GEOWEB® Slope Protection System

Project Name: Spotsylvania Solar Farm

Project Location: Spotsylvania, VA

Solar energy has become an increasingly hot topic over the past decade as it has turned into the fastest-growing electricity source in the United States. According to the Solar Energy Industries Association (SEIA), solar has experienced an average annual growth rate of 42% since 2010. We can attribute this growth to a variety of factors, including federal policies incentivizing renewable energy programs, rapidly declining costs, and increasing demand for clean energy.

While these factors provide myriad opportunities for future development in solar, the industry continues to face challenges when it comes to the construction and operation of solar farms. When designing and constructing solar farms, erosion control and stormwater management must be considered to ensure long-term project success.

Such was the case at Virginia’s Spotsylvania Solar Farm—the largest solar farm east of the Rockies to date (2021). The solar farm spans approximately 3,500 acres, with the project site including an additional 2,000 acres preserved as undeveloped, conserved land. Once complete, the solar farm is expected to generate enough power to supply the equivalent of approximately 111,000 homes and offset 340,000 tons of carbon dioxide emissions each year. It is not surprising that a project of this scope posed some unique geotechnical challenges which required economical, long-term solutions.

Spotsylvania Solar Farm

The Challenge: Stabilizing a Slope Vulnerable to Erosion

Stormwater runoff is a concern during the development and operation of solar farms, especially large-scale operations like the Spotsylvania Solar Farm. To meet local stormwater regulations, the Spotsylvania project required the construction of hundreds of retention ponds. The solar site included a sloped area that led into one of the larger detention ponds, and due to its location and large volume of runoff it had to handle, the slope required a permanent stabilization solution.

Located at the back of the site, the 2:1 slope that extended down into the stormwater pond was experiencing severe erosion. Situated next to an access road and downslope from a solar array, the slope failed several times, despite multiple efforts to stabilize the slope surface.

The solar site topography included a gradual decline over approximately 23 acres to the pond slope. Because of this, the slope’s surface had to support a large volume of concentrated flow, with the neighboring access road causing the flow to gain velocity as it approached the area. Stone check dams and other traditional erosion and sediment control practices were not recommended because they would block the access road, which was needed for construction.

Conventional Erosion Control Methods Fail to Stabilize Slope Surface

Initially, the contractor tried hydroseeding the sloped area and covering it with a single-net straw matting—however, this system eventually failed. Then the contractor installed a straw coconut matting over the slope surface, but this also failed. After the second slope failure, they placed a tri-net turf reinforcement mat (TRM) on the slope; however, this failed due to runoff undermining the anchor trench next to the access road.

The large volume of runoff was getting under the TRM, creating soil loss and causing large gullies to form. The contractor evaluated rip rap, articulated concrete blankets, and the GEOWEB system to solve the issue permanently.

The Solution: The GEOWEB® Slope Protection System

The contractor opted for the GEOWEB Slope Protection System, citing cost and performance as the major determining factors. The perforations in the GEOWEB system cell walls allow water to flow throughout the system if it reaches the area below the anchor trench, while the cell walls hold the soil in place, preventing the soil loss and gullies that resulted with the use of a TRM.

Another significant advantage to using the GEOWEB system is the ability to use on-site fill. Often solar projects are Net Zero sites that prohibit the use of imported fill. Additionally using on-site, low-quality fill often amounts to considerable overall project cost savings.

The GEOWEB GW30V6 (mid-size cell, 6-inch depth) system was securely installed over the 2:1 slope utilizing TP-225 tendons (woven polyester, 5100 lb. break strength) tied to a buried deadman pipe and the patented ATRA® Tendon Clips, which provide twice the pull-through strength of any other load transfer device. The ATRA Tendon Clips lock into the GEOWEB cell wall for the most secure connection on the market. Tendons secured with the locked components allow preassembly off-slope for easier installation.

After installation, the slope was hydroseeded and covered with a straw-coconut erosion control blanket. The GEOWEB® 3D Slope Protection System provides a structurally stable environment for topsoil and sustainable vegetation through a structured network of interconnected cells. The 3D GEOWEB system confines and reinforces the vegetated upper soil layer, increasing its resistance to erosive and sliding forces.

Project Results

To date, the 3D GEOWEB system has held up to multiple high-intensity rain events without any slope failures or gully erosion formation. Once the system was installed and the area seeded and matted, the slope immediately began to show vegetation. Given the size of the slope and the amount of runoff directed to the area, the 3D GEOWEB system is performing better than anticipated.

Michael Wills, CESSWI, CPESC, Environmental Manager – Mortenson (General Contractor)