Thin-Walled Concrete Pipe Works Well in Flood-Control Project
As the tunnel was being bored, however, the quantity of groundwater infiltrating into the excavated tunnel led the contractor, Kajima-Marra/Majestic (K-M/M), to believe cast-in-place was not a viable alternative. More than 900 gallons of water per minute was flowing through the tunnel as opposed to the expected rate of about 300 gpm. Trying to cast concrete under such wet conditions would have been extremely difficult, according to Kendall Marra, project manager for the contractor.
Unfortunately, because of space constraints, it was considered impossible to bore a large enough tunnel through the rock with adequate space for a pipe with a wall thickness of 12 inches and the necessary 12-foot inside diameter. "We wanted to go with a thicker pipe, but there just wasn't room given the inside diameter requirements," said Steve Kingsland, project engineer for Chase Precast Corp., the project's North Brookfield, Mass., pipe supplier.
After studying pipe geometry and inherent strength, K-M/M proposed prefabricated pipe with wall thickness of eight inches for the project. Initially, the Army Corps of Engineers, acting as project owner, was not convinced of the viability of the thinner pipe. Chase Precast, along with consultants Simpson Gumpertz & Heger, of Arlington, Mass., tested the pipe for strength and quality, convincing the Corps that the thinner pipe was indeed an adequate alternative.
The pipes were tested using a simulated 20,000-pound load on the roof and 30,000 pounds on the floor. In addition, a prototype was enclosed in a 42-foot-long water-filled bladder to check joint gaskets for leakage during grouting.
Strength and integrity were not the only concerns, however. The Corps also questioned whether the pipes would be too fragile and could be damaged during transport or installation. So in addition to the welded reinforcing cages and lifting brackets built into to the pipe, the Corps called for support struts to be installed at the factory inside each 10-foot long section. The struts, which are attached to the lifting brackets, help prevent deformation and cracking during shipping and installation.
The pipes were lowered sideways into the outlet shaft where they were transferred to a tracked carrier. The carrier moved the pipe to the installed segment, beginning at the inlet end and moving backwards toward the outlet. The pipe was held in place by the carrier as the area surrounding the pipe was filled with embedded pea stone. Every 300 feet the liners were grouted, creating a reinforced ring of concrete around the pipe joint. The struts were then removed from the section, and the process was repeated for the next segment.
In addition to the tunnel pipe, 42 sections of vertical shaft pipe (roughly 340 feet) were installed as entrance and egress to the tunnel. The shaft pipe is 8 feet long, with 12-inch thick walls. The project team determined there was a need for 12-inch-thick pipe at the top of the shaft because there wasn't enough rock surrounding the pipe to support it adequately, according to Kingsland. He said the overburdening of an 8-inch pipe wall in the soft soil could compromise the pipe strength.
Construction was carried out on three shifts around the clock to keep the operation moving at a steady pace. That turned out to be a wise decision since heavy rains during the project caused another flood risk for the area. Kingsland summed it up by saying: "The tunnel was not supposed to come on-line yet, but the flooding pushed it into service."
Editor's Note: This article first appeared in the Spring 1997 issue of Concrete Pipe News, a publication of the <%=company%> . It has been edited by Ian Lisk to Water Online's and Public Works Online's journalistic style.