Crosshole Sonic Logging (CSL)
The method, Crosshole Sonic Logging, can be applied to verify the homogeneity and integrity of concrete in cast-in-situ shafts. The size and location of anomalies or defects in the concrete can be determined by sending ultra-sonic pulses between access tubes installed on the rebar cage.
Crosshole Sonic Logging provides information about:
- Identification of anomalies or defects within the shaft.
- Variations in concrete quality.
Measuring principle
The CSL-test is performed by sending ultra-sonic pulses through concrete from one probe to another (probes located in parallel access tubes installed vertically along the rebar cage), moving both the transmitter- and receiver probe through the tubes in the same horizontal plane at an appropriate speed. The measurements are performed for all combinations of paths between the access tubes in the full length of the shaft.
Both the time between pulse generation and signal reception (First Arrival Time or FAT) and the strength of the received signal give a relative measure of the quality of concrete between transmitter- and receiver probes. Dividing the distance between transmitter- and receiver probes by the FAT value yields the approximate concrete wave speed which also is an indicator of concrete quality.
Equidistant tubes and uniform concrete between them yield consistent FATs with consistent wave speeds and a good signal strength. Anomalies such as contaminated or weak concrete, voids or inclusions exhibit delayed arrival times and/or reduced signal strength.
Access Tube Preparation
The number of access tubes for each shaft and the time of testing shall be specified for each project.
In general one access tube for every 0.25 to 0.30 m of drilled shaft diameter however, a minimum of three and a maximum of eight access tubes is recommended for installation in each shaft. The access tubes shall be located at equal distance along the rebar cage inner perimeter and be arranged parallel throughout their entire length.
The access tubes shall be straight, any couplers tightened, and they shall be sealed at the bottom with permanent end caps and fitted with removable caps at the top to prevent contamination with debris. The tubes shall be terminated 1.2 to 1.5 meter above platform (access) level from where the test procedure is performed.
The access tubes shall be filled with clean freshwater within an hour after casting and refilled prior to performing the test procedure. Furthermore, the tubes shall be checked for debris by e.g., a dummy probe and if required needed be cleaned by hydro jetting to ensure a complete set of measurement paths and the best conditions for acquiring high-quality signals for a subsequent effective analysis procedure.
Information about access tube as-built configuration around the rebar cage perimeter as well as shaft-, access tube- and ground level elevations shall be provided prior to performing the test procedure to aid planning and subsequent analysis.
The Crosshole Sonic Logging should not be performed earlier than three to seven days after casting depending on shaft diameter and the concrete strength development over time.
Analysis and Reporting
In the analysis of Crosshole Sonic Loggings the horizontal path between two access tubes along their entire length is called a ‘profile’, hence a shaft prepared with three access tubes allows for three profiles, while a shaft with four access tubes will provide six profiles – etc. Each profile holds data of FAT values and signal strengths with a few centimeters of vertical resolution.
The concrete quality of each profile is evaluated based on the measured FAT-values, or rather the derived wave speed and the measured signal amplitude, in their own right as well as in combination. If an anomaly is observed in a profile at a given elevation, all profiles are compared to identify the location and extent of it.
The resolution of the shaft integrity evaluation by means of Crosshole Sonic Logging is typically around 5 cm. The accuracy with which the extent and location of anomalies can be stated is therefore highly dependent on the quality of the as-built information provided about access tube configuration and elevations of platform, access tube heads, shaft base and cut off levels.
A preliminary evaluation of the results from the analysis can be forwarded, as soon as the analyses are performed. The final report is subsequently prepared including the supporting appendices. The integrity of the shaft is evaluated based on the stipulated project verification criteria.
References
NOTE: Crosshole Sonic Logging is currently not covered by Danish or European standards.
Standards:
ASTM D6767 – 16: Standard Test Method for Integrity Testing of Concrete Deep Foundations by Ultrasonic Crosshole Testing. https://webstore.ansi.org/Standards/ASTM/ASTMD676016
Scientific journal publications about Crosshole Sonic Logging:
Likins, G. , Rausche, F. , Webster, K. , Klesney, A. (2007). Defect Analysis for CSL Testing. Geotechnical Special Publication No. 158 Contemporary Issues in Deep Foundations, Denver, CO. https://www.pile.com/wp-content/uploads/2017/03/GeoDenver2007_DefectAnalysisCDROM.pdf
Likins, G. , Webster, S. , Saavedra, M. (2004). Evaluations of Defects and Tomography for CSL. Proceedings of the Seventh International Conference on the Application of Stresswave Theory to Piles, Selangor, Malaysia, pp 381-386. https://www.grlengineers.com/wp-content/uploads/2004/08/SW2004-046-Eval_of_Defects_and_Tomography_for_CSL.pdf
Sellountou, A. et al (2019). Terminology and Evaluation Criteria of Crosshole Sonic Logging (CSL) as applied to Deep Foundations. Deep Foundations Institute. https://www.grlengineers.com/wp-content/uploads/2022/01/Terminology-and-Evaluation-Criteria-for-CSL-of-Deep-Foundations-Final-Oct-2019.pdf