Causes of Foundation Heave

Foundation heave is an upward movement of the foundation due to soaked expansive soil.?

The factors that lead to the saturation of soil beneath a foundation are seasonal changes in moisture, underground aquifer, or leakage in underground pipes. It is stated that financial losses due to heaving in the United States are larger than those caused by tornadoes, hurricanes, floods, and earthquakes combined.?

The major cause of foundation heave in loose soil, like expansive soil, is the variation in soil moisture. Variations in soil moisture occur due to changes in the field environment due to natural as well as man-made conditions, and changes related to construction work.

Lighter buildings with thinner foundations are more likely to suffer from foundation heave than heavier structures with thick foundations. The International Association of Certified Home Inspectors states that soil can exert pressure of up to 260 KN/m2?against the?concrete foundation.

Causes of Foundation Heave

Environmental Condition

Environmental condition is one of the reasons that leads to foundation heave of a structure. The following environmental changes are found to be contributing to the heaving of foundations:

• Dramatic climatic changes, for instance, long droughts and heavy rains, cause cyclic moisture variations, resulting in an upward movement of the edge of a structure.
• Variations in the level of the?water table lead to changes in soil moisture, causing soil swelling.
• Frost heave?and chemical reactions like oxidations of iron pyrite make the soil underneath the foundation expand, creating pressure against the foundation.

Figure-1 through Figure-3 illustrate how clay soil expands in the presence of moisture, leading to foundation heave.

Figure-1: Clay Particles Exhibit a Net Negative Charge and Pack Tightly when dry

Figure-2: When Water Becomes Available, It is Attracted by Clay’s Negative Charges, and Bonds Tightly to the Surface of the Clay

Figure-3: Because Water is Incompressible, It Pushes the Clay Particles Apart, Causing an Expansion or Swelling of the Clay

Construction-Related Changes

The following changes in the construction process lead to increased moisture in soil:

• Natural evaporation of moisture of the area covered by the structure is reduced, leading to increased soil moisture under the foundation.
• Soil moisture of the area occupied by the structure is also increased due to a reduction in transpiration of moisture from vegetation, see Figure-4 through Figure-6.
• Removing large trees from the construction site may increase soil moisture because the soil moisture was previously depleted by the trees' root system. So, elimination of the source of moisture depletion leads to accumulation and raise of moisture in the same soil mass.?
• Insufficient?drainage to keep surface?water from the structure creates ponding and subsequent increase in moisture. For instance, damaged rain gutters and downspouts create ponds around the structure, and the water finds its way toward the soil under the foundation, which increases soil moisture. In this case, the clay soil will expand and may cause foundation heave.
• Seepage into the soil beneath the foundation at soil-foundation interfaces and through excavations drilled for basements or shaft foundations results in increased soil moisture under the foundation.
• When an aquifer is tapped, underground water moves upward and saturates the soil beneath the foundation, leading to soil expansion and the generation of pressure against the foundation.?
• Drying of exposed foundation soil in excavations and reducing soil surcharge weight increase the likelihood of foundation heaving.

Figure-4: Trees Before Construction Process Begin

Figure-5: Removing Tree from Construction Site Stopes Transpiration and Moisture Migration to the Root Zone Continue

Figure-6: Soil Rehydrates and Heaves

Effects of Activities in the Building on the Soil Moisture?

1. Drying of soil beneath heated areas of a foundation like furnace rooms causes soil shrinkage. As a result,?foundation settlement?occurs, which can be problematic for serviceability and, to a great extent, the structure's safety.?
2. Leakage of?underground water?and sewer lines saturates clay soil around these utilities, leading to foundation heave.?
3. Planting and growing trees at distances less than 1 to 1.5 times the mature tree's height from the structure exacerbate cyclic edge heave.
4. The water from lawns can increase soil moisture.

Foundation heave is an upward movement of the foundation due to soaked expansive soil. The factors that lead to soil saturation beneath the foundation are seasonal changes in moisture, underground aquifer, or leakage in underground pipes.

The Major Cause Of Foundation Heave

The major cause of foundation heave in susceptible soil, like expansive soil, and variation in soil moisture.

Type Of Soil That Is Susceptible To Heave

Clay soil expands when it comes in contact with water and exerts pressure on the foundation and slab constructed on the ground.

Limit To Which Soil Can Exert Pressure Against A Concrete Foundation

The Association of Certified Home Inspectors states that soil can exert pressure up to 260 KN/m^2 against a concrete foundation.

Signs Of Foundation Heave

1. Cracks in foundations and walls

2. Uneven floors

3. Sticky doors and windows

Credit: The Constructor