Cross Hole Sonic Logging: A Complete Guide to Pile Integrity Testing

Author : Pile test | Published On : 26 Oct 2026

Deep foundations are essential for supporting buildings, bridges, highways, and major infrastructure projects. Because piles transfer structural loads into the ground, their quality and structural integrity are critical to the safety and long-term performance of a structure. However, defects within a concrete pile may not be visible from the outside, making reliable testing methods an important part of construction quality control.

Cross hole sonic logging is a non-destructive testing method used to evaluate the quality and uniformity of concrete in drilled shafts and bored piles. By transmitting ultrasonic signals between probes positioned inside access tubes embedded in the concrete, engineers can identify potential abnormalities and investigate areas that may require further assessment.

Understanding how this method works, the relevant testing standards, and the available equipment helps construction professionals establish reliable foundation inspection procedures.

What Is Cross Hole Sonic Logging?

Cross hole sonic logging is a non-destructive method for assessing the integrity of concrete foundations, particularly large-diameter bored piles and drilled shafts.

The technique evaluates concrete quality by measuring how ultrasonic signals travel between transmitting and receiving probes positioned inside access tubes installed within the pile reinforcement cage before concrete placement.

When the concrete is uniform and properly consolidated, ultrasonic signals generally travel through it with predictable characteristics. Changes in signal arrival time, energy, or waveform may indicate variations in concrete quality or potential defects.

These variations can be associated with problems such as concrete inclusions, voids, segregation, or areas of poor consolidation. However, the test does not automatically establish the exact nature or dimensions of a defect. Engineers must interpret the results alongside construction records and other relevant information.

Cross hole sonic logging is particularly useful when inspecting deep foundations where visual examination cannot provide sufficient information about the internal condition of the concrete.

How Does a Cross Hole Sonic Logging Test Work?

A cross hole sonic logging test involves several stages, beginning before the pile is concreted and continuing through data collection and interpretation.

1. Installing Access Tubes

Before concrete placement, access tubes are attached to the pile reinforcement cage. Their number, arrangement, and spacing depend on the pile dimensions and the project's testing requirements.

The tubes must remain secure and suitable for probe movement during testing. They are generally filled with water to provide acoustic coupling between the probes and the surrounding concrete.

Proper installation is important because damaged, obstructed, or incorrectly positioned tubes can interfere with testing.

2. Inserting the Ultrasonic Probes

After the concrete has reached an appropriate age and the pile is ready for testing, a transmitting probe and a receiving probe are lowered into separate access tubes.

The transmitter generates ultrasonic pulses, while the receiver measures the signals travelling through the concrete between the tubes.

The equipment records the arrival time and other characteristics of the received signals.

3. Collecting Measurements

The probes are moved through the access tubes in a controlled manner, collecting measurements at successive depths.

The resulting data provides information about the concrete between each tested pair of tubes. Depending on the access tube arrangement, engineers can evaluate several paths through the pile.

Testing multiple tube combinations helps create a more comprehensive picture of concrete quality within the accessible portions of the foundation.

4. Analysing the Results

The recorded measurements are processed to identify changes in signal behaviour.

Longer signal arrival times, reduced signal energy, or missing signals may indicate potential anomalies. However, such changes can also be influenced by tube conditions, probe positioning, water coupling, and other testing variables.

Engineers therefore review the measurements carefully before deciding whether a result indicates a potential defect.

5. Preparing the Report

The final report typically documents the testing procedure, access tube combinations, measured depths, signal characteristics, and areas requiring further investigation.

These records support foundation quality assurance and provide useful information for engineers responsible for evaluating the pile.

Why Is Cross Hole Sonic Logging Testing for Piles Important?

Cross hole sonic logging testing for piles helps engineers investigate the internal condition of concrete foundations that cannot be assessed adequately through surface inspection alone.

Concrete placement in deep foundations can present challenges. Reinforcement congestion, difficult site access, concrete delivery interruptions, and unsuitable placement procedures may contribute to inconsistencies within the finished pile.

Testing provides an additional source of information that can help identify areas requiring engineering review.

Identifying Potential Concrete Defects

The method can help identify anomalies associated with poor concrete consolidation, inclusions, segregation, and other irregularities along the tested paths.

Detecting such indications allows engineers to investigate potential concerns before making decisions about foundation acceptance.

Supporting Construction Quality Control

Testing results can be compared with construction records, including concrete delivery information, installation logs, and reinforcement details.

This helps project teams assess whether the completed foundation meets the specified quality requirements.

Improving Confidence in Foundation Assessment

Cross hole sonic logging provides information about the concrete between the installed access tubes. This can be particularly valuable for large-diameter piles where internal concrete quality is difficult to evaluate using other methods.

However, the method does not examine every part of a pile equally, and results must be interpreted within the limitations of the testing arrangement.

Supporting Further Investigation

If the results indicate a potential anomaly, engineers can determine whether additional testing, specialist assessment, or other investigative methods are necessary.

This evidence-based approach helps avoid premature conclusions about the condition of a foundation.

Understanding ASTM D6760

ASTM D6760 is a recognised standard covering the integrity testing of concrete deep foundations using ultrasonic crosshole testing.

