Most people are impressed when looking up at a skyscraper. But geotechnical engineers think that is just scratching the surface, because the most interesting details are found looking down.
Tom Cooling, principal at URS Greiner Woodward Clyde, explains that the geotechnical engineer gets involved after the architect has developed a building’s concept. Then designing a foundation can take several months or years. “Our job is to determine what it will take to place an economical foundation under the building and keep costs in line,” Cooling says. “There are technologies to build on most any site, but the economics may not work out for sites with very difficult ground conditions.”
Cooling says a solid foundation depends on several key considerations. First is column load, which is determined by a building’s weight, and that depends on the building’s height. “Every building has columns,” Cooling says. “The columns carry the building’s weight to the foundations. The ones you see are connected to the ones you don’t see.” Southwestern Bell’s Data Center, for example, weighed in at about 54,000 tons per column.
Basement depth is another important factor, Cooling says, especially in relation to the underground water table. “The water table varies with river levels and rainfall. There are several different water levels in downtown St. Louis, due to variations in soil conditions and distance from the Mississippi River.”
Basement depth determines excavation depth, or how deep a hole has to be dug, and what type of bracing has to be designed to keep the ides from caving in. The Thomas F. Eagleton Federal Courthouse provides an interesting example of excavation bracing. Cooling explains the building’s foundation is the deepest in the city, about 60 feet. It’s also the first building in St. Louis to use a structural slurry wall anchored directly into the bedrock, a technique developed in Europe about 20 years ago.
Urban areas present special challenges in regard to foundation design. Richard Frueh, vice president/St. Louis office manager of Shannon & Wilson, says geotechnical engineers always look at how a city site was developed in the past. “In our St. Louis office, we have topographic maps dating back to l875, plus photographs and histories that can tell us what was on the site before, and equally important, what was nearby, like a tunnel.”
Other maps indicate environmental uses of a property. “If a filling station or insecticide factory was there, you may want to consider a different site,” Frueh says. “But since location is key in real estate, many owners feel it’s worth it to clean up a hazardous site and build.”
Not everything shows up on the maps, however. Cooling again cites the Eagleton Courthouse, where work was held up for several weeks while uncharted underground tanks were removed. Other city projects can reveal abandoned utilities, old lead smelters and buried cisterns, from which residents got their water. “They may be filled with interesting artifacts but are a real nuisance to clean out,” Cooling says, adding that sometimes an archaeologist goes ahead of the builders. “For example, when I-255 built on the east side, a significant amount of historical Indian artifacts were recovered.”
Recently, as urban sprawl has advanced, finding sites with good ground conditions for new construction has become increasingly difficult, says Chris Mueller, associate/manager of the Geotechnical Division of Geotechnology. “The result is booming development in the Chesterfield Bottoms and Riverport/Earth City areas, which are right next to the Missouri River and very flood prone.” Soils in these valleys are very poor, he says, “and although you can always engineer a solution to a problem, the challenge is to develop a cost-effective solution.”
Another consideration regarding new development is seismic load and the region’s proximity to the New Madrid Fault. “A lot of soils assume a consistency like liquid under certain stress conditions,” Mueller says. “We’ve developed some innovative approaches to improve the ground for development.”
A case in point is the six-story United Healthcare building in Riverport. “Normally a building that size would require a deep foundation,” Mueller says. “But given the soil conditions, this was a big problem. So we improved the ground by dropping a 20-ton weight on the site for six to eight weeks to densify it to a depth where we felt if the New Madrid shook the soil, it wouldn’t liquefy it.” As a result, Mueller was able to design a shallow foundation–a 30-inch-deep trench filled with steel or concrete supports–and offset the costs of improving the soil.
Ground can also be improved before building by introducing a cementatious or chemical grout. “The material is injected under high pressure into holes drilled into the ground,” Mueller says. This technique is being used throughout the area in conjunction with numerous Metropolitan Sewer District projects.
Mueller has worked on several memorable projects, mostly involving transit systems, such as in San Juan, Puerto Rico, “where in some parts of the city things date back to Christopher Columbus’ era,” he says. “These projects are always challenging, because they’re in dense urban environments where the ground is poor and there are significant structures to protect.”
Frueh fondly recalls designing the foundation for Station Casino St. Charles. “The interesting aspect was creating a dry area in the Missouri River in order to build. We put in a sheet pile cofferdam, with wells to pump out the water.”
He also enjoyed working on the roller coaster at Busch Gardens in Williamsburg. “A roller coaster doesn’t look heavy, but the cars move so fast they exert tremendous loads on the foundation, and they also change direction all the time, so one part needs to be held up sometimes and held down other times.” The ride was built on a 70-foot foundation, “literally tied down with bolts cemented into the bedrock,” he says.
Modern foundation construction is only about 50 years old, Mueller explains, and the technology is constantly changing. “Thanks to computerization, we can monitor performance during construction. We can send energy through foundation elements into the ground and measure capacity. We have sophisticated tools we can take to the field to document and verify that a deep foundation can support the load it’s intended to support.”
Still, despite advanced technology, “you can encounter things you never anticipated,” Cooling says. “That’s what makes this type of work interesting and far from ‘cookbook.’ Every time you think you have it all figured out, Mother Nature throws you a curve.”
