Ground Engineering

Electrokinetic geosynthetic (EKG) technology has been used to successfully stabilise a failing clay embankment in London resulting in a 26% cost reduction and a 47% reduction in carbon dioxide emissions over conventional methods; described in detail in the case studies page.

EKG technology offers significant benefits in numerous ground engineering applications, including:

• Slope (embankment and cutting) stabilisation
• Ground consolidation
• Reinforced soil
• Sports turf conditioning

The benefits of EKG technology can be directed at stabilising both new and existing slopes. These benefits arise from the short and long term effects of the technology. The technology mobilises several distinct effects which can be adapted to suit individual ground situations and timescales. These include:

• Electroosmotic dewatering and reduction in pore pressure
• Consolidation of soft materials
• Increase in shear strength (drained and undrained)
• Reduction in plasticity and shrink swell characteristics
• Continuing passive drainage
• Reinforcement

Owing to the multi-component nature of the EKG approach, the treatment can be adapted to suit geotechnical settings with significant geological, geomorphological and hydrogeological heterogeneities.

Electrokinetic processes

Electrokinetic processes active in the ground

The above characterises the electrokinetic processes that are activated when ground is treated with EKG materials. These include:

• Electroosmotic flow from the anode areas towards the cathode
• Pore pressure reduction spreading out from anodes
• Cementation around the cathodes
• Precipitation around the cathodes

The key consequence of harnessing these effects is the overall strengthening of the ground.

In addition the soil between the electrodes can be further improved by the use of chemical conditioners, whose application is ideally facilitated by the design of EKGs.

All of these effects can be used in various combinations in ground engineering settings including slope stabilisation, ground consolidation, construction of reinforced soil and sports turf conditioning.

Slope stabilisation

EKG technology has been used to successfully stabilise a failing clay embankment in London resulting in a 26% cost reduction and a 47% reduction in carbon dioxide emissions over conventional methods; described in detail in the case studies page.

EKG used for slope stabilisation combines active electroosmotic drainage and pore pressure change with passive drainage and soil nail reinforcement.

In using EKG to stabilise clay slopes, EKG treament offers immediate effects and  and long-term benefits, namely:

Immediate effects

• Reduction in porewater pressure
• Reduction in water content
• Reinforcement

Long-term benefits

• Cementation
• Reinforcement
• Drainage

An example of a slope treatment is shown in case studies.

Immediate effects

Reductions in porewater pressure and electroosmotic flow are given by equations 1 and 2.

u = - k_e/k_h. g_w. V   Equation 1 (Mitchell, 1993)

Q = k_e.V/L. A          Equation 2, (Mitchell, 1993)

Where:

u = porewater pressure

k_e = coefficient of electroosmotic permeability

k_h = coefficient of hydraulic permeability

g_w = density of water

Q = electroosmotic water flow

V = voltage applied between electrodes

L = spacing between electrodes (anode – cathode)

A = area

During soil treatment, electroosmotic flow is independent of hydraulic permeability and the degree of negative porewater pressure or suction that builds up is proportional to the ratio of the coefficients of electroosmotic and hydraulic permeabilities. Therefore, electroosmosis is most effective in fine grained soils such as clays and silts.

When designing a treatment it is important to establish the ke and the kh which can then be used to determine the three working parameters of treatment time, voltage and electrode spacing. These parameters can be adapted to the economic needs or urgency of the project. This is illustrated below.

Relationship of electrode spacing applied voltage and treatment time for a simulated slope stabilisation project

By adjusting the parameters, different factors can take priority. For example, if treatment time is critical then the use of a closer electrode spacing is appropriate. On the other hand, if cost is the main driver a wider spacing of electrodes can be used to reduce the number of electrodes and spread the treatment out over a longer duration.

Long term benefits

By choosing the appropriate combination of anodes and cathodes and a specific electrode array or style of installation, the treatment will provide the following long-term benefits:

Reinforcement – anodes remain in situ as bonded soil nails. The effects of the soil nail array is to further increase the factor of safety of the slope.

