Underpinning is usually used to increase a foundations depth or for repairing faulty foundations. It might be the case that your foundations need underpinning if you are planning to add additional stories to an existing structure or when your foundation have been damaged. One of the visible signs that your building may need underpinning is if cracks are visible. A building will need underpinning if cracks are wider than 6mm – especially diagonal cracks. Foundation failures could also be to blame when your home has cracked or buckled walls or cracked concrete floors.
The most common method of underpinning is concrete underpinning. The method itself is nearly 100 years old, and things haven’t changed much in that time. This method strengthens an existing foundation by digging boxes by hand or machine underneath your foundations and sequentially pouring concrete in a specific order. The final result is a brand new foundation built under your existing foundation. This method is used when your existing foundation is at a shallow depth, however, may still be suitable for depths up to 10 meters deep.
There are other methods of underpinning for specific scenarios such as:
Beam and base underpinning – Construction of a reinforced concrete beam below/above/as replacement of the existing foundations. The beam then switches the load of the building to mass concrete bases, which are built at different locations and sizes, depending on ground conditions.
Underpinning by resin injection – A mix of resins and hardeners are usually injected into the ground beneath footings. Upon injection, the resin and hardener mix and begin to expand, filling any voids and crevices whilst compacting any weak or loose soil.
Mini-piled underpinning – This is usually the best option when access is restrictive or where structural movements must be kept to a minimum.