UK-based geotechnical specialist Bachy Soletanche has developed and tested an efficient low-carbon grout mix solution for use in cut-off wall construction. Over the past year, Bachy Soletanche has trialled 32 low-carbon binders for potential use in two cut-off wall construction projects: the Lowdham Flood Alleviation Scheme, led by Jackson Civil Engineering, and the Arne Moors Flood Defence project, led by Kier Construction.
The aim of the trials, which were part funded by the Environment Agency (EA), was to investigate if the latest cementitious binders could be used to reduce the environmental impact of the cut-off wall construction process. Both sets of trials investigated the suitability of these mixes for use with Bachy Soletanche’s Trenchmix® method.
Bachy Soletanche was tasked with developing mixes that not only offer an effective low-carbon solution, but meet the strength, durability and permeability requirements for use in the cut-off wall process. Both projects required a grout mix with a target compressive strength of 1 MPa and a target permeability less than or equal to 1×10-8 m/s at 28 days.
Using its in-house laboratories in France, Bachy Soletanche trialled binder mixes ranging from pure CEM I (100% Portland cement), which was the reference mix design, to binders containing up to 95% ground granulated blast-furnace slag (GGBS). Bachy Soletanche also trialled the use of other by-products from the industry, including fillers and alkali-activated materials.
All the cement replacement alternatives investigated by Bachy Soletanche offer a much lower carbon footprint than traditional CEM I concrete.
Bachy Soletanche’s trials demonstrated that the cement replacements routinely used in concrete production (GGBS, fly ash and fillers) can be added to the grout mixes used in deep soil mixing solutions, such as Trenchmix, to reduce carbon emissions.
During Bachy Soletanche’s trials for the Lowdham scheme, two low-carbon mixes were deemed appropriate for use in cut-off wall construction: a CEM III/C mix and a mix containing an optimised ternary blend of GGBS, CEM I, and limestone filler. Bachy Soletanche calculated that these mixes have a carbon footprint approximately six times lower than the reference mix.*
Bachy Soletanche’s trials for Arne Moors concluded that there were three mixes that met both the environmental and performance requirements: a mix of pure CEM III/C, a CEM III/C mix blended with limestone filler and a mix of pure GGBS combined with an in-house activator. These mixes had a carbon footprint two to five times lower than the reference mix.*
*All carbon savings are based upon current Environmental Product Declaration (EPD) figures.
Bachy Soletanche’s trial report noted that the different properties of the soil on a site will impact the mix performance. Therefore, thorough ground investigations and laboratory trials such as these must be carried out for any future cut-off wall projects wanting to incorporate these mixes. Conducting these trials will help identify what is attainable for each site, depending on the project specification.
The geotechnical specialists also highlighted that anyone wanting to incorporate these low carbon binders on their cut-off wall project should consider extending the specified curing lengths beyond the traditional 28 days, where possible.