Alternative aggregates

Ground engineering professionals face a growing number of challenges relating to sustainability and expanding government targets to achieve net zero. Within the ground improvement sector, vibro stone columns (VSCs) are the most commonly used technique. With this approach, vertical ‘reinforcing’ columns of compacted aggregate, which are installed using specially designed vibrating pokers, improve and stabilise the ground prior to foundation and building construction. The VSC sector still relies heavily on primary quarried aggregates, yet such sources are unsustainable, and their extraction and transportation (typically by road) can have significant environmental impacts, particularly in the context of CO₂ emissions. Increased recycling and reuse of materials, particularly close to source, are increasingly becoming more essential.

Historically, there was little incentive within the VSC sector to use alternative aggregates, which includes recycled aggregates (those derived from reprocessing materials previously used in construction, such as recycled crushed concrete and spent railway ballast) and secondary aggregates (the by-products of industrial processes that have not been previously used in construction, such as well weathered metallurgical slag). Such alternative sources were frequently perceived as inferior and there was little or limited awareness of the benefits (Serridge, 2005). However, the current levy on primary quarried aggregates provides a financial incentive for alternative aggregate use, whilst continuing research and quality control testing protocols for alternative aggregates ensures parity of performance with primary quarried aggregates.

The sustainability credentials of alternative aggregates in VSC projects have been extensively investigated (e.g. Serridge, 2005; Egan and Slocombe, 2010; Jefferson et al., 2010). For example, Egan and Slocombe (2010) found that the embodied CO₂ is halved when recycled aggregates are used instead of primary quarried aggregate for VSCs.

The redevelopment of brownfield sites with a legacy of industrial activity perhaps offers the greatest potential for recycling. Through early involvement on a project, specialist VSC contractors can assist in identifying potential sources of recycled aggregate. Materials such as old concrete floor slabs can be crushed and screened on site, then reused as VSC aggregate to stabilise heterogeneous made-ground (anthropogenic) deposits and any underlying weaker natural soils prior to construction. Such in-situ recycling reduces the need for quarrying, magnitude of processing, and transport, thereby significantly improving the sustainability credentials of a project. Thomas and colleagues (2009) suggested that such an approach can reduce CO₂ emissions by one third compared to trafficking in primary aggregates.

Yet, despite these advances the procurement process for VSC projects in the UK remains predominantly price driven, based on competitive design-and-build contracts that favour the cheapest proposal, typically with less emphasis on the environmental and sustainability aspects of the tender bid. Indeed, VSC project specifications in the UK often do not include the option for alternative aggregate use.

The ground improvement community, can increase awareness (amongst specifiers and procurers of VSC projects), of the great potential and suitability of alternative materials by: sharing and disseminating published research and case history information; facilitating visits to recycling plants to permit inspection of product quality and the operation at first hand (modern plants are increasingly comparable to those seen in modern quarries); and push for the development of a definitive database on both the geographical location and available volumes of alternative aggregate sources. Data must be regularly collected and statistics compiled to track sustainability progress, including volumes of alternative aggregates relative to primary aggregates consumed, to better quantify and inform on the positive achievements of the VSC sector in contributing to a circular economy. If the UK ground improvement sector is to become more sustainable, all stakeholders must be actively engaged in promoting use of alternative aggregates.

Author

Colin Serridge,  Principal Engineering Geologist in the Specialist Ground Improvement industry and a Lecturer in Earth Sciences, at Edge Hill University, UK.

Further reading

• Egan, D. & Slocombe, B.C. (2010) Demonstrating environmental benefits of ground improvement. Proceedings of the Institution of Civil Engineers, Ground Improvement163(1), 63–69, Thomas Telford, London; https://doi.org/10.1680/grim.2010.163.1.63
• HMSO (2002) The Aggregates Levy (General) Regulations. Richmond, UK; Volume 761, pp. 1–20.
• Jefferson, I. et al. (2010) Emissions assessment related to vibro stone columns. Proceedings of the Institution of Civil Engineers – Ground Improvement 163(1), 71–77, Thomas Telford, London; https://doi.org/10.1680/grim.2010.163.1.71
• Perkins, L. et al. (2021) The Use of Recycled and Secondary Aggregates to Achieve a Circular Economy within Geotechnical Engineering Geotechnics 1(2), 416–438; https://doi.org/10.3390/geotechnics1020020
• Serridge, C.J. (2005) Achieving sustainability in vibro stone column techniques . Proceedings of the Institution of Civil Engineers – Engineering Sustainability 158(4), 211–222, Thomas Telford, London; https://doi.org/10.1680/ensu.2005.158.4.211
• Thomas, A. et al. (2009) Estimating carbon dioxide emissions for aggregate use. Proceedings of the Institution of Civil Engineers – Engineering Sustainability 162(3), 135–144, Thomas Telford, London; https://doi.org/10.1680/ensu.2009.162.3.135

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