What Causes Concrete Foundation Issues?

CAUSES OF FOUNDATION PROBLEMS

 Water is the greatest threat to the issues of vast soils. Whether too much water is present, causing the soil to swell, or not enough, causing the soil to shrink.

When all the soil under a base uniformly swells or shrinks uniformly it is unlikely to create a problem. Yet differential movement causes cracks and other damage when only part of the base heaves or settles.

Most of the differential movement is due to soil moisture differentials. Soil underneath a portion of the base is wetter or drier than the rest of the soil after building.

Here is why this happens:

When there is a gain in soil moisture:

This is the most significant threat, as the potential for vast soils to swell is much greater than the potential for shrinking. The increase in moisture may result from plumbing leaks, subsurface water such as wet weather or a high water table, or surface water. Surface water is an improper runoff of water or rainwater from the ground.

Low drainage may be a significant contributor to increases in soil moisture. Runoff from the roof should be guided away from the house using gutters. Gutter downspouts should not be allowed to spill water alongside the base. Surface drainage next to the foundation should be about 1⁄4 “per foot away from the house.

When there is a loss of soil moisture:

When the foundation is constructed the soil may be at or near its optimum moisture content, but during a drought, it may lose enough moisture to cause the foundation to settle. Typically, settlement is highest near the edge of the base where the soil dries the fastest.

Extremely low or high soil moisture during construction

If the soil content is very low when a slab-on-grade base is poured, soil to the edges of the slab first regains moisture as it is exposed directly to rainwater or irrigation water.

If the soil moisture during construction is extremely high, the slab should keep in the moisture except at the edge, where it is more exposed to wind and heat. In situations like this, the edge of the slab loses moisture at a rate different from the soils under the foundation, and the building must settle down.

Poor Pre-Construction Compaction of the building pad

Slab-on-grade foundations depend on the uppermost layers of soil to have bearing power to support the structure and to strengthen the foundation. Unless the bearing soil has not been sufficiently compacted during grading, the base is subject to settling as deposited by the supporting soil.

Also of interest is when specific soil conditions surround a structure. The house can, in this case, settle differently. For instance, if one half of the foundation sits on expansive clay and the other half on selected fill and/or rock, the amount of seasonal movement varies from half to half. The differential movement can cause damage to the foundation and structure if the foundation system is not properly built.