Advancing the ALCOSAN Clean Water Plan Tunnel System from Concept to Design

Key Takeaway: The implementation of a tunnel program is an evolving process that must continually adjust to changing conditions. Learning Objective: Optimizing ALCOSAN's CSO control approach considered multiple issues and challenges associated with the deep tunnel system proposed as an integral part of ALCOSAN's Clean Water Plan. A key consideration was the determination of the cost effectiveness of the primary components given the necessary flow capture to meet the water quality objectives. Additional issues and challenges included flow management for capturing, storing, and conveying CSO to the tunnel and then through the tunnel system to the Woods Run Wastewater Treatment Plant (WWTP) for treatment. The tunnel system and the WWTP are connected at a new dewatering pump station must be hydraulically matched for various flow scenarios. Horizontal and vertical alignments of the tunnel and near surface facilities were optimized considering economic, community, property, and subsurface conditions. This paper will summarize the activities started under ALCOSAN's Preliminary Planning project and continuing through design to optimize these factors in preparation for the implementation of their CSO tunnel control system. Abstract: Background The Allegheny County Sanitary Authority (ALCOSAN) servicing the greater Pittsburgh area has a mandate to control combined sewer overflows (CSO) along its three rivers (Ohio, Allegheny and Monongahela) utilizing a network of storage tunnels, collection system improvements, and expansion of wet weather treatment capacity at their Woods Run WWTP as illustrated in Figure 1. ALCOSAN completed a Preliminary Planning project that optimized and further defined the tunnel system to a 20% design level in a basis of design report (BODR) provided to the regulatory agencies on October 1, 2020. The tunnel network includes three tunnel stretches: the Ohio River Tunnel (ORT), the Allegheny River Tunnel (ART), and the Monongahela River Tunnel (MRT). The ART and MRT connect to the ORT which extends to the Woods Run WWTP. The tunnels utilize dynamic storage via a 120 MGD dewatering pump station that will pump during and after wet weather events thus reducing the overall storage needs of the tunnel network. ALCOSAN continued the advancement of the first leg of the tunnel system through the design of the ORT. During the design of the ORT, the preliminary design was advanced based on further optimizations and updated property acquisition negotiations. Method The effort included evaluating and improving the hydraulic flow management for the diversion structures, consolidation sewers, drop structures, and tunnel facilities. This evaluation considered a range of storm conditions for both a 'typical year' of rainfall as well as a larger collection system design storm. The work also balanced hydraulic flow management with tunneling and construction challenges that need to be addressed in the highly urbanized area surrounding Pittsburgh. The tunnels were evaluated with advanced transient modeling to identify required air management and surge mitigation measures to control the development of geysers. The basis of design included inflow control systems using gated controls on the highest flow sources to mitigate these conditions. During final design, the surge management approach was further refined to simplify the control approaches based on discussions with ALCOSAN operations staff. The preliminary planning effort also evaluated the optimal methods for capturing flows through diversion structures, consolidation sewers, and/or drop structures; ventilation; as well as configuring the tunnel system to address the flow conditions anticipated for both typical year control storm events as well as larger storm events that exceed the combined conveyance and storage capacities. Comparing different strategies for capturing CSO outfalls through diversion and consolidation sewer systems feeding a consolidated drop structure, or direct diversion and drop to the deep tunnels, were conducted at over 20 shaft sites situated mainly along the three rivers in the Pittsburgh area. The BOD layouts of these facilities were further advanced during final design based on updated existing facilities information, property acquisition discussions and discussions with ALCOSAN operations staff. These strategies also assisted in the development of horizontal alignments that considered property constraints as well as community impacts, construction risks, and operational efficiencies for maintenance and tunnel access. During the final design, the horizontal alignment was optimized to minimize the number of required easements and to adjust for near surface facility location modification based on property considerations. Vertical alignments were developed by considering a phased geotechnical exploration program conducted as part of the project. Over 60 borings were performed to assess the overburden and rock conditions along the planned alignments. An additional 60 boring were performed during the final design of the ORT which confirmed the BOD vertical alignment. Close coordination with the dewatering pumping strategies conducted under ALCOSAN's WWTP Program management effort was also required as the sizing of the tunnel is based on a dynamic storage concept. The tunnel will operate with wet weather pumping occurring as the tunnel begins filling during rain events. The plant will ramp up its wet weather treatment capacity to receive flow from both the existing interceptor through the main pump station and the tunnel dewatering pump station. This 'dynamic storage' enabled the tunnel to be sized efficiently by taking advantage of additional wet weather treatment capacity at the plant. Close coordination was continued during the design of both the ORT and the wet weather pump station to confirm the tunnel sizing. Lastly, the program will take advantage of potential flow reductions experienced through adaptive management over the course of the tunnel implementation planned out to 2036. This will include green infrastructure and other stormwater management strategies such as targeted combined sewer separation opportunities being implemented and/or planned in the future by ALCOSAN and their tributary communities. Implementation of this program will be one of the largest capital improvement projects to occur within Allegheny County and the City of Pittsburgh with construction costs estimated to be over $1.2 Billion. The project will result in a reduction of over seven billion gallons of annual CSO discharge to Pittsburgh's famous three rivers.