Deficiency of concrete materials is a common problem in Bored Pile construction. Integrity Problems of Concrete Piles is a major problem n in large diameter piles. This is specially true, because integrity problem can affect the load distribution properties of concrete piles. Piles and deep foundations transmit top structure loads to strong substrate through friction and/or standing on bed rock. This load transmission is performed effectively when no major issue is accompanied by substructure such as important cracks, voids, soil intrusion, etc. The consequences of such deficiencies might be challenging when the top structure stands on weak substructure (pile and/or foundation). This might vary from partial settlement to significant damage or collapse of the top structure.
How To Evaluate Integrity Problems?
ASTM D5882 introduces a standard method for integrity testing of piles and deep foundations using low strain impact. This method includes generating acoustic waves using a hammer impulse and transmission of acoustic probe waves through the test area (i.e. piles or deep foundations). The transmitted waves are reflected off boundaries (e.g. pile toe) and internal anomalies, recorded by a transducer (e.g. accelerometer, geophone) on the tip of pile. The recorded signal is further analyzed using different signal analysis features (e.g. FFT, Low-Pass Filter and High-Pass Filter, etc.) to find out possible anomalies, and pile toe for pile length testing.
A knowledge about possible sources of deficiencies increase pile testing resolution. Like other non-destructive testing methods, the resolution of pile testing results significantly increase when site observations (e.g. soil profile, design length of pile, concrete quality) are available. The following briefly presents some major issues that should be considered during pile testing and signal analysis.
Integrity Problems of Concrete Piles
1- Major Cracks and voids
Acoustic probe waves are fully reflected off major cracks and voids boundaries. This is because of high acoustic impedance between concrete and empty space. Therefore, acoustic waves provide no information regarding lower levels.
2- Soil Intrusion
During pouring drilled borehole, the hole walls slide into hole and causes gaps over length of pile. It is obvious there is acoustic impedance between soil and hardened concrete, but the difference is not as high as concrete and empty space. So it is not easy to distinguish acoustic waves reflected off soil intrusion from pile toe.
3- Major Change in Soil Profile
Pile integrity testing relies on compression deformation over length of pile. The wave travelling along the pile loses energy as a results of pile friction and surrounding soil. Major change in soil profile (e.g. change from loose soil to high strength soil) may highly attenuate probe wave energy, causing a situation where no toe reflection is visible. This may happen in very long pile.
Rapid change of cross section along the pile causes probe waves are partly reflected. Depending on necking or bugling, it is expected the polar shape of probe signal changes in location of necking or bulging.
5- High Roughness
very high roughness along the pile may highly attenuate the probe wave energy, making difficult to detect the location of pile toe over the recorded signal. Applying low-pass filter for signal analysis might reduce the noise, increasing the resolution of the pile toe reflection.
Other Challenges of Integrity Testing
Some concrete piles stands on bed rock. The acoustic impedance between concrete and certain rocks is not significant. This is important in long piles where the toe reflection is weak because of high energy attenuation of probe waves.