Shoreline Storage Tunnel Shafts and Near Surface Structures Support of Excavation

The Shoreline Storage Tunnel (SST) Project in Cleveland, Ohio is part of EPA-mandated Project Clean Lake, Northeast Ohio Regional Sewer District’s program designed to reduce pollution in Lake Erie by 4 billion gallons per year over the next 25 years. The $3-billion program includes construction of large ‑diameter storage/conveyance tunnels, shafts at key combined sewer overflow (CSO) pick-up points, underground pump stations, improvements to wastewater treatment plant capacity and treatment, and expansion of green infrastructure. When completed, SST will comprise over 2.5 miles of 23-ft-diameter tunnel installed 87 to 135 ft below ground. The storage tunnel will manage higher flow volumes and keep stormwater out of the combined sewer system to reduce overflow occurrences. As part of the project, three deep shafts (SST-1, SST-2, and SST-3) were constructed to provide access to the tunnel, launch and retrieve the tunnel boring machine, and provide ventilation. Several additional structures will serve as diversion structures, drop manholes, gate structures or gate control structures. A short connection tunnel will connect the SST to the Dugway Storage Tunnel, which was completed earlier in the Project Clean Lake program. The shaft support of excavation system was designed using diaphragm wall methods. This innovative solution provided watertight, continuous shafts keyed into rock that significantly reduces risks as compared to traditional secant piles combined with a rib and liner plate system. Unreinforced concrete diaphragm walls were constructed using mechanical clamshells for shafts SST-2 and SST-3, and clamshell equipment and a hydromill were used for SST-1. All the SST shafts were lined with reinforced cast-in-place concrete and include baffle drop structures. Outside of the SST-1 and SST-3 shafts, several ground improvement blocks were installed using overlapping cement bentonite panels. These ground improvement zones were created to provide increased strength and water cutoff in the soil at locations where the TBM broke into and out of the shafts. A gate structure attached to both the SST-2 and SST-3 diaphragm wall shafts was supported by a soldier pile reinforced cement bentonite wall. The design of the gate structure SOE was handled collaboratively with the owner and their engineer, McMillen Jacobs Associates. The team worked together to resolve challenges surrounding tying in the originally designed bracing scheme to the new SOE method and methods for exposing and supporting a 100-year-old existing brick sewer, as well as excavating through an abandoned brick sewer. This case study will present the design for the shafts and gate structures, the diaphragm wall construction methods, the unique challenges overcome for each shaft, and the benefits of the final solution to the overall project.