Pseudo three-dimensional imaging of creep failure in a highway embankment using two-dimensional electric earth resistivity

In May 2017, we carried out an electrical resistivity tomography (ERT) survey of the downslope embankment of a state highway in central Illinois where creep failure has been observed for over 50 years. The fill is over 15 meters (~50 feet) at its deepest point and sits partially in an old stream bed. Water is currently conveyed through the channel in a box culvert underneath the embankment. The fill has been steadily creeping since initial construction and has caused severe damage to the highway above on several occasions. The purpose of this study was to image the failure associated with the subsurface creep in the fill and to identify the subsurface engineering works that may be interacting or moving with the creep. We surveyed 10 parallel ERT profiles using an ABEM Terrameter SAS 4000. Nine profiles were 80 meters long and one was 40 meters long, with 10 meter spacing between each profile. We used an inline dipole-dipole array with electrodes spaced 2 meters apart (with a minimum 'a' spacing of 2 meters and a maximum was 4 meters; the minimum 'n' value was 1 and the maximum was 8). These 10 profiles were processed using Geotomo's Res2DInv software. Topographic corrections were made using GPS and elevation data collected the same day as the ERT survey. The data was interpolated between the survey lines using ArcScene to create a pseudo 3-dimensional model to identify key components of the slope creep. Several underground objects were identified, including metal and plastic drainage pipes, a chimney drain, the box culvert, and possible groundwater plumes from recent rainfall events. The failure surface associated with the creep was also identified, and mapped in three dimensions.