Danbury's Deep Sewers

In spite of unusually deep trench cuts, a sewer contractor has been able to attain high production rates for a much-needed sewer in a residential section of Danbury, Conn.

Underground Builders Inc. (UBI) of Southington, Conn., is nearing completion of its $2.5-million contract to install sewers

While the central portion of this city of 75,000 in southwestern Connecticut is already served by sewers, much of the outlying areas rely on septic systems. When neighborhoods are plagued by septic system failures, they can petition city fathers to have sewers installed. The Great Plains Avenue project is one of those petitioned projects, and follows a similar sewer contract in the Jackson Drive area recently completed by M. Rondano Inc. of Norwalk, Conn.

The present contract is administered by the Engineering Division, part of the Danbury Public Works Department headed by Director Antonio Iadarola. Supervised by City Engineer Farid Khouri, P.E., the division provides engineering review of all public works projects (including the multimillion-dollar construction of artificial turf and synthetic track surface for Danbury High School, covered in the October 28, 2002 issue of New England Construction).

UBI's contract calls for installing about 12,000 linear feet of 8-inch PVC sewer plus more than 180 6-inch PVC lateral house connections. In addition, the contractor is setting in place 46 precast concrete manholes and laying about 1,000 feet of 15-inch HDPE drainage pipe. Superior Products Distributors Inc. of Millvale, Conn., is supplying all pipe for the job, while United Concrete Products Inc. of Yalesville, Conn., is providing the manholes.

Jim Sorvillo, who co-owns UBI with wife, Pam, is personally running the job. He said they have been fortunate that ground water has not been a major problem considering the depths of trench cuts.

"About 1,800 feet of the sewer runs between 18 and 24 feet deep, but we've had some cuts down 27 feet," said Sorvillo. Traffic on Great Plains Avenue could have been problematic, he said, but the city allowed him to close the road to through traffic.

"And the local residents have been good," he added. "We keep them informed about what we're doing and they appreciate that."

UBI employs two excavators — a Komatsu PC400 and a Volvo EC360 — as the main line diggers. Volvo L70 and L90 loaders handle backfilling while a Volvo 160 excavator equipped with a landscape bucket is used for cleanup work.

To support walls of the deepest trenches, the contractor has rented an SBH Series 750 Double Slide Rail System from SBH Trench Shoring Company of Southington, Conn. According to Bill Edgerly, president, the Series 750 system can accommodate trench depths up to about 30 feet. Other Series made by the German manufacturer work in even greater depths, he said.

The Slide Rail System consists of vertical beams, or rails, horizontal struts, and side panels. Usually, the contractor assembles two rails and a strut, and lowers this assembly into the trench. A backhoe bucket pushes the rails into the soil, one at a time, while the rolling strut keeps the rails parallel. When the desired depth is reached, side panels are slipped into grooves in the rails and lowered in place. The 8-foot-tall side panels can be stacked to attain total depths of about 30 feet.

For the Danbury project, Sorvillo rented three sets of slide rails. Each assembled set measures 16 feet in length, so the crew works within a 48-foot-long trench support structure. Once the trench is dug and the slide rails installed, the crew uses one excavator to lay and backfill pipe. A second excavator at the forward end digs the trench. As the pipe advances, the rear excavator lifts out the strut, rails and side panels of the rear section and the components are leapfrogged forward and attached to the front end of the system. In this way, the trench support structure keeps advancing while providing continuous protection for the crew.

Sorvillo said the slide rail system has expedited the Danbury project.

"This system has saved us time and money. It allows us to control the backfilling and there's no compromise on safety. And we've been able to lay and backfill up to 70 feet of pipe a day in cuts greater than 20 feet," he said.

By mid-October, UBI had finished its deepest cuts, and had completed about 90 percent of the sewer installation. Barring any unforeseen difficulties, such as early storms, the contractor expects to complete pipe work before winter. The contractor is patching the sewer trench surface with bituminous concrete using a paving box and a Dynapac roller. In spring, a subcontractor will pave the entire street as part of the contract.