Our last blog piece looked at cracking in houses and we mentioned the nasty “U word” – underpinning. As we touched on in that article, underpinning is a widely misunderstood exercise, and many builders and inspectors often nominate underpinning as a solution to house defects caused by footing movement when, many times, it’s an inappropriate response or ineffective solution. In some cases, it can even exacerbate the situation and make the damage worse. When it comes to treating and/or repairing footings and foundations, it’s vital to first identify the cause of the movement/damage and treat that, rather than merely treat the symptom.
So what is underpinning and when should you employ it? Importantly, when should you not employ it? To answer those two questions, we first need to define and explain a few concepts….
In simple terms, underpinning is the strengthening or improving of either a footing, or the foundation material directly underneath the footing. Some tangible examples of underpinning are:
- Pouring concrete underneath an old brick or stone footing to bridge any joints and allow the footing to sit on a more stable platform.
- Injecting resin or cement powder into the loose sand underneath a footing to strengthen the foundation soils and prevent them from consolidating (settling) or washing away.
- Installing bricks or mass-concrete directly underneath a footing to extend the footing down to a more stable foundation material. In many cases in Sydney, this might involve taking shallow footings that were originally built at high-level on loose material and underpinning (extending) the footing down to solid rock. (This is what is taking place in the photograph shown above.)
Underpinning can be effective and necessary if there’s a particularly localised portion of a house’s footing (or the soil material underneath the footing) that is defective or simply different to the rest of the house. A classic example might be the corner of a house where an old, disconnected downpipe discharged stormwater from the roof eaves gutters down into the ground and flushed away or consolidated the loose material underneath the footing at that location. The rest of the house’s footings have remained strong and stable, but the corner of the house has subsequently settled because the underlying soil at the location has been affected. In such a situation, underpinning the footing in this corner of the house would be an appropriate response to re-instate the foundation material to match the rest of the house. (But only after FIRST repairing the downpipe and connecting it up to the stormwater system, or the problem will simply return with ongoing rainfall!)
Situations requiring underpinning usually arise with houses that have been renovated and added to over the years. Typical examples include:
- An old 1920’s house that was built on sandstone block footings on reactive clay at shallow level, but the rear extension added in the 1980’s was built on concrete strip footings that were dug deeper into more stable ground. Since the two different parts of the house are on different footing systems on different foundations, differential settlement is likely to occur, which is likely to cause nasty cracking in the walls where the original construction meets the more recent add-on. Underpinning at this location may be a solution.
- A single-storey house may have had a First Floor addition added to it, which has increased the loads on the original walls and footings. There may be a concentrated load or an accumulation of stress in one location that is greater than what the underlying soil can support, and so underpinning the foundations at that location will help the ground to adequately support the increased load.
Another common problem is houses built on sloping sites. The slope and topography of the underlying rock below the surface may not match the slope of the natural ground at the surface, meaning that a portion of the house might be sitting on rock, whilst another portion is not. (See illustration below).
Similarly, the underlying rock might suddenly dive away at a localised section, leaving a small portion of the house on foundations that will behave differently to the rest of the house. (Again, this is differential settlement).
In both the situations above, underpinning can be utilised to re-instate support to those footings which are on the softer material, and to transfer the building at those locations down to the same rock level as the rest of the house.
The above discussion all revolves around remedial underpinning – that is, underpinning to solve and rectify existing building defects. An altogether different reason for underpinning is when you’re excavating immediately adjacent to an existing footing and you need to protect and prevent the footing (and the structure above it!) from collapsing into your excavation. This is a common scenario in house renovations when excavating to create a new basement or Lower Ground Floor level, or when carrying out landscaping works and levelling on a sloping site.
When excavating next to an existing wall and footing, the footings are underpinned down to the same level as the proposed excavation BEFORE the excavation commences. It obviously isn’t possible to underpin the entire length of footing in one go, and so underpinning is typically undertaken in a segmental hit-and-miss fashion. The maximum width of each individual underpin segment is typically 600-900mm, depending on the construction of the footing above and the stability of the soil. As you can appreciate, underpinning is thus a slow and awkward procedure. It is also a specialist building discipline and can be very expensive. You should only engage experienced contractors to carry out such works.
So if you’ve got foundation movement at your house and cracks have appeared, make sure you engage an experienced and knowledgeable structural engineer to investigate the situation first before undergoing repairs.