A water channel in Coppell, Texas, that connects to lakes at Andrew Brown Park needed a new erosion control system to direct high flow levels caused by heavy rainstorms and runoff. The water level was pumped down with three diesel pumps (Barco) for about seven months during construction, and segmental retaining walls with an 18-inch footing and a pervious cement backfill were installed at the mouth of the channel to control the flow.

The project engineer formed energy dissipating blocks in concrete (right) to slow the water as the channel reaches the lake. The retaining wall system was installed to prevent erosion, rather than manage a grade transition. Due to potential scour from high velocity flow, increased embedment and protection of the leveling pad was critical to the project's long-term performance, leading to a vertical embedment depth of more than two feet.

JDK Associates, Inc., based in Pilot Point, Texas, installed the modular blocks and stabilized aggregate at the park. According to Steve Kenney from JDK, the installation process with stabilized aggregate was quicker than using walls reinforced with soil. The structural backfill was delivered to the jobsite in ready-mix trucks and placed with the bucket of a backhoe.

Coppell, Texas, is a thriving residential community located in the center of the Dallas-Fort Worth metroplex. Approximately 40,000 residents occupy the area, and surveys indicate high satisfaction with the local amenities. One reason for the town's approval is the Andrew Brown Park, a 160-acre space that is constantly active with jogging, tennis, fishing, and other recreational activity.

Andrew Brown Park frames the north border of the surrounding residential neighborhood and includes more than 30 acres of lakes, which host a variety of wildlife species and provides a great setting for cookouts. In addition to the serene setting, the lake also serves as a large detention facility, and is responsible for the stormwater runoff from 900 acres and more than 2,000 houses in the development. The lake's primary source of flow is a channel that meanders through the neighborhood, approximately 60 feet wide by six feet deep. The relatively small channel responsible for this large development can yield high velocity flows, which can exceed 15 feet per second during torrential storms due to significant impervious areas. The design engineer incorporated energy dissipaters as the channel terminates into the lake. Whenever high hydraulic velocities and associated critical shear stresses occur, it is important for engineers and landscape designers to create solutions that satisfy the performance and aesthetic needs of the stakeholders.

Enclaves on the Parkway is an upscale community that borders the south side of Andrew Brown Park. The development's proximity to the park required a more natural, appealing solution than traditional erosion countermeasures, which include rip rap, concrete lined channels and shoreline. Multi piece segmental retaining walls (SRWs) in blended earth tone colors offered the perfect aesthetic solution to satisfy hydraulic challenges presented by the neighborhood and park area.

About 26,000 square feet of three-piece Diamond Pro stone cut blocks in a Milsap blend (Pavestone) were installed for the retaining walls, and Diamond Pro wall caps in the same color were used to top the walls. Construction took as many as three 4- or 5-man crews about six months to install, as numerous rain events forced delays during the installation.

Segmental retaining wall installers can facilitate wall construction up to 12 feet high by placing structural backfill immediately behind the block facing. In the case of the Andrew Brown project, the depth of the structural backfill was 35 percent of the wall height.

Traditionally, segmental retaining walls are constructed using in situ soils and, given certain design parameters, often require geosynthetic reinforcement to stabilize the soil mass behind the block. Depending on site configuration and wall parameters, the geosynthetic reinforcement depth can vary from 60 percent to more than 100 percent of the wall height. The Dallas-Fort Worth area is notorious for some of the softest clay soils in North America however, and combined with a water application that required the design of saturated soils and rapid drawdown in the reinforced zone, traditional reinforcement depths would have extended into property owners' yards.

Structural backfill was used as an alternative to traditional geosynthetic reinforcement, forming a cohesive structure with the SRW units that maintained the desired facing solution. Structural backfill, also known as "no-fines" concrete, is a readily available, easily workable, highly permeable mixture of clean crushed stone, cement, and water. The resulting material has at least 25 percent voids after curing. This construction method eliminates the need for the mechanically stabilized earth zone and requires substantially less excavation and compaction than is usually necessary in a grid-reinforced wall project.

The ability to handle water flow makes structural backfill SRWs an ideal solution for the challenges in Andrew Brown Park. During flood events, fast moving waters will be absorbed by the new retaining wall and take the pressure off the channel, energy dissipaters, and lake. The new wall towers over the streambed, protecting the community and lake while providing an attractive addition to the backyards.