ABSTRACT
Electrical resistivity is a physical property that defines how a material resists the flow of electricity. The resistivity method is therefore based on the principle that the potential drop across a pair of electrode (P1 and P2) associated with DC or low frequency current injected into the soil using another pair (C1 and C2), as shown in Figure 1, is proportional to the soil resistivity distribution, so that:
ρ = K V I* / (1)
where, ρ is the soil resistivity (Ohm ⋅ m), ΔV is the voltage difference (Volts), I is the current (Amps), and K is a geometric factor (metre) that accounts for the electrode arrangement. As the cracks are
1 INTRODUCTION
Unsaturated clay soils are inevitably subjected to wetting, drying and cracking processes that adversely impact on their engineering properties and behavior. In particular, desiccation cracks alter the macro porosity, inltration and runoff and create pathways for water that reduce soil strength and stability (Kodikara et al. 1999). However, cracks have complex patterns that are difficult to measure. Although surface crack networks can directly be described by measuring cracks geometry (Ringrose-Voase & Sanidad, 1996), or imaging cracks morphology using 2D surface imaging analysis (Vogel et al. 2005), these techniques are largely based on inadequate visual inspections. Field measurements of cracking dynamics are difficult and have largely been limited to soil pits (Bouma & Dekker, 1978), or pushing a probe wire or measuring tape into the crack (Abedine & Robinson 1971; Kishne et al. 2009). Obviously, these techniques are destructive and prohibit repetitive measurements. In a recent review paper, Dinka & Lascano (2012) concluded that, in the field, none of the available techniques can provide sufficient information on cracking dynamics continuously, non-destructively and with a reasonable certainty. Clearly, an accurate understanding of cracking dynamics requires a non-invasive technique that can offer in-situ continuous monitoring of cracking development below the surface. Electrical Resistivity Tomography (ERT) has been suggested as a non-invasive tool that can be used to map cracking of soils
normally filled with air, a dielectric material that is infinitely resistive, cracks form barriers that alter the soil resistivity distribution significantly (Amidu & Dunbar, 2007). In the first reported ERT experiment, Samouëlian et al. (2003) demonstrated the efficiency of the method to detect an artificial crack of 2 mm wide in a compacted soil. Samouëlian et al. (2004) used resistivity anisotropy index AI of a square array defined as the ratio of resistivity at two perpendicular directions, conventionally named as α and β-resistivity, to provide information on the presence, position and extension of the cracks. Using 2D ERT, Sentenac & Zielinski (2009) imaged fine fissuring in clay induced by natural desiccation. Greve et al. (2010) measured temporal changes in the anisotropy proles which allow monitoring of cracks dynamics. Jones et al. (2012) used 3D ERT to map desiccation fissure networks forming in compacted clays.
