Subsidence Repair Adelaide

Subsidence is when the ground beneath your home sinks or settles, pulling the foundation down with it. It’s one of the most serious foundation problems an Adelaide homeowner can face — but when it’s caught and treated properly, the damage can be stopped and the foundation stabilised permanently.

Through ADL Underpinning, you can connect with local contractors who specialise in diagnosing and repairing subsidence damage across Adelaide. If your home is sinking, cracking, or showing signs that the ground beneath it is giving way, this page explains what’s happening, what’s causing it, and how contractors fix it.

What Is Subsidence? Subsidence is downward movement of the ground beneath a building. Unlike normal settlement (which happens gradually in the first few years after construction), subsidence is ongoing ground movement that the original foundation wasn’t designed to handle.

The key distinction: settlement is the building compressing the soil directly under its weight (expected and usually minor). Subsidence is the ground itself sinking due to external factors — moisture changes, soil erosion, decomposition, or removal of material below the surface.

What Causes Subsidence in Adelaide? Adelaide’s soil and climate create several common subsidence triggers:

Reactive Clay Shrinkage This is the number one cause. Adelaide sits on extensive deposits of Keswick and Hindmarsh Clay — some of the most reactive clays in Australia. During dry summers, this clay can shrink by up to 50–70mm vertically, pulling the foundation down with it. When winter rains arrive, the clay swells again, but rarely returns to exactly where it was. Over years, this ratcheting effect causes progressive subsidence. Suburbs most affected include Burnside, Unley, Mitcham, and Campbelltown.

Tree Root Moisture Extraction Large trees — especially eucalypts, willows, and poplars — extract enormous volumes of water from the soil through their root systems. This localised drying causes the clay to shrink beneath one part of the foundation while the rest stays put, creating differential subsidence that’s particularly destructive.

Water Erosion and Washout Leaking water mains, broken sewer pipes, poorly directed stormwater, and inadequate drainage can wash soil out from beneath foundations. This creates voids that the foundation eventually collapses into. Common in older suburbs with ageing infrastructure like Prospect, Norwood, and Adelaide City.

Poor Fill and Compaction Some Adelaide homes — particularly in the outer suburbs developed in the 1960s–1990s — were built on poorly compacted fill material. This fill settles over decades, causing the foundations to drop unevenly.

Adjacent Excavation Construction next door, particularly basement excavation or deep trenching, can remove the lateral support that keeps the soil beneath your foundation stable, triggering sudden subsidence.

Signs of Subsidence Watch for these warning signs:

Diagonal cracks — typically radiating from the corners of doors and windows, wider at the top than the bottom (indicating the foundation has dropped on one side) Widening cracks — cracks that visibly grow wider over weeks or months, particularly during dry periods Sticking doors and windows — frames distort as the structure moves, making doors hard to open or close Sloping floors — a marble or ball rolls consistently to one side, or you can feel the slope when walking Gaps at skirting boards — the wall is pulling away from the floor as the foundation drops External cracking — stepped cracks in brickwork following the mortar joints, or vertical cracks through bricks (more serious) Separation of building elements — extensions pulling away from the main house, verandah posts no longer meeting the beam, gaps at expansion joints If you’re seeing several of these signs, it’s worth getting a professional inspection sooner rather than later.

How Subsidence Is Repaired in Adelaide The right repair method depends on the cause, severity, and type of foundation. Contractors use several approaches:

Screw Pile Underpinning Screw piles are a common recommendation for subsidence caused by reactive clay. Steel piles with helical plates are screwed through the reactive layer into stable ground below, then connected to the existing footing. This permanently anchors the foundation to ground that doesn’t move with the seasons.

Mass Concrete Underpinning Mass concrete involves excavating beneath the existing footing and pouring new concrete pads that extend down to stable soil. It’s well-suited for straightforward subsidence repair where access is good and the reactive layer isn’t too deep.

Resin Injection For minor subsidence caused by soil voids or washout (rather than deep clay movement), resin injection can fill the voids, compact the soil, and lift the slab back toward level — all in a matter of hours.

Beam and Base Underpinning For severe subsidence affecting multiple walls, beam and base provides the most comprehensive solution — a new reinforced concrete foundation system that redistributes loads evenly and prevents future differential movement.

The Subsidence Repair Process Inspection and diagnosis — the contractor inspects the property, documents all damage, identifies the cause of subsidence, and assesses whether movement is ongoing or stabilised Engineering assessment — for significant subsidence, the contractor coordinates with a structural engineer to design the repair. This includes soil assessment, footing investigation, and a detailed engineering specification Fixed-price quote — a clear, written quote covering the full scope: underpinning, any necessary drainage corrections, relevelling, and certification Underpinning works — the contractor carries out the foundation stabilisation using the specified method, following the engineer’s design exactly Address contributing factors — if the subsidence was caused or worsened by drainage problems, tree roots, or plumbing issues, these need to be resolved alongside the underpinning to prevent recurrence Relevelling — where possible, the foundation is lifted back toward its original level using hydraulic jacks Certification and documentation — the engineer inspects the completed work and issues a compliance certificate. You receive a full documentation pack for your records and for any future property sale Subsidence Repair Costs in Adelaide Costs vary significantly based on the cause, extent, and chosen repair method:

Minor subsidence (localised, one area, resin or small-scale piling): $3,000–$12,000 Moderate subsidence (one or two walls, screw piles or mass concrete): $12,000–$30,000 Severe subsidence (entire perimeter, beam and base, extensive relevelling): $30,000–$70,000+ Visit our cost guide for detailed breakdowns, or use our cost calculator for a property-specific estimate.

Don’t Wait — Subsidence Gets Worse Foundation subsidence compounds over time. Every dry Adelaide summer shrinks the clay further. Every wet winter swells it back unevenly. Cracks widen. Doors jam harder. Floors slope more. What starts as a $10,000 repair can become a $50,000 project if left for several more seasons.

The good news: catching it early means simpler, cheaper repairs and better outcomes. Send photos of the damage and your suburb through our contact form, and we'll pass them to a local contractor. The contractor can give you an honest assessment of the situation — whether it needs urgent attention, monitoring, or something in between. No obligation, no pressure.