NON-INVASIVE CHARACTERIZATION AND MONITORING OF EARTH EMBANKMENTS USING ELECTRICAL RESISTIVITY TOMOGRAPHY (ERT)

Earth structures, such as embankments, require ongoing monitoring and maintenance to identify potential failure zones and to compensate for the effects of settlement. Extreme weather events leading to prolonged periods of desiccation or saturation are becoming more frequent and threaten embankment stability. Recent research has shown that geoelectrical methods can be successfully applied to the investigation of these structures (Jackson et al., 2002; Sjodahl et al. 2006). In this paper we further develop electrical resistivity tomography (ERT) as a non-invasive tool for characterising and monitoring earth embankments. A study is described in which ERT was applied alongside intrusive techniques to investigate and monitor a section of Victorian Great Central Railway embankment, near Nottingham, UK. A number of modes of deployment were considered including linear 2D ERT arrays both parallel and perpendicular to the longaxis of the embankment and 3D imaging arrays. The resulting ERT images, when calibrated using intrusive geotechnical testing and core samples, revealed the spatial variability of the embankment soils. Parallel ERT sections were used to identify major discontinuities between material types at locations associated with poor track geometry. Perpendicular ERT sections also revealed significant internal heterogeneity, and were used to monitor seasonal changes in the moisture content within the embankment. ELECTRICAL SURVEY DESIGN The test area (Figure 1) covers a 100 m length of embankment (y = -20 to 80 m), which includes the crest and the two flanks (x = 0 to 31 m). Geotechnical tests (trial pits, boreholes & cone penetration tests), described by Gunn et al. (2007), were restricted primarily to the crest of embankment (x = 8 to 24 m). However, resistivity sections extended the full length and width of the area (Figure 2). Prior to the field survey synthetic modelling studies were undertaken to quantify the effects of a conductive metal rail on electrical resistivity tomography (ERT) measurements made along a line running parallel to it for a range of standard ERT electrode configurations (e.g. Dahlin and Zhou, 2004); it was considered that conductive rails could potentially short-circuit injected current, thereby distorting the inverted resistivity images. These studies revealed that the dipole-dipole array configuration was less effected by the conductive rails than other standard array types; moreover, it was found that given a sufficiently high contact resistance between the rail and the ground the influence of the conductive rails on the inverted resistivity image would be small; in this case assessment of the field data showed that rail-ground contact resistances were sufficiently high to have eliminated the problem of current short circuiting through the rails. All resistivity data were collected using the dipole-dipole array configuration, and were inverted using Res2DInv software (Loke, 2006). Data collected parallel to the embankment long-axis (y) used electrode arrays with dipoles of 1.5, 3, 4.5 and 6 m, and unit dipole separations of 1 to 8, whilst measurements collected on the perpendicular sections employed dipoles of 1, 2, 3, and 4 m, and unit dipole separations of 1 to 8. Resistivity data collected during an initial reconnaissance survey of the site during September 2005 is shown in Figure 3. Figure 2. ERT survey line positions. EMBANKMENT CHARACTERISTICS Intrusive investigations within the test area have shown that the embankment is approximately 5.5 m high, and is underlain by Mercia Mudstone bedrock. The embankment comprises weathered mudstone material between y = -20 and 40 m, which was excavated from a nearby cutting and is characterized by relatively low resistivities of < 100 m (Figure 3, ERT line x = 12.5 m). Beyond y = 40 m the composition of the embankment changes to gravel, sand and silt, and is characterized by resistivities significantly in excess of 100 m. At the interface between these two distinct lithologies significant distortions in track geometry can be observed, which are likely to be the result of differential settlement. The compositional changes shown in the resistivity section parallel to the embankment long-axis are also seen in the perpendicular sections at y = 0, 20, 40 & 60 m (Figure 3). However, these sections also show significant features associated with variations in moisture content within the embankment. In particular, the flanks are dominated by relatively high resistivities due to moisture loss from evaporation and transpiration during the preceding summer months.