The International Information Center for Geotechnical Engineers

Wednesday, 21 September 2016 06:55

PDI-TOMO Helps GRL Evaluate Shaft

Crosshole Sonic Logging (CSL) evaluates the integrity of the concrete of drilled shafts and other cast-in-place deep foundations. Ultrasonic transmitters and receivers are inserted in water-filled tubes pre-installed in the foundation.



A signal generated by the transmitter travels through the concrete and is picked up by the receiver in another tube. An instrument such as PDI’s CHAMP-XV collects the data as the probes are raised from bottom to top. The test is repeated for all pairs of tubes; data collected from each pair is called a profile. An important result is the time when the signal first arrives at the receiver, called First Arrival Time (FAT). The speed at which the signal travels in the concrete, which is distance between tubes divided by FAT, gives some indication of concrete quality. The signal strength at the receiver, typically called “Energy”, is also helpful in assessing concrete quality; a large reduction in signal strength indicates a potentially serious defect in the shaft.

The interpretation of CSL results benefits from comparisons with nearby similar shafts, knowledge of soil conditions and installation details, engineering judgment, and experience. The standard CHAMP-XV processing software displays FAT and Energy versus foundation depth and the location of potential defects for each profile, but does not provide an estimate of the extent of the defect. Tomography analysis was therefore developed to supplement standard CSL analysis. It combines FAT data from all pairs of tubes, interprets the combined profiles and produces a representation of the shaft, with different colors indicating ranges of measured data. This allows a better visualization and estimate of the scope of potential problems.

Software for tomography analyses of concrete foundations had been available for several years, but the recent release of PDI-TOMO considerably improved their speed, ease, and reliability. GRL offices in Texas and Colorado used PDI-TOMO to enhance the results supplied to their clients in recent projects.

GRL tested all four drilled shafts of a bridge foundation pier in the Southwestern United States. The 152 cm (60 inches) diameter, approximately 6 m (20 ft) long shafts were each installed with four steel access tubes. Subsurface conditions were mostly silty sand and sandysilty clay over sandstone. CSL was performed within 40 days of concrete placement. Four perimeter profiles and two diagonal ones were collected.

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