This report provides estimates of the carbon impacts of four remedial techniques used to stabilise a notional 25o highway slope 10m high and 100m wide for failure depths of 1m and 2.5m. The four techniques are Granular Rock Fill Replacement, which was used as a control), Willow Pole planting, Fibre Reinforced Soil (FRS), and Electrokinetic Geosynthetics (EKG). These were compared using a cradle-to-site life cycle assessment. Five life cycle steps were defined and assessed, from the acquisition and processing of each installation’s constituent materials, through to the construction of the final product – a stabilised slope.

A cradle-to-site Life Cycle Assessment (LCA) boundary was adopted (using the cut-off approach), as opposed to a full cradle-to-grave assessment. This approach was adopted for a number of reasons. Firstly, each of the techniques has different maintenance requirements, occurring in uneven time frames. Secondly, the expected lifetimes and end-of-life scenarios for each technique vary in ways that would not allow for a balanced comparison within equal time frames. Data was compiled from a number of representative sources and where possible was selected for UK specific values. Transport was calculated using factors for a range of delivery vehicles, depending on the material and scenario. All transport was calculated for two-way (full outward-empty return) journeys.

This study aimed to provide a range of general estimates, and as the slope under assessment was hypothetical, it did not have a real-world location. As a consequence, the transport impact calculations are based on assumed low-medium-high distances for transport impact calculations. The functional unit of analysis of the assessment was the ability of each technique to stabilise 1m2 of failed slope.

It was found that at both failure depths, and for all transport cases, the greatest environmental impact was for the Granular Rock Fill Replacement. At 1m and 2.5m failure depths this technique resulted in an impact of 51 to 174 kgCO2e/m2 of failed slope respectively, depending upon the transport distance assumed. Willow Poles had the least impact, resulting in 4 to 12 kgCO2e/m2 of failed slope respectively. For the 2.5m failure depth EKG was the second best performing technique (EKG was not assessed for the 1m failure depth scenario), with an impact of around 14 kgCO2e/m2. FRS had an impact ranging between 16 and 35 kgCO2e/m2. For techniques requiring large quantities of materials and movements of these materials across substantial distances (e.g. Granular Rock Fill Replacement and Willow Poles), it was found that transportation accounts for more than half of the total impact.

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