Article : January 2013 LC/DBM: The Great Wall of Fort Lincoln

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The Great Wall of Fort Lincoln

Located in Washington DC’s Fort Lincoln neighborhood, the Dakota Crossing project included five wall segments totaling 21,000 square feet to support the 42-acre retail project. On the 28-foot high hill, the contractor planted one plug in each brick module. The plant material included: Virginia Bluestem, Muhly Grass, and Fountain Grass plants. This material was chosen for its high evapotranspiration characteristics and aesthetic appeal. In preparation for the plant material, workers packed in more than 800 cubic yards of specialized bioretention planting soil.

Just across the Anacostia River in Washington DC’s Fort Lincoln neighborhood, a large parcel has been ripe for development for more than 20 years. When the $52 million, 42-acre project finally began, Hardscapes Construction, Inc. came onboard as the primary landscape contractor for the newly named Dakota Crossing project.

The project called for 430,000 square feet of retail, including a town center shopping center with a Costco.

Prior to installing the next series of modules, workers temporarily pulled filter fabric back and prepared to fill the modules with bioretention soil. A 5-man crew from Hardscapes Construction, Inc. installed the wall and irrigation tubing, while a 4-man team from Furbish Company did the planting. A crew from Saco Construction, Inc. used heavy equipment for the more extensive site work.

The Project
The site plan required significant retaining wall infrastructure, including five segments totaling 21,000 square feet (Phase I) to handle grade change. While traditional retaining walls were considered, the designers chose a SmartSlope-based design. These modular cells create a vegetated/structural retaining wall system that also acts as an integral component of stormwater management.

Moreover, the plan includes capturing stormwater runoff from the roofs, parking lots, and impervious surfaces, which are then channeled into in a series of 10,000-gallon cisterns buried below grade. The water can later be pumped through the retaining wall through drip irrigation tubing. The tubing attaches to the wall modules through a notch on each sidewall. The system can manage 0.5 gallons of stormwater per square foot per day.

Workers installed irrigation-tubing headers through the front of the wall in order to connect a future water supply. Drip irrigation tubing was specified to run a maximum distance of 1,000 feet per supply source. Hardscapes Construction installed more than 14,000 linear feet of Netafin polyethylene piping.

The installation of a modular wall is similar to that of a traditional structural retaining wall. Due to the scale of Dakota Crossing, a larger than normal 2-foot wide by 4-foot deep poured concrete footing was installed as per design. Next, two rows or courses of wall modules were set and filled with gravel. Workers stacked the modules 14 inches apart, and additional courses rest on the sidewalls of the modules below. Strap reinforcement is belayed through a true mechanical connection in the base of the modules. The project was engineered to employ strap reinforcement every third course, with maximum strap lengths of 32 feet due to the excessive top load. Site soil was used in the reinforcement zone and compacted over the strap with heavy construction, vibratory rollers. The modules were then filled with the bio-retention planting soil before starting the next course.

The contractor installed irrigation tubing before filling modules with soil. A notch built into each sidewall of the block holds the tubing in place. The tubes have small drip emitters that allow water to be released into the planting soil. 11,350 modules were used in the project (21 truck loads). Workers installed more than 22,500 linear feet of 4-inch ADS drainage pipe.

The actual wall installation took five laborers 1,000 man-hours to complete. These types of modular walls can be installed at a rate of around 700-1000 square feet per day in normal site conditions with an experienced 5-man crew. Additional workers were called in for planting and excavation.

Dirt used as backfill and for compaction was sourced onsite from the excavation. The site contractor used large earthmovers to move more than 25,000 cubic yards of dirt for the wall construction.

Plants & Soil
The plants were chosen specifically for their high evapotranspiration characteristics. Andropogon virginicus (Virginia Bluestem), Muhlenbergia capillaries 'white cloud' (Muhly Grass), and Pennisetum alopecuroides (Fountain Grass) were planted, one plug in each module.

Once fully grown out, they grow three feet high. Special bio-retention soils were chosen for their ability to retain water longer and their ability to aid in the quality treatment of the runoff.

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