Engineers choose CSL for large-diameter bored piles, where a pile-head test such as the Seismic Integrity Test (low strain) cannot give enough detail about the concrete across the whole section. This article explains how CSL works, which tubes must be installed in advance, what it can detect, which standard to reference now that ASTM D6760 was withdrawn in 2025, and how it differs from the Seismic Test.
The first figure sums up the whole principle in one image. On the left, the transmitter and receiver are raised together in a pair of tubes; on the right is the Waterfall Diagram, where a defect shows up as a zone of delayed arrival and weak signal.
How Does Cross Hole Sonic Logging (CSL) Work?
CSL works by sending an ultrasonic pulse from a transmitter in one tube to a receiver in another tube at the same depth. The system raises the transmitter and receiver together from the pile toe to the pile head, while a depth-measuring pulley records the level of every reading. The output is the first arrival time (FAT) and the received wave energy along the full pile length.
The interpretation principle is simple: sound concrete transmits sound waves quickly and with consistent strength, while at voids, cracks or soil-contaminated concrete the waves travel more slowly and lose noticeable energy. Results are plotted as a Waterfall Diagram or contour versus depth, so both the location and the depth of a defect can be identified.
The main parameter for interpretation is wave velocity, calculated as the tube spacing divided by the arrival time. Good concrete typically has a wave velocity of about 3,500–4,500 m/s. If the velocity in any zone drops by more than 10–20% from the average for that pile, it is a signal that needs further evaluation. The actual acceptance criteria must be set by the responsible engineer according to each project's specifications.
Which Tubes Must Be Cast into the Pile Before Concreting?
CSL must be planned before concreting, because access tubes — steel or PVC pipes — have to be tied to the reinforcement cage and lowered into the hole with it. The number of tubes depends on pile size; common practice is:
- Piles up to 0.60 m in diameter: at least 2 tubes
- Diameter 0.60–1.20 m: 3–4 tubes
- Diameter over 1.20 m: 4 tubes or more
The more tubes, the more test pairs (profiles) and the more complete the coverage of the cross-section. The number of profiles is n(n−1)/2, so 4 tubes give 6 profiles and 6 tubes give 15. The tubes must be capped at the bottom so that concrete or slurry cannot block them during the pour, and filled with water to act as the coupling medium between the probes and the tube wall.
The tubes should be straight and parallel over their full length, because the tube spacing is used to calculate wave velocity. Steel tubes are preferred over PVC, because PVC tubes may debond from the concrete and weaken the signal in a way that resembles a defect.
The figures below show the CSL field setup as a 2D model (side section) and a 3D model. Note the access tubes tied to the reinforcement cage, the depth-measuring pulley at the tube top, and the measurement planes where waves cross all 6 tube pairs in a 4-tube pile.
ASTM D6760 Was Withdrawn in 2025 — Which Standard Does CSL Reference Now?
CSL testing today follows the guidance of ACI PRC-228.2 together with the procedure of ASTM D6760-16 (with its status noted as withdrawn by ASTM in 2025) and the acceptance criteria in the project specification. The long-standing standard for CSL was ASTM D6760-16 (Standard Test Method for Integrity Testing of Concrete Deep Foundations by Ultrasonic Crosshole Testing), but ASTM withdrew it from active status in 2025.
The withdrawal was procedural — not because the test method is wrong or unusable. Under ASTM regulations, a standard that is not reviewed or reapproved within 8 years is withdrawn automatically. D6760-16 was not reviewed in time, so it was withdrawn, and no new ASTM standard has yet been issued as a direct replacement.
In practice, the test method is unchanged. Engineers and testers still use the D6760-16 procedure as guidance, while ACI PRC-228.2 (Report on Nondestructive Test Methods for Evaluation of Concrete in Structures, formerly ACI 228.2R) covers Crosshole Sonic Logging as recommended practice. Acceptance criteria are set by the specific requirements of each project or owner.
When writing a specification or TOR today, state clearly "tested in accordance with ACI PRC-228.2 guidance and the procedure of ASTM D6760-16 (withdrawn 2025)" rather than simply "per ASTM D6760", to avoid later disputes about citing a standard that no longer has active status. The acceptance criteria should also be stated directly in the specification.
For project owners, SPN's CSL reports state the test method as ACI PRC-228.2 guidance and the ASTM D6760-16 procedure, with the edition year and status (withdrawn by ASTM in 2025) clearly marked, alongside the acceptance criteria agreed in the project specification. Reports are certified by a licensed civil engineer (Council of Engineers Thailand) and can be submitted to the relevant authorities.
Which Piles Should Be Tested with CSL?
