When a concrete slab sinks or settles noticeably over time, the reasons for it are either due to poor soil conditions, poor compaction, or poor drainage. The soils underneath an outdoor concrete slab can fail to support the weight of the slab due to three main causes:
WHAT CAUSES SETTLEMENT
WHAT CAUSES A CONCRETE SLAB TO SINK?
POORLY COMPACTED SOIL
During installation of a concrete sidewalk, patio, pool deck, or garage floor, if the new concrete is poured on top of loosely compacted soil, the slab is more likely to settle. The concrete is only as good as the base it was built on. As the fill soil underneath the concrete slab compresses and settles, this creates voids under the slab. With nothing supporting the slab, the concrete cracks and sinks.
EXPANDING & SHRINKING SOILS
Changes in the amount of moisture in the soil around your home can affect the soil underneath your concrete slab. When the soil gets wet, it retains water, becomes soft, and expands in size. This is especially true for clay soil. Soft soil does not bear loads well, causing heavy objects to sink into it. During prolonged dry periods, clay soils shrink considerably, creating voids (or empty space) under the slab, resulting in uneven, cracked concrete. Also, clay will freeze and expand during cold weather and shrink when it heats up creating voids.
Soil washout occurs when water from heavy rains and plumbing leaks find their way underneath your concrete slab, washing away the soil that supports the weight of the concrete. As the soil gets washed away, voids or empty spaces are left underneath the slab. Over time, with nothing left to support it, your concrete slab may begin to sink or cave in.