Nondestructive Assessment Of Pile Tip Elevation Using Flexural Waves

Abstract

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