Gregory H. Wilson Engineering

The soils in North Texas are characterized by highly active clays, which exhibit a high degree of expansion when wet and shrinkage when dry.  This situation can result in severe vertical and/or lateral movement of structures.  Repeated changes in soil moisture content can cause differential movement and undue stress to the supported structure.  These movements may result in unlevel floors, cracked or separated masonry, interior cracks, misaligned doors and windows, and cracked foundations.  An easy way to think of how soils react to moisture is to think of the changes that take place when one adds water to a dry sponge.  Maintaining constant soil moisture content around the perimeter of your foundation throughout the year will result in less stress and a longer service life to the structure.  This task requires special detail and attention to DRAINAGE, TREES/VEGETATION, WATERING AND PLUMBING LEAKS.

DRAINAGE

Maintain a positive slope away from the structure approximately 5% for a distance of 5 feet (3" in 5') where possible.  The soil near the foundation should be predominantly clay so that excess water will flow away from the foundation.   Sandy loam or other porous material should not be used. Soil levels against the concrete perimeter grade beam should be approximately two to three inches below the brick line on a slab foundation and about halfway up on a pier and beam.  A swale or gently sloping channel is normally included between structures.  These features should never be altered by the addition of fill material or blocked by landscaping bed, other structures or fences.  While not absolutely necessary, gutters and properly dispersed downspouts can help control moisture problems.  The use of downspout extensions and splash blocks, or underground piping to reduce erosion directing the excess water to flow away are highly recommended.  Flowerbeds, curbs or sidewalks near the foundation may trap water.  These beds should be filled with soil to prevent ponding and in some cases area drains may be required.

TREES/VEGETATION

Overgrown vegetation can affect soil conditions around and under a structure in negative ways.  The damage is most frequently the result of large bushes and trees planted too close to the building, causing the soil to dry-out and shrink.  This condition is magnified during dry conditions, which force the roots to consume more moisture and to extend further under a structure for their own survival.  Large trees, such as Weeping Willows, Cottonwoods, or Oaks can transpire hundreds of gallons of water a day.  These trees should not be planted any closer to a building than the distance of their mature drip line, or safer yet, their mature height.  However, if trees do exist in close proximity to the foundation, there are effective measures which can be taken to minimize the effects of moisture consumption.  Tree pruning, root barriers, or even tree removal may be necessary and sometimes a decision between the tree and the structure will have to be made.

WATERING

Whenever cracking of soil occurs or soil is noted to be pulling away from the foundation, it is an indication that soil moisture levels are too low.  In this case, the entire landscape could probably use water if there are trees and vegetation in the vicinity of the foundation.  I recommend that the entire landscape area be watered regularly.  Watering soil near the foundation for the purpose of increasing moisture levels to the bearing soil should be applied in a slow systematic manner using a properly designed automated sprinkler system or a soaker hose with the holes placed downward approximately 18" from the foundation.  During hot and dry weather, water application may be required every other day or more, depending on slope and runoff.  Soil should be moist whereas over watering can cause heaving of the foundation, just as under watering may result in excessive settlement.  If a consistent watering program is followed where an inch of water is added weekly, there should be little need for additional foundation watering.

PLUMBING LEAKS - Concrete Slab Foundations

A slab-on-grade foundation acts as a barrier by resisting soil variations due to  evaporation and be shielding the under-slab soil from rainfall.  Under optimum conditions, the moisture content under the slab will achieve equilibrium.  When a plumbing leak occurs under a slab, the moisture equilibrium is disturbed.  As moisture is added to the soil from the leak, soil and foundation movement often result,  The amount of movement depends upon soil type, soil expansiveness, soil density, soil moisture content before the leak, and other factors.  Sanitary sewer systems and domestic water supply tests should be performed if distress symptoms are present  This may establish the basis for possible filing of insurance claims.  These tests should also be conducted whenever significant foundation lifting is performed where plumbing exists.