Why Do Concrete Slabs Settle?


Poor Construction Practices And Changing Soil Conditions – The Most Common Causes Of Settling Slab Foundations

When concrete floor slabs settle, they can cause serious damage to the structures they support. In most cases, slab settlement can be traced to the dehydration and shrinking of the soil under a slab, poorly compacted fill soils and the washout of soil. In each situation, a void is created under the concrete slab by the shifting of the soil or by a reduction in soil volume. The concrete may not be sufficiently strong to span the void, causing the compromised slab to crack, break, and eventually settle to fill the void.


Dehydrated Soil Beneath A Slab

Changes in the level of moisture content in the soil are a major cause of slab settlement. During extended periods of dry weather, the soil beneath a concrete slab foundation can dry and shrink, creating a void. Leaking HVAC systems can also create dry, shrinking soil, as can other conditions. The resulting void will cause the slab to sink, settle or crack. In some cases, the settling slab may pull interior partition walls along with it, causing additional settling or cracking in attached drywall.


Poorly Compacted Fill Soils

During the building of a structure, layers of soil are commonly added or shifted to assure a level surface for the construction. During construction, the footings may be driven deep enough be housed beneath the fill soils, but the foundation slab often remains on a layer of fill soil. If the fill soils are not tightly compacted, shifting can occur beneath the weight of the slab, resulting in open spaces or voids.


Soil Washout — Plumbing Leaks

Many contractors run plumbing lines beneath concrete foundation slabs. Under the right set of conditions, these plumbing lines can leak, causing soil to flow out and away from the concrete slab. When this soil is transported, it creates a void beneath the slab, causing it to crack, sink or settle.

If you are experiencing one or more of these issues, don’t hesitate. Foundation problems do not improve on their own with time – the situation will only worsen. Call or email BL Bennett Construction today for a free home inspection and repair estimate.