Innovative SOE Solutions for Shaft Construction: Getting to Depth with Diaphragm Walls at Westerly Storage Tunnel-Paper

Paper

Pdf Summary

The Westerly Storage Tunnel (WST) is part of the Northeast Ohio Regional Sewer District’s Project Clean Lake, a long-term program to reduce combined sewer overflows into Lake Erie. The project includes a nearly two-mile, 25-foot-diameter tunnel deep beneath Cleveland and three access shafts.<br /><br />The original shaft designs relied on secant piles, deep depressurization wells, and—in some areas—steel ribs and liner plates. During bidding, Nicholson proposed a value-engineered alternative using unreinforced concrete diaphragm walls keyed into bedrock. The continuous, watertight walls eliminated the need for deep dewatering and internal support systems, enabling faster, safer excavation in a controlled environment. A soldier-pile-reinforced cement-bentonite wall was used for the adjacent WST-3 gate structure.<br /><br />The WST-1 and WST-3 circular shafts were excavated in panels with a crane-supported clamshell; WST-2, the smallest and deepest shaft, required both a clamshell and a hydromill to excavate and socket panels into rock. All three shafts were built under bentonite slurry, with panels concreted by tremie. The diaphragm walls were unreinforced and designed to act as compression rings.<br /><br />Construction challenges included maintaining panel verticality in small-diameter shafts, connecting the gate wall to the WST-3 shaft, protecting a century-old sewer, restricted site access, and severe winter conditions at WST-2. The team developed a guide extension to improve clamshell alignment in small shafts, while hydromilling helped address tight geometry and panel joints.<br /><br />The alternative was accepted after collaboration among the owner, designer, contractor, and specialty subcontractor. It provided continuous structural cutoff walls, reduced reliance on risky in-shaft installation work, and avoided additional subcontractors and complex dewatering and support systems—saving project time and cost.

Keywords

Westerly Storage Tunnel

Project Clean Lake

combined sewer overflows

Lake Erie

diaphragm walls

secant piles

bentonite slurry

hydromill

tremie concreting

Cleveland sewer project