Nondestructive Assessment Of Pile Tip Elevation Using Flexural Waves

For approximately 6000 of Georgia’s 14,500 bridges, pile tip elevations are unknown because design and<br>construction records no longer exist. This lack of information is critical because it is diEcult to determine the<br>capacity of these piles - particuhuly in the presence of scour. The unknown bridge foundation problem is an<br>important problem for federal and state agencies and has been the focus of several studies (Olson et al., 1995;<br>Douglas and Halt, 1993) The unknown bridge foundation problem is one in which either the type and depth of<br>foundation is unknown, or the type of foundation is known but the depth is unknown. Although soil borings and<br>other intrusive tests are capable of determining pile tip elevations, the time and cost of performing these tests on a<br>large number of bridges is prohibitive. Nondestructive tests are an effective alternative for assessing pile tip<br>elevations.<br>This study focuses on bridges which are supported by exposed pile foundations that have unknown lengths. The<br>purpose of this study is to evaluate the nondestructive use of flexural waves to determine the length of these bridge<br>pile foundations. Many nondestructive test methods rely on the use of longitudinal waves excited axially down a<br>pile. This type of testing is not feasible since the bridge superstructure prevents access to the top of a pile. Although<br>the theory behind flexural wave testing is more complicated than that of the traditional longitudinal wave testing,<br>flexural or bending waves can be excited laterally on the side of the pile with no physical interference from the<br>bridge superstructure. The pile in most cases can be modeled as a long slender member since its ratio of length to<br>section depth ratio is large. The propagation of the flexural waves within the beam is a function of its length, mass<br>density, moment of inertia, elastic modulus and end conditions. When dealing with bridge piles, the primary<br>unknown is the embedment length since all of the other variables can be assumed or measured.<br>The nondestructive test method developed in this study uses modal analysis techniques to interpret the pile length.<br>A three-step approach is used. First, the response model of the pile is determined by measuring a set of frequency<br>response functions for the pile. Second, the modal model consisting of the natural frequencies, modal damping<br>parameters, and mode shapes is calculated from the response model. Finally, the spatial model is determined from<br>which the embedded length of the pile can be obtained.