Concrete piles and drilled shafts are an important category of foundations. Despite their relatively high cost, they become necessary when we want to transfer the loads of a a heavy superstructure (bridge, high rise building, etc.) to the lower layers of soil. Pile integrity test (PIT), or as ASTM D5882 refers to it as low strain impact integrity test, is a common non-destructive test method for the evaluation of pile integrity and/or pile length. Pile integrity test can be either used for for forensic evaluations on existing piles, or quality assurance in the new construction. The integrity test is applicable to driven concrete piles and cast-in-place piles.
What is Pile Integrity Test?
Low strain impact integrity testing provides acceleration or velocity and force (optional) data on slender structural elements (ASTM D5882). Sonic Echo (SE) and Impulse Response (IR) are employed for the integrity test on deep foundation and piles. The test results can be used for evaluation of the pile cross-sectional area and length, the pile integrity and continuity, as well as consistency of the pile material; It is noted that this evaluation practice is approximate.
The PIT method works best for column type foundations, such as piles and drilled shafts. The method provides a rapid and simple tool for evaluation of a number of piles in a single working day.
How to Perform PIT?
The pile head surface should be accessible, above water, and clean of loose concrete, soil or other foreign materials resulting from construction. Any type of contamination should be removed (using a grinder) to reach to solid and sound concrete surface. This step is so vital, because then connection between the sensor and concrete should be solid (firm contact). The location of the sensor should be selected away from the edges of the pile. The integrity testing should be performed no sooner than 7 days after casting or after concrete strength achieves at least 3/4 of its design strength, whichever occurs earlier.
A hammer (with or without force measurement unit) is used for impacting pile top; the impact should be applied axially with the pile. Motion transducer should be capable of detecting and recording the reflected echos over the pile top. Acceleration, velocity, or displacement transducers can be used for this purpose. At the minimum, acceleration transducer should have an Analog to Digital Converter with 12 bit resolution; and a Sample Frequency of at least 25 KHz.
The distance between the impact location and the sensor should be no larger than 300 mm. Several impacts are applied to the top of the pile. The reflected echos are then recorded for each individual impact. As an alternative, the average can be determined and used. As mentioned earlier, acceleration transducer can be used for the purpose of this test. In this case, the apparatus shall provide signal conditioning and integrate acceleration to obtain velocity. The apparatus shall balance the velocity signal to zero between impact events.
What Information Does Pile Integrity Test Provide?
The Pile Integrity Test (PIT) provides information about:
+ Continuity of pile
+ Defects such as cracks
+ Soil incursions
+ Changes in cross section
+ Approximate pile lengths (unless the pile is very long or the skin friction is too high).
Limitations of Pile Integrity Test
The PIT provide an indication of soundness of concrete; however, the test has certain limitations:
+ PIT can not be used over pile caps.
+ It does not provide information regarding the pile bearing capacity.
+ Test should be undertaken by persons experienced in the method and capable of interpreting the results with specific regard to piling.
+ This test is not effective in piles with highly variable cross sections
+ It is not effective in evaluating sections of piles below cracks that crosses the entire cross sectional area of the pile.
The primary shock wave which travels down the length of the shaft is reflected from the toe by change in density between the concrete and the sub strata. However, if the pile has any defects or discontinuities within its length these will set up secondary reflections which will be added to the return signal.