Pile Integrity Testing (PIT) is a non-destructive testing method for evaluating the unknown length and integrity of piles and deep foundations. The basic concept behind the technique is determining the velocity (required) and force (optional) response of pile induced by an impact device (normally, a handheld hammer). Pile Integrity Testing works best for long structural component such as driven concrete piles, cast-in place concrete piles, concrete filled steel pipe piles, timber piles, or slender structural columns.
The procedure for low strain impact integrity has been well established, and standardized in ASTM D5882. The test has some inherent limitations, and requires an experienced technician to conduct a successful test and interpretation of results. In this article, we will briefly describe some practical considerations regarding pile integrity testing, and obtaining reliable measurements.
Considerations in Pile Integrity Testing
1- Selecting Proper Hammer
Low strain impact integrity testing is performed using a hand held hammer. The hammer can be as light as couple of hundred grams, to relatively heavier options. Hammer can be a basic one, or instrumented (optional). The impacts induced by smaller hammer have higher frequency content, and shorter rise time. Larger hammers on the other hand, induce higher energy. Sharp and narrow input pulses are reported to be better than wider ones. However, when the size is reduced, the frequencies contained in the impact increases; these waves attenuate faster, and are tend to decrease the ability of investigating longer piles. The hammer tip should be made of material that does not damage concrete during the impact, as this will impact the test results. The use of instrumented hammer when measuring the impact force is of interest to the engineer. For example, detecting deficiencies in near top sections of the pile is better to be done by instrumented hammer.
2- Surface Preparation
A firm connection between the sensor’s tip and concrete surface (pile tip) is needed for successful application of the test method. The test surface should be prepared before performing any measurements. The pile surface should be accessible, and above water. All loose concrete, soil or other foreign materials resulting from construction should be removed from pile surface. If there is any type of contamination on the surface, it should be removed (using a grinder) to reach to solid and sound concrete surface.
3- Placement of Acceleration/Velocity Sensor
The acceleration sensor should be placed at or near the pile head using a suitable, or temporary, thin layer of bonding material (that is, wax, vaseline, putty etc.) so that it is assured that it correctly measures the axial pile motion (transducer axis of sensitivity aligned with the pile axis). For circular and rectangular cross sections, place the sensor near the center of the pile. If the diameter of the pile is larger than 500 mm, additional locations should be considered to obtain useful integrity information. The low strain impact should be applied to the pile head within a distance of 300 mm from the sensor. Make sure that the impact is applied axially.
4- Number of Impacts
ASTM D 5882 requires a minimum on 10 impacts for integrity test on each pile. The reflectograms from each impact should be displayed and/or saved. It is recommended to use the average of the 10 impacts for evaluation purpose.