Why peatlands are prone to subsidence

Peat forms in wetland areas, where waterlogged conditions slow the decomposition of organic matter. As a result, peat typically consists of up to 95% water and just 5% solid material. That solid material is mostly partially decayed vegetation that has accumulated over hundreds or even thousands of years.

Peat behaves very differently from other soils such as clay or sand; it’s much less predictable, and its strength can vary greatly from one spot to another. The thickness, density, and composition of peat depend on the water content, type of vegetation and its rate of decomposition. Some peat layers are only a few centimetres thick, while others can be several metres deep.

It is this composition that makes peat-ground extremely challenging to build upon. The problem with building on peat is that it is:

• Compressible and spongy, with a very low bearing capacity, so that even lightweight buildings can result in excessive settlement and bearing failure.

• Susceptible to long-term settlement as the organic material continues to slowly decompose.

• Its properties can be inconsistent across a site, which can lead to differential settlement, where parts of a structure sink at different rates.

In addition, human activities such as drainage, peat extraction, or agricultural reclamation can accelerate settlement. When peat is drained, it shrinks and the rate of decay of organic increases, causing further ground loss and instability.

Common signs of peatland-related subsidence

Peatland subsidence can affect any property, often causing damage to foundations and infrastructure. One well-known example is the Radharc An Seascan estate overlooking Dungloe Bay, Co. Donegal. Built in the early 2000s, the homes began to suffer visible movement within a year, including:

• Sinking driveways

• Detached steps and porches

• Broken sewer and water pipes

• Cracked walls and uneven floors

The problems became so bad that many of the properties have now been abandoned.

In addition to the issues listed above, other typical pointers of peatland-related subsidence include:

• Cracks appearing in walls or ceilings

• Sloping floors or extensions

• Windows and doors sticking

• One side of a building gradually sinking

Treating peatland subsidence

Successful treatment of peat-related subsidence begins with a comprehensive site investigation. This will help identify the cause of the subsidence, such as leaking drains or tree roots, which will need to be addressed before remediation can commence.

The right remediation solution will depend on the depth and composition of the peat, the type of structure subsided, and the extent of the settlement.

Screw piles

Screw or helical piles are usually the most effective way to deal with structural subsidence on peat soils.

A screw pile is essentially a giant metal steel screw, up to several metres long, which is screwed into the ground, passing through the soft peat layers, to anchor into the firm ground below.

A series of screw piles are usually required to provide a strong and stable support for a structure. At the top of each pile is a bracket, which is attached to the building's foundations to supports the building, transferring its weight safely to stable ground.

The benefit of a screw-piled solution is that:

• It is quick, cost-effective and environmentally friendly

• With no need for trenching or waiting for concrete to cure, there is minimal disruption to property owners and minimal spoil removal

• Screw piles can be installed in tight spaces

• Piles can be positioned to avoid the need to move underground services

Resin injection

Where peat layers are shallow and subsidence is minor, less invasive ground improvement techniques like resin injection may be appropriate. These methods involve injecting engineered resins or grout into the ground beneath a building's footings or floor slabs, filling voids and transferring structural loads to firmer ground below.

Mainmark’s holistic approach to peatland remediation

Not every project is the same, which is why we take a holistic and evidence-based approach to treating peatland subsidence. Each project begins with detailed site investigation and monitoring to identify the underlying causes of ground movement. From there, we design a custom remediation plan that addresses both immediate structural stability and long-term ground performance.

No matter the required solution, our aim is always the same: to restore stability quickly, safely, and sustainably with minimal impact on the surrounding environment.

What should you do?

Peatland subsidence is manageable but early action is essential. You should look out for early warning signs such as cracks in walls, floors developing a slope, or doors and windows sticking. If any of these appear, it’s important to seek professional advice as soon as possible.

Mainmark can assess the cause and recommend the most effective, non-invasive remediation solution. Early intervention not only prevents more serious structural damage but can also save homeowners significant time and cost in the long run.