Use Of Gpr In 2D And 3D Imaging Of Bridge Footings And Scour Studies

GPR was used at two different Massachusetts bridges to help assess the stability of their foundations. At<br>one highway bridge, GPR was used to confirm the lateral extent of a bridge footing, while at another it was<br>used to help evaluate the extent of river scouring at the bridge’s piers.<br>During the bridge footing study, data were collected continuously along a 2-foot survey grid using a GSSI<br>SIR System-2 and monostatic 900, 400, and 200 MHz antennas. CMP gather measurements were made<br>using bistatic 400 MHz antennas with the intent of producing CMP stacked sections of each survey line in<br>the grid and imaging the footing using tomography.<br>Because of its unexpected shallow depth, the 900 MHz antenna provided the best 2D resolution of the<br>bridge footing’s transverse and longitudinal sections. 3D data imaging from the data collected continuously<br>failed to produce a clear image of the bridge footing, although this is partially attributed to a heterogeneous<br>mixture of metal scraps, boulders, and cobbles in the topsoil covering the bridge footing. The CMP gathers<br>helped establish the interval velocity through the topsoil, and hence give accurate depths to the footing.<br>However, because CMP gather measurements were very time-consuming using only two antennas and 3 cm<br>offsets, we were unable to collect a sufficient number of CMP gathers to produce a meaningful CMP<br>stacked section. Imaging the bridge footing using radar tomography in the future may provide the highest<br>resolution 3D data, but is currently impractical and costly using a two-antenna array.<br>A monostatic 200 MHz antenna was used to map sediments up to 3 feet below the Charles River waterbottom.<br>Data were collected continuously between, and upstream from, existing bridge piers along survey<br>lines spaced approximately lo-feet apart. The water velocity, and hence the depth to the river-bottom, was<br>derived from existing Cole-Cole Distribution Models (Cole and Cole, 1941) assuming a resistivity of 1,000<br>ohm-meters and a temperature of 55 F. Because of the strong water-bottom reflector and an adequate line<br>spacing, 3D images derived from compiling 2D data were adequate in defining at least one potential area<br>of scour immediately upstream from a bridge pier. Due to a lack of velocity information below the water-<br>‘bottom, however, other areas which had been potentially scoured and replaced with fine river-bottom<br>sediments may not have been identified. The application of radar tomography, with use of a multi-antenna<br>array, could enable the determination of river sediments velocities, and help more reliably identify those<br>areas which have been scoured and subsequently filled in by fine river sediments.