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Borehole Nondestructive Test (NDT) Methods

Parallel Seismic (PS)

The Parallel Seismic (PS) method is a borehole test method for determining depths of foundations. The method can also detect major anomalies within a foundation as well as provide the surrounding soil velocity profile. The method requires the installation of cased borehole close to the foundation being tested. The method can be used when the foundation tops are not accessible or when the piles are too long and slender (such as H piles or driven piles) to be testable by sonic echo techniques.

Basic Concept: PS method involves hammer impacts at any part of the exposed structure that is connected to the foundation (or impacting the foundation itself, if accessible). A hydrophone or a three-component geophone located in a nearby borehole records the compressional and/or shear waves traveling down the foundation. Therefore, the PS test requires drilling a 5- to 10-cm-diameter hole as close as possible to the foundation being tested (preferably within 1.5 m). The borehole should extend at least 3 to 5 m below the expected bottom of the foundation. If hydrophones are used, the hole must be cased, capped at the bottom, and the casing and hole filled with water. For geophone use, the hole must usually be cased and grouted to prevent the soil from caving in during testing.

PS tests can be performed on concrete, wood, masonry, and steel foundations. Some portion of the structure that is connected to the foundation must be exposed for the hammer impacts.

Data Acquisition: The field setup for PS tests is shown in Figure 11. In a PS test, a hammer strikes the structure, and the response of the foundation is monitored by a hydrophone or a geophone receiver placed in the borehole. A signal analyzer records the hammer input and the receiver output. The receiver is first lowered to the bottom of the hole, and a measurement is taken. Then, the receiver is moved up 30 or 60 cm, and the second measurement is made. This process is continued until the receiver has reached the top of the boring. If needed, the hammer strikes can be repeated multiple times at each location in order to stack the records and thereby improve the signal-to-noise ratio.

Data Processing: Analysis of the PS data is performed in the time domain. In PS tests, one relies on identifying direct arrival times of compressional and shear waves at the receiver locations, as well as the wave amplitudes. The PS tests are performed at 30 to 60 cm vertical receiver intervals in the borehole. Figure 11 shows the pile and borehole configuration used for the test. The first arrival times are plotted as a function of depth, and the depth where a change of slope occurs is observed to find the foundation depth. Also, the foundation depth can be obtained by observing the depth where the signal amplitude of the first arrival energy is significantly reduced. In addition, geophysical processing techniques can be used to help optimize the Parallel Seismic data. These techniques include Automatic Gain Control (AGC) and frequency filtering to enhance weak events.

Parallel Seismic survey setup.
Figure 11. Parallel Seismic survey setup.

Data Interpretation: For hydrophone data, the time arrival of the first compressional waves is picked from the data for all receiver locations. A plot of the time arrival-versus-depth is prepared, an example of which is illustrated in Figure 12.

In Figure 12, the velocity of the concrete in the shaft is 5,155 m/s. A break in the graph occurs at a depth of 8.5 m indicating the depth of the shaft.

For uniform soil conditions, two lines are identified in the plot as shown in Figure 12. The slope of the upper line is indicative of the velocity of the tested foundation, and the second line is indicative of the velocity of the soil below the bottom of the foundation. The intersection of the two lines gives the depth of the foundation. For nonuniform soil conditions, the interpretation of data from hydrophone use can be difficult due to the nonlinearity of the arrival time of the first compressional waves. For geophone data in uniform soil conditions, the data can be interpreted in a way similar to the hydrophone data. When variable soil velocity conditions exist, an alternative to the first arrival time in data interpretation is used. All the traces are stacked, and a V-shape is searched for in the data because the bottom of the foundation acts as a strong source of energy (a point diffractor and a reflector), which produces upward and downward traveling waves. When a geophone is used, the borehole is generally not filled with water. As a result, tube waves are minimized so that later arrival of reflected and diffracted shear and compressional waves can be identified.

Parallel Seismic data and velocity lines.
Figure 12. PS data and velocity lines.

Advantages: The PS method is more accurate and more versatile than other nondestructive surface techniques for determining unknown foundation depths. The accuracy of the method depends on the variability of the velocity of the surrounding soil and the spacing between the borehole and the foundation element. Depths are normally determined with 95% accuracy or better.

Limitations: A borehole is needed for PS tests, which adds to the cost of the investigation (unless borings are also required for other geotechnical purposes). The borehole should be within 1.5 m of the foundation, which sometimes cannot be achieved. Note that for very uniform soils (such as saturated sands), a successful test can be performed with up to 4.5 to 6 m spacing between the source and the borehole. As the borehole moves away from the foundation, interpretation of the PS data becomes more difficult, and the uncertainty in the tip depth determination becomes greater.

