The City of Atlanta’s Department of Watershed Management identified the need for an underground storage vault to increase the storage capacity of the existing combined sewer system in the upper section of the Clear Creek West Basin. The purpose of the storage vault is to intercept combined sewage flows from an existing 80” x 120” brick, egg-shaped trunk sewer and an existing 78” overflow sewer. Storing combined sewer flow in the vault will reduce the peak flow in the downstream combined sewer. The project included analysis to verify the hydraulic design provided by the owner and construction of a 4-million-gallon cast-in-place concrete buried vault. The vault was designed to allow loading from a possible future parking deck to be built above. The design included means and methods to break into the existing old brick sewer and to stabilize the sewer within the limits of the project, particularly at its intersection with the new vault.
A critical element of this project was maintaining continual flow and use of the egg-shaped brick and 78” overflow sewers in both the baseline and flood conditions for the entire 18-month construction period. This was achieved with the use of a gravity bypass culvert which ultimately became the outside section of the vault, negating the need for temporary structures or pumps. Using gravity flow for the bypass is an important feature, as it eliminates the risk and noise associated with the magnitude of pumping required to pump the anticipated flows during major storm events.
The 4 million gallon underground concrete storage vault structure consists of hydraulic monitoring facilities, ‘knock-out’ panels along the east wall which can be knocked out in the future if the vault is expanded, and hydrostatic one-way relief valves (PRVs) to ensure against failure in the event of an extreme loading condition with the vault empty and the groundwater table high. Surface features include landscaping.