Bypass Aids Greenbush Project

Edited by Paul Fournier | September 28, 2010

One of the challenges facing the design-build team for the new $250-million Greenbush Line of the Massachusetts Bay Transportation Authority (the "T") was the relocation of a 48-inch combined sewer of the Massachusetts Water Resources Authority (MWRA).

Joint venture contractor Jay Cashman Inc./Balfour Beatty Construction Inc. (CBB) has the design-build contract for the project, which will restore commuter rail service on the existing right of way known as the Greenbush corridor through the towns of Braintree, Weymouth, Hingham, Cohasset, and Scituate, on Massachusetts' South Shore.

The Greenbush Line is being built to mitigate the impact of the Central Artery/Tunnel Project (Big Dig) in downtown Boston, and is expected to reduce automobile traffic on the congested highways leading into downtown Boston by diverting commuter traffic from the South Shore from automobiles to transit.

Beginning with the connection with the T's existing Old Colony Main Line in Braintree, the project extends 18 miles easterly along the former New Haven Railroad Greenbush Branch to the terminus in the Greenbush section of Scituate. One of the first obstacles confronted by CBB was a section of MWRA's 48-inch combined sewer located on the route of the new Greenbush line in Braintree. In order to complete the rail extension as planned, an existing sewer line beneath the railroad right of way would have to be lowered 5 feet. And in order to do this, wastewater flows in the pipeline would have to be bypassed while the work went on — without disturbing the local population of the area or causing environmental impact. There was a tremendous amount of water that would have to be re-routed before re-construction of the pipe could begin. CBB contacted Baker Pumps to handle the problem.

Because of the size and importance of this bypass, Baker Pumps would have little margin for error. "With a bypass, what we do is interrupt the line," Carroll Hunnewell, a regional manager at Baker Pumps' Boston Branch office in Oxford, Mass., explained. "We plug it off and it has no place to go. If we can't pump it, it backs up, and then there is a chance that it can come out in the street or back up into homes."

According to Hunnewell, the average dry-weather flows passing through the pipeline was between 13 and 15 million gallons per day (mgd). However, the flow could rise dramatically during rainstorms and this had to be taken into account.

Potential issues with pumping also put fiscal responsibility of the project in danger. Julie Power, a project engineer who worked on the Greenbush project for CBB, pointed out, "It was an active construction site. Any problem would have been a big mess to clean up. And with the environmental issues at stake, we could have faced fines for any violation. Once the pumps were installed, we needed to ensure that they would remain reliably operating to their maximum capacity so the construction team would be able to go in and complete their work in a timely fashion."

Pump selection was critical, and eventually, after considering several different makes, the project team opted for The Gorman-Rupp Company's Prime Aire pumps. The pumps operate with a patented priming system engineered to eliminate weepage associated with some vacuum-assisted primers, and also have dual-suction side capability, a compressor-over-pump and an abrasive handling seal.

Hunnewell designed a bypass that lifted the wastewater 22 feet from the existing combined sewer into the temporary pipeline, which crossed over the railroad track work on a temporary utility bridge. The operation implemented seven 12-inch pumps — six primaries and one backup — which attached directly to seven 14-inch suction lines, with water being discharged 250 feet away into an existing siphon that crossed beneath a local brook.

The first obstacle in the pumping stage came courtesy of Mother Nature. With the pumps selected, the plan in place and the project team ready to go, the Boston area suddenly found itself in the midst of unexpected and extreme weather.

"When we were getting ready to start pumping, we were in the midst of a pretty dry time," said Power. "That was a big plus for us. Then, once we started pumping, we hit a period of several back-to-back storms." In fact, the Boston area, at that time, was experiencing back-to-back, 100-year storms. Flows in the bypass reached a peak of 42 mgd, according to Hunnewell.

"We were aware that flows could be considerably greater than we were asked to design around," added Hunnewell. "But, by building a safety factor into the design of the pumping system, we were assured that the operation would continue to run smoothly during the times of heavy rainfall." The built-in safety margin enabled the pumps to handle flows of 10 million gallons per day more than expected.

Furthermore, since the presence of homes and shops in the area around the bypass made noise a primary concern, Baker Pumps developed a soundproof barrier around the project through the use of sound-absorbing curtains, which cut down noise level by more than 30 percent at a distance of 30 feet.

And finally, the pumps proved to be more fuel efficient than anticipated over the 75 days they were in use.

Power explained, "Our fuel costs were dramatically less than calculated. We weren't quite certain what to expect in terms of the amount of fuel we would burn running the pumps." However, based on past experience, Baker Pumps and Gorman-Rupp anticipated that the authority would burn 3 gallons per hour per pump. But in the end, according to Hunnewell's calculations, the bypass used 1.5 gallons per hour per pump — a 50-percent reduction in fuel costs resulting in a savings of nearly $15,000. n

(Ed.: Information provided by RMD Advertising for client Gorman-Rupp Pumps, together with information from other sources including Carroll Hunnewell of Baker Pumps, and the T, was used in this article.)