Sanitary Sewer Overflows: Big Prevention in a Small Footprint

The Racine Wastewater Utility has safety sites which are defined as predetermined locations for sanitary sewer overflows (SSO) to prevent backups into residential and commercial basements. One such safety site has historically experienced several significant SSO events, most recently a 500,000 gallon overflow. The utility wanted to eliminate the risk of overflows at this location for typical storm events. AECOM completed smoke testing, flow and rainfall monitoring, and developed a hydraulic model to evaluate two options to prevent the SSO events for typical storm events. The two options were to increase the downstream capacity of the sanitary interceptor sewer system which was deemed not cost effective due to major street reconstruction and the distance between the safety site and the wastewater treatment plant. The second option was to store the SSO volume for the duration of the design storm event. This solution was selected and the limited available sites were evaluated and prioritized based on design and construction issues.The design SSO volume of 600,000 gallons (for the 25 year storm event) would be stored until it could be pumped back into the interceptor sewer. The difficulty was finding a site to accommodate a large storage basin that would accommodate the required volume and the location and depth of the interceptor sewer. This presentation will focus on the preliminary and final basin design and construction methods to determine the requirement and size of storage volume, site the storage basin, and determine the basin configuration and operational requirements to resolve the site limitations and construction issues.The site chosen was approximately 120 feet by 80 feet. Due to neighboring residential houses and commercial businesses, no driving of piles or sheeting would be allowed. This was a problem due to a high groundwater elevation, sandy soil profile, and the deep excavation required. The basin would be circular, with an outside diameter of 66 feet. The overall depth of the required excavation would be approximately 50 feet below existing grade to accommodate the required volume and peak gravity overflow from the interceptor system. The entire depth of excavation could not be performed at one time due to soil support and dewatering issues. The basin design would allow the construction of the concrete basin in stages using top down construction as the excavation progressed. Secant piles were drilled in and grouted, around the perimeter of the basin, to provide soil support and to seal out groundwater. The concrete basin wall was constructed in three lifts, starting with the upper third. As the concrete wall lifts were poured, they became part of the soil support system to allow further excavation from within the basin footprint. All this construction, within feet of property lines, was to be performed with no measurable settlement or damage to neighboring structures.The top down construction method is typically used for large buildings where the simultaneous construction of the substructure and the superstructure is advantageous. In this example, the method of construction allowed the use of a small site for a large storage basin despite the construction related issues.This presentation will be of interest to engineers and operators trying to increase collection system capacities and prevent overflows with limited available site space.