SRW Water Applications

Edison Towne Square uses Allan Block walls to combine functional stormwater management with attractive, durable design for a complex 100-acre watershed.

Watershed Preserved with Style

When the city of Edison, NJ approved plans for the Edison Towne Square project, they knew it would bring more than just a new shopping and retail center—it also provided a solution to local stormwater management challenges. City planners hired Menlo Engineering Associates to develop the site plans, while Land Tek Consulting handled the final designs and site inspection of the retaining walls.

The detention basin required by the city had to be both functional and visually appealing to complement the new development. The city chose Allan Block’s AB Collection Ashlar Blend, known for its aesthetic four-block pattern, supplied by local manufacturer Clayton Block Company of Edison, NJ. Clayton Block representatives collaborated closely with developers and city planners to select block colors, while Land Tek and Menlo guided the design process.

Complex Site Conditions

The project site encompasses roughly 100 acres of watershed draining into the basin, including the new development and an adjacent interstate highway. Complicating matters, the local water table was just a few feet below the surface. This required careful planning to prioritize water management.

Land Tek followed standard water application design and construction procedures provided by Allan Block Corp. and the NCMA for all basin walls. Walls were constructed using high-quality clean gravel infill from the wall face to the back of the reinforced geogrid mass, installed in 8-inch (200 mm) compacted layers, extending about 1 foot (25 mm) above the high-water mark. Using washed rock promotes rapid drainage and prevents hydrostatic pressure buildup as water levels fluctuate.

During design, water storage volume and overflow management had to be reevaluated due to site changes. Land Tek increased basin capacity by designing a larger leveling base under the walls, which Menlo connected to the underdrain system. The city and DOT coordinated to connect the overflow system to the interstate highway drainage network.

Construction faced challenges due to the high water table. Argus Construction, the wall builder, employed aggressive dewatering techniques. Land Tek modified the design to include additional buried block courses, ensuring a stable construction base. Once walls were set, construction proceeded smoothly to completion.

Designing SRW Water Applications

Segmental retaining wall (SRW) units are widely used for water applications including detention basins, ponds, lakes, rivers, and wetlands. Designing walls for water applications follows the same principles as standard walls, assuming no hydrostatic pressure exists within the reinforced mass.

The key is using free-draining gravel, often called wall rock, with virtually no fine-grained particles. This allows water to rise and fall both inside and outside the mass, keeping pressure balanced. During rapid drawdowns, the wall rock allows water to evacuate quickly, maintaining pressure equilibrium.

Key Design Details

Beyond the free-draining gravel, designers should consider:

• Embankment protection fabric to prevent fines from entering the reinforced zone as water levels fluctuate.
• Filter fabric above the wall rock, below topsoil, to stop soil migration into the reinforced mass.
• Drain pipe placement just above the low water mark, enabling efficient water evacuation.
• Wall embedment and rip-rap at the toe for erosion protection, using boulders, cobbles, or prefabricated mats based on site-specific conditions.

Understanding the type of water surrounding the wall is critical for proper detailing and long-term durability.

Construction Tips for Water Applications

Building SRW walls for water applications is straightforward if the site is dry or dewatered. Dewatering may involve cofferdams using sandbags or sheet piling, along with pumps, which can be costly. Accurate bidding is essential for project success.

Walls are often constructed during dry seasons or winter, using gravel-based soils resistant to freezing and easily compactable. In some cases, an alternate base of impermeable granular material may be used to prevent water from ponding in lower wall sections.

Additional Considerations

• Culverts: Flexible culverts, like corrugated steel, require symmetrical wall construction to avoid deformation.
• Stormwater pipes: Surround pipes with extra washed rock in the reinforced zone to prevent leaks from damaging the wall. Evaluate outflow and provide rip-rap or other surface protection to control erosion.