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Return to: 2002 Feature Stories
CLIENT: MACTEC, INC.
October 2002: Structure
Tall buildings have inherently large foundation loads. If soil conditions are not suitable to support footings, either due to insufficient bearing capacity or excessive settlement, pile foundations are necessary. Drilled piles are frequently used to support tall buildings. If unfavorable soil conditions are present, drilled pile foundations will likely require the use of shell casings and/or drilling mud. In these cases, direct observation of the drilled pile shaft is not possible and, therefore, other means to evaluate the adequacy of the piles may be required. Since tall buildings will have higher foundation demands, including higher seismic overturning forces, integrity of the pile foundations is critical.
Several non-destructive testing methods may be employed to evaluate the adequacy of drilled piles (and other structural members constructed using similar techniques). Some of these methods include sonic testing of piles by seismic echo, impulse response, cross-hole sonic logging, and density testing by downhole gamma-gamma logging. General criteria for acceptance of piles for the various test methods are available.
In addition to the specific test methodology, a major part of the integrity testing procedure is the determination of the appropriate level of testing required.
Non-Destructive Testing Methodology
Non-destructive testing methods can generally be separated into two categories: 1) methods that require access for equipment within the pile, and 2) methods that rely on external excitation of the pile and utilize wave propagation relationships to estimate physical pile characteristics.
The first category of includes gamma-gamma logging (GGL) and cross-hole sonic logging (CSL). These methods each require access for probes to be inserted within the pile. The second category includes sonic echo (SE) testing and impulse response (IR) testing, and can be employed where access to the top of the pile is available without the use of tubes within the piles.
Each of these methods provides results immediately. However, interpretation of test results by the engineer and determination of mitigation measures (if necessary) require adequate time.
Downhole Gamma-gamma Logging (GGL)
GGL testing provides a means to evaluate the consistency of a drilled pile shaft by giving an estimate of the bulk density of the concrete. GGL is typically performed within PVC tubes that are pre-installed to the steel-reinforcing cage of cast-in-drilled-hole (CIDH) piles. In cases where the testing is not initially planned, GGL can be performed within holes cored in the pile. In general, the tubes (or cored holes) should be located at least two inches from longitudinal reinforcement, as the density of the reinforcing steel will affect the results of the logging.
Depending on the intensity of the source and the spacing between the source and receiver, GGL can evaluate the density of the concrete for a radial distance of about 2 to 3 inches using a low-energy source, and 6 to 8 inches using a high-energy source. Typically, several tubes are installed in the piles to allow multiple readings. Several public agencies, including the California Department of Transportation (CALTRANS), recommend one access tube per foot of pile diameter.
The results are reported on a density versus depth plot. The plot typically indicates the mean density and mean plus one, two and three standard deviations. CALTRANS has suggested that acceptance criteria for axially loaded piles could be based on the measured density of the concrete being greater than the mean density minus three standard deviations.
A major limitation is that radiation sources are subject to Nuclear Regulatory Commission (NRC) regulations, including special training and licensing for handling and transporting the devices. Because of the radioactive source in the probe, the probe cannot be left in the pile. Retrieval and recovery of the probe can be very costly. In one instance, two weeks of effort was required to retrieve a probe lodged in an access tube.
Blockages of the access tubes can occur, limiting the depth to which the probe can be advanced. The length of the probe limits measurements to approximately below the upper 18 to 24 inches and above the lower 18 to 24 inches of the PVC tubes. If the access tubes are too close to the longitudinal reinforcement, the higher density of the steel will affect the results.
Whether or not PVC tubes are out of plumb after the pile is constructed can also limit results. In some cases, PVC tubes can be pushed outward, allowing the lesser soil density to affect the measurements. Finally, GGL is relatively slow when compared with CSL and SE Testing.
Cross-Hole Sonic Logging (CSL)
CSL evaluates the uniformity and continuity of concrete by recording the velocity of signals from an emitter to a receiver, each inserted into the pile in preset tubes or pipes, typically Schedule 40 PVC pipe. Steel pipes can also be used and, in fact, are preferred for CSL. If the concrete is consistent and uniform, the velocity (wave travel time) at different depths should be constant, assuming that the horizontal distance between the PVC tubes (or steel pipes) is constant. Variations in the velocity indicate irregularities in the concrete between the PVC tubes (or steel pipes).
Shortly before or after the concrete is placed, water is added to the access tubes to allow transmission of the sonic signal from the tubes to the concrete. When PVC tubes are used, the water will also help minimize the effects of heat given off during the concrete curing, which can result in de-bonding between the concrete and PVC tubes. De-bonding of the PVC tubes from the concrete diminishes the quality of the downhole logging to the extent that the logs may not be useful. Thus, steel tubes, which are much more resistance to de-bonding effects, are preferred.
GGL cannot be performed if steel tubes are used. Downhole testing would be limited to CSL in this case. Applying an anti-de-bonding agent to the PVC tubing could also be employed to minimize the potential for de-bonding. Typically, de-bonding of PVC tubes will occur about 7 to 10 days after the placement of concrete and after about 45 days in steel pipes.
