Railway Embankment Stabilisation
EKG stabilisation of a railway embankment
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 footprint over conventional methods.
There are 20,000 km of earth structures (cuttings and embankments) on the UK highway and rail networks. Few were built to modern geotechnical engineering standards. The ongoing maintenance and remediation that these structures now require has become a major engineering issue for many UK infrastructure owners.
Toe weighting and/or slope regrading is commonly used to tackle the problem, but these do not address the problem of shrink-swell or pore water pressure changes and typically delay failure rather than prevent it. In addition, these methods can consume large quantities of primary aggregate and energy and are becoming less environmentally and economically viable.
Network Rail identified EKG ground treatment as a novel slope treatment method which could:
• stabilise the slope
• require only modest access owing to the absence large plant
• involve low relative energy consumption
• reduce cost
A trial was conducted on a 22m stretch of a 9m high Victorian embankment. The embankment had been constructed by end tipping a mixture of weathered London Clay and other material such as brick and stone fragments onto underlying alluvium and terrace gravels. An assessment of the embankment identified several sections as unstable. Inclinometer readings indicated a slip surface at approximately 2.5m depth, which could either be a shallow translational slide or a deeper circular failure. Stability calculations indicated a factor of safety (FoS) for the slope of only 1.0.
EKG treatment was designed to accommodate either of the identified failure mechanisms. The treatment was based around an array of EKG electrodes installed at 2m centres in the form of tessellating hexagonal cells, with the hexagon being defined by anode stations and a central cathode.
Upon application of a DC potential (60-80V) electroosmosis forced water to flow from the soil adjacent to the anodes to the cathodes. The treatment took only six weeks and resulted in
• dewatering from the cathodes >25 times that from control drains.
• a reduction in plasticity and shrinkage characteristics.
• an increase in groundwater temperature from 10°C to 20°C.
• a modest DC power consumption of only 11.5kWhrs/m³ of soil treated.
• improvements in shear strength parameters (c’ and Φ)
• a 263% improvement in the bond strength of the anodes acting as nails
a cessation of slope movement.
Following EKG treatment the anodes have been retained as permanent soil nails and the horizontal cathodes retained to act as permanent drainage.
Slope stability analyses were undertaken pre and post treatment. The analytical results are shown in the table below.
Longevity of treatment
The use of EKG to stabilise slopes is a long term solution because:
• Soft weak embankment materials consolidate and improve in shear strength with EKG treatment. This consolidation is permanent.
• Additionally, EKG treatment works best on these soft materials, which are critical to the stability, and in this way the treatment can be considered ‘self selecting’
• Modifications in soil clay chemistry such as cementation and plasticity occur under conditions induced by electroosmotic flow. Given the fine grained nature and very low transmissivity of the soil, the probability of the reversal of these changes is negligible and hence the effects are considered permanent.
• Enhancement of soil/reinforcement bond is a long term effect.
• Passive drainage (de-activated cathodes) is retained in the slope.
A cost analysis comparing slope stabilisation using the EKG method with the lowest cost alternative of gabion baskets and slope slackening, indicated that the EKG treatment produced total project cost savings of 26%.
A carbon footprint comparison of the EKG and conventional treatment options showed 47% lower emissions by using EKG.
Issues have been raised regarding the possibility of ‘stray’ currents. For clarification, this term is used to denote electric currents which do not flow where intended and are caused by two mechanisms:
• Direct conduction
• Induced currents
An analysis of the EKG treatment indicated that such currents are negligible.
Benefits of EKG treatment
In summary, the benefits of EKG treatment include:
• Effective method for slope stabilisation
• Reduced cost
• Reduced access requirements for labour and plant and materials
• Reduced health and safety risk
• Rapid deployment and low labour requirements
• The treatment can proceed whilst maintaining the railway in service (as occurred during the trial)
• Long term drainage of the slope can be provided for by the filtration and drainage functions of the EKGs in the passive mode.
• Sustainability benefits including reduced carbon footprint and elimination of the use of primary aggregates.
Other benefits include:
• The treatment is gradual, only acts when current is on and does not induce rapid changes in ground conditions especially settlement. This therefore provides the option to cease treatment immediately if ever it were deemed necessary.
• The treatment can be flexible in approach by varying voltage, electrode spacing and duration of treatment.
Further flexibility will be possible by manipulating the electrode array and angle of electrode installation to accommodate in situ obstacles such as trees.
More details of the above work may be found in Lamont-Black et al 2009 in the downloads page.
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