<title>Nondestructive determination of pile tip elevation using modal analysis</title>

Abstract

Pile tip elevations are unknown for nearly half of Georgia's 14,500 bridges. Since the length of a pile is directly related to its capacity, and ultimately, to a bridge's load capacity, the need for a straightforward, effective, and inexpensive method to determine pile embedment lengths is apparent. If the top of the pile is exposed and free to be impacted and instrumented, the task is relatively simple. The exposed pile top is impacted axially, and the resulting longitudinal wave motion is monitored and used to calculate the pile lengths. However, such piles rarely exist in bridge structures, and the more common case is that the tops of the piles are cast into the bent cap. This lack of access poses a challenging problem when trying to nondestructively ascertain the length of pile that is embedded in the surrounding soil. Although soil borings and other intrusive tests are capable of determining pile tip elevations, the time and cost of performing these tests on a large number of bridges is prohibitive. The research project described here uses flexural waves, induced by exciting piles laterally, to determine the unknown embedment lengths. Modal analysis techniques are employed to quantify the difference in modal characteristics (natural frequencies, damping, and mode shapes) of piles with differing lengths, to create a modal model of the system, and finally to back-calculate the unknown pile embedment lengths.