Week of Sept. 11, 2006

Keeping up with King County’s culverts – a never-ending job

Within a few weeks, the rainy season will arrive and that means King County road crews will be responding to water over roadways and other weather-related problems. But, maintenance work the crews have done underneath the roadways this summer should cut down on many of the problems they will have to deal with this fall and winter.

There are more than 1,800 miles of roadway in unincorporated King County, but amazingly there are more than 17,000 culverts that cross underneath those roads. And, if the culvert is not healthy, roads engineers begin to worry about the stability of the road above – roads that can be undermined or even collapse if the culverts beneath them fail to do their job. So far in 2006, the King County Road Services Division has replaced several thousand feet of culvert, and still has a long list of future projects.

A culvert is a conduit used to enclose a flowing body of water. They have been used for centuries to allow water to pass underneath a roadway or embankment without building a bridge. The King County Road Services Division’s inventory of culverts range in age from 100-years-plus to barely one-month-old. The oldest still-existing culverts are made of wood and clay, while the newest are constructed of reinforced concrete – such as those constructed this summer in the Snoqualmie Valley.

The biggest concern about culverts for the Roads Division is when they begin to deteriorate or leak and cannot handle higher flows from fall and winter rains. Jon Cassidy, a managing engineer for the county’s Roads Maintenance section, says wooden culverts eventually rot and collapse. The old clay culverts become very brittle as they age, and crack easily. Metal culverts only last about 40 years before corrosion sets in. Concrete pipe will last 100 years, but has to be installed correctly.

“The problem you have with old concrete pipes is how the three-foot sections were fit together,” says Cassidy. “If those seals disintegrate, you get settlement in the joints, water can leak out and mix with the surrounding soil. That causes a misalignment between the sections, which pulls soil into the pipes and that can cause voids underneath the roadway – just like you see with a mineshaft collapse.”

Voids under a roadway are the worst-case scenario. In one location several years ago, a road collapsed and created a hole larger than a car.

“Sometimes the voids caused by culvert failure will just leave a thin ribbon of asphalt on the surface that looks normal,” says Assistant Roads Maintenance Operations Manager Leo Griffin. “They are very dangerous.”

Another problem with older culverts is that they are often under-sized. During the rainy season, a stream that normally trickles through the culvert suddenly becomes a torrent. When it is forced into a small culvert, it creates a high-velocity flow that can wash away the road or shoulder on the downstream side. Similar problems can happen on the intake side of the culvert when they become clogged with debris. In steep areas and along slopes, the whole road can slide away if these washouts are not quickly repaired.

Cassidy and Griffin say undersized culverts have caused reoccurring problems for years along Issaquah-Hobart Road, because sediment from small creeks coming off Tiger Mountain flow into the culverts and then send water up over the roadway.

Finding the problems require constant monitoring of the culverts countywide to check for signs of failure, scouring, or plugging. Repeated checking was how county crews caught problems early on in the Cherry Valley area east of Duvall, where they began seeing cracks in the road surface and found the joints of pipes installed in the 1920s were separating.

That’s why the Roads Division has a long-range plan to replace aging and undersized culverts, which is more complicated and costly than when most of the culverts were originally installed.

New culverts – such as those installed in Cherry Valley this summer – arrive at the work site in sections 5-8 feet across and 10 feet long. These sections can weigh more than 20,000 lbs., and it requires a good-size crane and deep excavation to position them below the natural depth of the stream that will flow through them.

And, the culverts cannot be replaced randomly.

“These days, you have to look at the whole system that connects a series of culverts,” says Griffin. “You have to maintain the flow throughout that waterway. If you try to replace one culvert in the middle, you could cause flooding downstream where the culverts are still under-sized. So, we try to start at the most downstream pipe and work our way upstream to fix the problems.”

The projects must also be coordinated with modern-day environmental regulations.

“The new environmental rules have changed our whole design process in a good way,” says Cassidy. “If it’s a stream crossing, the pipe has to match the width of the 100-year flood plain for that creek, so you can direct all the flow under the road without restricting it. This reduces localized flooding.

“And, culverts cannot be designed just for hydraulics anymore, they have to be designed for habitat,” he says. “In some cases, we’ve restored salmon runs that have been gone for years by providing a larger passageway through the new culvert. But, it also improves the habitat for more than just fish. Some of our culverts are big enough for even deer, elk and bear to use, which means you have a lot less road kill in the area.”

Subscribe to DOT Dash

Sign-up to receive an e-mail text version of "Transportation Today," along with other significant DOT news by sending an e-mail to us with subscribe King County DOT Dash in the subject line.