Foundations disrupt ground and release carbon, and damage biodiversity. Less invasive methods are effective so let’s use them, says James Mickelburgh
Healthy soils can act as a carbon sink absorbing carbon and storing it. Up to 20% of the world’s CO2 emissions come from the release of carbon due to of ground disturbance; an estimated 133 billion tonnes of carbon have been released from soil since measurements began. The fact that this is mainly due to tilling and ploughing for agriculture does not let construction off the hook.
Site strips and excavations in construction contribute to the release of carbon as they disturb the ground. This increases the rate of decomposition of dead plants and animals, roots and soil organisms and so speeds the release of carbon into the atmosphere. And the existing biodiversity of soil has a critical role to play, contributing to the function of all ecosystems.
‘It isn’t just topsoil disturbance that needs consideration, it also matters where that soil goes,’ says James Gilroy, lecturer in ecology at the University of East Anglia. ‘If soil is left exposed to the air or water on the surface carbon emissions will be much higher. If the soil is immediately buried again somewhere else the carbon may remain stored although that depends on whether the soil organisms survive. So an important question is what you do with the material you dig up.’
Most carbon stored in the ground is in the top 1m, and predominantly the top 0.3m, explains Gilroy. This is particularly problematic as even on small projects the typical sub-structure working zone lies within these depths.
Early design to minimise ground disturbance can reduce carbon release, while re-purposing of excavation spoils can reduce the amount of carbon emission from a site, helping maintain some of the soil’s sensitive ecosystems.
The starting point on a construction project is to understand the ground we are working with. Then we can protect its ecosystem, mobilise its inherent load bearing properties and develop efficient, low-impact designs that work with it.
A detailed site investigation is critical. Early geotechnical inquiry may include in situ testing and collection of samples for laboratory testing. These should give a sense of the organic make up of the ground. Francis Williams, director at Ground and Water, explains: ‘Data can be accumulated that tells an engineer the type of ground, its load bearing capacity, susceptibility to movement and whether it is contaminated.’
But even after a detailed site investigation, foundation designs are often conservative. The greater the foundations and the more the site is stripped, the more the soil’s structure is destroyed and sequestered carbon released into the atmosphere – not to mention costs and the embodied carbon of the materials used. Conservative engineering of foundations isn’t the way to create a more robust foundations; costly mistakes are more often the result of misunderstanding ground conditions.
Ensuring a building is designed to suit its specific ground conditions allows the best foundation solutions to be chosen.
Low rise, single or two-storey projects with lightweight superstructures which employ materials with a degree of malleability will permit the use of a number of low-impact, no-concrete and even no-dig techniques.
Happily, solutions with a lower impact on biodiversity also tend to have lower embodied carbon. This is important as new foundations can produce 17%-31% of the total embodied carbon of a building project, according to LETI’s Embodied Carbon Primer.
At the Nest House designed by Studio Bark, structural engineer Structure Workshop showed one way. It used fully demountable Jackpad foundations, on the Wye Valley scheme, which are both recycled and recyclable and crossed with a grid of reclaimed railway sleepers.
In France, in a regional nature park in Limousin, architecture practice Cigue used removable steel foundations and floated its simple wooden house above the ground at Saint-Julien le Petit.
Designers should not be afraid to pursue innovative techniques or suggest alterations to the building design to reduce impact and ground disruption. Various suitable alternatives include steel screw piles, vibro stone columns, timber piles, jack pads and stone trench footings. A comparison of embodied carbon in foundations suitable for low to medium rise construction by the Institution of Structural Engineers shows that significant carbon reductions (in the materials) can be made by using vibro stone columns – at the bottom of the list at 5-10kgCO2e/m² – rather than concrete strip footings at 100-125kgCO2e/m².
They are often easier to re-use than standard concrete foundations, so help a building to move towards being circular – following LETI’s verdict that buildings can only be considered whole life net zero if all components are reused or demountable and reusable.
James Mickelburgh is a director of structural engineer JensenHunt Design
More on foundations and ways to engineer carbon out of your project