August 12, 2010

DEFIANCE COUNTY ENGINEERING

Completed abutments awaiting arrival of the deck. They take three days to build.

FOCUS | Roadbuilding

Geotex reinforcement: An old idea that works

KORKY KOROLUK

correspondent

A county in northwestern Ohio is using geotextile-reinforced aggregate to build abutments and wingwalls for bridges and culverts, in an adaptation of an old, old, idea.

And it works.

Warren Schlatter, the county engineer in Defiance, Ohio, says there have been no failures or problems with the bridges.

“They do what I want them to do.”

Some ancient civilizations used reeds to stabilize structures made of sun-dried brick, even compacted sand. And some of those structures still stand. Some of the oldest dikes in the Netherlands were reinforced with sticks. So were many levees in China.

Most people today, however, consider Henri Vidal, a French architect and engineer, to be the pioneer in mechanically stabilized earth. He did his work in the 1960s, using steel straps, although geotextiles have since come into common use to reinforce and stabilize slopes.

It’s geotextiles that Schlatter uses, although he uses them for vertical walls, and a few simple, but specialized techniques.

“Whether you’re building a wingwall or a bridge abutment, you dig down to some stable level of soil, he explained. “At that point, you start with a piece of high-strength woven fabric, so the holes in it are very small.”

He said the fabric serves both as a separation and a reinforcement.

“We drape it across the hole so when you fill with stone you can wrap the fabric back over the top of the stone,” he says.

He uses 3/8-inch aggregate level and compacted inside the fabric and there are two basic rules.

“Use a quality aggregate that is well compacted everywhere; and you put reinforcing layers of fabric between every eight-inch layer of compacted stone.,” he adds.

“We have found you can carry loads safely all day every day in the range of two tons per square foot on that abutment, which is a much higher load than you’ll get in typically mechanically stabilized earth wall.”

The system offers several payoffs. It’s quick, it’s cheap, and Schlatter can do it with his own crews.

With no poured-in-place concrete involved, there is no cure time, no waiting for concrete to achieve its design strength. It’s possible to build a pair of abutments in a week, leaving nothing to be done but hire a truck to bring in the deck slab and a crane to place it.

An even greater saving, he said, is “since we can build a vertical wall instead of a slope in front of the abutment, we can shorten the length of the beam we need to buy.”

The total cost saving is between 30 and 50 per cent, but it’s not mainstream.

“Civil engineering is a very conservative industry,” Schlatter said. “A lot of people don’t want to go out on a limb and try something that hasn’t been widely done before. Another reason is there’s no salesmen.

“You’d be amazed how many new technologies come to our industry and are pushed because a salesman comes in and says ‘I can save you money.’ ” Another cost factor, he said is none of these ingredients — the fabric, the stone, the facing— are proprietary.

“You can go to any masonry manufacturer and buy the block. You can go to probably 20 different manufacturers and buy the fabric,” he said. “And you can go to quarries across the world and buy stone. And because they are all products that are mass produced, they are just commodities. There’s not much margin for the manufacturer, so he can’t afford to spend a lot on marketing them.”

Schlatter said there are salesmen making their rounds “selling wall systems that cost two or three times what this does,” but there’s no one out selling these generic products, and that’s why this method doesn’t take off.”

It may sound cynical, but it’s echoed by Bill Gille, the county engineer in Yamhill, Ore., who is planning a fabric-reinforced job for next year.

“This is a kind of homespun thing, so to speak, so it’s almost word of mouth,” he said.

The system “lacks the glitz of some of the pile systems. Nobody is selling it or making a huge profit off it, so the advertising dollar is used for other systems.”

Gille had hoped to get a small bridge done this summer, but the project was delayed because of budget, even though he expected to get the job done “for half price or less.”

He got some second-hand slabs free from the state department of transportation. The abutments he planned to do “with our own folks, using the equipment that we have available, instead of hiring a contractor.”

He would have to “hire some folks to haul the bridge slabs in, and a crane to set them and that sort of thing.

“But the majority of the work I think we can do ourselves with our own folks.”

Print | Email | Comment