Peat is a notoriously difficult soil on which to construct developments, one that has conventionally led to increased construction costs and longer duration of groundworks, writes Chris Eccles of Redstart Northwest.
There is an increasing awareness of the importance played by peat in the removal of carbon from the atmosphere and its role in biodiversity.
Peat is a huge carbon sink, present across 3% of the world’s surface. It holds more than twice the amount of carbon than all of the world’s forests. If you drain or excavate peat, you release huge amounts of carbon. Each hectare of drained peat will emit as much CO2 in a year as driving three million miles in a petrol car and that will continue for every year while the peat is dried out. On-site relocation of peat into a raised landscaping feature will cause it to dry out, oxygenate and release carbon dioxide for an indefinite period.
Many people think of peat either in a horticultural sense, or the dark boggy ground encountered on walks in more remote areas of moorland and in upland areas such as the Pennines. Lowland peat deposits, which we have right across our region, are just as important too. Many of these areas are called mosses, such as Ashton Moss, Mossborough / Rainford Moss, Moss Side and Danes Moss.
With increasing urbanisation, many of these areas are around or within urban sites in the North West where developers are considering construction. Mosses were often not built on due to the poor ground conditions, with existing development often spreading to the edges of these areas, but as land becomes more valuable, more peatland is considered for development. An example is Jet Parks Ringway and Jet Parks 3 car parks at Manchester Airport in an area called Moss Nook.
Future developments need to consider how they manage peat, and where peatland cannot be retained, there needs to be a move from drainage and excavation to retention and re-wetting. But how?
In recent years there has been an increasing use of innovative solutions including:
- Shallow cement stabilisation: Mix cement (and sometimes sand) into the peat to depths of about one to two metres and then construct using conventional methods. The strength of the stabilised ground can be made the same or better than the surrounding ground.
- Deep cement stabilisation: There are a couple of different techniques, with one method based on a mixing attachment on an excavator which can operate down to a depth of about four metres. Another is based on using a rotary piling rig to form columns of stabilised ground up to 1.5m in diameter and to depths of more than 15m.
- Use of lightweight fill: Use of large blocks of expanded polystyrene as fill has been used successfully for some structures such as bridge approach embankments. This enables the peat to remain in the ground and the embankment to ‘float’ above the peat.
The above methods enable the peat to remain in the ground without it releasing damaging amounts of carbon. The embedded carbon in the cement is acknowledged, but this has a much lower footprint than the excavation of peat.
Carbon sequestration and biodiversity
With a steady move towards net zero carbon for developments, any sites with peat present will have to consider how the land is managed and whether construction should avoid it altogether.
If you have an area of drained peat, it may be possible to re-flood the area and use this for stormwater attenuation. Re-flooding of peat will help off-set the carbon footprint of other aspects of developments. If additional measures are taken, the peatland can be rehabilitated so it starts sequestering carbon again for the lifetime of the development and beyond.
The coming Environment Bill will require developments to have a 10% biodiversity net gain so we ‘leave the environment in a better state than we found it.’ Flooding and rehabilitation of peatlands will have an important role in achieving this goal.
The protection and rehabilitation of peat bogs is much more effective than planting trees. On an equal area basis, peat bogs store 10 times more carbon than forests.
Where there is construction in areas of peat, there needs to be a move away from ‘dig and dump’ to improve the sustainability of developments and reduce carbon emissions. Developers must consider this in the construction planning and programming stages.