The Springfield Water and Sewer Commission (SWSC) rehabilitated its diversion tunnel high-head outlet works facility at Cobble Mountain Reservoir Dam, which retains the water supply for 250,000 residents in Springfield, Massachusetts, and its surrounding communities. The work was prompted by the replacement of more than 200 needle valves—the same needle valves in operation at Cobble Mountain—at dams across the United States due to reported failures. These cases had resulted in worker deaths and millions of dollars in damages.
Valve failure at Cobble Mountain would have been similarly overwhelming: loss of the system's entire water supply—and thus drinking water and fire protection—for the residents, as well as likely onsite worker fatalities. Yet, over the past 15 years no practical solution had surfaced for rehabilitating the valves, more than 200 feet below ground in a confined space and under enormous reservoir pressure. To be rehabilitated, the valves had to be worked on in place, 'in the dry,' even though there were no isolation gates to allow dewatering prior to disassembly.
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SWSC and CDM developed a first-of-its-kind mechanical plugging system that isolated the outlet valves in the dry. With the plug installed, restrained, and inflated, the rotary valve could be drained and depressurized, allowing safe disassembly of the auxiliary valve components.
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Protecting the sole water supply for Springfield and surrounding communities—both safely and affordably—was the primary consideration. CDM employed sustainable design choices where feasible to meet budgetary constraints and overall project goals.
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The Cobble Mountain project, which was preceded by 15 years of discussion on its safety issues and challenges, required complex design and construction coupled with excellent planning, engineering, and sound decision making.
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Despite its high stakes, the Cobble Mountain project was completed without one incident or disruption of service and zero leakage. All improvements were completed for $1.99 million-less than Springfield's $2.1 million design and construction budget.
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The Cobble Mountain project has, not surprisingly, drawn significant attention throughout the country, given the number of similar, potentially serious situations that exist. Having learned about the Cobble Mountain success, the Metropolitan Milwaukee Sewerage District recently selected CDM to implement an innovative plugging system to shore up its system.