hastened by the fact that Loadtest had become a company focused primarily on O-cell testing, a unique situation in the foundation industry. Conventional deep foundation testing had always been a sideline of engineering or construction firms. Eventually automation, along with the Internet, led to setting up tests in the remote tar sands of Alberta and in places like Tajikistan in central Asia, and then running the tests (controlling pumps and valves plus collecting data) from the comfort of the office in Gainesville, Fla. Most of the publicity for the O-cell test has been about its ability to apply and mea- sure high loads. Almost from the beginning, O-cell tests broke existing loading records. A test on one of the drilled shafts in the main pier of the William H. Natcher Bridge, Owensboro, Ky., produced a nominal loading of 6,000 tons (53 MN) in 1993. The following table illustrates the pro- gression of record loading since that time. measured such high capacities for bored piles. This new knowledge and better understanding of potential pile capacity unleashed a torrent of cost-saving foundation re-designs that typically provided savings in the range of 10 to 20 Workers checking O-cells at the Incheon bridge site World Record Osterberg Cell Load Tests 2010 Mississippi River Bridge, St. Louis, MO 36,067 tons (321 MN) 2010 Incheon 2nd Link, Incheon, Korea 2003 Pomeroy OH - Mason WV, Ohio River 2006 Amelia Earhart Bridge Kansas City, KS 17,800 tons (158 MN) 2001 Tucson, AZ 2002 San Francisco 1997 Apalachicola River, FL The ability to determine the ultimate capacity of drilled shafts capable of carrying very high loads, especially in rock sockets, led to the following discovery. This measured relationship between design engineers assumed or estimated ultimate capacity (E) and the actual measured capacity (M) showed clearly that such estimates had almost always missed the mark on the low side. We usually explain this underestimating of foundation capacity as a natural reaction of geotechnical engineers to the uncertainty inherent in dealing with natural materials. Paradoxically, however, the data showed that the stronger the founding material, the more conservative engineers became. We believe that most of this behavior stems from the fact that prior to the O-cell test no one had ever actually 31,350 tons (279 MN) 18,400 tons (163 MN) 17,000 tons (151 MN) 16,500 tons (146 MN) 15,000 tons (135 MN) times the cost of the related O-cell test. According to Jim Cahill, vice president of the Case Foundation Division of Keller Industries, “Use of the O-cell was the best thing that ever happened to the drilled shaft industry in that it gave us a way to prove the value of our product to … Departments of Transportation.” Figure 2 illustrates another major technical achievement of the O-cell method; the bi-directional nature of the Figure 1. M/E ratio DEEP FOUNDATIONS • NOV/DEC 2012 • 47
