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How Structural Damage Happens to Slab Foundations

Oct 11, 2013 • By Matthew Stock.

Slab Foundation

A slab foundation or, more accurately, a slab-on-grade foundation, is often used in large residential developments where construction is done quickly and homes are built for sale at reasonable prices. 

The process begins with stable soil that, in keeping with best construction practices, should be compacted before work begins.  Shallow footings are dug around the perimeter and a bed of gravel is poured on the interior space, smoothed and compacted.  A plastic vapor barrier is laid on top of the gravel and reinforcing grid (rebar) is set into place.  The concrete is then poured and finished, creating a flat pad for construction that also forms the floor, or sub-floor, of the house.

Of course, any utilities such as electrical wiring, plumbing or HVAC ducts must also be put into place before the concrete is poured.

Typically, the slab foundation is used more in the southern half of the U.S. where temperature ranges aren’t as broad, but slabs are also used in colder climates with footings that extend below the frost line.  Slabs are not very common in the Chicago area but can be found in Schaumburg and other newly developed communities.

3 Ways Slab Foundations Can Be Damaged

Slab foundations can crack and drop, causing damage to floors and interior walls.  This damage occurs when a void develops under the floor, which can happen for one of 3 reasons:

1.  Shrinking Soil – Much as a full-basement foundation can drop when soil dries out and shrinks, this is also a common affliction for slab foundations.  Drought conditions can cause desiccation of the underlying soil when trees and shrubs send their roots deep and wide in search of water or when prolonged dry conditions cause a significant drop in the water table.  When this occurs in what are called “expansive” soils, such as the clay soil that is common to the Chicago area, the shrinkage is worse because of the higher initial water content of the underlying soil, compared to soils like sand and loam that offer better drainage.

2.  Poor Compaction of Fill Soil – It is very common in residential construction for soil to be brought to the construction site and used to fill in areas where the existing soil is unsuitable or where large trees or rocks have been removed.  A conscientious builder will compact each layer of soil as it is added but even the best attempts at proper compaction sometimes leave all or part of the soil prone to further compaction.  When this occurs after construction, due either to the weight of the slab itself or of fill layers above, the slab will no longer be supported and will crack and drop.

3.  Soil Wash-out – Another common building practice is to run pipes for plumbing under a slab foundation before it is poured.  If household plumbing develops a leak, it can cause soil under the foundation to wash out from under the slab, leaving a void where the slab is no longer sitting directly on soil.  Because of the location of the plumbing, it is easy for such a leak to go undetected for a long time, allowing the erosion under the slab to become severe enough to create a void and cause cracking and dropping of the slab.

When any of these conditions occurs and the slab foundation needs structural repair, the homeowner will need the advice and assistance of a full-service foundation repair contractor.  At U.S. Waterproofing, our foundation experts are experienced with all types of residential construction and employ the best of modern technology and engineering to stabilize foundations and save the home from destructive damage.  Why not ask for a free consultation, no matter what type of foundation is under your home?

Tags: structural foundation damage, slab foundation, slab foundation damage

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