Crosshole Logging Test of Piles
Crosshole logging (CSL), designated Ultrasonic Crosshole Testing in ASTM D6760, is a non-destructive method for assessing the homogeneity and integrity of concrete in bored piles, drilled shafts and other deep foundation elements. It detects anomalous zones by measuring ultrasonic pulse transmission between access ducts installed in the pile.
Principle
A transmitter probe sends an ultrasonic P-wave through concrete to a receiver probe in a parallel, water-filled access duct. The probes are normally maintained at the same elevation and moved together along the pile.
Good-quality concrete typically has a P-wave velocity of about 3600–4400 m/s.
Poor-quality zones may show lower velocity and reduced signal energy.
The system records:
- First arrival time
- Signal amplitude/relative energy
- Probe depth
These data are processed into an ultrasonic/CSL profile.
Access Tubes
Access tubes are installed inside the reinforcement cage before concreting.
Key requirements:
- Mild-steel tubes are preferred for crosshole testing.
- Typical internal diameter: 38–50 mm.
- As a guide, use approximately one tube per 0.25–0.30 m of pile diameter; a minimum of three tubes is preferred.
- Tubes should be straight, unobstructed, parallel and securely fixed to the reinforcement cage.
- Bottoms should be closed and tops capped.
- Tubes should be filled with water before or within one hour of concrete placement.
- Tube lengths and spacing should be recorded.
Equipment
The main requirements are:
- Transmitter frequency: 30–100 kHz
- Waterproof probes to at least 1.5 × maximum test depth
- Probes capable of reaching within 125 mm of the duct bottom
- Depth measurement accuracy: 1% of duct length or 0.25 m, whichever is larger
- Data acquisition: minimum 12-bit amplitude resolution and 250 kHz sampling frequency
- Pulse interval typically 50 mm or less.
Test Procedure
- Remove tube caps, measure tube lengths, check for blockage and fill tubes with clean water.
- Place the transmitter and receiver in a selected tube pair.
- Lower both probes to the bottom while keeping them approximately level.
- Set transmitter power and receiver gain at an intermediate zone of apparently good concrete.
- Raise both probes together at a controlled speed while continuously recording ultrasonic pulses.
- Repeat the test for all access-tube pairs.
- Repeat any depth where a usable pulse was not obtained.
Testing is generally performed 3–7 days after casting, depending on concrete strength and pile diameter. Plastic-tube installations should preferably be tested as soon as practical because tube debonding can affect the data.
Interpretation of CSL Results
The output is generally presented as a waterfall diagram and processed CSL profile showing:
- First arrival time / apparent wave velocity
- Relative signal energy or amplitude
A simultaneous delay in arrival and loss of signal strength provides stronger evidence of an anomalous zone. Energy/amplitude is commonly presented on a logarithmic scale, with attenuation potentially expressed in dB.
Important: CSL identifies an anomaly, but does not by itself determine whether it is honeycombing, a void, soil inclusion, segregation or another defect.
Try the CSL Profile
How does a concrete anomaly appear in a crosshole logging profile?
Use the interactive visualizer below to move the anomaly through the pile and observe the corresponding changes in apparent wave velocity and signal energy.
Investigation of Anomalies
Suspected zones can be investigated using:
- Fan-shaped testing
- Fixed-offset testing
- Tomography
For detailed investigation, the probes should extend at least 1 m above and below the suspected anomaly. Confirmation may require coring, excavation or other integrity tests.
Limitations
CSL primarily evaluates concrete between the access tubes and provides limited information outside the reinforcement cage when the tubes are installed internally. Poor tube installation, blockage, misalignment or debonding can also affect results.
Interpretation should not rely on a single wave-velocity threshold. The final integrity assessment requires appropriate engineering judgment and experience. Excessive filtering or smoothing should be avoided because it can conceal defects.
CSL Report
The report should include pile identification and geometry, construction details, tube layout and spacing, equipment, test date, tube limitations, CSL profiles for each tube pair, suspected anomaly depths and any follow-up investigation. The original/waterfall data should preferably be retained for future review.
Conclusion
CSL evaluates pile integrity by measuring ultrasonic travel time and signal energy between water-filled access tubes. Changes in velocity, arrival time and signal attenuation identify zones requiring further investigation.
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