Why is one section of my Ottawa concrete driveway sinking?

A sinking section of your concrete driveway is almost always caused by soil settlement or washout beneath the concrete slab, creating a void that allows the heavy concrete to drop into the space. This is particularly common in Ottawa due to our clay soils, freeze-thaw cycles, and heavy spring runoff that can erode base materials.

The most likely culprit is poor drainage causing soil washout. Water flowing under your driveway — whether from downspouts, poor grading, or spring snowmelt — gradually washes away the granular base or erodes the clay subgrade beneath the concrete. Once enough material is removed, the unsupported concrete section cracks and settles into the void. This often happens along driveway edges where water runs off the surface, or near downspouts that discharge too close to the driveway.

Clay soil movement is another major factor in Ottawa. The clay soils prevalent in areas like Barrhaven, Orleans, Gloucester, and Riverside South expand when wet and shrink when dry. If your concrete was poured directly on clay without adequate granular base (minimum 150mm of compacted Granular A in Ottawa), seasonal clay movement creates uneven support. Wet clay swells and pushes up; dry clay shrinks and creates voids. Over multiple seasons, this differential movement causes sections to settle.

Inadequate base preparation during original construction often becomes apparent years later. Many Ottawa driveways built in the 1980s and 1990s were constructed with minimal base — sometimes just sand or a thin layer of gravel. Ottawa's 1.2 to 1.5-metre frost depth requires substantial granular base to prevent frost heaving and settling. Without proper base depth and compaction, the concrete essentially "floats" on unstable soil that shifts with freeze-thaw cycles.

Utility line settlement can also cause localized sinking. If water, sewer, or gas lines run beneath your driveway, the excavated soil around these utilities may not have been properly compacted during backfilling. Over time, this loose soil settles, creating a depression that the concrete follows.

Immediate steps: Stop water from reaching the settled area by redirecting downspouts, improving surface grading, and sealing any cracks that allow water penetration. Mark the settled area and monitor whether it continues dropping — active settlement indicates ongoing soil movement that needs professional attention.

Professional repair options include slab jacking (pumping grout or polyurethane foam under the slab to lift it back to level) or mudjacking for smaller areas. Costs typically range from $800 to $2,500 depending on the size and access. If the settlement is severe (more than 50mm) or the concrete has cracked extensively, replacement of that section may be more cost-effective.

When to hire a pro: Any concrete settlement requires professional assessment to determine the underlying cause and appropriate repair method. A structural engineer or experienced concrete contractor can evaluate whether the settlement is stabilized or ongoing, and recommend the most durable repair approach for Ottawa's challenging soil and climate conditions.