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Crosshole Sonic logging test


Cross-hole sonic logging test is more advanced than the integrity test. Similarly, it is used to examine pile concrete quality. Before pile casting, access tubes shall be fixed inside steel cages. The defects in pile concrete shaft can be determined by measuring the time (FAT: first arrival time) between transmitting and receiving of ultrasonic wave and the energy of received wave between transmitter and receiver. The velocity of the wave is equal to the distance between transmitter and receiver divided by FAT. Then the calculated velocity compared to wave velocity in concrete. Weak wave energy and lower speed spring from defects in concrete piles such as discontinuity or voids.











 Test procedure:

1.sonic tubes shall be attached to the steel cage. Attached sonic tubes shall be free from any rust or dirt, so strong adhesive with concrete attained. The spacing between tubes shall be equal. Tube shoe usually used to seal the bottom opening of tubes to avoid ingress of concrete or mud. Similarly, the top of the tubes shall be sealed temporarily until conducting the test. 


Figure 1


3.before beginning with test sonic tubes shall be filled with water, and in the event of a blockage, it shall be cleared before beginning with the test. Two probes will be lowered inside tubes. The transmitter probes will be lowered in the first tube, and at the same time, the receiver probes will be lowered in the second tube.

4. ultrasonic pulse will be transmitted between both probes. Then both probes will be raised up at the same time and level by an accurate winch which measures the depth of probes inside the tube. The measurement shall be taken for each 50 mm.

5. The transmitted characteristics as a function of depth will be displayed on the computer screen, and the data will be recorded and recovered later.


                                      
                                                                           Figure 2


 Analysis of results

The obtained results shall contain the First Time Arrival or wave speed and the energy of wave relative to the depth of the pile. Analysis based on wave speed may produce less accurate results related to change in spacing between pair of tubes under testing. Thus, evaluating pile based on wave energy is more accurate and providing more reliable results.





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



Good
FATincrease0-10%
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 number of sonic tubes increases with increasing size of piles. Figure no:3 showing number of sonic tubes for pile size ranges. Also, the arrangement of testing.

Figure 3






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