Expansive soil changes volume significantly with moisture content, swelling when wet and shrinking when dry. These continuous changes exert pressure, damaging structures and infrastructure.
The Science Behind Expansive Soil
Expansive soil’s behavior stems from specific clay minerals like smectite, including montmorillonite, which absorb large amounts of water. These minerals have a layered structure, allowing water molecules to enter between layers. As water enters, layers push apart, increasing soil volume. When dry, water leaves, causing particles to pull closer and the soil to shrink.
This swelling and shrinking cycle, known as shrink-swell potential, generates significant forces impacting structures. Some expansive minerals can expand by 30% of their original volume, and the soil itself by 10% or more.
Recognizing Expansive Soil
Signs of expansive soil around a property include specific crack patterns in foundations and walls, such as stair-step cracks in brickwork, vertical cracks, or horizontal cracks in basement walls, often from differential movement. Cracked or uneven concrete in driveways, sidewalks, and patios also indicates expansive soil activity.
Inside, signs include uneven floors, and sticking doors and windows. Outside, deep, wide polygonal cracks in the ground surface during dry periods indicate expansive clay soils. Regions with clay-rich geology and distinct wet/dry seasons are more susceptible.
Impacts on Structures and Infrastructure
Expansive soil damages structures and infrastructure. Its cycles exert pressure on foundations, causing upward heave and downward settlement. This movement leads to structural damage, like cracks in concrete slabs, walls, and upper floors.
Beyond buildings, expansive soil impacts roads, pavements, and utility systems, causing cracking and buckling of asphalt and concrete roads, driveways, and sidewalks. Underground pipes and utility lines, including sewage and water lines, are vulnerable, as soil movement can bend or break them, leading to leaks. Damage from expansive soils causes billions of dollars in losses annually in the United States, often exceeding combined damage from floods, hurricanes, tornadoes, and earthquakes.
Managing Expansive Soil
Managing expansive soil involves controlling moisture and reinforcing structures. For new construction, site investigation and soil characterization are important. Techniques include soil modification, like replacing expansive soil with non-expansive fill or stabilizing it chemically with additives such as lime or cement. Deep foundation systems, like piers or helical piles, can also extend below the active zone into more stable layers.
For existing properties, maintaining consistent soil moisture around foundations is a primary approach. This involves ensuring proper site drainage by grading ground away from the foundation and keeping gutters clear to direct rainwater at least 10 feet away. In dry periods, watering the soil can prevent excessive shrinkage; during wet periods, avoid overwatering. Consulting qualified geotechnical engineers or structural professionals is recommended for assessing conditions and determining appropriate management and repair solutions.