Bridge Pier Requires Massive Tremie Pour October 2003
Putzmeister pumps used exclusively in high-volume project
STURTEVANT, Wis. (Oct. 20, 2003) – What is believed to be the largest tremie, or underwater pour ever on the East Coast was completed in mid-June with a staggering 7,200 cubic yards of concrete placed during a 52-hour period.
Putzmeister concrete pumps were used exclusively to complete the pour, which occurred during construction of the Brightman Street Bridge. The bridge will cross the Taunton River in southeastern Massachusetts, linking the communities of Fall River and Somerset via Route 6. Scheduled for completion in 2007, the $200 million structure will span 1,750 feet and reach a maximum height of 60 feet. It is the largest bridge project and fourth-largest construction project in Massachusetts history.
The new bridge will replace a structure built more than 90 years ago and that is slated for demolition. Main features of the new bridge include a center drawbridge, median strip, roadside shoulders and pedestrian access.
Because the new bridge will be twice as high as the old structure and better aligned with the river channel, larger ships will be able to pass under, thus furthering the goal of Fall River officials to transform their city into a world-class port. In addition, project planners say that proposed landscaping will enhance the aesthetics of the area by screening Route 6 from nearby residential areas.
The tremie pour occurred on one of the center bascule piers for the drawbridge, during the third of five phases. Crews were challenged to pour concrete through 50 feet of water in a cofferdam and to form a slab measuring 83 x 140 feet.
Engineers initially calculated that a slab 26 feet thick would be necessary for the pier to withstand the immense hydrostatic pressure created after the cofferdam was drained. Project general contractor Modern Continental Construction Co. Inc. attacked the problem by awarding a subcontract to Treviicos Corp. to drill a series of shafts down into bedrock, to anchoring a slab “only” 16 feet thick.
Treviicos drilled 18 drilled shafts, each 8 feet in diameter, and down approximately 185 feet into bedrock. With a design load of 2,500 tons, the shafts were needed to support the massive loads of the pier.
To prevent cold joints in the tremie seal, it was necessary to pour all 7,200 cubic yards continuously. A special tremie mix was used so the concrete would flow easily and stay fluid for extended periods. Rounded aggregate, a high-cement content (650 lbs/yd3) and six ounces per cubic yard of a concrete retarder called RecoverTM were incorporated into the design of the mix.
To handle the pumping, Modern Continental teamed up with D&M Equipment Co., an authorized Putzmeister dealer, with D&M operators handling the pour.
“We’ve never heard of a tremie pour of this size or complexity,” said Dan Kelley, project manager for Modern Continental. “Because it was critical to pour non-stop for 52 hours, reliable equipment and experienced operators were a must. We had worked with D&M on other projects such as the Big Dig (in Boston) and found their concrete pumps very dependable and their operators highly capable. Again on this large pour, they performed to our high expectations.”
Thirty-five, 12-inch diameter tremie pipes, each 60 feet in length, were spaced 10 feet from the edge of the cofferdam and approximately 20 feet apart in a seven- by-five grid. Three different pump and slick line configurations were set up to complete the job in the shortest amount of time.
One pump was set up on a temporary pier. From there, Modern Continental’s recently purchased Putzmeister BSA 2110 trailer pump delivered concrete through 500 feet of slick line, which was positioned atop flexi-floats. The slick line reached out to a Putzmeister 36-Meter placing boom, mounted to the drilled shafts inside the cofferdam. The high pressure of the trailer pump and the convenience of the placing boom made this an effective method of concrete placement.
A second configuration used truck-mounted concrete boom pumps from D&M’s fleet. From another temporary pier, a Putzmeister 52Z-Meter stretched over 150 feet horizontally to pump concrete directly to the deck pipe of a Putzmeister 42X-Meter situated on a barge. Acting like a placing boom, the 42X-Meter then extended to reach selected pipes within the cofferdam.
A third configuration involved a Putzmeister 32Z-Meter boom pump with 500 feet of slick line. A crane that held a large elbow at the end of the slick line placed the concrete directly into the tremie pipes. This configuration was positioned on shore, between the two temporary piers, and was used only part time.
“The beginning of the pour was the most critical since the greatest chance of losing the seal and contaminating the concrete is most likely at the start of the pour, when there is no concrete in place to surround the tremie pipes,” Kelley said.
Ray Peck of D&M Equipment was involved in the setup and organization of the job. Peck was onsite for most of the pour. “Initially, we used only one pumping configuration to place about 250 yards into the first pipe and get concrete flowing around the adjacent pipes,” Peck said. “Then we gradually added the other two pump setups. All the pumps did an outstanding job throughout the two and a half days of non-stop pumping.”
The overall approach was to create a sloping or leading edge of concrete and to place the 16 feet of concrete in four lifts. Crews allowed the concrete to flow around each succeeding row of pipes ahead of the pour, while forcing contaminated concrete (laitance) to the leading edge. Eventually, the laitance was pushed to the end of the cofferdam where it was airlifted out of the water with the help of divers.
A placement sequence was developed where each row in each lift was completed consecutively, in stepwise fashion. This allowed the natural angle of the concrete to be mimicked as close as possible by the advancing slope of the tremie.
Engineers determined it was vital for the concrete to maintain its natural slump angle during placement, that the material never be forced to flow farther than 30 feet and that each successive row of pipes be embedded in concrete prior to placing more concrete.
Because it also was important to evenly distribute concrete within the sloping edge, occasional leap-froging from one pipe to the next occurred with divers directing the process. The longest lag time in returning to an individual tremie pipe was seven hours, although the retarder allowed the concrete to flow up to 12 hours.
The Putzmeister pumps on average placed 140 yards of concrete an hour. This average was achieved despite a slow start when only one pump was used; the shutting down of one pump because of a lower-than-expected supply of concrete and a slowed pace at the end of the pour to bring low points up to grade.
In the end, the concrete flowed even better than anticipated at a nine-to-one slope. This made the area more level so that only 28 tremie pipes were used out of 35 that were available.
The pour was completed during the weekend of June 20th, with crews working against heavy rainstorms. A tremie pour of similar size for a second bascule pier is scheduled next year.
JOB SPECS: General contractor: Modern Continental Construction Co. Inc. – Boston Pumping contractor: D&M Equipment Co. – Fall River, Mass. Ready-mix supplier: Morse Sand & Gravel – Attleboro, Mass. Equipment supplier: D&M Equipment Co. – Fall River, Mass. Equipment: Putzmeister 52Z-Meter, 42X-Meter, and 32Z-Meter truck-mounted concrete boom pumps; Putzmeister 36-Meter separate placing boom; high-pressure Putzmeister BSA 2110 trailer-mounted concrete pump