What is Underpinning?
Underpinning is the process of reinforcing a current foundation or footing when the proposed works are to bear further loads on the wall or because the existing foundations are failing.
Why Underpin?
You may want to underpin a wall if you plan to build on it, such as add an additional floor or extension. You may want to repair a wall by underpinning it because there are signs of subsidence (i.e. cracks in walls or uneven floors).
Underpinning can be done by extending the foundation in depth and/or width so the wall either rests on a more supportive soil stratum or distributes its load across a greater area. The use of micropiles and jet grouting are becoming common methods in modern underpinning. There are alternatives to underpinning, the strengthening of the soil by using special grout, including expanding urethane-based engineered structural resins.
Types & Methods of Underpinning
There are five main types of underpinning used by the construction industry:
Mass Concrete Underpinning – This is the most traditional method, 100 years on and it’s still in use. The method includes digging boxes by hand underneath a wall and sequentially pouring concrete in a strategic order. The result is an additional layer of foundation built underneath the existing foundation.
Beam and base underpinning – This method is based on the Mass Concrete method mentioned above. But, in this method rather than pouring concrete underneath the existing foundation, a reinforced concrete beam is constructed below, above or in replacement of the existing foundation. The beam then transfers the load of the building to mass concrete bases, which are built at specific load bearing locations.
Mini-piled underpinning – This is a much more versatile method, it is used where ground conditions are variable, where access is restrictive, where environmental pollution aspects are significant, and where structural movements in service must be limited.
Mini-piled underpinning schemes – Like the above method, Mini-piled underpinning schemes include pile and beam, cantilever pile-caps and piled raft systems. Cantilevered pile-caps are usually used to avoid disturbing the inside of a building, and require the construction of tension and compression piles to each cap.
Underpinning by expanding resin injection – For this method a mix of structural resins and hardener is injected into the foundation ground beneath existing footings. The expansion element of the process occurs when a chemical reaction happens between the resin and hardener on entering the ground.
Underpinning a Party Wall
Quite often a wall shared between neighbours will need underpinning, either for repair or to strengthen. The same Party Wall Act rules apply, generally, the Building Owner would engage a Party Wall Specialist to ensure they are legally compliant. Be aware of the potential risk of having no Party Wall Agreement as this could lead to significant legal costs by way of an injunction and claims of damage.
Are you or your neighbour looking to carry out underpinning work? Do you need assistance with Party Wall notices and Surveying? We at Seven One Associates are here to help with our expert Party Wall Surveyors in London.