SITE INVESTIGATIONS failed to pick up a sinkhole which caused a motorway viaduct to collapse in Tampa, Florida last month.
Ground investigations involved borehole probes to 3.5m below the base of the 19.5m foundations for each of the viaduct's 212 piers.
Even so, on 13 April a 6m high pier for the 10km highway sank suddenly into the ground during construction. It disappeared almost to its top, but the collapse was slow enough for workers to get clear.
The cause of the collapse is thought to be a limestone sinkhole more than 30m below the site.
A spokeswoman for the Tampa-Hillborough Expressway Authority said Florida was largely underlain by limestone and sinkholes are prevalent. It was not possible to determine the location of every one.
Ground investigation was by Dames & Moore, a subcontractor to the authority's general engineering consultant URS, with additional input by Williams Earth Science.