Managing stormwater runoff and preventing erosion are critical challenges on building sites with significant grade changes. When a site is raised out of a flood plain, steep embankments can create drainage complications that threaten both structural integrity and landscape stability. Innovative concrete systems that combine the load-bearing capacity of reinforced concrete with the permeability of vegetated surfaces offer a practical path forward. One such solution, Grasscrete, has been deployed on demanding sites to solve drainage issues while maintaining a natural appearance. For builders tackling similar site conditions, understanding how vegetated concrete systems work alongside other drainage strategies is essential. A well-designed approach to site water management, such as Building an Affordable Site Built Deck Drainage System, provides useful context for integrating surface stabilization with broader drainage planning.
The Drainage Challenge at Raised Building Sites
When a building site must be elevated to meet flood plain regulations, the resulting grade change introduces cascading engineering considerations. The case of the Town of Jean Lafitte, Louisiana, illustrates the complexity. After Hurricane Ike destroyed the town auditorium in September 2008, the Federal Emergency Management Agency required the replacement building to be raised out of the flood plain. The site received more than nine feet of River Sand fill, and an 18,000-square-foot pile-supported auditorium was designed for the location.
Raising the site created a steep embankment along the parking lot where the elevated grade transitioned down to a ball field. The slope descended approximately seven feet over just twelve feet of horizontal distance, producing a gradient vulnerable to severe erosion under Louisiana’s heavy rainfall.
Site Factors That Increase Erosion Risk
Several factors combined to create a high-risk scenario at this site:
- Steep gradient: A 7-foot drop over 12 feet represents roughly a 58 percent slope, far exceeding the 3:1 ratio considered the maximum for stable, unreinforced vegetated slopes
- High precipitation: Louisiana receives over 60 inches of annual rainfall, with intense thunderstorms capable of delivering several inches per hour
- Fill material: River Sand has low cohesive strength and is prone to surface erosion when exposed to overland flow
- Traffic exposure: The slope bordered a parking lot in a high-traffic zone adjacent to schools and a library, adding pedestrian and vehicular loads
Without intervention, this combination would lead to rill erosion, gully formation, and progressive slope failure, creating both a safety hazard and an aesthetic problem in a prominent community location.
Constraints That Shaped the Solution
FEMA funding required the site to be raised above the base flood elevation, which dictated the final grade. The stabilization method had to accommodate continued public access to the adjacent ball field and parking areas. Solutions requiring additional land for low-angle terracing or extensive retaining walls were impractical. For building sites with similar considerations, Solar Access Building Site Design examines how site layout decisions interact with grading and surface water management to create more resilient projects.
How Grasscrete Works: Design and Structural Properties
Grasscrete is a cast-in-place concrete paving system that creates a continuous reinforced concrete lattice with void spaces that can be filled with soil and planted with vegetation. Unlike traditional concrete that creates impervious surfaces requiring separate drainage infrastructure, Grasscrete functions as both a structural pavement and a permeable surface treatment. Robby Oswald, owner of Bomanite of New Orleans, the contractor who installed the system at the Jean Lafitte project, describes it as the ideal solution because it forms a concrete lattice that allows grass to grow within the voids.
Former System and Installation Process
- The subgrade is prepared and compacted to meet load-bearing requirements
- Molded-pulp formers are laid out across the prepared surface
- Continuous #4 rebar reinforcement is placed through and around the formers
- Concrete is cast monolithically across the entire area
- Concrete cures, then formers are removed leaving a lattice pattern
- Voids are filled with topsoil and seeded or sodded
The monolithic placement and continuous #4 rebar reinforcement give Grasscrete structural properties comparable to conventional reinforced concrete flatwork.
| Property | Specification | Notes |
|---|---|---|
| Slab thickness | 5.5 inches | Standard Grasscrete former depth |
| Compressive strength | 4,500 to 12,000 psi | Depends on concrete mix design |
| Reinforcement | #4 rebar, continuous | Provides structural integrity across the lattice |
| Vegetated coverage | 40 to 60 percent | Open void area for vegetation |
| Permeability | Fully permeable at voids | Stormwater infiltrates through vegetated cells |
| Load capacity | Equal to conventional concrete flatwork | Supports emergency vehicle loading |
Vegetation and Drainage Integration
The vegetated component provides multiple functions beyond aesthetics. Grass root systems penetrate through the soil-filled voids and help bind the surface layer, adding biological reinforcement. The vegetation also intercepts rainfall, reduces runoff velocity, and promotes infiltration. At the Jean Lafitte project, the Grasscrete surface was covered with sod for immediate erosion protection. By allowing water to infiltrate through the entire surface, the system eliminates the need for edge drains, catch basins, and piped conveyance that conventional pavement would require.