CSL suits large bored piles, or critical piles that require high confidence in the concrete across the whole section, where the low-strain Seismic Integrity Test does not give enough detail. Typical cases for CSL include:
- Bored piles with diameters of about 0.80–1.00 m or more, where Seismic Test waves spread out and become hard to interpret
- Very long bored piles, where low-strain reflections from the toe are unclear
- Piles for critical structures such as bridges, high-rise buildings or heavily loaded foundations
- Piles with problems during concreting, such as interrupted concrete flow or doubts about mix quality
CSL needs tubes installed in advance, so the test must be specified before piling starts. If a pile has already been cast without tubes, CSL cannot be performed and another method must be used instead. Where the concrete cover outside the cage also needs checking, some projects use Thermal Integrity Profiling (TIP) to ASTM D7949-25 together with CSL; its equipment must also be installed before concreting.
What Can Cross Hole Sonic Logging Detect?
CSL detects defects inside the concrete of bored piles in more detail than pile-head test methods, identifying both the type and the depth of the problem. CSL can detect:
- Voids and honeycombing from concrete segregation or incomplete flow
- Cracks and concrete discontinuity (necking) between pour stages
- Concrete contaminated with soil or slurry (soil inclusion) from inadequate hole cleaning
- Localized poor-quality concrete where wave velocity is clearly lower than in adjacent zones
The causes of each defect type and other detection methods used alongside CSL are explained in Pile Defects: Causes and Detection
An important limitation is that CSL tests only the zone "between the tubes". Defects outside the tube ring — such as the concrete cover outside the cage or outer faces not spanned by tubes — may go undetected, so the number of tubes must suit the pile size. In addition, CSL indicates only the integrity of the concrete; it does not indicate the pile's load capacity.
Test results are summarized in a report certified by a licensed civil engineer (Council of Engineers Thailand), with signature and registration number, suitable for submission to the relevant authorities.
How Does CSL Differ from the Seismic Integrity Test?
CSL and the Seismic Integrity Test (low strain) are both pile integrity tests, but they differ in measurement method and resolution. The Seismic Test strikes the pile head and measures the reflected wave — fast and economical, but hard to interpret in large piles. CSL requires tubes to be installed in advance, but gives a far more detailed picture inside the cross-section.
← Scroll to see the full table →
| Topic | CSL | Seismic Integrity Test |
|---|---|---|
| Principle | Sonic waves between tubes cast in the pile | Wave reflection from a hammer blow at the pile head |
| Advance preparation | Tubes must be installed before concreting | None — test directly at the pile head |
| Best for | Large-diameter bored piles | General piles / driven piles |
| Resolution | High — locates defects | Moderate — overall continuity |
| Speed / cost per pile | Slower and higher | Faster and more economical |
| Reference standards | ACI PRC-228.2 guidance + ASTM D6760-16 procedure (withdrawn 2025) | DPT 1551-51 (method per ASTM D5882-16, withdrawn by ASTM in 2025) |
In short, the Seismic Test suits testing large numbers of piles for initial screening, while CSL suits large or critical piles where the internal concrete quality must be confirmed with confidence. Many projects therefore use the two methods together.
See Seismic Integrity Test for details and an overview of all methods in the Complete Guide to Pile Testing Methods. If the piles are already built and their length is unknown, see Parallel Seismic Test
How Much Does Cross Hole Sonic Logging Cost?
CSL has no fixed price. The fee depends on the number of piles, the number of tubes per pile, the pile size and depth, and the distance to the site. Contact us for a free quotation within 24 hours based on the actual project conditions.
The cost per pile for CSL is higher than for the Seismic Integrity Test, which is about THB 100–150/pile, because the probes must be lowered and raised through every tube pair in each pile. Supply and installation of the access tubes is another item that should be agreed clearly at the specification stage — whether or not it is in the piling contractor's scope.
Note: this price excludes travel and equipment mobilization, which are charged extra according to the distance to the site. SPN provides Cross Hole Sonic Logging as part of its pile testing services; see the Pile Testing Services page for details.
Summary
Cross Hole Sonic Logging is one of the pile integrity test methods that gives the most detailed picture of a bored pile's interior, at the cost of having to plan the access tubes before concreting and of testing only the zone between the tubes. Testing today follows ACI PRC-228.2 guidance together with the procedure of ASTM D6760-16, which ASTM withdrew in 2025 for procedural review reasons — not because the method is unusable. The choice between CSL and the Seismic Test should be based on pile size, the importance of the structure and what needs to be confirmed, with the responsible engineer making the decision.
SPN Soil Engineering provides pile integrity testing — both CSL and the Seismic Integrity Test — nationwide across all 77 provinces of Thailand, with more than 20 years of experience and reports certified by a licensed civil engineer (Council of Engineers Thailand). Contact us for a free quotation within 24 hours.