Evaluation Of Resistivity Imaging (RI) Method For Determining Unknown Deep Foundation Depth

Abstract

There are approximately 580,000 highway bridges in the National Bridge inventory and 104,000 of these bridges are estimated to have unknown foundations type and/or depth. Determining unknown foundation length and type of bridge foundation is becoming an important issue for State DOTs to ensure the safety of the bridge. There are several methods available to determine unknown foundation depth based on specific site conditions and type of foundation to be tested. Resistivity Imaging (RI) is a non destructive method to investigate the subsurface condition. The objective of current study was to determine unknown bridge foundation depth of three bridges at North Texas using RI method. Two of the investigated bridges were supported by driven steel H-pile and the other bridge had drill shaft as foundation. Based on RI results, unknown foundation depth were determined from the variation of resistivity below the foundation. Parallel Seismic (PS) and Sonic Echo (SE) method were also utilized for one bridge site supported by steel H pile to verify the depth determined by RI method. PS method was performed in two casing backfilled with sand and cement grout. The investigated unknown bridge foundation depths using RI method were compared with the actual foundation depth. The determined unknown foundation depths from RI method were found very close to actual foundation depth for two steel H-pile sites. However, no significant variation of resistivity was observed below the foundation of third bridge supported by drill shaft and the determined foundation depth for third bridge site did not match the actual foundation depth. PS method also predicted the depth of the foundation very close to actual foundation depth. However, the SE test was determined to be not suitable for determining the unknown steel H-pile foundation depth.