SONIC LOGGING - CONSTRUCTION FIELD

CONSTRUCTION FIELD

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METHOD STATEMENT FOR SONIC LOGGING

General

Sonic logging is a technique used to detect defects in the concrete structures such as bore piles, caissons, barrettes and diaphragm walls.

The method involves the installation of 4 numbers of vertical tubes, preferably steel, in the shaft before placing the concrete. The tubes are filled with clean water and some time after the concrete has set, a transmitter is lowered in one tube and a receiver in a neighboring tube. The transmitter emits pulses of ultrasonic waves while the receiver functions to detect the arrival of the waves. Ultrasonic waves passing through the structure are delayed or absorbed depending on the physical properties of the structure. The properties affecting the travel time between the tubes include:

• The distance between the tubes
• The wave speed
• The camping caused by the concrete

Plant and Equipment

The field equipment consists of 3 components

-          A signal transmitter

-          A signal receiver

-          A sonic pulse generator and logger

Installation of Sonic Logging Tubes

The sonic logging tubes, preferably of steel of internal diameter between 40 to 60 mm will be securely fixed to the reinforcement cage. Care will be taken to ensure that each of the tubes are aligned in a straight line, spaced at a constant distance and oriented vertically within the pile.

The ends of the tube are sealed and the joints must be water tight to ensure that there is no ingress of grout or slurry.

Conducting the Sonic Logging Test

For the test, the tubes will be filled with clean water that will serve as a couplant between the probes and the tubes.

Relevant information on the structure to be tested and the project is keyed into the system. The information includes:

• Project name
• Pile number
• Profile number
• Depth of logging
• Approximate distance between tubes for profile
• Date of test

The transmitter and receiver will then be lowered into the tubes to check the depths and calibrate the system.

After checking that the lengths of the tubes are clean, the transmitter and receiver are then positioned at the lowest accessible location. The analyzer is then activated to emit ultrasonic waves from the transmitter. Electrical pulses generated by a pulse generator are converted to ultrasonic waves by the transmitter probe and are reconverted back to electrical signals by the receiver probe. After conditioning, signals are digitized and stored. The encoder of the meter wheel, acting as a depth sensor provides the vertical (Y) axis for the storage display and the signal is displayed on the horizontal (X) axis of the storage display. Variations in delay and signal strength indicate the presence of non-homogenous concrete.

The transmitter and receiver are then raised simultaneously for the entire length of the tubes while the transmitting and receiving of signal takes place.

The logged data is then stored on the memory device on the logger and retrieved for processing and analysis in the office.