Induction Field (IF)

Induction Field (IF) method is used for the determination of the unknown depth of steel or continuously reinforced concrete piles.

Basic Concept: This is an electrical method that relies on detecting the magnetic field in in response to an oscillating current impressed into a steel pile. In order for this method to work, the pile must, therefore, contain electrically conductive materials. For reinforced concrete piles, this usually implies that reinforcing rebar extends along its full length.

A sensor is placed down a drillhole located close to the pile and detects the changing magnetic field strength. This sensor could be a magnetic field sensor or a coil. Along the length of the pile, the magnetic field strength will be relatively strong. However, the magnetic field strength will be significantly diminished at levels in the drillhole beneath the bottom of the pile to a residual conductivity value of the soil or bedrock. This change in the magnetic field strength is used to determine the depth of the pile.

Data Acquisition: An electrical contact must be made to the pile in question when conducting an Induction Field survey. Another electrode must be placed some distance from the pile. This can be another pile or simply an electrode placed in the ground. Oscillating current is then made to flow between these two electrodes. Figure 13 shows the layout of the pile, borehole and electrodes.

Data Processing: The detector measures the voltage and the data stored in the recorder. The magnitude of the measured voltage is plotted against the depth of the detector. This plot will show a significant decrease in magnetic field strength when the detector is beneath the toe of the pile.

Data Interpretation: Interpretation of the Induction Field method simply requires a visual observation of the voltage decay with depth down the borehole. The voltage will usually stabilize beneath the pile at a low voltage whose magnitude will depend on the resistivity of the ground.

Advantages: The Induction Field method is a proven technology for the determination of unknown depth of piles containing electrically conductive material, such as rebar. If the reinforcement is continuous in a concrete foundation, it can be used to detect the presence of piles underneath a footing.

Limitations: The interpretation of data is complicated by the existence of conductive materials in the bridge structure and the surrounding ground (including the water-table). Probably the most restrictive requirement is that there must be metal (usually reinforcing rebar) inside the pile, and it must extend continuously along the length of the pile. In addition, an electrical contact with the metal must be possible near the top of the pile. A PVC cased borehole is required. No signal would be received through a steel-cased borehole.

Induction Field method setup.
Figure 13. Induction Field method setup.

Borehole Logging Methods

Two borehole geophysical logging methods, magnetic logging and electromagnetic induction logging methods, can be used for the determination of unknown depth of steel of reinforced concrete piles.

No information was found on the use of these methods for measuring pile depths. However, it seems clear that these methods should work if the conditions are appropriate. In view of their simplicity and ease of use, it is included erein as methods that could be tested.

A borehole magnetometer could be used to find the depth of piles if they contain reinforcing rebar along the length of the pile. The reinforcing rebar will have induced magnetization due to the influence of the Earth's magnetic field and will radiate a secondary magnetic field. A magnetometer will respond to both the Earth's magnetic field and the secondary field. Beneath the toe of the pile, the secondary field will rapidly diminish resulting in a decrease in the magnetic field strength. A magnetometer in a nearby drill hole will detect this magnetic field change from which the toe of the pile can be deduced.

Alternatively, borehole electrical methods, such as induction logging, can be used. In the induction logging method, an AC current is transmitted into the ground by the source coil and another coil is used for receiving the returning signal. The transmitted AC current generates a time-varying primary magnetic field, which induces eddy currents in the conductive ground or steel reinforcing rebar. These eddy currents set up secondary magnetic fields, which induces a voltage in the receiving coil. The magnitude of the received current can be used to determine the pile toe.

These two geophysical logging methods were not found in any of the references and are under investigation by Blackhawk GeoServices.

Dynamic Foundation Response

The Dynamic Foundation Response uses the resonant frequencies of structures to differentiate foundation types. The vibration response of a bridge substructure will exhibit lower resonant frequency responses when excited for a shallow foundation versus the comparatively higher resonant frequency response of a deep foundation system.

Basic Concept: The method is unproven for this use in bridges, but is based on the dynamic analysis theory for vibration design of foundations (soil dynamics) and geotechnical analyses of foundations subjected to earthquake loading.

