What is underpinning and when is it needed?

Underpinning is required when a building’s foundation becomes unstable or unable to adequately support the structure. There are several common reasons for this, including changes in soil conditions, poor original foundation design, nearby excavation or construction, building modifications, and natural disasters. Over time, the soil beneath a building may shift due to factors such as subsidence, moisture fluctuations, tree root activity, or poor drainage, leading to foundation movement or weakness.

Additionally, if soil properties were not thoroughly assessed during the initial construction phase, the foundation may not have been designed to accommodate site-specific conditions, resulting in long-term instability. Large-scale excavation projects near an existing structure can also disrupt soil composition and compromise foundation support. Furthermore, substantial renovations or the addition of extra storeys can place additional stress on the foundation, necessitating reinforcement to manage the increased structural weight. Natural events such as earthquakes, floods, and droughts can significantly impact soil stability, leading to foundation movement and the need for underpinning.

Mainmark provides underpinning in, Canberra, Hobart, Townsville  NSW, QLD, SA, WA, Tasmania, Victoria plus the major cities of Sydney, Melbourne, Perth, Adelaide, Brisbane, Newcastle

Mainmark also provide the following in Australia: wall crack repairs, sinking floor repair, Concrete slab repair, sinking foundation repair, underpinning, ground improvement, re-levelling, subsidence repair, void filling and slab lifting

The type of soil beneath a building plays a significant role in determining the stability of its foundation. Some soils, known as reactive soils, undergo substantial volume changes due to variations in moisture content. These fluctuations can cause expansion and contraction, which may compromise the integrity of a building’s foundation.

A building’s foundation is composed of both the footing system and the flooring system, working together to ensure structural stability. In residential construction, there are two primary types of flooring systems: slab-on-ground foundations and suspended floors.

Slab-on-ground foundations include raft slabs, waffle pod slabs, reinforced slabs on fill, and slabs with dropped edge beams. Suspended floors, on the other hand, consist of stumps or piers that support timber or steel bearers and joists. Common footing systems include continuous footings, which are concrete strips or slabs that evenly distribute loads, and pad footings, which are square or round concrete pads used to support concentrated loads, often in conjunction with stumps. Other types include stumps made from hollow steel, timber poles, or machined timber posts designed for elevation and stability, as well as piles and piers such as bored piers, driven piles (timber, steel, or concrete), and steel screw piles, which provide deep foundation support.

There are several reasons why building foundations may fail, leading to structural damage and necessitating underpinning. One major factor is reactive soil, which expands, and contracts based on moisture levels. In dry conditions, these soils shrink, causing settlement, while in wet conditions, they swell, leading to heaving. Both of these movements can result in foundation instability and structural cracking.

Poorly compacted fill is another common issue. If fill material was used during construction and not adequately compacted, the foundation may settle unevenly, leading to structural cracks and movement. Additionally, erosion caused by water leaks, poor drainage, or flooding can wash away soil supporting a foundation, causing instability. Buildings constructed on slopes are also susceptible to gradual downhill movement (creep) or sudden landslides, necessitating underpinning for stabilisation. Tree root activity can further contribute to foundation problems, as large trees extract moisture from the soil through transpiration, causing shrinkage and movement.

There are various methods of underpinning that can be used to address foundation issues, depending on the severity of the problem and the site conditions. One common method is concrete slab underpinning, which involves expanding the foundation using reinforced concrete. Screw pile underpinning is another effective technique, using steel piers combined with concrete footings to provide stable support and lift structures back into position.

Them modern alternative to traditional underpinning is, resin injection, wherein a polyurethane resin is injected into the soil to provide ground improvement and level correction. 

Several telltale signs may indicate the need for underpinning. These include Cracked Walls, particularly if they are widening over time, as they can signify foundation movement. Uneven or sloping floors are another indicator of foundation instability, as are doors and windows that have become misaligned and difficult to open or close. Additionally, if gaps or trenches appear around a building’s foundation, this may be a sign of soil subsidence, requiring professional assessment.

If you suspect that your foundation may be compromised, it is essential to consult a professional before the damage worsens. You can contact our engineering experts at 1800 623 312, or you can fill out a quick enquiry form. 

A structural engineer can conduct a thorough site analysis to determine the extent of foundation issues and recommend the most suitable underpinning method. Some underpinning companies offer in-house engineering services, streamlining the process for property owners.