Causes of Slab Settlement and How to Fix It

Slab settlement has several different causes including drying of the soil under the slab, washout of the soil, or poor compaction of fill soil.  The soil under the slab may dry and shrink due to leaking ductwork that is installed under the floor slab.  As the ductwork leaks, the leaking air can dry out the soil.  This causes gaps to form under the floor slab where the floor is no longer supported.  Then the floor begins to crack and sink into the empty gap.

Plumbing leaks usually cause washout of the soil under the slab.  If the leak is severe and there is an open path for the water to flow, the soil may be washed out from under the slab, creating an empty space.  Once again, the soil is no longer supporting the floor slab, and it begins to crack and sink into the void.

Poor compaction of fill soil may have occurred when the home was built.  During new construction, layers of soil are removed to get the desired grade level.  Footings may be deepened to extend below weak fill soils.  However, the slab remains on the fill soils.  The fill soil will then compress and settle if the fill was poorly compacted.  It then creates an empty gap, where the slab cracks and settles.

Signs of slab settlement include cracks in the concrete floor and walls separating from the floor or ceiling.  Also, interior cracks off the corners of doors or walls pulling away from other adjacent walls are other common signs.

The best solution to slab settlement is to install slab piers.  Slab piers are steel piers that are driven deep into a stable layer of soil below the floor slab.  There are two types of slab piers: hydraulically-driven and helical slab piers.  Both types are equally as effective and merely depend on the unique situation.  They both use small installation equipment, so they can be installed in small areas such as laundry rooms or bathrooms.  Slab piers provide an opportunity to lift the concrete slab back toward its original position, oftentimes closing the cracks and therefore eliminating trip hazards.