Reactive Soils
Description and Categories

 Description of Reactive Soils

DEFINITION OF REACTIVE SOIL - Generally speaking, reactive soils can be defined as any type of soil that, when exposed to certain physical or geological conditions, will undergo changes in shape and structure. These soil changes can lead to serious foundation and structural damage to both residential and commercial buildings, if appropriate preventative measures are not taken in both the pre-construction and post-construction phases.

NATIONAL PROBLEM - It's estimated that approximately one half of the homes built in the United States today are constructed on reactive soils. Nationally, structural damage caused by reactive soils is estimated at $6-$11 billion each year. Due to its wide-spread occurence and overwhelming damage expense, reactive soils rate as the highest risk of any geological hazard, by a large measure. Therefore, reactive soils are a serious concern to anyone looking to buy or build a home.

GENERAL TYPES OF REACTIVE SOIL - The most common type of reactive soil is expansive clay which, following changes in moisture level, has the ability to shrink or swell. Some expansive clay can expand or contract up to 15 times its original size, thus releasing an extreme amount of stress upon the surrounding environment, including the foundation of your home or other building structure.

 Categories of Reactive Soils

MAJOR CATEGORIES - There are two major categories of reactive soils: expansive and non-expansive. In addition to expansive reactive soils, there are three major categories of non-expansive reactive soils, which can present a threat to your home or commercial building.

1. Expansive or swelling reactive soils expand under various conditions. This is the category of reactive soils that presents that greatest risk for building foundations.
   
   
2. Non-expansive or shrinking reactive soils that contract under various conditions, including:
   • Collapsible Soil - Soil that collapses in the presence of water
   • Uncompacted Fill - Construction fill, including a variety of soils from landfills to peat
   • Chemically Reactive Soil - Soils that contain chemically reactive minerals, such as gypsum and other sulfates

 Potential For Change

The potential of any reactive soil to undergo a change in volume depends on five primary factors:

1. Type of mineral: Certain types of minerals such as smectite (expansive soil) and loess (non-expansive soil) have a higher shrink or swell potential.
   
   
2. Concentration of minerals: The more reactive soil particles there are in a piece of land, the greater its swell or shrink potential.
   
   
3. Density: A dense material containing reactive soils will have less air-filled voids and thus a greater potential to undergo volume change.
   
   
4. Change in moisture levels: A dry soil can absorb more moisture than a wet soil, and thus can exhibit larger volume changes. The amount of moisture change in a soil depends on the original moisture content of the soil (natural moisture content), the ability of the soil minerals to chemically attract water molecules, and the amount of drainage available to the soil.
   
   
5. Restraining pressure: The swelling potential of a soil is restrained by the weight of the overlying soil. If the overlying soil applies more pressure than the soils swelling pressure, uplift will not occur.
   
   Referenced from Noe, pgs. 17-18