The reversible lanes viaduct project on the Tampa Crosstown Expressway was planned to allow heavy, rush hour traffic to move more efficiently within the busy east-west corridor of Tampa. The elegant design of this 5-mile long elevated structure called for concrete box segmental structural units supported on relatively slim mono-piers. Each of the piers was supported on a single drilled shaft foundation. The entire project included 218 piers (up to 60 feet high), each supported on a 6-foot to 8-foot diameter drilled shaft installed as deep as 105 feet below ground.

During construction, one of the piers plunged 11 feet causing a collapse of the two sections of the structure. Measurements taken shortly after the collapse indicated that at least one additional pier had experienced settlement (although much less than the failed pier). Tetra Tech was brought onto the project by the Expressway Authority to evaluate the potential causes of the collapse and to determine whether the remaining drilled shaft foundations on the project (more than 200 piers) could support the design loads.

Tetra Tech’s engineers oversaw an extensive exploration of the highly variable subsurface conditions, employing numerous data gathering devices such as Standard Penetration tests, Cone Penetrometer Tests, and cross-hole seismic testing. The analyses of the data included detailed and advanced statistical methods. Tetra Tech recommended modifications for those foundations determined to be insufficient for the anticipated loads, including constructing “sister” shafts adjacent to previously constructed shafts and constructing a series of “mini-piles” around existing shafts to provide additional carrying capacity.

A panel of nationally recognized university researchers and the Florida Department of Transportation reviewed Tetra Tech's report for this project and agreed with the evaluation and recommendations. Tetra Tech’s response to this high-profile project exhibits a high level of technical capability along with an innovative approach and represents a clear advancement of the state-of-the-art of drilled shaft design in Florida. The new portion of the Tampa Crosstown Expressway opened to the public in September 2006.