Having established that Mabel’s Farmhouse, our Grade II-listed, 18th-century property, is definitely moving, we soon realised that she is continuing to move as the freeze/thaw cycle affects the building. However, while all buildings move to a degree, and movement isn’t necessarily an issue — Mabel’s movement certainly is.
One of the walls of Mabel’s Farmhouse which will need to be secured to stop further movement
When the internal and external fabric of the building are showing signs of damage from the movement, and walls are so out of plumb that you can’t hang a picture off them, then you might want to try and limit the movement. If the movement is in the ground then there is an accepted solution to limiting it and stabilising the building: underpinning.
What is Underpinning?
This word can cause people many sleepless nights, but it is actually quite a simple concept and is not necessarily difficult if you use an approved contractor with an engineered solution. Essentially, underpinning involves creating a new concrete foundation for the building to sit on which will be deep enough to negate the movement caused by the unstable ground conditions, in effect anchoring the building on solid ground.
However, underpinning is just one solution to unstable ground conditions and it is unlikely that a structural issue is one-dimensional; often a combination of different causes manifests itself in a range of cracks and bulges.
For Mabel’s there are concerns regarding the ground stabilisation but there are also places where the structure is not performing as one — the upper level joists are loose and not tying the external walls together properly.
Therefore, the structural solution is a hybrid of a range of techniques, which are solving different issues, that work together to form a holistic structural design. In addition, the sequencing and timing of these various structural solutions also has a bearing on the project timeline.
Neil, our structural engineer, has recommended the first item on the list is to rebuild the external wall in the north-west corner of the house. This is because, due to the condition of the wall in this area, the underpinning would undermine the wall and therefore there is a risk of structural collapse. Typically, in conservation terms, rebuilding is the last resort, and normally only adopted after all other pinning and consolidating options have been exhausted.
Essentially, the rebuilding at Mabel’s involves the stone mason repairing the external leaf of the wall in this area and re-keying the stones together using a lime mortar. The next stage is to insert several metal tie rods into the cracks; these are grouted in to ‘stitch’ the building back together.
At this point the structure will be stable, but will still be affected by the ground movement and this is where the underpinning is vital. The underpinning will involve digging a series of trenches and backfilling them with concrete. The trenches are dug in a specific sequence to ensure that only a small segment of the wall is left unsupported at one time. It is an arduous process and requires the expertise of an experienced groundwork contractor.
The depth of the underpinning is typically specified by the engineer, and is fundamental to the cost and complexity. At Mabel’s the depth of the underpinning relates to the influence of the mature walnut tree at the edge of the garden as shown in the tree influence drawing.
Tree influence only affects foundation depth on buildings that are located where the soil has a high clay content. The depth ranges from three to four feet and it is required around the whole perimeter of the building — that’s a lot of concrete!