Expansive soils have a relatively high percentage of clay minerals and are subject to changes in volume with changing moisture conditions. Two of the most common expansive soils are the adobe soils found predominately in the Southwestern U.S.. and Lea (highly frost-susceptible) soils, found mostly in Eastern Canada. Furthermore, all types of clay soils are found throughout North America and may cause significant damage to a structure. It is estimated that 50% of the land in North America contains expansive soil.
The soil under a house swells and shrinks with the seasons. This movement is not a problem as long as it is uniform or not enough to damage the foundation and/or house. Damage to the house may appear and disappear on a regular basis as the seasons change. Significant defects occur when the movement is uneven or localized.
Movement in foundations is caused primarily by; isolated movement or uneven settlement of sections of the structure, additional lateral pressure at foundation walls, sliding surface layers of soil, vibration or additions to the structure.
Moisture can move under the foundation through suction (similar to dry sponge absorbing water). Under moist conditions water moves vertically and horizontally through the soils under the foundation – as the clay soils draw water, they grow in volume (swell or heave). The opposite is true as the soils around the foundation dry out – the moist soils lose volume (shrink) as the moisture moves out from under the foundation.
Movement occurs because the soils expand so forcefully. Foundations can actually be displaced by the forceful movement. In a structure, cracks are usually caused by this movement at different rates and distances. Changes in the water content of clay soils cause up to 90% of the cracking problems in a house. Uniform changes in soil moisture are less damaging than localized changes. These cracks are evident as:
- CRACKS IN THE EXTERIOR OR INTERIOR WALL COVERING AND CEILINGS
- CRACKS IN GARAGE OR BASEMENT SLAB, DRIVEWAY, PATIO, OR WALKWAY
- SEPARATION OF DRIVEWAY, PATIO, OR WALKWAY FROM FOUNDATION
- BOWING, DISPLACEMENT OR ROTATION OF EXTERIOR WALLS
- SEPARATION OF WOOD TRIM AT CORNERS
- TILTING OF FENCES AND RETAINING WALLS
- BINDING DOORS AND WINDOWS
- UNEVEN FLOORS/SEPARATION OF WALLS FROM FLOOR
There are things you can do to protect your foundation and home from movement. Tune in to next week’s blog for some tips and things to pay attention to. If you are thinking of selling your home and you have some of the items listed above, it would be a good idea to have a structural engineer come and take a look. It is much better to take care of issues before your home is listed than to deal with them under the pressure of a real estate contract.