Most of he most water movement is from 10 feet to 30 feet below the surface, the divers said.

"We've found we can swim against it," he said of the current. "That's not enough to pick up the soil from the bottom and take it away, and that would be scour."

Ongoing inspections below and above the water provide the bridge authority with information for the annual maintenance plan, said Mr. Sweeney, and major projects are included in a 20-year plan.

Other specialized bridge inspections include a fracture critical inspection, which takes place every two years and involves looking at bridge components that, if they were to fail, would close the bridge. The next fracture critical inspection is scheduled for next year.

An in-depth cable inspection is performed every two to three years by the bridge maintenance staff. Eight locations have been inspected over the last 10 years, said Mr. Sweeney, and all of the cables have been found to be in excellent condition. The crew has built its own equipment, which re-compacts the cables and wraps them with a protective coating after the inspection. The next cable inspection is scheduled next year.

Maintenance workers also inspect the bridge as they work throughout the year, and, in addition, the bridge is inspected annually by Parsons Corporation of New York, the same firm that designed the bridge more than 50 years ago. That inspection takes three weeks.