Does Underpinning Really Work?

Visible cracking in walls, uneven floors, or doors that no longer close properly can be distressing for any property owner. These are common signs of structural movement, often leading to concerns about foundation failure. One of the most talked-about remedies is underpinning a method designed to strengthen or stabilise the existing foundations of a building. While it is widely used and effective in the right circumstances, underpinning is not always the ideal or only solution.

Whether underpinning will work for your property depends on several factors. These include the type of soil beneath your home, the original foundation design, the cause and extent of movement, and how the underpinning is carried out. In many cases, it is a valuable structural intervention that delivers long-term stability. However, its success hinges on proper diagnosis and engineering design. Without this, you risk investing in a solution that may not address the root problem.

At OB Engineering, based in Adelaide, we help clients navigate this process with confidence by providing evidence-based assessments and tailored engineering solutions.

Will Underpinning Fix the Cracks in my House?

One of the most common questions we receive is whether underpinning will eliminate the cracks already visible in a home. The answer is both yes and no depending on expectations. Underpinning is intended to stabilise the building by addressing the movement that caused the cracking. In doing so, it can prevent further damage, and in some cases, minor cracks may close slightly as the structure settles.

However, underpinning does not act as a cosmetic repair. Cracks that have already formed typically need to be repaired separately, once stability has been achieved. These repairs might include joint sealants, or replastering and repainting. More importantly, if the original cause of movement, such as moisture fluctuations, tree roots, or leaking pipes, is not also addressed, new cracks may appear even after underpinning has been completed.

This is particularly relevant in areas with reactive clay soils, such as many Adelaide suburbs, where seasonal changes in soil moisture can lead to significant foundation movement. If underpinning is applied to only part of a structure, it may change how the building responds to those soil conditions. This can inadvertently shift stress to adjacent areas, resulting in new cracking elsewhere. For this reason, underpinning should be part of a broader strategy, not a standalone fix.

What Is Underpinning?

Underpinning is the process of reinforcing or strengthening the foundation of an existing structure. This is typically done to halt ongoing movement, correct differential settlement, or prepare the structure to bear increased loads, such as from renovations or additional storeys.

The method involves either extending the existing footings deeper into more stable soil layers or increasing the strength and stiffness of the soil beneath. The approach used depends on several site-specific factors, including soil composition, access to the work area, the design of the original structure, and the severity of the movement.

In residential settings, underpinning is often required when movement has caused visible signs of distress such as cracks in masonry, separation of cornices, or uneven internal floors. For commercial or industrial buildings, the focus may be on performance, safety, or protecting assets and operations from interruption.

What are Some Common Underpinning Techniques

Two of the most used methods in South Australia are concrete underpinning and resin injection. Each has its own advantages and is suited to specific conditions.

Concrete underpinning is a traditional and robust method that involves excavating beneath the foundation and pouring reinforced concrete piers down to more stable soil layers. These piers are often installed several metres deep, especially in reactive soils, and are designed to bypass the upper, less stable ground. A pile cap is then formed to transfer the building load onto the new piers. This method is well-suited to sites with significant or long-term movement, particularly where access is available for excavation.

Resin injection, by contrast, is a modern technique that uses an expanding polymer resin injected into the ground under pressure. As the resin expands, it fills voids, compacts loose soil, and can gently lift settled areas of the foundation. This method is less disruptive and quicker to implement, making it ideal for sites where access is restricted, such as beneath internal floors. However, it is not always suitable for deeper movements or highly variable soil profiles.

Selecting the right method involves a combination of engineering judgement, soil testing, and an understanding of the structure’s behaviour. At OB Engineering, we conduct detailed site investigations to ensure the most appropriate technique is selected for each project.

How do I Know if Underpinning is Right for my Property?

Choosing to underpin a building is not a decision to be made lightly. It’s important to first understand the cause of the structural movement. Without this knowledge, any repair underpinning or otherwise may fail to resolve the problem.

A professional structural assessment should be your first step. This may involve visual inspections, floor level surveys, crack monitoring, and soil investigations. In some cases, external factors such as poor drainage, plumbing failures, or nearby trees may be contributing to the issue. These should be addressed in conjunction with any underpinning to ensure a comprehensive and lasting solution.

Once the cause of movement is understood, your engineer can determine whether underpinning is necessary, and if so, what method is best suited to your site and structure. In some cases, underpinning may not be required at all; instead, correcting drainage or removing tree roots may be enough to stabilise the foundation.

Is Underpinning the Final Fix?

While underpinning can successfully stabilise a structure, it should be viewed as one part of a broader remedial strategy. Without addressing the environmental and site-related factors contributing to the movement, further problems may arise.

For instance, if tree roots continue to draw moisture from reactive soils near the foundation, movement may continue in areas that were not underpinned. Likewise, poor surface drainage or broken plumbing may undermine soil strength over time, even after underpinning.

In many cases, additional works such as root barrier installation, drainage improvements, or regrading of the site may be necessary. Your engineer may also recommend staged repairs to monitor how the structure responds over time. This holistic approach ensures that both the symptoms and causes of movement are addressed.