Overview

EKG Geochemical Soil Improvement

Environmental Sustainability

environmental sustainability

The EKG drainage and soil nail slope stabilisation system combines ground improvement, reinforcement and drainage in such a manner as to accommodate a wide variety of ground and groundwater conditions with a low environmental impact, low carbon footprint and reduced cost. Subsurface electroosmotic ground improvement combined with lightweight materials and plant used during construction mean that the required workforce is small and the exposure to potentially hazardous working conditions e.g. roadside or trackside, is greatly reduced. Installation of the lightweight electrodes using a small lightweight plant saves time, generally obviates the requirement for traffic management and helps to reduce energy consumption and preserve precious resources such as wildlife habitats, topsoil and tree cover.

Examples of the sustainability benefits using EKG were provided by two projects; A21 Stocks Green Kent, and South Greenford railway embankment, London Borough of Ealing.

Embankment stabilisation on the A21 Stocks Green (site 1) near Tonbridge, Kent
(owner: Highways Agency)

The embankment stabilisation at Stocks Green provided the opportunity to assess the impact of the EKG slope stabilisation system on both habitat preservation and carbon footprint.

Preservation of habitat

The area is an important habitat for an endangered species (common dormouse) and it was important that the remediation design minimise the environmental impact in terms of removal of trees and disturbance of topsoil.

In 2010 a number of soil nailing repairs, including the adjacent site: Stocks Green (site 2), had been carried out on the embankments of the A21 Tonbridge Bypass. Previous remediation designs had used conventional soil nails aimed to retain as many trees as possible in these locations, in order to maintain dormouse habitat connectivity. However inevitably there was the need for significant tree removal, which is unsatisfactory for designers and local residents alike. The photograph below shows the two sites side by side. In the EKG example the total earthmoving as part of the slope stabilisation amounted to 4m³ of soil, which represented the arisings from installing the cathodes by auger.

Photos taken October 2011

	Stocks Green (site 2) Conventional soil nailing.		Stocks Green site 1 (EKG) note DC power supply.
	Note the clearance of trees using conventional soil nails and the preservation (>90%) using EKG.
	Slope during electrode installation. Note the preservation of trees.		Installed cathode (prior to electrical connection).

The EKG method requires that sufficient space is available for the installation of electrodes. This is created by removing undergrowth such as brambles and dogwood, coppicing hazel and trimming tree branches lower than 2m above ground level.

Because the EKG method improves the soil strength and provides drainage, the role of the reinforcement is reduced such that the stripping of topsoil is not required. This means that soil macro and micro fauna are preserved along with the seed bank to promote rapid re-growth. The latter is import in re-establishing wildlife habitats.

The photographs below were taken in January 2012 whilst the electroosmotic process was active.

	Anode		Cathode
	Arum maculatum (Lords and Ladies), an ancient woodland indicator which established as a result of the canopy being thinned allowing light into the understorey. Their growth immediately adjacent to anodes and cathodes during a period of active electrokinetic treatment indicates the low impact of the treatment.

Carbon footprint

The project offered an excellent opportunity to compare the carbon footprint of the technique with conventional soil nailing, which was used at the adjacent Stocks Green 2 site. Owing to the difference in dimensions of the two slopes the comparison is made on the basis of the slope area; this indicated that the Stocks Green 1 EKG site (160m long 8m high) resulted in carbon emissions of 29.6kg/m2 whereas Stocks Green 2 (105m long 5 m high) with conventional soil nails resulted in carbon emissions of 49.7 kg/m2. This equates to a 40% reduction in the carbon footprint of the remediation method by the use of EKG technology. Comparing EKG as used on Stocks Green 1 with a design that would have used conventional soil nailing on the same site indicated that the reduction in carbon emissions would have been around 48% (see below).

Electrokinetic solution (Stocks Green 1) against soil nailing solution (Stocks Green 2)

This comparison examines the embodied CO₂/m² of slope which was derived through materials and site works on each project.

The Stocks Green 2 project used a conventional soil nailing approach to treat a 105m wide embankment with a slope length of approximately 14m. Nailing was only used on the uppermost part of the slope which was cut back from the original profile to an angle of 45 degrees. Three rows of nails were then used at a spacing of 1m vertical and horizontal.

