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Saving a local treasure

Products | December 6, 2011 | By:

Geogrid technology helps restore Wisconsin lake and wetlands

Lake Belle View was once the pride of south-central Wisconsin. Older residents remembered its glory days, when the natural beauty of the 92-acre lake and its surrounding wetlands drew visitors from miles around for music festivals, picnics, and sporting events.

But as it filled with farm and city runoff carried in by two connecting rivers, its depth dropped to just 2ft due to the erosion. Once host to swimmers, anglers, boaters, and birdwatchers, the dying lake was bereft of fish and swimmers shunned its algae-filled high bacteria levels. An adjacent forest flooded frequently, killing lakeside trees.

Spurred by their memories, local residents began a grass-roots effort in the 1980s to restore the lake to its former beauty.

Planning the lake’s restoration

After decades of planning and lobbying by local officials and lake supporters, authorities approved a restoration plan in October 2009. Ultimately, the $4 million project was funded by grants, private contributions, and a local tax referendum that raised $1 million.

The goals of the Lake Belle View project were to improve and increase:

  • water quality.
  • lake depth.
  • diversity of aquatic habitat
  • diversity and quality of wetland habitat at the perimeter of the lake and beyond.
  • the natural beauty of the area’s ecosystem.

To refill the lake and separate it from the Sugar and West Fork Sugar rivers, engineers decided to construct a 3,200-ft berm. The berm would decrease the rate of surface runoff from the rivers and reduce the amount of sediment that reached the lake. In addition to this erosion control, the berm would serve as a pedestrian walkway and bike path.

Enhancing the load-bearing capacity of the soil

To follow green construction guidelines and keep costs down, the project engineer recommended building the berm on top of the existing grade. However, the berm subgrade consisted of saturated, silty lake sediments with low shear strength. Removing the sediments was not an option because it would widen the footprint of the berm construction in the wetlands and perhaps damage the environment. It also would be expensive to remove and replace the sediments.

Instead, the contractor built a mechanically stabilized layer (MSL) using geogrid layers at the base of the berm.

“We chose [geogrid] because it reduced construction costs and improved the feasibility of the overall design by allowing us to build the berm on the existing grade, minimizing our labor and equipment costs,” said Glenn Joski, the general contractor.

Together, the geogrid and the aggregate layer formed the MSL, creating a construction platform over soft soils. Mechanical interlocking increases the load-bearing capacity of the aggregate, distributing the load in all directions, 360 degrees.

Although the Lake Belle View berm traffic is restricted to pedestrians and bikers, it was a different story during construction, when vehicles put extra weight on the soft subgrade soils beneath the geogrid. But the contractor wasn’t concerned, since the stiffness of the geogrid creates a “snowshoe effect,” which spread the load over a larger area and reduced the pressure on the soft soils.

Geogrid separates subgrade and aggregate

The contractor also reduced costs by using geogrid as a separation material as well as a structural component of the aggregate section.

The key was selecting a well-graded aggregate so that both large and small aggregate particles locked into place in the open, triangular apertures of the geogrid. This forms a natural filter that allows water to pass through while preventing the subgrade particles from migrating into the aggregate layer. Furthermore, the snowshoe effect of the geogrid resists pumping of the subgrade fines into the aggregate layer and the punching or pushing of the aggregate particles downward into the subgrade.

Mechanically stabilized layer supports the berm

Choosing an MSL gave the contractor a stable construction platform as the berm embankment was built.

First, workers placed a layer of geogrid directly at the base of the berm, bridging the lake sediment across a 17,000 square-yard area. Second, they unrolled adjacent strips of geogrid, overlapping them by 3ft and temporarily securing them with nylon cable ties. Third, they covered the geogrid layer with approximately 24in. of well-graded sand, creating the MSL that supports the berm. Fourth, they added the granular fill to build up the berm to about 5ft above grade.

Slashing construction costs

Geogrid requires less aggregate than conventional construction—reducing construction costs by as much as 50%, according to industry experts. That’s because it reduces both labor and equipment costs. It also increases the life span of the surface and enhances the reliability of trafficked surfaces.

Costs were further reduced because there was no need to over-excavate or haul in additional off-site aggregate. Another plus in this environmentally sensitive area is that the use of geogrid kept the construction footprint to a minimum.

“This project is an example of how geogrid technology can make the construction of a berm not just possible, but also economically and environmentally feasible,” Joski said. “Ultimately, the geogrid improved the slope stability of the berm embankment, reduced its overall and differential settlement, and separated the lake sediments from the granular fill that we used to construct the berm.”

A beautiful transformation

By summer 2011, the Lake Belle View restoration was complete, and local citizens happily resumed their long-delayed recreation at the lake and community park. Lake Belle View has become a renewed source of pride for area residents.

SOURCE: Tensar Intl. Corp.

EDITED: Geosynthetics magazine

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