Deficiency of concrete materials is a common problem in Bored Pile construction. This is mainly because integrity problems of concrete piles can affect the load bearing capacity and load distribution properties. 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. Integrity Problems of Concrete Piles can become a huge challenge in large diameter piles.
The inspection and evaluation of integrity problems of concrete piles are often challenging, mainly because these elements are not easily accessible for visual inspection. The process of quality control for this group of elements is very much through indirect measurement of other parameters, i.e. resistance of pile to driving or drilling, or other non-destructive testing techniques (NDT). NDT methods can shed light on this hidden part of concrete piles.
How To Evaluate Integrity Problems?
According to the ASTM D5882, pile integrity refers to qualitative evaluation of the physical dimensions, continuity of a pile, and consistency of the pile material. 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.
Integrity Problems of Concrete Piles
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.
1- Major Cracks
Major cracks can affect the quality and reliability of piles. Cracks might be a results of poor construction (precast piles), or can be a results of damages during transportation, and installation. Pile integrity can provide valuable information about the presence of such cracks in the pile. Acoustic waves are fully reflected off major cracks boundaries. This is because of high acoustic impedance between concrete and empty space. Therefore, acoustic waves provide no information regarding lower levels.
2- Major Voids
Similar to cracks, voids can affect the consistency and quality of pile materials. Voids can affect the load bearing capacity of concrete piles, as they reduce the effective cross-sectional dimension. pile integrity testing does not provide any useful information about the portion of the pile that locates underneath the major voids or cracks.
3- Soil Inclusion
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.
Necking in concrete piles can happen during casting of pile shaft in soft clay. This rapid change of cross section (as a results of necking) can be a source of integrity problem. Necking can affect the load bearing capacity of concrete piles.
While bulging may increase pile ultimate load, it is still considered as a pile defect and should be investigated. Pile integrity testing can effectively help identify bulging and necking in concrete piles.
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