You may be wondering about what options are available to you in case a pre-construction soil test revealed that expansive soils (soils containing large amounts of clay) exist on your lot. This article discusses some of the remedies that can be implemented in order to prevent the problems that can result when you build on expansive soils.
The engineer in charge of your project may recommend that the expansive soil be excavated to a certain depth. That excavated soil can then be replaced with more stable soil so that the variations in the moisture content of the soil don't trigger the subsidence of the foundation. This option may be costly depending on how far the replacement soil has to be transported to your site. The excavated expansive soil is also costly to dispose of.
Use of a Structural Slab
Another option that can be considered is to construct the foundation with a structural slab. A structural slab is one that is several inches thicker than normal slabs. It also has rebar within it so that the slab is better able to withstand the forces generated as the expansive soil expands or contracts when its moisture content varies. This option should be used as a measure of last resort because it does nothing to address the soil movements at the site. It simply tries to make the building less susceptible to being damaged when the expansive soil moves.
The expansive soil can be stabilised by adding a mixture of lime and water to the clay soils. This soil amendment causes the expansive soil to form a hard crust that will be less prone to expanding and contracting as the level of moisture in the ground changes. The downside of this option is that you will need very costly equipment to break up that soil if you ever desire to use that site for another purpose later on.
As already noted, the changes in the moisture content of expansive soils cause the soil to expand or contract. Foundation problems can be prevented by ensuring the moisture content of the soil is kept uniform throughout the year. This can be done by installing sub-grade pipes through which regulated amounts of water can be introduced into the soil during dry spells. The challenge of this method is that it requires frequent moisture content tests to be done in order to decide how much water should be added into the soil underneath the foundation. Other ongoing costs, such as the cost of the water pumped into the soil, exist as well.
Discuss the options above with professionals, such as the engineer who tested your soils. That expert will advise you about the most cost-effective way to build on that site without exposing your structure to the risks posed by the expansive soils there.