Project Spotlight: Residences at the Fairfax County Government Center

Published by Carissa Alexander

This project is highlighted by an underground storm water detention system composed primarily of 72” pipe and fittings assembled in a three-barrel loop. It was designed to handle the intense rainfall runoff typically generated during heavy rain showers. The arrangement of the three barrels of 72” pipe provides the necessary short term storage of the rainfall runoff, preventing harmful flooding and washouts.
The specifics of the detention system include three lines of 72” pipe x 220’ long. Within these parallel runs of pipe, a multitude of fittings were included to produce curb and gutter inlets, access for inspection and cleaning, and connection for flow between each of the three pipe barrels. The fittings accomplishing this purpose included three 72 x 48 Manhole Tees, one 72 x 4×4 Box Access Tee with a 4×4 hatch, two 72 x 72 Pipe Tees, and four Precast Junction Boxes.

During the submittal process, a need for greater flexibility for field construction was recognized and a suitable redesign was recommended by CP&P engineers and W. B. Hopke to change 72” – 90 degree Bends on the four corners to Junction Boxes. This allowed for both the ease of construction of a “closed loop” detention system, and for multiple CP&P plants to work together to meet the necessary delivery dates for the jobsite installation schedule. The Salem Precast Plant produced the junction boxes while the Jessup Pipe Plant simultaneously made the 72” RCP and fittings. The 72” pipe, the JB-1’s on the outfall side, and the 72” Pipe Tees were shipped three weeks from the submittal approval date and each piece shipped continuously for two weeks to complete the system on schedule.