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Crosshole Sonic logging test

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Cross-Hole Sonic Logging (CSL) Test

The Cross-Hole Sonic Logging (CSL) test is a more advanced method compared to conventional integrity testing. It is primarily used to assess the quality of concrete in deep foundations, particularly in cast-in-place piles.

Principle of CSL Testing

Prior to pile casting, access tubes are securely fixed inside the steel reinforcement cages. The CSL test evaluates potential defects in the pile concrete shaft by measuring:

  • First Arrival Time (FAT): The time taken for the ultrasonic pulse to travel between the transmitter and receiver.
  • Wave Energy: The strength of the received ultrasonic wave.

The wave velocity is calculated using the formula:

This calculated velocity is then compared to the expected velocity of sound waves in concrete. Lower wave speeds and weak energy signals indicate potential defects, such as voids, honeycombing, or inclusions within the concrete pile.










 Test Procedure

1. Preparation of Sonic Tubes:
    • Sonic tubes are securely attached to the steel reinforcement cage before casting.
    • Tubes must be free from rust and debris to ensure proper bonding with concrete.
    • The spacing between tubes should be uniform.
    • Tube shoes are typically used to seal the bottom openings of the tubes, preventing the ingress of concrete or mud.
    • The top of the tubes is sealed temporarily until the test is conducted.


Figure 1



2.Filling the Tubes:
    • Before testing, the sonic tubes are completely filled with water.
    • If any blockage is present, it must be cleared before proceeding.
3. Probe Insertion:
    • Two probes (one transmitter and one receiver) are inserted into separate tubes.
    • The transmitter probe is placed in the first tube, while the receiver probe is placed in the second tube.
4. Testing Process:
    • An ultrasonic pulse is transmitted from the transmitter probe to the receiver probe.
    • Both probes are raised simultaneously at a controlled rate using an accurate winch system.
    • Measurements are taken at every 50 mm depth interval.
5. Data Recording & Analysis:
    • The transmitted wave characteristics as a function of depth are displayed on a computer screen in real time.
    • The data is recorded for later analysis.


                                      
                                                                           Figure 2


Analysis of Results

The test results include two key parameters:

  1. First Arrival Time (FAT) or Wave Speed: Measures the time taken for the pulse to travel between the probes.
  2. Wave Energy: Assesses the signal strength received by the probe.

While wave speed analysis can sometimes be affected by variations in the tube spacing, wave energy analysis provides a more reliable and accurate assessment of pile integrity. A significant reduction in wave speed and energy indicates potential defects such as voids, discontinuities, or weak concrete zones.



Based on test result pile shaft integrity can be classified into the following:



Good
FAT increase0-10%
Energy reduction <6 db
Questionable
FAT increase10%-20%
Energy reduction <9 db
Poor/flaw
FATincrease21%-30%
Energy reduction 9 to 12 db
Poor/defect
FATincrease31%
Energy reduction >12s db


Number of Sonic Tubes

The required number of sonic tubes increases as the pile diameter increases. Figure No. 3 illustrates the recommended number of sonic tubes based on different pile size ranges and their testing arrangement.


Figure 3









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