Slope failure is a major concern in New Zealand roads. Applying conventional slope stabilisation techniques, such as slope repair, benching, wire netting and soil nailing, is not always practical due to financial and environmental concerns. Electro-osmosis (EO) consolidation technique can provide engineers with a low impact and economical solution for that purpose. EO induces water movement in the fine-grained soil body due to external electric potential gradient (voltage) by establishing a net water flow toward the cathode and creating negative pore water pressure that produces soil consolidation. The external electric potential gradient is normally applied by means of conductive electrodes which can be laid horizontally for slope stabilisation. In this paper, a systematic review of EO consolidation case histories and large-scale experiments have been presented with the aim of establishing an initial cost-effective design framework for EO consolidation scheme in slopes. The hydraulic permeability, electrical resistivity and EO permeability of soil have been identified as factors controlling the shear strength improvement, EO efficiency and power consumption. In addition, depending on soil and system properties, up to approximately 300% increase in shear strength has been achieved.
Some parts of New Zealand are covered by fine-grained soils, such as Oamaru clay loam in Southland, Waikato silty clay and Taranaki/Manawatu humic clay. In many cases, these soils need to be improved for the purpose of slope stabilisation and structural support. The traditional preloading consolidation method is one of the effective ground improvement techniques; however, it is time consuming and is not applicable in all cases. For instance, this technique cannot be used to enhance slope stability. In fact, slope failure often occurs because of pre-loading and the effect of other factors, including pore water increases, ground water flow and seasonal climate changes. Several ground improvement and repair methods for slopes are available, such as changing slope geometry and using stabilising piles, though their feasibility depends on site situation and project budget. Hence, there is a need to find a method for slopes that could overcome most of the limitations of the conventional methods.
Electro-osmosis (EO) is one of the efficient techniques in geotechnical engineering that could be used for this purpose. In many situations, because of the limitation related to load application and environmental and financial constraints, this technique is identified as a unique option (Bjerrum et al. 1967). Simply put, EO works in the following way: generally, under the influence of an electric potential, the cations within the body of saturated soil mass are drawn to the cathode and the anions to the anode. Ions carry their water of hydration and exert a viscous drag on the water around them. When the soil is subjected to direct current (DC), electric potential gradient induces a net water flow toward the cathode and creates negative pore water pressure that produces soil consolidation (Mitchell and Soga 2005). This method has been proven as a low impact, environmentally-friendly and cost-effective technique. In addition, EO could consolidate the soil without clearing trees in forested areas (Jones et al. 2011). The characteristics of electrodes, properties of the soil, level and duration of applied potential difference and power consumption control the improved soil behaviour (Mimic et. al. 2001; Jeyakanthan 2011). Unfortunately, lack of knowledge in the field of electro-hydro-mechanical (EHM) behaviour of EO-treated soils, power consumption and, consequently, cost of the EO ground improvement technique restricts the applicability of this method. In addition, although the laboratory-scale understanding of EHM behaviour of the soil has improved recently, case histories of electro-osmosis consolidation in large-scale play a significant role in understanding the in-situ EHM behaviour. Therefore, to have a thorough understanding of EO consolidation effects and efficiency, laboratory and field scale tests should be studied simultaneously.
In this paper, the application of EO consolidation for slope stabilisation purposes in laboratory and field scale is investigated. In addition, the post-treated soil behaviour is studied in order to propose a more efficient EO consolidation set up and to determine the feasibility of EO system on slopes.
2 ELECTRO-OSMOSIS SYSTEM PROPERTIES AND APPLICATIONS
Generally, the electric field is applied by means of installing electrodes in the body of the soil in various directions. Based on electrode alignments, the application of electro-osmosis consolidation can be categorised into two general cases; (1) ground improvement applications and (2) slope stabilisation. As shown in Figure 1, for ground improvement purposes the electrodes are usually laid in vertical alignment.
Figure 1: Electro-osmosis ground improvement tests: (a) field test (Burnotte et al., 2004); and (b) laboratory test (Lefebvre and Burnotte, 2002)
Therefore, to accurately model the EO consolidation, the laboratory apparatus should be designed to accommodate the electrodes vertically in the testing cell (Figure 1b). However, in case of slope stability the electrodes are installed in horizontal alignment as shown in Figure 2. In addition, a self-climbing rig can be used to install electrodes on the slopes with no deforestation, as shown in Figure 2a. In such a case, the metallic electrodes can also be maintained in place to confine the soil and provide further improvement after EO treatment. Therefore, to accurately model the EO slope stability application in the laboratory, the electrodes should be installed in horizontal direction, as shown in Figure 3.
In addition to the electrodes alignment, electro-osmosis system properties consisting of level of applied electrical difference (V), electrode length (L) and electrode spacing (S), govern the behaviour of post-treated soil and economics of the EO improvement project. Therefore, depending on level of required treatment, the system properties and cost of the project can be estimated.
Figure 2: EO slope stabilisation (Lamont-Black et al., 2016)