The word 'subsidence' probably sends a shiver up every home owner's spine, and the changes in the contours of the land that occur during subsidence can cause extensive damage to your home's foundations. Subsidence can be caused by a number of processes, from natural limestone erosion to excessive groundwater extraction (a particular problem in the more arid parts of Australia), but when it comes to saving a home's foundations from crumbling, underpinning is almost always the best way to tackle the issue.
Underpinning involves reinforcing a house's existing damage foundations, and most residential underpinning projects use one of two distinct methods to achieve results. If your home has fallen victim to subsidence and you are casting around for repair quotes, it's important to know the advantages and disadvantages of each method.
Mass concrete underpinning
This traditional method of foundation reinforcement is primitive but undeniably effective, and essentially involves installing a second foundation underneath your home's primary foundations. During mass concrete underpinning, voids dug in the ground underneath your foundations are filled with liquid concrete; the voids are filled slowly and in a specific order, to minimise the amount of strain and axial forces placed on the building above.
The great advantage of this method lies in in its simplicity, as it requires a minimal amount of heavy equipment. The voids dug underneath your foundations are small enough to be dug out with manual digging tools, significantly reducing the amount you are likely to pay on labour and equipment costs. Mass concrete underpinning also allows the building above the damaged foundations to be used throughout the underpinning process, and barring any mishaps you will not be required to vacate your home at any point.
However, this type of underpinning relies on relatively consistent ground materials, and subsidence damage caused by limestone leeching or other natural processes may require a more complex approach to avoid the underpinning falling victim to the same problems as the original foundations. Mass concrete underpinning also becomes somewhat impractical when dealing with the particularly deep foundations of larger homes, as the difficult of digging and filling voids at such depths can necessitate days or even weeks of (expensive) labour.
Beam and base underpinning
A more advanced form of underpinning, this method involves excavation of the damaged sections of foundation. The building is supported by special load-bearing equipment while the foundation is excavated to maintain structural integrity of the building above. The damaged foundations are then replaced with a strong, complex network of reinforcing metal beams which extend to a significantly greater depth than the original foundations, and which are supported by a robust concrete base. The metal beams are then encased in poured concrete to provide a strong and stable foundation.
This method of underpinning is more flexible, and can be used on a wide variety of soil types. It is particularly useful in cases where subsidence has been caused by excessive groundwater extraction caused by aquifer use, tree roots etc., as it allows for the installation of layers of anti-heave material. This flexible material helps provide a buffer against damage caused by soils expanding or contracting due to changes in groundwater levels -- if your foundations are set in highly absorbent clay soil, mass fill underpinning may be totally unsuitable.
Unfortunately, this modern underpinning method requires modern equipment and expertise, and you can expect to pay a higher premium if you choose this option due to the costs of using heavy load-bearing equipment and hiring structural engineers to design the metal beam structure. Because the damaged foundations are often removed entirely, it may not be possible to remain in your home for the duration of the underpinning process, further increasing potential cost
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