A sand base course is easy to compress, but can rut easily during construction. Free Reformed Church of Southern River

The weight of the slab and anything on top of it is going to eventually be supported by the soil. When a building site is excavated, usually the soil gets moved around—high spots are cut and low spots are filled. Everything then should get compacted before you place the concrete (and subbase and base).

The type of soil determines what needs to happen before placing a slab. There are three basic types of soil and here's what you should know about each:

  • Organic soils, what you might call top soil, is great in your garden, but awful beneath a slab. Organic soils can't be compacted and must be removed and replaced with a compressible fill.
  • Granular soils are sand or gravel. You can easily see the individual particles and water drains quite easily from them. Just like at the beach when you make a sand castle, if you take a wet handful of granular soil and make a ball, as soon as it dries it will crumble. Granular soils have the highest bearing strength and compact easily.
  • Cohesive soils are clays. If you take a wet handful, you can roll it into a string just like with modeling clay. This soil has a greasy, smooth feeling between your fingers and individual particles are too small to see. Cohesive soils are often difficult to compact and take on a rock hard consistency when dry, but they have a lower bearing strength than granular soils. Some clays expand when wet and shrink when dry making them particularly difficult as subgrade materials.

Post tensioning is often the best solution for a slab on poor soil. J.C. Escamilla's Concrete

Most natural soil, of course, is a mixture and so is characterized by the type of material that is predominant.

The amount of weight the soil can support before it fails is its bearing capacity, typically given in pounds per square foot. The design, however, is based on the allowable soil pressure, which adds a safety factor to the ultimate bearing capacity. Let's look at the weight the subgrade soil will typically need to support. A 6-inch-thick slab weighs about 75 pounds per square foot. According to the International Residential Code, the live load (anything that is not part of the building itself), varies from about 20 to about 60 pounds per square foot--50 pounds per square foot in a garage. That gives us 125 pounds per square foot for the soil to support. A clean sandy soil might have an allowable soil pressure as high as 2000 pounds per square foot. Even a poor soil--silt or soft clay--might have an allowable soil pressure of 400 pounds per square foot.

We can see then that the allowable soil pressure for a slab is seldom a problem and we get back to the need for uniform support. If one part of the slab settles more than another, that's when we get bending in the slab--and potentially cracks and differential settlement. Knowing which areas have been cut and which filled is important—make sure the fill areas have been well compacted. In fact, any soil that's been disturbed during excavation must be compacted.

Probably the most difficult soil you'll have to deal with is expansive clay. These soils swell when they get wet then shrink when they get dry. The best way to counter this problem is first with good compaction, then to not let them get wet (by providing drainage). But as the ground beneath the slab dries over time, it will shrink and the slab will sink. That's not a big problem as long as the slab is isolated from the footings and columns and from any pipes that penetrate the slab so that it can settle a little and settle evenly. Often, with expansive clays, the best approach is a structural slab that doesn't bear on the soil at all or a post-tensioned slab that floats atop the soil but doesn't rely on it for structural support.

Now let's look at why and how we compact the soil.

Return to Subgrades and Subbases