Slab on Grade Foundation

| February 18, 2011 | Reply

sandy loam foundation

The basic slab-on-grade foundation is what builders call a monolithic slab. Wooden foundation forms are set to a grade which is usually 18” higher than the crown of the street in order to assure excellent drainage. Then a substrate is compacted inside of the forms and grade beams are dug. The slab or surface is graded and reinforcement steel is placed according to the engineer’s specifications. Then the concrete foundation is poured as a single structure known as a monolithic slab. The perimeter and interior beams will bear the weight of the load bearing walls.

No matter how well a builder has followed the engineer’s specification, unless the substrate was properly prepared, there is a good chance that within 10 years or possibly sooner there will be a foundation failure. In order to avoid this situation, once the unstable soil has been scalped and the forms built,  stable soil must be brought in to assure a good pad for the concrete slab to rest on. Because a clay soil is expansive, and will expand when wet and shrink during dry weather, it is undesirable to build a foundation upon. After a number of years a clay soil can deal a severe blow to a monolithic slab.

Sandy Loam substrate for the foundation, which is known in the construction world as “select fill”,  is a very stable soil that doesn’t expand and contract as the weather changes from a dry to a wet season. This soil will compact and support a slab foundation with very little heaving. It is a superior substrate and if properly placed and compacted, there will most likely never be a need to call for a foundation repair company (the “slab jacking man”).

It is imperative that the substrate fill is constant over the entire area of the slab. If there are different types of soils within the slab area,  they will react differently and once again you will run the risk of a foundation failure.

Assuming that the foundation has been properly prepared, the contractor or home owner must still be concerned with erosion. Another common cause of slab failure is dryness in the soil around the foundation. During the  hot, dry summer months when rainfall is sparse or nonexistent; you may notice the soil drawing away from your concrete slab at the edges.  You must  consistently wet down the perimeter of your foundation at this time. The simplest and most energy efficient way of doing this would be to install a timed irrigation system which comes on 4 or 5 days a week for about 15 to 20 minutes each day. The shrubbery you plant for its cosmetic addition to your home,  will become  lush and lovely with regular irrigation.

Last but not least, you want to make sure that water drains away from your foundation. It might benefit you to add and consistently maintain a gutter and down spout system to retain balanced wetness around your home.

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