If you find cracks in your walls, door or window frames coming away from the brickwork, or the building visibly leaning to one side, it could be signs of subsidence. Causes can vary from tree roots too near the house to old mineworkings below, but in many cases the best solution is underpinning.
What Is Underpinning?
Underpinning refers to various techniques of extending the foundations of a building. The usual approach is to take the foundations down to a lower substratum of the soil which can support it more securely, although sometimes the soil itself can be strengthened. Older buildings often have only very shallow foundations, but even a modern building with strong foundations can need underpinning if the ground no longer supports it adequately, for whatever reason.
Mass Concrete Method
This has been around a hundred years but can still be effective, though it’s a labour-intensive approach. It involves digging out sections of the area below the foundations, one at a time, and pouring in concrete. Eventually, you’ll have a complete extra layer of concrete beneath the original foundations, directing the load down to a more secure level. This is generally best with shallow foundations. Although it takes longer than other methods, it has the advantage of needing no heavy machinery.
Beam and Base Method
In this approach, a reinforced concrete beam is inserted, usually below the existing foundations, resting on a number of mass concrete bases. If the bases are placed effectively, this will spread the weight down to a firmer layer of subsoil.
Perhaps the most popular approach today, mini-piling involves either auguring or driving slender piles down to considerable depths and anchoring them in a stable level. This takes specialist equipment and a good deal more technical precision than the mass concrete method, but enables the piles to take advantage of stable levels well below five metres down — levels that would be beyond other underpinning techniques.
Expanding Resin Injection Method
A fairly recent innovation, this aims to harden the soil rather than add extra foundations. A mixture of resin and hardener is injected into the ground, compacting the soil and allowing it to support the building’s weight more effectively.
Which Is the Best Method?
All four methods have their advantages, and it will depend on the precise situation. Most underpinning nowadays uses either mini-piling or resin injection, though the former is usually more secure, especially if a strong substratum can be found more than five metres down.
If you need advice on what kind of underpinning your house needs, contact us to discuss your problem.