The Electrokinetic approach was used on a 160m wide embankment with a slope length of approximately 20m

Comparison of embodied CO₂ for Stocks Green 1 against Stocks Green 2

	EK Treatment (Stocks Green 1)			Soil Nailing (Stocks Green 2)
	Item	Embodied CO2 (tonnes)		Item	Embodied CO2 (tonnes)
	Anode tube	19.68		Nails	17.37
	Rebar	5.70		Face plates	2.81
	Face plates	2.94		Grout	12.46
	Grout	0.81		Geotextile	2.16
	Anode connections	1.52		Installation (diesel for plant)	25.18
	Cathodes	5.30		Cut and Fill	13.07
	Installation (diesel for plant)	1.90
	Cabling	7.70
	Treatment (diesel)	49.22
	Total (tonnes)	94.77		Total (tonnes)	73.06
	kg of CO2/m² of slope	29.60		kg of CO2/m² of slope	49.70

	EKG as a percentage of conventional method	59.59%
	Reduction through using EKG	40.41%

Electrokinetic solution against a soil nailing solution (both Stocks Green 1)

This comparison takes into account the actual figures from EK treatment against a theoretical conventional soil nail solution for the slope. The theoretical figures are based on a design which uses a nail array of 1.5m vertical and horizontal spacing. The design of the nails is based on what was used on Stocks Green 2.

• Nail length – 5m

• Nail diameter – OD = 30mm, ID = 16mm

• Grout annulus – 150mm

• Face plates – 250mm side length, 10mm thickness

It should be noted that calculations for the soil nailing approach do not take into account fuel used in plant that would be required for re-grading.

Comparison of embodied CO₂ for EK approach at SG1 against soil nailing approach at SG1

	EK Treatment			Soil Nailing
	Item	Embodied CO2 (tonnes)		Item	Embodied CO2 (tonnes)
	Anode tube	19.68		Nails	58.40
	Rebar	5.70		Face plates	9.47
	Face plates	2.94		Grout	17.67
	Grout	0.81		Geotextile	13.10
	Anode connections	1.52		Installation (diesel for plant)	25.18
	Cathodes	5.30		Cut and Fill	84.72
	Installation (diesel for plant)	1.90
	Cabling	7.70
	Treatment (diesel)	49.22
	Total (tonnes)	94.77		Total (tonnes)	183.41

	EKG as a percentage of conventional method	51.67%
	Reduction through using EKG	48.33%

Calculations used EA Carbon Calculator 2007 and Spon’s civil engineering and highway works price book 2008

Embankment stabilisation at South Greenford, London Borough of Ealing
(Owner: Network Rail)

EKG was used to stabilise a short section of 9m high embankment. A direct comparison of the EKG method with the alternative method used on the site of gabion baskets and reducing the slope angle indicated a reduction in the overall carbon footprint of 47% by using EKG.

This section makes a comparison of the EKG treatment with a treatment based on slope slackening using gabion baskets. The calculations are based on

The Section of slope treated is 22m in length and stretches the full length of the slope; the gabions are installed as a wall 2m high and 1m wide.

Illustration of the parameters for the calculationof the volume of soil for backfill

The carbon footprint consequences of the construction components associated with the conventional and EKG options are shown in the Tables below. The carbon footprint of the EKG treatment was compared with conventional remediation of gabion baskets and slope slackening using the ‘carbon calculator version 3.1.1’ (Environment Agency, 22/10/2009), which calculates the carbon dioxide associated with direct activities such as plant movement and electricity generation along with carbon dioxide embedded in materials and services.

Construction quantities and associated CO₂ emissions for the EKG option

	Gabion baskets and slope slackening (conventional)			EKG slope stabilisation
	Item	Embodied CO2 (tonnes)		Item	Embodied CO2 (tonnes)
	Gabion baskets	2.80		Anodes	5.70
	Gabion fill	7.40		Cathodes	1.16
	Soil Fill	16.00		Fuel¹	8.84
	Fuel¹	0		Small plant	1.00
	Small plant	5.00		Site Accommodation	0.67
	Site Accommodation	1.10		Travel of personnel	1.78
	Travel of personnel	3.60
	Total	35.80		Total	19.14

	EKG as a percentage of conventional method	53.46%
	Reduction through using EKG	46.54%

Note¹ No additional fuel is included in the conventional method as this is accounted for in the plant calculations.
The fuel associated with EKG treatment is included as a separate item over and above fuel associated with the installation plant.

Treating the slope with EKG results in a saving of CO₂ emissions of 16.7 tonnes or a 47% reduction compared to the installation of a 2m high gabion basket and backfill option (see also note 1 in the table above).

Origins of the reduced carbon footprint associated with EKG

The increased carbon foot print of the conventional treatment compared to the EKG treatment results from several factors including:

• Materials - importing of fill materials for slackening the slope and filling the gabion baskets.

• Labour - conventional treatment requires about twice the labour. This is owing to the fact that the labour requirement for the EKG option requires only installation and connection and security, whereas the conventional option requires extensive work throughout the construction period.

• Plant – the conventional treatment requires extensive ground preparation (benching), multiple material handling, compaction, and the construction, placement and completion of gabion baskets.

• Travel and accommodation – the larger workforce associated with conventional treatment requires larger welfare units and more transportation to and from the site.