Applications of NDT methods for the determination of unknown bridge foundation depths

摘要

This paper presents experimental test results of four nondestructive test (NDT) methods applied for the determination of unkuown depths of bridge foundations. The authors completed a research study sponsored by the National Cooperative Highway Research Program (NCHRP) for the determination of unknown depths of bridge foundations. In addition, the authors were involved in aplying the results of this research study in many consulting projects. A comprehensive evaluation was made of potential NDT technologies that have relevance to this problem including surface and borehole methods, of which only the surface-based Sonic Echo/Impulse Response (SE/IR) and Flexural Wave methods for timber piles, and the borehole-based Parallel Seismic (PS) and Induction Field methods had been previously used to determine foundation depths. The newly investigated methods by Olson included the Ultraseismic(US), Spectral Analysis of Surface Waves (SASW), Borehole Radar and Borehole Sonic methods. Discussed in this paper are results from four NDT methods performed on bridges with known foundation as part of the NCHRP project. Discussed also are results from an independent consulting investigation performed by the authors to determine the unknown depths of bridge foundations in Connecticut. The four methods discussed are the surface methods of Spectral Analysis of Surface Waves (SASW) and Ultraseismic (US), and the borehole methods of Parallel Seismic (PS) and Borehole Sonic. This study represents a new application of the US method, which was developed at Olson Engineering, a new application of the SASW method, which predicted the depth of a bridge abutment in Connecticut with an excellent agreement with the depth predicted by the PS method, and a limited research on the Borehole Sonic method. The US method is an extension of the SE/IR and Flexural Wave methods and requires exposed parts of the substructure to track wave arrivals at many receiver locations. The applicability of the SASW method is limited by the access to the top of the abutment and the lateral extent of the abutment. The use of clamped 3-D geophones in PS tests improve the data quality as compared to the conventional use of suspended hydrophones in cased, water filled boreholes. In case of geophone use, the entire waveform is used in the data analysis including first arrival of compression waves and subsequent events such as diffraction of waves from the tip of the foundation. The Borehole Sonic showed promising results on wall-shaped bridge substructures.