Project Name: Lemm Gully Erosion Control
Project Location: Montgomery County, Texas
In November 2021, the banks around a bend in Lemm Gully, located near a main sanitary line, were facing rapid erosion. Situated close to a lift station, this section of the gully required stabilization and reinforcement to safeguard against the impact of a 100-year storm event. Montgomery County, overseeing the project, emphasized the need to preserve Lemm Gully’s natural geometry while addressing the erosion issue.
The project faced constraints due to the presence of the sanitary line, which ran through the heart of the area, limiting the extent of excavation that could be undertaken. Additionally, the county expressed a preference for a solution that incorporated vegetation, aiming for a more natural approach to reinforce and control erosion on the banks.
Access limitations at the site made it impossible to use industrial-grade equipment. The soil in the area was particularly sensitive to moisture, posing a risk of washout during storm events if left exposed. This vulnerability was a key concern during the construction phase, requiring careful management to ensure the stability and integrity of the site.
Due to the steep side slopes and impending erosion from the next major storm event, the owner wanted a resilient solution. Given the project’s distinct constraints, the project team aimed to explore various mechanically stabilized earth (MSE) options. The combination of factors, including the critical slope angle of the banks, the softness of the channel bed, and the presence of a sanitary line bisecting the slopes, necessitated the consideration of multiple alternative approaches to ensure effective stabilization and durability.
The project team presented two final options to stabilize the slopes. The first option was a gravity wall consisting of aggregate-filled gabion baskets, tied back by reinforcing steel mesh into the embankment. The second option was a GEOWEB® Vegetated MSE Wall reinforced by uniaxial geogrid into the embankment. Both designs were well-suited for the application at hand, offering flexibility to build around the sanitary line, as well as the ability to construct the wall with small to medium sized equipment.
However, after reviewing both options for erosion control, the GEOWEB Vegetated Retaining Wall System was selected to maintain a permanent, green aesthetic and restore Lemm Gully back to its original state. 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. Uniaxial geogrid was used to reinforce the system’s face, providing adequate load distribution behind the GEOWEB System. Strips of turf reinforcement mats were used to protect the soil within each of the exposed GEOWEB facing units until vegetation was established. The project’s challenging site conditions proved to be no match for the flexibility and ease of construction that the GEOWEB Vegetated Retaining Wall System provided.
The GEOWEB System’s ability to confine material within each of the system’s cells allowed the contractor, Triple J Enterprises, to effectively compact the face units of the wall as they worked their way up, constructing the system from the bottom of the slope to the top. The soft belly of the channel was combatted by burying the first few lifts of the GEOWEB System to establish a firm footer for the wall. Once vegetation was established, the root system grew through the face cells to produce an interconnected, monolithic matrix of roots. In turn, this will mitigate long-term soil loss and provide additional surficial reinforcement for the banks. Upon final application, a concrete apron was poured upstream and downstream, vertically along the slopes to ensure high velocity events have no chance in undermining the system.
The GEOWEB Vegetated Retaining Wall System was custom-cut on-site to ensure a snug fit around the sanitary line, effectively mitigating erosion near this critical obstruction. This tailored approach allowed for precise adaptation to the specific contours and challenges posed by the sanitary line’s presence.
The contractor constructed a custom-engineered frame from lumber, following the specified plans, to pre-stretch the GEOWEB sections before placing and filling the system. This enabled the installers to move quickly, filling the front cells with organic material for vegetation purposes, and the back cells with aggregate to improve drainage and stabilize the embankment. The GEOWEB Vegetated Retaining Wall System significantly reduced the required quantity of aggregate and concrete, allowing the owner to build a long-term erosion-control solution for the once failing banks of Lemm Gully with minimal material costs.
The GEOWEB installation was phenomenal. The way the product connected together was so easy with the ATRA® Keys. Creating a jig/frame made for a smooth installation. The crew loved using it so much that they built another one on-site so they could fill one up while they were pre-stretching the next.