Cementation – anodes chosen and operated with a partially sacrificial function will produce several distinct effects:

• dissolved ions in solution cement the clay around the anode thus stiffening the clay itself forming a ‘mini pile’.
• a very strong bond with the anode
• additionally if required, anodes can be conditioned with materials such as lime or calcium chloride to foster cation exchange and further improve the soil.

Drainage – EKG electrodes are designed to function as filtration drainage elements. These can be installed in sympathy with the electrokinetic treatment in such a way as to provide long term drainage and stability after completion of the active electrokinetic treatment phase.

Slope stabilisation using EKG technology can be applied to cuttings and embankments in railway and highway infrastructure and other earth structures such as tailings dams and flood protection embankments. Soft fine grained materials such as clays and silts can be improved by electroosmotic flow consolidation and cementation. The presence of high permeability materials such as sands and gravels does not present a difficulty because these materials can be accommodated by the long term drainage function. All materials are subsequently reinforced by the reinforcing function of electrokinetic nails.

Electrodes for slope stabilisation can be installed to accommodate the dominant failure mode and can also be orientated to maximize drainage from wet areas and optimise orientations for reinforcement as shown below.

Example of and installation of multifunctional anodes and cathodes

 

Soft ground consolidation

EKG materials formed as prefabricated vertical drains can increase the speed of consolidation of soft ground by employing electroosmotic flow. In fine grained soils, such as tailings and sludges, flow rates induced by electroosmosis can be 3 - 4 orders of magnitude higher than equivalent hydraulic flow rates.

EKG materials, in this application, are constructed as band drains and can be inserted into the ground by lance and mandrill. The treatment can reduce consolidation time by 50 – 80% compared to conventional wick drains.

A typical application for EKG technology is consolidation/dewatering of lagooned materials such as mine wastes of sewage sludges. These materials tend to have high water contents (good for electroosmosis) and low hydraulic conductivities (also good for electroosmosis). An approach is illustrated below:

Example of a sequential lagoon consolidation using EKG 

 

Reinforced soil

Reinforced soil EKGs can be used to construct vertical soil walls using very poor material in the form of a clay slurry. An example is given in the downloads page.

The design philosophy is based upon achieving short term stability by raising the drained shear strength of the soil and combining this with stiff reinforcement provided by the EKG material. The relationship between water content and undrained shear strength is used to determine a target shear strength which will provide stability.

EKG materials are deployed in dual polarity modes as:

• Agents of electroosmosis
• Drainage paths
• Reinforcing elements

Sports Turf conditioning

In the conditioning of sports turf EKG is used to:

• Control the water content of the material near the playing surface and thus influence it mechanical behaviour (ball bounce, shear strength, traction etc.)
• Evolve oxygen gas in the root zone to provide well aerated conditions to develop healthy and robust plants.

The conceptual scheme is shown below.

Conceptual arrangement of EKG grid anode (upper) over band drain cathodes (lower)

A fully developed application of the EKG turf system would permit different areas of a playing surface to be treated according to the conditions of the soil and intensity of play or variations in microclimate within a stadium as shown in the football example below.

 

Examples of how an EKG turf system could be installed in a new pitch and individual areas controlled (top) of installed only in areas of high wear (bottom).

An example of this approach is shown here.

Summary

General features of EKG technology

• Active water flow through clays
• Long term (post – EK treatment) drainage
• Long term  (post EK-treatment) reinforcement
• Enhanced soil –reinforcement bond
• Reduction in shrink swell behaviour
• Rapid increase in undrained shear strength
• Permanent increase in drained shear strength
• Oxygen production near anode

Benefits of EKG technology

• No requirement for large plant
• Greatly reduced access considerations
• No need for large volume material haulage
• Require minimal levels of manning and supervision
• Rapid deployment
• Reduced overall project time
• Permits use of conventionally unacceptable material
• Lower the environmental footprint
• Reduced overall project costs