Lemm Gully Slope Stabilization Project
Not only were Lemm Gully's eroding banks threatening a nearby sanitary line, but also the worksite had major constraints. Ferguson Waterworks assesses the project, provides recommendations, and helps facilitate a successful outcome
Project Overview
Montgomery County tasked VGOT Engineering and Triple J Enterprises to address Lemm Gully’s eroding banks. Unfortunately, this particular bend of Lemm Gully was near a lift station and was quickly deteriorating the earth around a main sanitary line. Montgomery County requested minimal impact on the geometry of Lemm Gully. Furthermore, excavation efforts were restricted because the sanitary line cut through the center of the project.
Challenge
Both stabilize and reinforce the eroding banks around the Lemm Gully bend nearest the sanitary line. Furthermore, the erosion control measures need to withstand a 100-year storm event.
Additional Considerations:
- Limited site access prevents industrial-grade equipment from entering.
- The soils were moisture-sensitive and, if left exposed to the elements, were prone to washout during storm events throughout the construction phase.
- Lastly, Montogomery County preferred a vegetated option for reinforcement and erosion control.
Multiple stabilization and erosion control products would need to be considered to accommodate the critical slope angle of the banks, soft belly of the channel, and sanitary line splitting the slopes.
Solution
After considerable assessment, the team presented two slope stabilization solutions.
- A gravity wall consisting of aggregate-filled gabion baskets tied back by reinforcing steel mesh.
- Alternatively, a vegetated MSE wall consisting of GEOWEB and reinforced by uniaxial geogrid embedded into the embankment.
Both designs were well suited for the complex slope stabilization project. This is because they provided flexibility to build around the sanitary line, as well as the ability to construct the wall with small to medium-sized equipment. Ultimately, the client chose to proceed with the GEOWEB vegetated retaining wall system. The GEOWEB wall solution offers a terraced structure so vegetation can effectively grow within each of the cells—much like a series of isolated planter beds.
How was the GEOWEB Vegetated Retaining Wall System Installed in order to ensure long-lasting vegetated slope protection?
To establish a firm footer for the wall, the team buried the first few lifts of the GEOWEB System. As a result, it compensated for the soft belly of the channel. In addition, they incorporated strips of turf reinforcement mats in order to protect the soil within each of the exposed GEOWEB-facing units until vegetation was established. At that point, the root system will grow through the face cells. This creates an interconnected, monolithic root matrix. As a result, it mitigates long-term soil loss and provides additional surficial reinforcement. Upon final application, the team poured a concrete apron upstream and downstream, vertically along the slopes. This ensures high-velocity events have no chance of undermining the system.
The site’s challenging conditions proved to be no match for the GEOWEB Vegetated Retaining Wall System.
- Flexible design
- Does not require large construction equipment for installation
- Offers natural aesthetic
- Not to mention, reliable and long-lasting stabilization and erosion control
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