A big problem for McCarthy was discovered about 40 feet below grade while testing foundation soils for the Acute Care Pavilion, Rady Children's Hospital and Health Center, just north of San Diego in the Kearny Mesa area. They hit cobbled soils just where you don't want them – where foundation anchors would stop. McCarthy is the construction manager and general contractor, and is doing structural concrete placement for the $260-million, 300,000-square-foot project.
“Calcs by the structural contractor – KPFF – showed we didn't have enough dead-load on the building for the seismic uplift,” said Steven Van Dyke, senior project manager for McCarthy. This was a curve ball thrown late in the game, during OSHPD back check of the permit plans.
“When the issue with the foundation design was discovered, rather than having to redesign the complete structure and foundation system, the solution was with the vertical tie-backs. We considered using base-isolators, but the costs were prohibitive,” he said.
“We did six test anchors, with 8-inch-diameter holes. But the cobbles appeared at the 30-40-foot level, collapsing the shafts. This required the drilling contractor to custom-make eight-inch-diameter steel casings that we used on about 95 percent of the holes to allow a threaded Dywidag rod to be set to the 60-foot design depth.
“We tried rotating the casings the same direction as the drill bit, but this was destroying the bit,” Van Dyke said. “So, we went to a reverse thread case and it worked. The driller also had to import a special bit from England to make the drilling process work. Once the Dywidag rods hit the 60-foot-depth level, the casings were removed and the holes were grout-filled.”
Next, rebar was placed, and the footings poured; a pull-stress test to 200 percent of design strength was performed on each configuration. After that, the Dywidag tops got locked off with a nut. Then there was a 28-day wait for the concrete to cure. Finally, a fiberglass covering was placed on top and grout-sealed.
“It's simply a vertical tie-back system for seismic uplift on the foundation,” said Van Dyke. “And I believe it is the first time it has been used at a health care facility in California.” The project is using some 150 of these tie-backs throughout the foundation. “The whole process caused a 15-day delay in the work schedule, which we were able to overcome with our self-performed structural concrete,” Van Dyke said.
“Expanded shale aggregate (hydrolite) is on allotment in California – there is only one plant in California that produces it,” said Van Dyke. “It is hard to get right now; everyone is being set back by its lack of availability. Its use is for UL floor assemblies for fire rating in buildings.”
“We were able to purchase all the structural steel prior to construction at a time before recent price jumps worldwide,” Van Dyke added. “We also got many early subcontractors on board with a design-assist, design/build basis; they factored-in potential price increases of materials during the course of the job.”
“Bottom line: This project is really about the kids and the community. You think about all the good that will be happening for children stricken with cancer or other traumatic events, and it makes this project a bit more special to our team,” said Van Dyke.