ABSTRACT
This paper focuses on the applications of new developments in stress wave techniques for the identification and quantification of defects in bridge foundations. The two methods discussed in this paper are the Crosshole Sonic Logging (CSL) and Crosshole Tomography (CT) methods.
The Crosshole Sonic Logging method measures the travel time of stress waves as they travel between tubes or coreholes in concrete or stone structures. A single CSL log requires 5 minutes or less to test the material between a pair of coreholes or access tubes with lengths to 100 feet and greater. CSL testing identifies soil intrusions, honeycomb, voids, poor quality concrete and other defects between tubes. The most common application for CSL is quality assurance of underwater (or slurry) concrete placement in drilled shafts, slurry walls and seal footings where access tubes are cast-in-place. CSL is also used to locate defects between coreholes in foundations and other large concrete structures. Presently, Crosshole Sonic Logging has been specified for drilled shafts on Department of Transportation construction in 10 states in the USA, and many other states are following.
While CSL technology is very effective in detecting anomalies and defects, it also provides a good indication of defect size, position, and severity within a foundation. In the cases of a critical foundation with a defect, a more precise determination of the defect may be needed. Crosshole tomography is a technique that produces a ‘slice’ or a plot of the concrete condition between the tubes. This plot clearly shows the defect size, severity, and position between the tubes. Crosshole tomography uses multiple CSL logs taken around a defect and a powerful computer program to calculate the position and severity of the defect. Although this is a new technology, Crosshole Tomography has been successfully applied on several projects.
Discussed in this paper are two case studies on two drilled shaft foundations in California, where Crosshole Sonic Logging and Crosshole Tomography tests were performed.
