The Use of Two Dimensional (2D) Multichannel Analysis of Surface Waves (MASW) Testing to Evaluate the Geometry of an Unknown Bridge Foundation

The evaluation of subsurface geometry continues to be a major issue for the thousands of bridges across the country with unknown foundations. These bridges pose a threat to the vitality of the nation's infrastructure, particularly given the difficulties in establishing their vulnerability to scour. In addition, unknown foundations present a major barrier to foundation reuse and rehabilitation. In many cases, geophysical methods can provide a suitable measure of confidence regarding the geometry of these unknown foundations. The multichannel analysis of surface waves (MASW) technique presents a number of potential advantages to evaluate unknown foundation geometry, including increased signal to noise ratio and ability to resolve stiffness inversions. However, very few studies have examined the use of MASW to specifically evaluate unknown bridge foundations. This gap in the literature may be related to concerns regarding vertical and lateral resolution. For example, vertical resolution decreases as the surface waves sample deeper material and the phase velocities are determined by materials over a greater depth range. Additionally, MASW largely averages out lateral variations in stiffness because a layered earth model is used in the data processing. MASW was utilized in this study to determine the two-dimensional (2D) shear wave velocity (V-s) profile over a bridge foundation. The purpose of this testing was to examine the viability of MASW when evaluating unknown bridge foundations, particularly with regards to vertical and lateral resolution. Generally, the results demonstrated that MASW is capable of providing a conservative estimate of depth. MASW proved more capable in accurately assessing the lateral dimension of an embedded foundation, though the results were a function of the survey parameters used during 2D testing.