Why Is My Concrete Slab Sinking

Sinking concrete is a common problem caused by changes in the soil beneath the slab. The main causes of sinking concrete include poor soil compaction, erosion, water intrusion, and shrink-swell cycles. Tree root growth and decay can also create voids, leading the concrete to sink. Inadequate preparation before pouring, such as failing to compact the soil properly, further contributes to a slab that is sinking.

These soil-related issues result in sunken concrete, uneven surfaces, and structural damage. To fix sinking concrete effectively, it’s important to understand what’s happening below the surface and address it directly.

Key Takeaways

• Poor soil conditions are the leading cause of sinking concrete: Loose soil, erosion, and expansive clay beneath the slab often lead to voids and settlement over time.
• Proper installation prevents long-term slab issues: Inadequate soil preparation and lack of compaction before concrete is poured are common construction-related causes of slab failure.
• Environmental factors accelerate concrete settlement: Droughts, heavy rain, and freeze-thaw cycles destabilize the soil, leading to cracks, uneven surfaces, and trip hazards.
• Water management is critical for slab stability: Poor drainage and uncontrolled runoff wash away soil and create conditions that allow concrete to sink.
• Concrete lifting is a reliable repair solution: Techniques like polyurethane foam injection and mudjacking restore sunken slabs with minimal disruption and long-term durability.
• Regular inspection and maintenance are essential: Early detection of cracks, gaps, or pooling water helps prevent costly repairs and ensures surface safety.

1. Soil Issues

One of the most common causes of a slab that is sinking is the condition of the soil beneath your concrete. If the soil is unstable, weak, or improperly compacted, it can shift or collapse under the weight of the slab. Loose soil, erosion, and expansive soil types often create voids that reduce support and lead to uneven surfaces.

These conditions can cause the slab to crack, settle, or shift unexpectedly. Over time, gaps and cracks appear as the soil compaction fails, allowing the concrete to drop into the empty space. Identifying and correcting these issues is critical to prevent further damage and restore stability.

Poor Compaction

If the soil beneath the slab isn’t compacted properly before the concrete is poured, it can’t provide the support needed to keep the slab stable. Inadequate soil compaction allows the slab to settle over time, creating uneven surfaces and structural stress.

Soil Erosion

Soil erosion caused by runoff, a poor drainage system, or even burrowing animals can gradually remove the support beneath the slab. When water isn’t directed away from the foundation using proper grading or a functioning gutter system, the soil begins to erode, leaving the slab unsupported and prone to sinking.

Shrink-Swell Cycles

Expansive soil types like clay expand when wet and contract when dry. These repeated shrink-swell cycles cause constant movement beneath the slab. If not addressed early, this preventable issue will lead to shifting, settlement, and long-term damage, making preventing your concrete from movement a priority.

Tree Roots

Tree roots can interfere with the soil’s stability as they grow, pushing against the slab or displacing soil. When those roots die and decay, they leave behind voids, and the lift is lost beneath the slab. This is a common cause of the sinking and can lead to uneven surfaces and structural imbalance.

2. Installation and Construction Problems

Improper installation practices are another major cause of slab settlement. When new concrete is poured on top of an unstable or poorly prepared base, it can shift, crack, or sink over time. Concrete lifting contractors may overlook essential steps like clearing debris, grading, or compacting the base material, leading to long-term failure.

Placing concrete without proper site preparation increases the risk of uneven settling. Even if the slab looks solid initially, weak support underneath makes preventing your concrete from future movement much more difficult. To avoid these issues, preparation must be prioritized before any concrete is poured on top.

Inadequate Soil Preparation

Not properly preparing the soil before pouring a slab can cause it to fail over time. Failing to excavate correctly, remove debris, or ensure the soil is compacted properly allows the concrete to sink due to unstable support beneath. Even a well-finished surface will eventually shift if the foundation isn’t solid.

Key preparation steps often overlooked:

• Excavate to a uniform depth
• Remove organic material and debris
• Compact soil layers in lifts
• Use proper base material before placing concrete back

Poor Drainage

Poor drainage is a leading cause of soil erosion and slab settlement. When water is not redirected effectively, it saturates the ground beneath the slab, weakening support. Without a proper drainage system, downspouts, or grading to move water away, the soil will eventually erode.

To prevent water damage and soil loss:

• Install or maintain gutters and downspouts
• Extend downspouts away from slab edges
• Grade the landscape to direct water away from the structure

3. Environmental Factors

Environmental conditions play a major role in the long-term stability of concrete surfaces. Changes in temperature, moisture, and weather patterns can all impact the soil beneath a slab, leading to movement and damage. This is especially common in outdoor areas like a concrete driveway, garage floor, or walkway, where exposure is constant.

Over time, these conditions can create sunken sections, uneven surfaces, and trip hazards that require repair. In regions with seasonal shifts, freeze-thaw cycles are one of the most damaging environmental factors, accelerating soil movement and slab deterioration.

Droughts and Excessive Rain

Extreme weather patterns can cause serious problems for the soil supporting concrete on your property. During droughts, the soil dries out and shrinks, creating voids that allow the concrete to sink. In contrast, heavy rain can saturate and shift the soil, weakening its ability to support the slab. These changes are often preventable with proper grading and drainage but can lead to long-term structural issues if ignored.

Freeze-Thaw Cycles

In colder climates, freeze-thaw cycles are a major cause of movement beneath concrete surfaces. As moisture in the soil freezes and expands, it pushes against the slab. When it thaws, the soil settles again. This repeated process creates stress that can cause the concrete to settle over time, resulting in gaps and cracks that lead to uneven surfaces and trip hazards.

Addressing the Causes of Sinking Concrete Slabs Effectively

Understanding what causes a slab to sink is the first step in choosing the right repair method. Whether it’s your walkway, pool deck, or driveway, uneven concrete is often the result of soil movement or water washout. Concrete leveling methods like polyurethane foam injection or mudjacking are commonly used to restore stability and eliminate hazards.

These techniques lift and support the slab without replacement. Polyurethane offers minimal disruption and long-term durability, making it ideal for sunken surfaces. If you notice signs of concrete sink, addressing the issue early can prevent costly repairs and further damage.Restore stability to your sunken concrete with fast, effective solutions. Contact Superior Polylift today for professional concrete lifting and leveling services.