What is underpinning of a building?
Underpinning is the process of supporting or strengthening the foundation of an existing house, building or similar structure. This is accomplished by reinforcing the existing foundation, strengthening the soil by introducing an expanding filler, or extending the foundation so that the load is distributed over a greater surface area.
When is underpinning needed?
For most homeowners, underpinning is required when the original foundation is not strong enough to support the house. This is usually a result of the soil supporting the foundation changing in some way (through subsidence, expansion or contraction due to moisture, large trees nearby, or damaged plumbing left unrepaired), or the properties of the soil not being adequately understood during the original design of the foundation.
In less common cases, underpinning is also required following a major renovation that changes how the structure is used, new construction nearby that excavates supporting soil, a desire to increase the capability of existing foundations (such as adding a storey), or natural disasters such as earthquakes, floods or droughts.
Soil types and site classifications
The soil type plays a key role in the stability of foundations. Certain types are prone to more significant changes in soil conditions (for example during extended periods of wet or dry weather) and thus contribute to structural foundation problems. We call these soils "reactive". When it comes to building foundations, we characterise the soil by its Site Classification, in accordance with Australian Standard AS 2870/2011, Residential slabs and footings.
Class P covers problem sites, including soft soils such as soft clay, silt or loose sand, varying depths of fill, landslips, mine subsidence, collapsing soils, soils subject to erosion, and sites that cannot otherwise be classified. The "D" inclusion refers to deep movements in soil due to deep variances in moisture, mostly found in dry areas.
Types of building foundations and footings
Technically, the "foundation" is the earth or strata upon which the "footings" for a building are constructed. In Australia the word "foundation" is regularly used to refer to the footing system and flooring system together. In residential construction there are two common flooring systems: slab on ground (raft slab, waffle pod slab, slab with dropped edge beams, or reinforced slab on fill) and suspended floors (framed with stumps or piers, supported by bearers and joists).
Common footing systems include continuous footings (a concrete strip or slab for uniformly distributed loads), pad footings (square or round concrete pads for a concentrated load), stumps (hollow steel or timber poles or machined timber posts), and piles and piers (driven or bored into the ground, including poured concrete piers, bored piers, driven piles and steel screw piles). Underpinning is most commonly carried out on slab-on-ground foundations.
Why do building foundations fail?
Reactive soils are the most frequent cause. Movement involves shrinkage (leading to settlement) or expansion (causing heaving). When dry conditions persist, soils lose moisture and shrink; when moisture is elevated, soils swell, sometimes by several hundred percent. Both can compromise the integrity of the foundation, resulting in heaving, subsidence and visible cracking.
Other causes include poorly compacted fill, site erosion (from a burst water pipe, uncontrolled water flow or inadequate drainage), slope failure (creep or landslides), transpiration from trees drawing moisture from the soil, and, to a lesser degree, inadequate original foundation design (less of an issue under modern building codes).
Types of underpinning
When repairing foundations on buildings with stumps, the method used is referred to as restumping or reblocking, which is not considered underpinning. Within underpinning, there are three methods in use today:
Concrete slab. Traditionally used to increase the size of foundations and, in the process, strengthen them. Still widely used today.
Screw pile. The method used by Foundation Solutions, combining steel piers with concrete footings to secure the building and lift it back to its original position, closing up gaps and cracks. The use of piers is considered a permanent solution, one that will not be affected by further changes in the ground surrounding the house, which is why we use this method.
Grout / resin injection. The newest method available, though it is not actually underpinning. It involves injecting resin or grout into the ground to fill voids under the slab and expand, compressing the ground. It is the least quantifiable method in terms of permanency and final cost, and is not suited to all ground conditions.
Do I need underpinning?
Subsidence occurs on many properties to varying degrees, but underpinning is only required where subsidence is actively occurring. Sometimes after subsidence initially occurs, the structure reaches equilibrium and there is no further danger. Signs to look for include cracks in floors or walls (observe them over weeks or months to see if they widen), floors that are not level (a lean, misaligned doors, or irregular trenches forming around the slab edge), and doors and windows out of alignment (widening gaps, jamming, or frames pulling away from walls). If you are unsure, it is best to ask a professional, which is why we provide a complimentary Home Check-up service.
When to get help, and do I need a structural engineer?
None of the above indicators guarantee you will require underpinning, but you don't want to wait until symptoms become severe. Do some research, speak to a few people willing to provide helpful advice, and get someone to investigate firsthand. Our complimentary Home Check-up is an excellent first step and avoids you having to pay for engineers when they may not be needed. During our Home Check-ups we determine whether a structural (forensic) engineer is required for a more detailed assessment. We use our own structural engineers, so you don't need to ring around for separate quotes.
Will underpinning provide a permanent fix?
The answer depends on the soil your home is sitting on (the site classification) and the type of underpinning you choose. Not all types are suited to all foundation problems. When we do our complimentary Home Check-ups, we let you know up front if our method can provide a permanent fix. If not, we won't quote you and will recommend an alternate course of action. We exclusively use screw pile underpinning because it is the most reliable method available.