APPLICATION OF ELECTRICAL RESISTIVITY IMAGING TO EVALUATE THE GEOMETRY OF UNKNOWN BRIDGE FOUNDATIONS

Abstract

Electrical Resistivity Imaging (ERI) was utilized to image the subsurface at two bridge sites owned by the Pennsylvania Department of Transportation. Data were acquired using 28 electrodes and an Advanced Geosciences (AGI) SuperSting R8/IP resistivity meter, which allowed multiple array configurations (e.g., dipole-dipole, Schlumberger, and Wenner). Site conditions, hardware configuration, and testing procedures are presented, followed by a discussion of data analysis and interpretation. The purpose of the ERI testing was to determine the bridge foundation geometry, including configuration, depth to bottom, and dimensions, at two bridges where design information was available to determine the feasibility of using this method for bridges with unknown foundations. Typical surface methods for evaluating unknown foundations are often unable to recover information from below a pile cap. ERI presents an opportunity to address this limitation and provide geometry information without the need for drilling operations. The ERI results were promising at one of the sites and provided a reasonable estimate of pile cap dimensions (8.5 m x 7.0 m) compared to actual dimensions (7.3 m x 5.5 m). ERI also detected the presence of battered piles at an approximate 1:2.5 batter angle, which compared favorably with the 1:5 batter angle shown on the foundation plans. Interpretation of the ERI images at the second site proved problematic. This was likely due to issues with data inversion when creating the resistivity section since the resistivity values of the soils were extremely low throughout the site. Moreover, foundation bottom predictions were negatively impacted at both sites by poor signal to noise ratio likely resulting from high levels of background electrical noise from nearby utilities and fences. Despite these issues, ERI shows potential as a tool for evaluating unknown bridge foundations, particularly in less urban areas where noise levels could be more manageable.