Sonic logging is based on measuring the propagation time of an ultrasonic sonic signal between two vertical metal tubes cast into the pile during construction. Concrete containing soil inclusions, gravel, bentonite or honeycombing has a much lower propagation velocity so that the presence of these irregularities is immediately obvious.
In homogenous concrete, free of defects and variations in quality, the velocity ultrasonic waves is constant and in the order of 4000 m/sec. Concrete containing soil inclusions, gravel, bentonite or honeycombing etc., has a much lower propagation velocity so that the presence of these irregularities is immediately obvious. Sonic Coring is based on measuring the propagation time of an ultrasonic sonic signal between two vertical tubes cast into the pile during construction These tubes are filled with water to act as a coupling medium.
Methodology
Testconsult uses the very latest sonic logging equipment – the SC-XT2000, developed by Testconsult Equipment. The system comprises a rugged site computer with a data acquisition module built in, an electronic, hand operated winch, which can control the whole system remotely, and piezo electric emitter and receiver probes on cables. Software analysis includes 2D & 3D tomography modules for quick visualisation of defects.
To carry out a test, the probes are first lowered to the toe of the pile in adjacent tubes. Cables are then raised using the electronic logging winch. As the winch wheel turns, the emitter sends an ultra-sonic signal every 20mm of vertical movement, which travels between transducers in adjacent tubes. The signal, which is about 50 – 60KHz, is captured and stored as a time/amplitude trace every 1cm of the pile length. Thus for a 20m pile there would be over 2000 measurements.
Each trace will consist of a time lag followed by the received signal, the time lag representing the time taken for the signal to travel from the emitter to the receiver through the concrete between tubes, being dependent on the path length and the concrete property between the tubes.
Each signal is modulated to a series of black-and-white lines, to build up a waterfall plot or sonic profile of the pile over depth for each profile measured. Any defects show up as an increase in signal transit time, between the affected tubes. The operator can choose to display the first arrival time (FAT) and signal energy simultaneously if needed.
The number of tubes required is typically four for piles between 700 – 1200mm diameter. This enables six profiles to be taken, including two across the core of the pile. In smaller diameter piles however, the number of tubes is restricted and may be only allow three tubes to be used. It should be noted in this case that information about the core of the pile is limited.
Testconsult has over 25 years experience with sonic logging and was the first company in the UK to utilise this method. We use the latest SC-XT2000 sonic logging system, which can view and store each individual impulse signal, display a waterfall plot, calculate and display first arrival time (FAT), calculate percentage increases in FAT and percentage relative change in signal energy. The software used can also produce Waterfall Plots. Pile cut-off level can also be marked on the plot to if it is known.