Wetlands are considered some of the most biologically productive ecosystems in the world. However, these areas are being compromised by erosion and increased exposure to toxins. The impacts of erosion and sedimentation in wetlands are a serious problem in the United States and across the world, causing habitat loss for plants and wildlife.
Erosion control and restoration projects in delicate habitats such as lakes and wetlands require site design innovations to avoid harming the local ecosystem. A primary objective of such projects focuses on minimizing the impacts during the construction process while providing the best possible long-term solution for the surrounding plants and wildlife.
Lake Belle View restoration
Recently, a victory for wetlands and lake habitats was won after the town of Belleville, Wis., took steps to successfully restore its lake and the surrounding ecosystem. The 94-acre Lake Belle View had been unusable for decades as a result of sedimentation and agricultural contamination that caused the body of water to slowly silt up to an average depth of less than 2 feet. The lake was finally refilled in the summer of 2011 after more than 30 years of planning.
Completion of this $4 million project, which was funded through grants, private contributions, and tax allocations, represents decades of planning and lobbying by local officials and lake supporters. In fact, a special tax referendum was enacted that raised approximately $1 million to support the restoration process. The restoration of Lake Belle View means that residents and visitors can once again enjoy bountiful fishing, quiet water activities, swimming, and playing football or picnicking in the park.
Not only was the lake restoration important to enhancing the leisure activities and enjoyment of area residents, it also created a more diverse and higher quality ecological habitat to the lake, the surrounding park, and the forested wetlands. To enhance the ecology of the area, an erosion control management plan was introduced that relied heavily on Tensar’s TriAx Geogrid.
Geogrid technology in practice
The environmental restoration required constructing a 3,200-foot-long berm and pedestrian path to separate Lake Belle View from receiving the flows from the Sugar River. Diverting Sugar River flows will prevent further accumulation of sediment in the lake now that it is restored. The primary objective of the berm construction was to build the structure without adversely affecting the river, the lake, or other environmentally sensitive areas. The project engineer determined that building the berm on top of the existing lake sediments was the simplest and most cost-effective solution.
Constructing the berm required using Tensar’s TriAx Geogrid, a geosynthetic material that is manufactured from a punched and drawn polypropylene sheet. Tensar geogrids are used in civil engineering applications to create a mechanically stabilized layer (MSL) that stabilizes soft soils when used in conjunction with an aggregate layer. The triangular design and rib profile of TriAx Geogrid forms the MSL by confining aggregate particles within its apertures to create mechanical interlock. Once mechanical interlock occurs, the stiffened aggregate layer is able to increase the load bearing capacity of the soil with 360 degrees of load distribution. TriAx Geogrid was particularly applicable for Lake Belle View because the site consisted of a subgrade of saturated silty lake sediments with very low shear strength. Using the MSL allowed for the creation of a stable construction platform for building the berm embankment.
Installation of the construction platform began with one layer of TriAx TX160 Geogrid placed directly at the base of the berm to cover the lake sediment soils over an area of 17,000 square yards. Next, adjacent rolls were overlapped 3 feet and temporarily secured with nylon cable ties. The geogrid layer was then covered with approximately 24 inches of well-graded sand to create the mechanically stabilized layer for supporting the berm construction operation. Subsequent lifts of select granular fill were used to build up the berm to approximately 5 feet above grade.
The Tensar Geogrid design proved to be a more viable alternative to the conventional construction method of over-excavation and removal of lake sediments since the variable depth of the sediment made it difficult to predict both the feasibility and cost of removal. Additionally, removal of the sediments would have created further environmental impacts with larger construction limits required for over-excavation and replacement of the lake sediments. The ability to use TriAx Geogrid with the existing sediments saved both time and costs that would have been incurred through further excavation.
TriAx Geogrid provides a number of benefits that includes simplifying construction by reducing labor time and the amount of necessary equipment. But most importantly, use of geogrids over soft soil can, in many instances, save as much as 50 percent in construction costs by requiring less aggregate, as well as increasing the lifespan of the surface and enhancing the reliability on trafficked surfaces. The cost savings for the Lake Belle View project came from the reduction of labor and equipment requirements because the contractor did not have to over-excavate or haul in additional aggregate from offsite.
Benefits of erosion control
Construction of the mechanically stabilized berm is a key step that will eventually restore the lake water quality, reduce thermal pollution, and improve river-system connectivity. The decades-long effort required to launch the project has been sustained by a strong public-private partnership among groups such as the Upper Sugar River Watershed Association and the residents and elected officials of the village of Belleville and town of Montrose.
Tensar provided application experience to the Lake Belle View restoration project and design. These contributions to the design team made certain that the proper aggregate fill gradation was in place to ensure stability and separation of the embankment fill, geogrid placement requirements, embankment construction, and equipment limitations. Tensar was present onsite throughout the process to assist with contractor questions and address site-specific application issues.
The use of Tensar’s TX160 Geogrid improved the slope stability of the berm embankment, reduced overall and differential settlement, and provided a separation barrier between the lake sediments and the granular fill used in the berm construction. The durable berm will benefit the lake restoration and wetland ecosystem for years to come.
Jim Howley, P.E., is North Central Region manager for Tensar.