Typically, one testing access tube is installed per each foot of pile diameter, with a minimum of two tubes for piles less than 24 inches in diameter. For larger piles, an added benefit is the ability to perform measurements between different tubes, and thus pinpoint a defect. In addition, staggering the depths of the transmitter and receiver can allow evaluation of different travel paths, providing additional data for evaluation. This method is known as CSL tomography.
Limitations include de-bonding of the PVC tubes after relatively short times, the inability to evaluate the concrete not located between two tubes, and the potential for blockages within the PVC tubes, limiting the depth to which the probe can advance. In addition, the length of the probes limit measurements to approximately below the upper 18 to 24 inches, and above the lower 18 to 24 inches of the PVC tubes.
Sonic Echo (SE) Testing
SE testing is a relatively simple non-destructive pile integrity test method that provides limited information. The principle of SE testing is that a compression wave generated by a hand-held hammer blow at the top of the pile will travel down the concrete pile shaft, and will be reflected at the bottom of the pile shaft where there is a significant change in the physical properties of the medium the wave is traveling through. Interpretation of the test results is generally difficult and often inconclusive, particularly for longer piles, as would be required for tall buildings.
Irregularities in the concrete, and/or increases or reductions in the pile shaft section (bulging or necking), are identified by changes in the resistance (impedance) to the traveling wave. Decreases in resistance indicate necking, reduction in the density of elastic modulus of the concrete, or a discontinuity in the pile. This will result in the wave echo received at the top of the pile prior to the time computed for the wave to travel the full length of the pile. Increases in resistance indicate increased cross sectional area, or an increase in the elastic modulus or density of the concrete.
An increase in the impedance will be shown by displacement opposite that of the displacement resulting from the hammer blow, where the pile is socketed into rock that is much denser and thus transmits the compression wave at a much higher velocity.
SE Testing Limitations
Minor defects generally are not detectable with SE testing. Recent studies show that it is difficult to determine defects that are less than about a quarter of the wavelength, which is typically about 4½ feet (1.6 meters). Therefore, defects less than about 15" are not detectable. Furthermore, some studies indicate that defects representing less that 50 percent of cross sectional areas are not detectable with sonic echo testing. As a result of the limitations, false positive test results may not be uncommon.
In addition, the length of the pile that can be tested is limited due to dissipation of the wave energy at the sides of the pile, resulting from friction between the soil and the pile. Typically, SE testing is reliable for piles that are less than about 30 times the pile diameter in length, although the range may vary from about 20 to 40 times the pile diameter, depending on the soil type.
Other limitations include the inability to evaluate the pile below a major defect, and the inability to determine which side of the pile a defect occurs.
Impulse Response (IR) Testing
IR testing is generally similar to sonic echo testing, however, the hammer used measures the force applied to pile head versus time. In addition, the data from the wave traveling down the pile shaft and the data from the hammer impact are processed using Fourier transform methods, resulting in a more sophisticated analysis of the data.
The distance to the base of the pile or to a major defect can be inferred by observing the increment of frequency between peaks, as this distance is in proportion to the distance from the top of the shaft to the point where the energy is being lost.
IR Testing Limitations
This method has the same general limitations as sonic echo testing, however, the IR method provides results in a format that is somewhat easier to interpret.
Evaluating the Need for Integrity Testing
Due to the sensitivity of each method, sonic echo test results may indicate a sound pile, while GGL and/or CSL results indicate significant defects in the same pile. Difficulty in interpreting SE and IR test results, due to variation in input signal and influence of pile geometry and soil characteristics, is especially true for longer piles that are typically required for tall buildings.
CSL and GGL test results generally do not correlate well, as each method essentially tests a different portion of the pile section. Thus, sound CSL results combined with poor GGL results could indicate an acceptable pile when considering axial loading, however, could indicate an unacceptable pile when considering lateral loading. As a result, it is necessary to select an appropriate test program that will provide acceptable confidence levels for the engineer, depending on the nature of the structure and the configuration of pile groups.
If periodic testing of piles is appropriate, it is generally prudent to perform testing on a pre-determined number of the initial piles. If the testing on the initial piles indicates an acceptable confidence level in the method of pile construction, and the contractor's execution of the construction methods, the frequency of the testing may be reduced.
Several non-destructive test methods are available to evaluate the integrity of drilled piles. These methods, when combined with proper geotechnical monitoring and observation of pile drilling and construction process, can provide confirmation of the adequacy of piles without performing more costly full scale pile load testing. CSL is the most effective for identifying significant defects in piles that carry primarily axial loads. GGL is useful for evaluating the density in the outside portion of the pile, but can provide little other information. SE testing is useful for shorter piles that support less critical or sensitive structures. IR testing provides a more sophisticated means of evaluating piles using surface echo testing methods, however, is generally subject to the same limitations of SE testing.
SE and IR testing are generally more economical than the downhole methods of GGL and CSL. GGL is typically much slower to perform, and thus more expensive than CSL, as it is necessary to allow more time to properly record the backscatter. Selection of the appropriate the test method is highly dependent on the nature of the drilled pile, including the geometry, the soil type, and the number of piles in a pile group. The time required for the engineer to evaluate the test results, and the cost of the testing with respect to the benefits should also be considered.
Return to: 2002 Feature Stories