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A problem-solving wall project in Michigan and preserving Stump Pass in Florida highlight two more progressive uses of geosynthetics.
First section by Jacob Manthei, second section by Jeff Barbian and Ron Bygness
Retaining wall, utilities merged in Michigan
Developers and engineers have long tackled the problems with hilly building sites and are ever in search of retaining-wall products to maximize usable space.
When Exxel Engineering in Grand Rapids, Mich. was charged with the task of specifying and designing three retaining walls at the Breton Woods North development, it faced several challenges. The most problematic was designing one section, which included geogrid placement, to accommodate utilities that had to be installed directly through and behind the wall.
Dennis Drier, a dealer with Grand Rapids Gravel, had originally introduced Exxel’s engineers to Redi-Rock wall options. When Exxel engineer Rob Berends examined the Breton Woods site in Kentwood, Mich., just south of Grand Rapids, he thought that this type of a retaining wall would fit this project.
Breton Woods is a multi-use development featuring single-family homes, multi-unit housing, retail and office space. The retaining walls were built across a location that was divided by an immense ravine. In addition, placement of all of the utilities including water main, electrical, storm sewer, and fiber-optic cables were intended within and behind the wall, making the design and construction difficult.
The top four courses, or six feet of the wall, is where the major challenges occurred, as this is where the utilities were tight behind the wall and where geogrid was required. To reinforce the wall and to leave room for utilities, the wall was designed with looped geogrid placement (Photo 1).
Jack Bergmann, Redi-Rock’s internal P.E., noted: “The layer of grid below the utilities was sized to handle the soil above as a surcharge and the looped grids provided local stability for the wall units and any future excavation for utilities.”
When completed, three retaining walls, the tallest a 27-ft. reinforced wall with utilities running through it and behind it, added to the beauty at Breton Woods (Photo 2).
Jacob Manthei of Redi-Rock International contributed to this article.
Breaking the waves in Florida
After years of pummeling by hurricanes and other storms, plus a history of erosion problems, it appears that a geotextile tubing system will help sustain the beaches at Stump Pass in Florida.
Stump Pass is a scenic stretch of sand beach—a natural inlet on the south end of Manasota Key—that connects the Gulf of Mexico and Lemon Bay near Englewood, Fla. It is part of Stump Pass Beach State Park in Charlotte County in the southwestern part of the state.
History
In 1998, an $11 million dredging and beach replenishment project commenced at Stump Pass. The ambitious erosion-control project included the nourishment of three miles of badly eroding beaches adjacent to Stump Pass. All told, more than 800,000 yd.3 of sand were pumped onto surrounding beaches, making the pass 150 ft. wide and at least 9 ft. deep.
But the sand would continually shift. Nautical markers became misleading. And boaters who weren’t familiar with the sandbars often went aground.
Last June, a team from Beach Restoration Inc. (BRI), Lebanon, Tenn., gathered just north of Stump Pass to measure beach profiles, wind exposure, tide patterns, and current flows. Using the data, BRI embarked on the assembly of a beach erosion-control system that was a first-of-its-kind in the United States, according to BRI president Tim Engle. BRI—working with Florida’s Department of Environmental Protection (DEP) and with Charlotte County—designed an eco-friendly system of submerged, low-profile, sand-filled geotextile tubes intended to reduce sand drift into the channel and to stabilize the eroding portion of the beach.
In 2003, county commissioners had budgeted $12 million for a 12-year plan to dredge the area every two years. But with the tubes in place, the expectation is that dredging will now be needed about every four years.
It’s working
It appears to be working. To slow the annual rate of sand filling into the channel, the experimental system tubes create sand-deposit zones and reduce cross-shore sand movement. The tubes, which run from the dunes of Stump Pass Beach State Park to a spot 350 ft. into the Gulf, are buried under the beach or submerged under water, so only a portion can be seen.
Since June, when three of six tubes were initially installed, 20 ft. of shoreline has been restored, according to Engle of BRI.
The tubes were manufactured by Ten Cate Nicolon, Pendergrass, Ga. The external armor fabric came from Twitchell Corp., Dothan, Ala., and was matched specifically for the sand color. Three tubes are currently installed, with another three going in this spring, Engle said. They range from 150 to 375 ft. long.