Pile Integrity Testing (PIT) is a nondestructive low strain method used to determine the either the integrity or the length of the pile based on a known or assumed wave speedfor testing pile integrity in concrete piles. This method involves mounting an accelerometer on the pile top and sending a force wave through the pile using a set ofsmall hammers varying from 1 lb to 8 lb. When the hammer impact sendsprovides the stress impact wave through the pile, the accelerometer response is recorded on the PIT.
Usually, tThe output is velocity over the time scale. Typically concrete shafts at construction sites which need pile integrity assessment have steel bar reinforcements in them and these reinforcements stick out 1. 5 to 3 m10 ft above the pile top. depending upon other factors. In general PIT records obtained on piles without reinforcements exhibit a smooth record where the velocity has an initial impact pulse. Beyond this the curve tends to shift downward due to soil resistance before it reaches the toe showing a rise in the curve again.
The above described is an ideal record rarely realized under field conditions. Small variations in the pile cross sections, bulges and defects tend to change the shape of the output record. While these can be analyzed and conclusions regarding the location of the defect or bulge or excessive soil resistance or lack of soil resistance can be arrived at, distortions caused in the record due to wave traveling through the reinforcement at a different frequency and the lateral vibration caused in the steel make it difficult to analyze records and arrive at conclusions.
When athe hammer is impacted is applied on the pile top, the stress waves generated there is atravel both downward inwave going to thedown the pile, and an upward wave into the steel bars. The upward waves into the steel bars aregets reflected back and then a set of secondary waves traveling at a different frequency interfere with the primary set of waves which started out in the downward direction.
The result is a velocity record which has characteristics of both the wave through the concrete and the steel. Due to different wave speeds of concrete and steel and the different lengths of teel bars of travel due to the reinforcement sticking out of the concrete top, the measured velocity responses become very complex and make the interpretation very difficultthe output record is sometimes very inconclusive.
This paper presents the results of numerical and experimental studies to understand the effect of reinforcements. To separate the steel bar effect on the response, Records collected with two sets of accelerometers have been used to perform tests on drill shafts:, one on the pile and one on the reinforcementbar lead to some useful findings.
Possible solutions to take advantage of two accelerometer measurement have been investigated. The tThree dimensional Finite Element simulations have been usedperformed to study how the stress waves travel through both in the steel and the pile. and pile and the interaction, andThis will help in determining the effect of the hammer weight, the and impact location of the hammer and the gage mounting location on the output,. whichThese results will They indicate that a possible way to remove the effect of the reinforcement on the PIT record is to introduce a shift factor in one of the velocities and add to the other after introducing a factor.
Though this does yield better records, the factors used don’t seem to follow a pattern. We then decided to perform a three dimensional Finite Element Analysis of this problem. The study enables us to decide the hammer weight, accelerometer positions and impact location of the hammer that will yield us maximum information. This outcome will helphelp in positioning the accelerometers better and impact the hammer at the right location thereby enabling a data set which can help us remove the reinforcement effect and judge the pile more effectively.
