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South Alex

Alexandria, Virginia


Combined Properties


Greenway Engineering


Total Civil


April 2019

Technical Description:

  • (18) Terre Arch™ 26 Chambers
  • Filterra® Biofiltration
South Alex (Formerly Penn Daw Shopping Plaza) is a new mixed-use property built on the site of an old shopping center.  The project consists of 400 apartments, an Aldi Store, 44,000 square feet of retail space, and 41 townhomes. 

The redevelopment of the Penn Daw Shopping Center presented a variety of engineering challenges due to the density of the proposed layout, and the limited footprint available for stormwater management facilities.  Also, as part of the entitlement process, engineers were required to not disturb the existing, on-site retention facility in the southwest corner of the site.   

Contech stormwater engineers developed a solution that used multiple facilities in series to meet the Alexandria, Virginia quantity control requirements and the Virginia Department of Environmental Quality Phosphorous reduction requirements. 

The Terre Arch system was selected for the upstream facility due to its shallow profile and flexibility to provide the required volume underneath the proposed plaza/surface parking of the site.  Terre Arch is a modular, multi-chambered, precast concrete stormwater storage system engineered for underground installation. Terre Arch provided the flexibility in footprint and incoming/outgoing pipe locations while minimizing earthwork required to install the downstream facilities. 

The Terre Arch systems consisted of 18 arches, two distribution boxes, two end caps, three distribution manifolds, one capping slab pair, an erosion grid and anti-scour mat, two risers, and one 48” corrugated metal pipe outlet control structure.  A 6’x4’ Filterra bioretention system was used to meet the phosphorous reduction requirements.

Contech provided sizing, routing, structural calculations, and installation estimates to help the engineer better analyze the best detention and water quality solutions for the site.

Installation began with a preconstruction meeting that included a Contech Field Consultant and representatives from the prime and installing contractor. Topics covered in the meeting included delivery time, truck spacing, recommended equipment, and safe offload and placement methods.

The contractor was able to reduce expenses by scheduling this delivery over two days and using equipment already on site. Having the option of using excavation equipment in lieu of a crane brought the project in on time and under budget.

“Contech’s consultants and coordination ability proved invaluable during the process of selection, siting, design, and construction of the Terre Arch system at South Alex,” said Lheep Kim, Principal at Greenway Engineering  “The flexibility provided by the product allowed for more flexibility in the design of on-site stormwater management and stormwater conveyance design, in addition to providing overall cost savings for the owner and contractor during the installation process.”

Technical Description:

  • (18) Terre Arch™ 26 Chambers
  • Filterra® Biofiltration

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