Data Acquisition: A hammer with a built-in dynamic force transducer is used as the vibration source. A triaxial block of seismic accelerometers records the resulting signals. Typically, a bridge is excited at five to six locations, and the triaxial response is measured at five to six locations giving rise to 25 to 36 source-receiver combinations. The bridges are impacted in the vertical and horizontal directions to excite these modes as well as rocking modes along the frame of the substructures. This type of testing is known as modal testing; when the impulse force is measured and the resultant vibration response is measured, the transfer function can be calculated as in the Impulse Response test.

Data Processing: In its most basic form, a transfer function is calculated by taking the Fast Fourier Transforms of the input impact force (F) and the output accelerometer receiver responses in acceleration units (A) as functions of frequency (f). The transfer function is obtained by dividing the output by the input (A/F). Plots of the transfer function versus frequency indicate the frequencies and amplitudes of resonance for a tested structure.

Data Interpretation: The dynamic foundation response of bridges, especially bridge piers, is much more complex than simple footing and pile foundation cases. Consequently, there will be many resonances present in transfer function results. Also, fundamental resonances of bridges are generally less than 20 Hz, and frequently less than 10 Hz. To determine the various resonances and their vibration mode shapes, the transfer function test must excite the range of frequencies of interest, and a number of locations must be tested to identify the mode shapes. The process of determining the full vibration behavior of a bridge abutment or pier then requires curve fitting of the experimental data, and can also involve theoretical dynamic analysis of the bridge with dynamic structural analysis programs.

Limitations: In tests, the Dynamic Foundation Response method showed some sensitivity in the response of the foundation as a function of depth and existence of piles, particularly for vertical vibrations. However, in practice, the bridges generally could not be excited with the 5.5 kg hammer at frequencies comparable to the natural frequencies identified in the theoretical modeling. More research is needed to explore different sources to generate the very low frequencies required for the experimental modal tests and then to perform curve fitting techniques to experimental data to be able to extract the mode shapes from the experimental data to compare with the theoretical mode shapes.

Practical tests showed that a 5.5 kg hammer was not sufficient to generate the required waves in bridges at the required frequencies. However, the method showed some response of the foundation as a function of depth and existence of piles. This was particularly true for vertical vibrations. It is possible that this method may be appropriate for monitoring over time, where changes in the response could be detected. However, no references were found indicating that this is currently done.

Borehole Radar

Borehole Radar uses a borehole ground penetrating radar (GPR) antenna to obtain reflection echoes from a foundation for the determination of unknown depth and geometry of foundation.

Data Acquisition: In borehole radar, an antenna transmits radar energy into the surrounding rock and soil, and a receiver then records reflections that occur as the radar signals encounter and reflect from interfaces with different dielectric properties. The method is very similar to the borehole sonic method, where seismic waves are used rather than electromagnetic waves.

Borehole radar can be used in reflection mode or in cross-hole tomography mode. The radar measurements are either directional or omnidirectional, depending on the type of equipment and antennas. Only the reflection mode will be discussed in this web manual.

Radar uses radio waves with frequencies varying generally between 10 and 2,000 MHz. These waves are influenced primarily by the dielectric properties of the medium through which they are traveling and the electrical conductivity of the medium. Highly conductive materials attenuate the radar signals and limit its depth of penetration. Although the lower frequencies penetrate more than higher frequencies, they have less resolution.

For radar frequencies of 100 MHz, penetration varies from 10 to 40 m in resistive rocks. In conductive, clay-rich rocks, penetration will be less than 5 m. Figure 14 shows the borehole radar system.

Data Interpretation: In the unknown depth of foundation application, the borehole radar signal will be reflected from the foundation until the bottom of the foundation is reached. There will be no reflections beneath the foundation, except for those emanating from geologic conditions. The observed change in the reflected signal is used to locate the bottom of the foundation.

Limitations: Borehole radar requires a PVC-cased borehole; the method will not work if the hole is steel-cased. The depth of penetration is significantly influenced by the electrical conductivity of the rocks and soil surrounding the borehole, which may not be known before the radar survey is completed. Penetration up to about 10 m may be achieved in resistive conditions. In conductive materials, since the penetration of the GPR signal will be limited, getting the borehole as close to the pile as possible will be advantageous.

Borehole Radar system.
Figure 14. Borehole Radar system.

Borehole Sonic

This method is similar to Sonar and is based on using borehole seismic sources and geophones to obtain reflection echoes from a foundation for the determination of unknown depth and geometry of the foundation.

