A carbon emission often overlooked in whole life carbon assessments is the release of carbon from ground disturbance, or, in the case of construction projects, site strips and excavations.
Up 20% of the worlds CO2 emissions come from the release of carbon stored in the ground as a result of ground disturbance (notably agricultural tilling and ploughing). Healthy soils can act as a carbon sink absorbing carbon and storing it. It is estimated that up to133 billion metric tonnes of carbon have been released from soil since measurements began.
Additionally, disturbance to the ground increases the rate of decomposition and reduces soil organic matter, damaging the soil structure, displacing nutrients thereby shifting organic matter deeper into the ground.
The Wooden House by French Studio Cigue via Dezeen
A carbon emission often overlooked in whole life carbon assessments is the release of carbon from ground disturbance, or, in the case of construction projects, site strips and excavations.
Up 20% of the worlds CO2 emissions come from the release of carbon stored in the ground as a result of ground disturbance (notably agricultural tilling and ploughing). Healthy soils can act as a carbon sink absorbing carbon and storing it. It is estimated that up to133 billion metric tonnes of carbon have been released from soil since measurements began.
Additionally, disturbance to the ground increases the rate of decomposition and reduces soil organic matter, damaging the soil structure, displacing nutrients thereby shifting organic matter deeper into the ground.
Typical construction site strip: Fineturf
James Gilroy, Lecturer in Ecology at the University of East Anglia says ‘It isn’t just the topsoil disturbance that needs consideration, it also depends where that soil goes. If the soil is left exposed to the air/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 I’d say an important question is what will you do with the material you dig up’
James also notes that the majority of the carbon stored in the ground is stored within the top 1m and predominantly the top 0.3m. This is particularly problematic as the typical sub-structure working zone lies within these depths, even for small scale projects.
Careful consideration of carbon release from ground disturbance and re-purposing of excavation spoils can lead to a significant carbon emission reduction and has an associated benefit to the soil’s sensitive ecosystems.
What does this mean for ground disturbance and foundation design and what can structural engineers do about it?
Ground investigations: Ground and Water
Knowledge is key: the more we can understand about the ground we are working with the more we can protect its natural ecosystem, mobilise its inherent load bearing properties and develop highly efficient, low-impact designs.
This starts with gaining a thorough understanding of the prevailing ground which requires detailed site investigations. Francis Williams Director at Ground and Water says that ‘getting a geotechnical specialist involved as early as possible is key to ensuring the site investigations provide the right level of information. Site investigations gather a suite of information relating to the ground below a construction site, through in-situ testing, the collection of samples and laboratory testing.
Data can be accumulated that tells an engineer the type of ground, it’s capacity for taking load, its susceptibility to movement and, if required, whether or not it is contaminated’
Therefore knowing what questions to ask to ensure the right data is gathered during the investigation stage is critical on order to avoid excessive conservatism in the design. It is also beneficial at this stage to understand the organic make up of the ground, the levels of any contaminates and to develop strategies to minimise ground disturbance.
We have the data, what should we do with it?
Even with a detailed site investigation, foundation designs are often overly conservative, leading in turn to large quantities of construction materials being used unnecessarily. This inevitably increases embodied carbon emissions and also cost to the project. Additionally, deeper foundations and site strips destroy the natural ecosystems of the soil that have sequestered carbon over time releasing it into the atmosphere. An emission that is usually not considered when undertaking whole life carbon assessments.
There are obvious reasons for this over-conservatism, when foundation design goes wrong it can be a very costly mistake to put right. Usually, failures in foundation design are rarely caused by incorrect foundation sizing but are more commonly the result of a misunderstanding of the ground conditions on the specific site.
In order to reduce carbon emissions in construction projects our approach to foundation design must change to ensure that low-impact choices are made.
Structural engineers are in a prime position to take a different approach to understanding the quality of the load bearing ground and therefore propose foundation types and sizes that reduce overall material usage and reduce the impact of the sub-structure on the ground and the natural ecology of the site.
So what solutions should we be looking for?
A holistic view should be taken when determining the appropriate foundation solutions and engineers should not be afraid to pursue innovative techniques or suggest alterations to the building design in order to reduce impact or ground disruption.
With the right data obtained from the ground investigations engineers should be trying to make the ground work to their advantage in terms of load bearing capacity, susceptibility to movement and ecological impact.
Crushed stone foundation: This Cob House
Looking at the ground first, understanding its potential through detailed site investigations and making it work as hard as possible are all crucial in developing the lowest impact foundation solutions and this should feed into the whole building design. Ensuring a building is designed to suit its specific ground conditions, such as a soil’s susceptibility to settlement, means optimised foundation solutions can be chosen.
According to LETI’s embodied carbon primer, the embodied carbon of new foundations can be between 17 and 31 percent of the total embodied carbon of a building project - a significant proportion.
LETI’s Climate Emergency Design Guide also states that, buildings can not be considered whole life net zero unless they are 100 percent circular meaning that 100 percent of their materials and products are made up of re-used materials and that the buildings are designed for demountability such that all of their materials and products can be taken apart and re- used.
Raised timber floor and screw piles: Techno Metal Post
Circularity is key, both in terms of the new structure and the inherent circularity of the ecosystems of the soil.
Low rise, single or double storey buildings with light weight superstructure and employing materials with a degree of malleability will allow the use of a number of low-impact, no-concrete and even no-dig solutions.
Concrete is understandably the most commonly specified foundation for small to medium rise buildings due to it’s familiarity, availability and cost but there are a number of suitable alternatives including steel screw piles, virbo stone columns, timber piles, jack pads and stone trench footings.
A comparison of the embodied carbon of foundations suitable for low to medium rise construction undertaken by the IStructE (ref: Scaling Low Carbon Construction Materials. IStructE) demonstrates that significant carbon reductions can me made by using alternatives to concrete strip footings, see below:
To avoid ground disturbance, a combination of these systems can be used to demonstrate demountability, circularity and minimal impact on the natural ecosystem of the ground, demonstrated successfully by Structure Workshop structural engineers and Studio Bark architects at the Nest House in the Wye Valley.
Jack pad foundations used at the Nest House: Studio Bark Architects/Structure Workshop
Act now
With the pace of industry change now happening at an unprecedented speed and with increasing regulation and legislation on the way (aiming to reduce carbon emissions) low-carbon foundation design and biodiversity net gain must become normal practice for every project.
Engineers and designers don’t need to wait until we are forced to make a difference, we should act now. We should use the knowledge, skills and ability already at our disposal to drive the data. Our understanding of the conditions we’re working with should be used to build in circularity and to maximise the possibilities for real, measurable carbon reduction and protection of the ecosystems that naturally store carbon in the ground.
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