The standard describes procedures for evaluating the concrete between access tubes installed within deep foundations. It establishes a framework for collecting and interpreting ultrasonic measurements to assess concrete integrity.

When planning a cross hole sonic logging programme, engineers should consider the applicable edition of ASTM D6760 and any additional project-specific requirements.

Following the relevant standard helps establish consistency in testing procedures and reporting. However, compliance alone does not guarantee that every defect will be detected. Test results depend on access tube installation, concrete conditions, instrument performance, data quality, and the expertise of the person interpreting the measurements.

The standard should therefore be applied as part of a broader foundation quality-assurance process rather than treated as a substitute for engineering judgement.

What Is a CSL Test?

A CSL test, short for crosshole sonic logging test, is another commonly used name for this method of assessing concrete pile integrity.

The technique uses ultrasonic pulses transmitted between probes positioned in separate access tubes. The measured signals help engineers evaluate the uniformity of the concrete along the tested paths.

CSL testing is frequently specified for drilled shafts, bored piles, and other suitable deep foundation elements where internal concrete quality needs to be investigated.

The main objective is to identify unusual signal characteristics that may indicate variations in concrete integrity.

For example, a significant delay in signal arrival time or a reduction in signal energy may warrant further examination. Such findings do not necessarily confirm a defect, because other factors can also influence the readings.

Engineers typically consider the depth, severity, and consistency of the observed changes when deciding whether further investigation is justified.

Cross Hole Ultrasonic Test: Equipment and Data Analysis

A cross hole ultrasonic test requires equipment capable of generating ultrasonic pulses, measuring the received signals, recording data against depth, and presenting the results for interpretation.

The main components generally include:

• An ultrasonic transmitter and receiver.

• Compatible access tube cables and probe assemblies.

• A data acquisition unit.

• Software for displaying and analysing measurements.

• Equipment for recording and exporting test results.

The quality of the equipment can influence the efficiency of fieldwork and the clarity of the resulting data.

Signal Quality and Measurement Accuracy

Reliable signal transmission and reception are essential for identifying meaningful changes in concrete behaviour. Equipment should provide consistent measurements and allow operators to recognise unusual readings.

Efficient Field Operations

Testing can involve numerous tube combinations and repeated measurements at different depths. A well-designed system helps operators organise this process and reduces unnecessary handling.

Clear Data Presentation

Software should make it possible to examine signal arrival times, relative energy, depth profiles, and other relevant measurements.

Clear visualisation helps engineers identify areas that require closer examination and prepare reports for project stakeholders.

Data Storage and Reporting

Digital data management allows testing records to be stored, reviewed, and shared with engineers responsible for foundation assessment.

A structured reporting process also supports traceability and future reference.

Introducing the CHUM System from Piletest

Piletest develops specialised equipment for non-destructive testing of deep foundations. Its CrossHole Ultrasonic Monitor, known as CHUM, is designed to support crosshole ultrasonic testing of concrete piles and drilled shafts.

The system collects ultrasonic measurements between probes positioned in access tubes, helping engineers evaluate the integrity of concrete foundations.

For projects requiring detailed investigation, equipment of this type can support systematic data collection and analysis across multiple tube combinations and depths.

When selecting a crosshole testing system, engineering teams should review its measurement capabilities, software features, compatibility with their testing procedures, reporting options, and suitability for site conditions.

It is also important to confirm that the proposed equipment and testing procedures meet the applicable project specification and the relevant edition of ASTM D6760.

To explore the system's capabilities and technical details, visit the Piletest CHUM product page.

Best Practices for Reliable Cross Hole Sonic Logging

Accurate results depend on more than the testing instrument. Appropriate planning and field procedures are essential for obtaining useful measurements.

Consider the following practices when organising a testing programme:

1. Plan access tube installation: Confirm the required tube number, spacing, and arrangement before concrete placement.

2. Check tube condition: Ensure the tubes remain accessible and suitable for probe movement.

3. Follow the specified procedure: Use the appropriate testing method and instrument settings for the project.

4. Review signal quality: Investigate unusual readings and repeat measurements where necessary.

5. Interpret results carefully: Consider potential equipment, access tube, and concrete-related influences before identifying an anomaly.

6. Maintain complete records: Document test conditions, tube combinations, measured depths, and relevant observations.

7. Escalate potential concerns: Refer significant anomalies to the responsible engineer for further assessment.

Following these practices helps improve data reliability and supports consistent foundation evaluation.

Conclusion

Cross hole sonic logging is an important non-destructive method for evaluating concrete integrity in bored piles and drilled shafts. By transmitting ultrasonic signals between probes inside access tubes, engineers can identify potential abnormalities and investigate areas that may require further assessment.

A properly conducted cross hole sonic logging test provides valuable information for foundation quality control, particularly when internal concrete conditions cannot be assessed through visual inspection.

ASTM D6760 offers a recognised framework for conducting this type of testing, while specialised equipment such as Piletest's CHUM helps support field measurements and data analysis.

🏗️ By combining appropriate equipment, careful installation procedures, reliable data collection, and professional interpretation, construction teams can make better-informed decisions and strengthen quality assurance throughout deep foundation projects.