Construction Practices & Soil Conditions That Lead to Settling Slab Foundations
Settlement of concrete floor slabs can be a major problem and can cause serious damages to a home or building. Some of the most common causes for slab settlement include drying and shrinking of soil under a slab, poorly compacted fill soils, and washout of soil.
Each of these conditions create a void under the concrete slab. If the concrete is not strong enough to span the void, the slab will crack, break, and settle into the void.
The following paragraphs offer some additional information for some of the more common reasons for slab settlement:
Drying and Shrinking of Soil Under the Slab
Concrete floor slab settlement is often the result of changes in moisture content or density of the supporting soils.
Soils beneath a concrete slab foundation can dry and shrink over time due to extended drought-like conditions. When this happens a void is created under the slab, causing the slab to sink and crack. Along with a settling slab, interior partition walls may settle and cracking in drywall can occur.
Poorly Compacted Fill Soils
During construction of a home or building, layers of soil are commonly moved around or spread out to bring grade to the desired level.
When the structure is built, footings may be deep enough to extend below the fill soils, but the slab may remain on the fill.
Over time, loosely compacted fill soils will consolidate beneath the weight of the slab or fill layers above, causing a void to form below the slab. When this happens, the slab will begin to crack and break as it settles into the void.
Soil Washout - Plumbing Leaks
A common building practice is to install plumbing lines below concrete slabs. In some conditions, plumbing lines under a slab can leak, causing erosion and soil is pushed away from under the concrete slab.
This displaced soil creates a void beneath the slab, causing the slab to crack and settle.