Basic Concept: This method is a borehole equivalent of the surface reflection seismic method. It requires a borehole close to the pile into which a sonde is lowered containing a seismic source and a seismic wave detector. The requirement for the method to work is that there is a seismic impedance contrast between the pile (for example, concrete) and the soil into which the borehole is drilled. It is likely that this will frequently be the case since the velocity of seismic waves in soil is typically much lower than those in concrete. Figure 15 shows the method and seismic waves.

Data Acquisition: The borehole sonic surveys are conducted using a single borehole utilizing borehole logging sondes with low frequency sources or alternatively using separate boreholes for the source and receiver that are held at the same measurement depths. The survey is conducted by taking measurements at various depths within the borehole. In many logging methods, readings are taken when the sonde is ascending.

Schematic of the Borehole Sonic method.
Figure 15. Schematic of the Borehole Sonic method.

Limitations: Seismic waves in soil are attenuated significantly with distance and may also be dispersive, thus limiting the higher frequency content of the signal. In addition, the waves are reflected from a curved surface (the pile), which may provide less returned energy than a plane surface as is common in surface seismic reflection methods. To this date, this method is not proven for the determination of unknown foundation depths.

Cross-borehole Seismic Tomography

Two- and three-dimensional tomography is used for the high resolution imaging of the subsurface between boreholes.

Basic Concept: Tomography is an inversion procedure that provides for two- or three-dimensional (2-D and 3-D) velocity (and/or attenuation) images between boreholes from the observation of transmitted first arrival energy.

Data Acquisition: Tomography data collection involves scanning the region of interest with many combinations of source and receiver depth locations, similar to medical CAT scan (Figure 16). Typical field operation consists of holding a string of receivers (geophones or hydrophones) at the bottom of one borehole and moving the source systematically in the opposite borehole from bottom to top. The receiver string is then moved to the next depth location and the test procedure is repeated until all possible source-receiver combinations are incorporated.

Tomographic survey design.
Figure 16. Tomographic survey design.

Data Processing: In the tomographic inversion technique, the acoustic wavefield is initially propagated through a presumed theoretical model and a set of travel times are obtained by ray-tracing (forward modeling). The travel time equations are then inverted iteratively in order to reduce the root mean square (RMS) error between the observed and computed travel times. The inversion results can be used for imaging the velocity (travel time tomography) and attenuation (amplitude tomography) distribution between boreholes.

Data Interpretation: Described below in Figure 17a is a tomographic survey designed to investigate the foundation of an existing bridge. In this example, cross-hole velocity tomography surveys were conducted by pairing a seismic source in one borehole and a string of receivers in an adjacent borehole to propagate and capture seismic signals transmitted between source and receiver boreholes. Steel or concrete piles that existed within the surveyed area were indicated in the tomograms as relatively higher seismic velocity zones than the surrounding ground. The pile group, as depicted in Figure 17a, appeared as relatively higher seismic velocity anomalies within the fill and soil material above the bedrock. There was also a clear indication of low-velocity anomaly pockets in the top of the bedrock where the piles were driven into the bedrock. The seismic tomography survey indicated that the piles were point-bearing on rock, and, in fact, the piles were driven into the bedrock surface when installed.

The example shown in Figure 17a is a tomographic survey designed to investigate the depth of four drilled shafts supported by a pile cap under a bridge column. The shafts were originally thought to be 2m socketed in the bedrock. The tomogram sections, however, show that the shafts rested on top of bedrock. The results were also confirmed using the parallel seismic method. In Figure 17b, the top of bedrock was well defined with no low-velocity anomalies apparent. In this Figure, the water-saturated alluvial sediments, shown in blue, lies above bedrock, shown in green.

Tomograms showing:  (a) socketed piles and (b) caisson on top of bedrock. (NSA Geotechnical Services, Inc. and 
		Blackhawk GeoServices, Inc., respectively)
Figure 17. Tomograms showing: (a) socketed piles and (b) caisson on top of bedrock. (NSA Geotechnical Services, Inc. and Blackhawk GeoServices, Inc., respectively)

Advantages: Tomography provides high-resolution two-dimensional area or three-dimensional volumetric imaging of target zones for immediate engineering remediation. Tomography can then be used in before and after surveys for monitoring effectiveness of remediation. Tomography can also be used in before and after surveys for monitoring fluid injections between test holes or for assessing the effectiveness of soil improvement techniques.

Attenuation tomography can be used for the delineating fracture zones. Wave equation processing can be used for a high-resolution imaging of the reflection events in the data including those outside and below the area between the boreholes.

Limitations: Tomography is data-intensive and specialized 3-D analyses software is required for true three-dimensional imaging. Artifacts can be present due to limited ray coverage near the image boundaries.