Applications for Grasscrete in Site Drainage and Erosion Control
While the Jean Lafitte project demonstrates Grasscrete’s effectiveness on steep embankments, the system has broader applications. Oswald notes that his firm has installed more than two million square feet of Grasscrete across diverse project types.
Overflow and Industrial Parking Areas
Parking lots that serve intermittently, such as overflow lots for schools and event venues, are ideal candidates. The vegetated surface maintains a natural appearance during low-use periods while providing structural capacity when needed. Surface drainage through the vegetated voids eliminates the need for separate stormwater management infrastructure, reducing both construction costs and long-term maintenance.
Roadway Shoulders and Embankments
Roadway shoulders frequently suffer erosion from sheet flow draining off the pavement. Standard solutions involve riprap, concrete curbs, or buried pipes, each with trade-offs. Grasscrete provides a vegetated shoulder that handles both traffic loads and hydraulic loads from runoff, without the edge erosion that plagues conventional vegetated shoulders.
Emergency Vehicle Access Routes
Municipal codes frequently require emergency vehicle access routes to support fire truck loading, which can conflict with goals for permeable, vegetated surfaces. Grasscrete resolves this by providing required structural capacity within a planted surface. Oswald notes that when a fire truck must drive or park on a surface, the underlying concrete must be strong and solid, and Grasscrete delivers that while maintaining a green appearance.
Coordination with Landscape Planning
Integrating Grasscrete requires coordination with the overall site plan, including grading, planting, and drainage design. Site Landscape Planning Green Building Construction provides guidance on integrating permeable systems with broader site design objectives for sustainable projects.
Installation Best Practices and Long-Term Performance
Subgrade Preparation
The subgrade must be prepared to the same standards as conventional concrete pavement. Proper compaction prevents differential settlement that could crack the lattice. On sloped installations, the subgrade should be benched to prevent downslope creep. Drainage beneath the slab must also be addressed, as water infiltrating through the vegetated voids collects at the subgrade level and must be directed away.
Concrete Placement and Curing
The concrete mix should be designed for the expected loading, with compressive strength selected from the 4,500 to 12,000 psi range. Workability is critical, as concrete must flow around the formers to create a monolithic lattice without voids. Placement should be continuous to avoid cold joints, which create weak points where cracking and differential movement can occur.
Vegetation Establishment
Vegetation can be established by seeding or sod installation. Sod provides immediate cover and is preferred for slopes where erosion control is needed from day one, as was done at the Jean Lafitte project. Key maintenance practices include:
- Filling voids with a planting mix appropriate for the selected vegetation and local climate
- Regular irrigation until root systems are well developed
- Standard mowing with conventional equipment once vegetation is established
- Annual inspection of the concrete lattice for cracks, spalling, or displacement
Cost Considerations
The installed cost of Grasscrete is generally higher than conventional concrete pavement due to the former system and additional labor. However, when the system eliminates the need for catch basins, underground piping, and riprap, the overall site development cost can be competitive. Additional savings come from reduced stormwater utility fees in jurisdictions that charge based on impervious surface area and from eliminated maintenance of separate drainage features.
Performance on Challenging Sites
The Jean Lafitte project demonstrates that Grasscrete performs effectively on extreme slopes with high erosion risk. The system has been in development since the mid-1970s with a proven track record across diverse climates and soil conditions. For building sites in wet or flood-prone areas, Building On Wet Site covers strategies for managing drainage on sites with high water tables and poor soil drainage, complementing the use of vegetated concrete systems for slope stabilization.
Summary. Grasscrete offers a proven solution for building site drainage challenges where steep slopes, vehicular loading requirements, and preferences for vegetated surfaces converge. The system combines the structural strength of 5.5-inch reinforced concrete flatwork with the permeability of a planted surface. The Jean Lafitte, Louisiana, auditorium project demonstrates successful deployment on a 58 percent slope that would otherwise have required extensive conventional drainage infrastructure. For builders and site designers evaluating slope stabilization options, Grasscrete provides a technically sound alternative that addresses multiple site constraints within a single integrated system.