What is Foundation Underpinning?

Foundation underpinning is a method of strengthening and reinforcing an existing foundation of a building or structure by increasing its depth. This is usually done by digging out the soil and replacing it with a more resilient material such as concrete. This is a complicated construction process that requires a geotechnical engineer and is often carried out in stages called ‘pins’.

The need for underpinning often arises when the ground conditions beneath your home shift, causing parts of your foundation to lose their bearing capacity. This can be caused by several factors including changes to the soil composition – clay soils expand and contract more than sandy or loamy soils, making them more susceptible to shifting – as well as excessive moisture and shrinkage. Poor design of the original foundations may also lead to a deterioration in soil properties.

Foundation Reinforcement: The Basics of Underpinning

Underpinning repair methods vary depending on the type of soil you have, and the structural integrity of your building. The most commonly used solution is pier underpinning which involves hand-excavating pits below the footing, and then placing concrete to form a series of piers that bear at a deeper level than the current foundation. This technique can be applied to transfer the support of both wall and column footings. Beam and needle beam underpinning methods are also possible, but these tend to be more expensive as they require more extensive excavation.

In more challenging situations, a more involved method known as mini pile underpinning can be employed. In this process, long piles (like those that hold up a clothesline) are similarly driven into the ground to traditional mass concrete underpinning. This can be a more cost-effective method, but still requires a professional geotechnical engineer to perform the work.