Highway Embankment Stabilisation
cutting the cost of stabilisation
The Highways Agency (HA) identified a problem with a 7 metre high failing embankment on the A21 in Kent. The particular section at Stocks Green, Hildenborough, had many mature trees and a rich wildlife habitat (including endangered species), which local people were keen to retain, so the HA sought a method which would deliver the essential stabilisation with minimal impact to the environment.
Balfour Beatty Mott MacDonald (BBMM) was commissioned to manage the project and engaged Electrokinetic to deliver its pioneering technology. The main contractor was Interserve who carried out the construction from September 2011 to March 2012.
EKG combines the reactive functions of conventional geosynthetics of drainage, filtration and reinforcement with the active functions of electrokinetic phenomena such as electroosmosis, and is applicable to most fine grained materials.
By installing a series of anodes (shown in white in the cutaway diagram above) and cathodes (shown in blue) this approach can provide:
• Improvement in soil shear strength over a period of 6-8 weeks through the application of a DC voltage (60-100 volts). Water is moved from anode to cathode by electroosmosis creating pore water suction causing the material to consolidate.
• Drainage in both the active electrokinetic treatment phase and long-term/post treatment passive phase as water accumulates around and then flows, under gravity, out of the cathodes.
• Reinforcement as the anodes act as soil nails with enhanced soil/nail bond after EKG treatment has finished.
For the Stocks Green embankment, a number of shallow slips along a 170 metre section of the embankment posed a risk to the integrity of the south bound carriageway. Analyses showed that, through the combination of shear strength improvement, reinforcement and drainage the progressive failure of the slope could be arrested.
The application used electrodes arranged in a series of contiguous cells across the site. Lightweight materials required only a small workforce and small plant for their installation and the array permitted stretching or compressing of the cells to accommodate trees. Active treatment took place over a period of 6 weeks.
The electrical energy caused water discharge from the cathodes (and accompanying consolidation) and the development of enhanced bond with the anode nails. Electrical current and water discharge were monitored continuously to compare with expectations from the design. Anodes were completed with the insertion of reinforcement tendons and grouting, and capped off with face plates. Load testing was carried out on some anode nails to confirm the development of the electrokinetic bond.
Results of the Electrokinetic method
The EKG slope stabilisation approach provided a multi-functional solution which included:
• Inclinometers demonstrate cessation of slope movement
• Long term stability secured due to improvements in soil shear strength together with the addition of horizontal drains and electrokinetic soil nails
• The ability to construct using the Observational Method i.e. continue to collect ground investigation data during installation due to methods used for cathode and anode installation (augering and driving respectively)
• Load tests on anode soil nails demonstrated achievement of reinforcement design including development of the electrokinetic bond between the anode nail and the soil
• Construction costs indicate a 30% cost saving compared to conventional soil nailing which was used on the almost identical adjacent site of Stocks Green embankment No 2.
• Minimal vegetation clearance with retention of all top soil (including seed bank and root bowls), resulting in better visual aesthetics and noise attenuation
• Habitat preservation – this particular project resulted in the preservation of an endangered species (common dormouse)
• Zero waste production in accordance with the Highways Agency’s zero waste policy
• Smaller material quantities and reduced diesel consumption resulted in a 40% reduction in carbon footprint compared to conventional soil nailing options
• The use of small plant ensured that lane closures were not required.
• Electrokinetic Ltd and Interserve Construction Ltd were highly commended in the category of Delivering sustainable value and solutions in the Highways Agency’s Supplier Recognition Scheme 2012 for the use of the new technology electrokinetic geosynthetics.
• The A21 Stocks Green project was awarded a Green Apple by the internationally acclaimed awards programme for sustainability and green endeavour in the built environment.
• The Institution of Civil Engineers (ICE) recognised the project with its 2012 Innovation Award in its regional Excellence Awards.
Jan Marsden, geotechnical expert at the Highways Agency said; “This is the first time that EKG has been used on a major road, and I am delighted that its potential benefits have been recognised in this way.”
The Green Apple judges commented; “What was particularly impressive about the Stocks Green scheme was just how transferrable the innovative technology is. The judges look for projects which can be developed and replicated as part of an ongoing programme of environmental best practice so we were impressed to hear the technology had already been implemented elsewhere in the country – a sure testament to its success.”
The project led to Interserve Construction Limited (main works contractor) and Electrokinetic being highly commended under the Highways Agency’s Supplier Recognition Scheme 2012.
The HA commented: “The HA recognises the contribution and roles played SMEs and new technology in maintianing the network and delivering sustainable and value solutions. Electrokinetic Ltd and Interserve Construction Ltd have been highly commended for delivering an embankment repair solution on the A21 in Kent which used the new technology of electrokinetic geosynthetics. The project came in on time with reduced costs to the Agency, reduced carbon footprint, the preservation of trees and habitats for endangered species and minimal disruption to local residents and motorists.”
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Electrokinetic is a trading name of Central Alliance Pre Construction Services Limited, Alliance House, Junction 41 Business Park, South Park Way, Wakefield, WF2 0XJ