Design and Construction of an Underground Cavern for CSO Control

The City of Atlanta, under Consent Decree with the U. S. Environmental Protection Agency (EPA) and the Georgia Environmental Protection Division, was required to implement long-term planning and combined sewer overflow (CSO) controls throughout the city's combined sewer areas. For the East Area Combined Sewer Basin, a 1-mile storage and transport tunnel was proposed to augment existing tunnel storage increase existing CSO storage capacity to comply with new state and federal regulations. The City conducted a value engineering study of the CSO Control Plan which resulted in modifications and projected savings of 150 million. Included in the VE recommendations was replacement of the tunnel with an above ground CSO storage tank. During preliminary design, a review of operational hydraulics, available property, protection of the mature trees in the area, impacts of construction and future operation to the area residents and cost, led to the consideration of an underground facility to provide the required additional CSO storage. The selected option was a 60-foot wide underground cavern with a storage capacity of 10 million gallons to be excavated into massive granite rock. Following a subsurface site investigation, the underlying bedrock was determined to be high quality, and well suited to construction of an underground opening.