The Allegheny County Sanitary Authority (ALCOSAN) provides wastewater treatment for 83 Allegheny County municipalities, including the City of Pittsburgh. Their service areas span approximately 310 square miles and ALCOSAN operates and maintains approximately 90 miles of sanitary system interceptors. ALCOSAN enhances the community’s quality of life and safety by working to protect drinking water, rivers, and streams.

In conveyance systems, diversion structures are the interface between the surface sewers and larger deep tunnels and provide the interface between the sewer systems and adjacent surface water bodies. These are vital for effectively conveying flows to the wastewater treatment plant and preventing backups of sewage into basements or onto city streets by allowing an overflow into the streams and rivers when the sewer’s capacity is exceeded. To keep the conveyance system operations reliable, ALCOSAN conducts ongoing preventative maintenance and implements strategic asset rehabilitation/replacement projects at diversion structures.

GAI was contracted to inspect the condition and operation of over 80 individual diversion structure assets to determine what needs to be repaired/replaced, and then to complete the engineering and permitting services for these critical improvement projects. GAI staff performed on-site inspections to assess the conditions and operability of the following types of diversion structure assets:

• Bypass valves
• Flap gates
• Stop logs
• Sluice gates

Inspections involved confined space entries into the diversion structures for the collection of data necessary to inform any valve rehabilitation or replacement design efforts. Field inspection data was collected and stored directly into a GIS geodatabase for organized reporting that can integrate readily with ALCOSAN’s existing system-wide base mapping —helping to streamline future inspection and/or maintenance activities at these diversion structures.

GAI also prepared a comprehensive assessment report summarizing the data collected, along with design concepts of rehabilitation options, anticipated project permitting requirements, a ranking of repairs/replacements, and planning-level capital cost and schedule estimates. GAI worked closely with ALCOSAN to develop construction package groupings that prioritize where more immediate repairs are needed and sites that would result in significant reductions of river intrusion. The packages streamline the overall construction sequencing and permitting coordination where possible.

Currently, GAI is developing fully engineered design drawing plans and specifications and is completing all necessary environmental and traffic control permitting for the first capital improvement construction projects under this contract. The first construction project bid package is projected to be completed in June 2025, with additional packages to be completed through 2025.

From the outset of this contract, GAI staff continue to remain flexible and steadfast in satisfying ALCOSAN’s goals and needs throughout changes in operational circumstances, field conditions, objectives, and/or priorities—a testament to the firm’s 10-year long relationship with this important client.