Typically there are two reasons for underpinning, subsidence or creating new deep basement beneath the existing building.
Reasons for subsidence
- Small foundations – before in 1964 building regulations were introduced there was no legal requirement to what depth and width foundation strip should be. As such we find many houses have small or no foundations.
- Soft soil – silt, soft clay, and sand are examples for weak ground that may cause subsidence.
- Coal mining – cavities in the soil beneath the building may collapse at any time taking the building down. Checking for coal mining report at Coal mining authorities may help to establish the risk of past mining. Alternatively, check maps at the British Geological Society for coal reserves
- Trees – vegetation near the house grow roots to under the foundations, extracting water and minerals causes soil shrinkage. Depending on a tree species and soil type we can expect up to 40% of soil volume reduction. See more at NHBC regulations
Deep concrete underpinning
To fix the subsidence, the most common solution is to dig 1m x 1m holes in 3 meters intervals under the existing foundation and to pour new concrete into these openings. This way we create new larger footing to prevent further sinkage of the building, but this method does not repair buckled and cracked walls, it only stops further subsidence.
The second method is to insert mechanical lifts, or expandable concrete filled balloons to lift the building up to the original positions. This method fixes the problem associated with masonry, and it prevents any future substance. The second method of underpinning is not as popular in the UK, but we work with contractors that can take such a project.
Deep basement project
The second reason for underpinning is to create new space, to deepen existing or to create a new basement. It is an expensive project, but with land and house prices going up, many people decide to increase the basement depth to create an additional room or entire flat beneath the existing house.
At CES working with Leeds ground contractor, together we developed a very original method that allows 1.5m long holes using sacrificial props. We provide detailed technical drawings as well as a project statement with graphical illustrations of the process. This method saves much time and money comparing to traditional underpinning using 1m x 1m holes.