Down-Hole Techniques
Down-hole techniques require placement of a number of metal or polyvinyl chloride (PVC) access tubes in the shaft before concrete placement. Alternatively, access tubes can be core-drilled after the concrete has set. Down-hole techniques are based on measuring the wave velocity between a pair of hydrophone probes: one emits an ultrasonic pulse, and the other is a receiver. The tubes are filled with water to provide for coupling between the probes and the shaft. The wave velocity depends on the density and modulus of concrete. Presence of voids, soil inclusions, or poor-quality concrete is manifested as a reduction in the wave velocity. Most state highway departments specify that testing be performed in 10 days or fewer of concrete placement for PVC tubes or 45 days or fewer for steel tubes to avoid problems associated with tube de-bonding. The most commonly used down-hole methods are CSL, single-hole sonic logging (SSL), and cross-hole tomography (CT):
- CSL is the down-hole technique most commonly used in the United States. The test is performed by lowering two probes to the bottom of two access tubes. Wave velocity typically is recorded every 50 mm (2 in.) as the probes are withdrawn from the holes. CSL tests typically are performed between all perimeter tubes, to evaluate the concrete condition at the outer edges of the shaft, and between major diagonal tubes, to evaluate concrete conditions at the inner part of the shaft. Offset source and receivers also are used to better characterize the nature and location of defects.
- SSL is similar to CSL, but it uses the same access tube for both the source and the receiver. The technique is best suited for small diameter drilled shafts, mini-piles, or auger-cast piles. It also can be used to further investigate a defect identified by using CSL.
- CT uses equipment similar to that of CSL but with multiple sources and receivers. Tomography is an analytical technique that uses the travel time between multiple sources and receivers, in an iterative process, to generate a two-dimensional image of the defect. When a defect is identified by using CSL, CT tests can be used to better characterize the defect.
CSL is effective in locating defects between tube pairs and determining defect depths but not the exact location of the defect between tube pairs. Also, CSL cannot locate diameter increases or provide information about the condition of the shaft below the bottom. CT is better than CSL for identifying the location and shape of defects. However, CT is time-consuming, and it has not gained wide popularity.
Integrity Testing Methods |
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Surface Techniques
Surface techniques are more commonly used in Europe. In the United States, they are used primarily when difficulties occur during the construction of drilled shafts where access tubes for CSL have not been planned. Nearly all surface techniques depend on the application of a stress wave at the top of the shaft by using a hand-held hammer and signal analysis of the reflected waves. Waves travel along the depth of the shaft and reflect back to the surface when they encounter an impedance change, which could be caused by changes in the material or cross section of the shaft. The force applied to the shaft is measured by using a force transducer mounted in the hammer, and the reflected waves typically are measured by using an accelerometer glued at the top of the shaft. The methods differ from one another in the way the force and acceleration time histories are processed. The most commonly used methods in the United States are (a) acoustic wave reflection, pulse echo, or sonic echo and (b) impulse response or sonic mobility.
Acoustic wave reflection, pulse echo, or sonic echo is based on observing the time taken for waves to reflect back from the tip of the shaft. Defects that exist along the length of the shaft will cause secondary reflections that could also be detected if one knows the compression wave velocity of concrete. The method is limited in that only the length of the shaft and the depth of the defects can be determined.
Impulse response or sonic mobility is based on transforming the force and acceleration time records to the frequency domain by using a fast Fourier transform algorithm. Next, velocity (integrated from acceleration) is normalized by force and plotted against frequency to obtain a mobility plot. Shaft length, diameter, stiffness, and depth of defects theoretically can be deduced from the mobility plot. One recent development in impulse response testing is the use of multiple geophones at the pile top, to improve the reliability of the technique.
Surface techniques are subject to a number of limitations (1, 7), including (a) the strength of the echo depends on the surrounding soil; (b) the signal-to-noise ratio decreases as the length-to-diameter ratio exceeds 20–30; (c) the size and lateral location of the defect cannot be determined; (d) defects near the bottom of the shaft are difficult to detect; (e) defects can hide below other defects located above them; and ( f) unplanned diameter changes, such as bulbs, which typically are acceptable, are indistinguishable from necking defects.
Primary Reference
- Iskander, M., D. Roy, C. Ealy, and S. Kelley, (2001). “Class-A Prediction of Construction Defects in Drilled Shafts,” Journal of Transportation Research Board, No. 1772, pp. 73-83, National Academy Press [link]
