Rock removal, concrete design, and working in a tight, urban construction site ... River City Construction is overcoming these and other challenges as it erects two new structures at the Boone Hospital Medical Plaza in Columbia, Mo.
River City's work consists of building a four-story parking garage and a three-story medical building on a strip of ground about 250 feet wide between East Broadway and Jack Estes Way. The cast-in-place, post-tensioned parking structure, built partially into a hillside just north of the existing Broadway Medical Plaza Building III, will accommodate more than 400 vehicles. The new Broadway Medical Plaza Building IV, which shares a portion of the parking garage's east wall, is structural steel on concrete.
Work on the buildings also includes a 25-foot-high retaining wall on the west side of the parking garage, a walkway bridge connecting the parking garage to the existing Building III, and a corridor between the parking garage and the new Building IV.
"It's fortunate that we have both jobs," said Wes Poppenhager, P.E., project manager for the parking garage. "Otherwise, I think it would have been more difficult to coordinate the work on this site. We have two different contracts and each project has its own project manager and superintendent. The garage contract includes all the site work for both projects so we also prepared the building pad for the medical building as part of the parking garage work."
River City Construction began its work in November of 2005 and completion of both structures is scheduled in May.
According to parking garage Superintendent Gary Hinners, the challenges began early on. "Our subcontractor Jeff Schnieders Excavation began excavation for the parking garage in April and that work included generating about 33,000 yards of dirt for the Building IV building pad," Hinners said. "In some places, the building is sitting on up to 10 feet of fill that was developed with soils from the garage excavation.
"During the excavation, they didn't dig very far until they hit about 2,000 cubic yards of rock that had to be removed. Because it's so near the hospital, we couldn't blast. We had to be very careful about the noise and vibration levels of the jackhammers, so it was chip and peck. It took about two full months of rock excavation before we could start the lagging and shoring system."
After the crews got down to the finished subgrade, caisson installation also proved to be a challenge. The caissons ranged from 30 inches to 60 inches in diameter and up to 50 feet deep and had to be socketed into 5 feet of stable rock. "Every one of the caissons had to be visually inspected with test bores," said Hinners. "For some of the caissons we had to go through 25 feet of rock to get a satisfactory socket."
Working with two rigs, it took about three months to get 48 caissons in place for the parking garage and 38 for Building IV.
River City began to get the concrete out of the ground with columns and slab on grade in August and started the structural deck by mid-October.
"The decks have to obtain 3,000 psi before we can stress them, so it has been a challenge to get the concrete mix to come up to strength quick enough that it doesn't impact the schedule," said Hinners. "We modified the mix design and bumped our strength up to about a 7,000-pound mix by adding a sack or a half sack of cement so that now we're getting our strength within two days. That was critical to keeping our schedule.
"Central Concrete has been good to work with on the project and we wouldn't be anywhere near where we are without them. One of the things they have done to help keep the loads within tolerance is to set up a plasticizing station on-site as we're pouring our decks so that they're calibrating and loading the trucks with equal amounts of plasticizer rather than the truck drivers doing it themselves. That has helped us a lot and now we're able to schedule a deck pour about every four days."
Post-tensioning has also been a challenge and has provided learning opportunities on the project. "There's not a lot of post-tensioning done in central Missouri," said Hinners, "so it was difficult to get enough experienced workers. We brought in some experienced people from out of the area and we have good foremen out there. The ironworkers from Local 396 have picked up on it so the learning curve is over and it should go very well to completion."
One of the important pieces of equipment on the site, a Liebherr tower crane, is a bit uncommon looking in that the boom is twice as long as the tower is high. "That is a little different," Hinners said. "It's 116 feet tall with about 230 feet of boom. We considered a rubber-tired or crawler crane but we don't have any access on the north or the west because of public roads and parking. If we would have used a standard crane we would have had double picks to get from the east end to the west. The tower crane covers the site and also allows us to use our laydown area to get equipment and material up on the deck."
As of late-November, River City Construction was on schedule. "We got behind at the beginning when the rock and the caissons took so long, but we're on track now," said Hinners. "We expect to have the superstructure completed by late January if the weather doesn't hold us back too much. We've got good subcontractors and workers on the project and the owners and architect have been excellent to work with."
River City Construction is self-performing column formwork, edge formwork, concrete, foundation work, and retention wall construction. Major subcontractors include: Jeff Schnieders Excavation, Sanders 1 Rebar/Reinforcing Service, Dun-Par, Meyer Electric, and Woods Mechanical.