Cross Hole Sonic Logging
Geo Dynamics is again at the forefront in introducing yet another technology in India that is getting popular worldwide. In 2006, we demonstrated that it is possible to generate a 3- dimensional image of the pile with sonic logging and using Tomography to the Indian deep foundation industry. We also demonstrated that the method can be applicable to any foundation type as long as pipes are left inside the pile before concreting.
The method is particularly useful for large diameter pile foundations for major structures like river bridges, towers, initial piles (before load testing). It can also be used for slurry walls or wall panels.
The Cross Hole Analyzer determines the quality and consistency of concrete between pairs of steel tubes pre-installed in bored piles, wall panels, concrete foundations. The output is in the form of waterfall map along with wave speed and energy or amplitude plot. Tomography software gives a good 2-D or 3-D image to visualize defects inside the pile for future corrective action. Typically one tube is installed for every 300mm diameter of pile and for diaphragm wall panels, the configuration is decided in advance based on the size of each wall panel. PVC tubes should be avoided as they lead to debording issues.
Single Hole Sonic Logging
Single Hole Sonic Logging is mostly used for smaller diameter piles where using CSL testing is difficult as multiple tubes cannot be installed inside such piles. Typically SSL is used for pile diameters upto 600mm by installing a 38mm-50mm steel pipe inside the steel cage before concrete is cast. Although limited information is available due to only one tube, the method may determine major defects inside the pile and if defect is closer to the tube. Currently, many specifications prefer SSL testing as a part of their contract documents. Geo Dynamics has the complete assembly and expertise to execute SSL jobs.
The method is obviously superior to the low strain integrity testing as it gives an image between two pairs of tubes. Geo Dynamics has done extensive testing at Bandra Worli Sea Link Project, Mumbai, Delhi, Cochin and Lucknow metro projects. Our current recommendation is to conduct 25% tests with this method for land piles and 100% tests for large diameter piles of 1.5m and above for both offshore and onshore projects.
Geo Dynamics complies with all requirements of ASTM D6760 and various international codes that specify these tests. There is no Indian code currently for this test and no plans yet by the BIS to introduce CSL testing as a part of code requirements.
It should be ensured that owners and consultants get all the data including water fall diagram during report submission. Use of sub-standard quality tubes, improper installation may lead to blockage of tubes and the result will then be obtained only upto the depth of tubes scanned. Training is required before proper data collection and interpretation of the reports which otherwise may lead to avoidable controversies.