What Causes a Concrete Driveway to Sink on One Side?

A concrete driveway sinking on one side is primarily caused by issues with the underlying soil, often due to improper subgrade compaction, inadequate drainage leading to soil erosion, or soil movement from expansive clays or tree roots.

A concrete driveway sinks on one side primarily due to poor subgrade compaction, soil erosion from inadequate drainage, or ground movement caused by expansive soils or tree roots.

A sinking concrete driveway usually points to problems occurring beneath the surface, specifically with the subgrade—the prepared soil layer directly under the concrete. The stability of your driveway depends entirely on this foundation, and when it fails to provide uniform support, the concrete slab will settle unevenly. For sidewalks, driveways, and patios, a firm and stable base is crucial, typically involving a 4-inch layer of gravel or crushed stone to improve drainage and provide extra support, thoroughly compacted with a hand tamper or plate compactor. (F9, F10, F12, F38)

Subgrade Issues and Poor Compaction

The most common reason for a driveway to sink is inadequate subgrade preparation. If the soil beneath the slab is not properly compacted before the concrete is poured, it can settle over time, creating voids or uneven support. This is especially true in Southern California, where diverse soil types, including expansive clays in inland valleys, can shift significantly with changes in moisture content. A key requirement for any concrete slab is to provide a firm and stable base by compacting the subgrade properly. (F38)

Water, Drainage, and Erosion

Water is a major culprit in driveway sinking. Poor drainage can allow water to seep under the slab, eroding the subgrade material. This washes away the soil, creating empty spaces that the concrete eventually sinks into. This erosion is often exacerbated by:

Improper Grading: If the ground around the driveway slopes towards it instead of away, water will naturally flow underneath.
Cracks in the Driveway: Existing cracks allow surface water direct access to the subgrade, accelerating erosion. If you're dealing with wide cracks, you might find our article on Repairing Wide Cracks in a Concrete Driveway helpful.
Missing or Damaged Drainage Systems: Clogged or insufficient downspouts and drains near the driveway can dump large volumes of water onto the concrete or directly into the subgrade.

Expansive Soils and Tree Roots

Southern California's geology includes areas with expansive clay soils that swell when wet and shrink when dry. This constant movement can shift the subgrade, causing sections of a driveway to lift or sink. Tree roots can also contribute significantly to uneven settlement. As roots grow, they exert pressure on the concrete from below, lifting it. If the roots later decay or are removed, the void they leave can cause the slab to sink. Homeowners often wonder if their insurance covers damage from tree roots; you can learn more about that here: Does Homeowner Insurance Cover Cracked Driveways from Tree Roots?.

Inadequate Reinforcement or Slab Thickness

While less common for isolated sinking, insufficient reinforcement or an undersized slab can contribute to a driveway's vulnerability. Rebar's purpose is increasing the concrete's tensile strength, helping it resist cracking from bending or movement. (F8) If the slab is too thin for the loads it carries (e.g., heavy vehicles) or lacks proper reinforcement, it may crack and allow the subgrade to be compromised, leading to settlement. Determining the right concrete mix design and reinforcement requirements for anticipated slab exposure and traffic conditions is essential. (F4)

Common Mistakes That Lead to Sinking

Skipping Subgrade Compaction: Not compacting the soil and gravel base sufficiently is the number one cause of long-term settlement.
Ignoring Drainage: Failing to ensure proper drainage away from the driveway allows water to undermine the base.
Pouring on Unstable Soil: Not addressing problematic soil conditions, such as highly expansive clays, before pouring.
Lack of Control Joints: While not directly causing sinking, improperly placed or missing control joints can lead to random cracking, which then allows water intrusion and subgrade erosion. After concrete placement, proper placement and spacing of control joints and adequate curing are important. (F5)

> Pro Tip: When planning a new driveway, communicate clearly with your contractor about soil conditions and drainage. A little extra investment in proper subgrade preparation and drainage upfront can save you significant repair costs down the road.

Addressing a Sinking Driveway

Once a driveway has begun to sink, traditional repairs often involve concrete lifting techniques like mudjacking or polyjacking, which inject material under the slab to raise it. For severe cases or widespread issues, full replacement might be necessary, ensuring the underlying problems with the subgrade and drainage are corrected beforehand. Efficient planning is crucial for any concrete project to ensure all elements are done correctly. (F3)

If you're observing your concrete driveway sinking on one side in Southern California, the first step is to accurately diagnose the root cause. Western Concrete can assess your situation and recommend the most effective repair or replacement strategy to restore your driveway's stability and appearance.