Background
As is well known by most homeowners in North Central Texas, the foundations for our homes are many times constructed on expansive clay soils. The problem is that clay soils shrink when they dry out and swell when they experience an increase in its moisture content and this shrink / swell of the soils can easily push a slab foundation around. This has cost Texas homeowners millions and millions of dollars in repairs.
In the North Texas area, the soils tend to become desiccated in the dry summer months. There is a depth below the surface which is called the “Moisture Active Zone”. This zone is typically considered to be about 15 (or so) feet deep but the actual depth varies depending on many factors (soil conditions, rain, drought, trees, exposure to sun/wind, etc.).
Below the active zone, the moisture content of the soil generally does not change. However, above the active zone, it does. As the soil moisture changes, so does the volume of the clay soils and a structure built in (or on top of) the active zone is prone to move. This includes almost all slab foundations.
So, in much of Texas, it is commonly said that to prevent (at least minimize) the movement of a slab foundation, a homeowner must maintain the soil moisture content around the slab foundation at a constant rate (year around). This means attempting to replace the moisture in the active zone that was lost because of desiccation caused by environmental influences. This is done in the dry seasons by adding water to the soils. Maintaining the soil moisture content at a constant value is an easy thing to say but is difficult to do, especially during times of extended drought.
One difficulty for homeowners is that there is no “formula” available to tell them how much water they need to add per day/week to keep the soil stable, i.e., to keep the soil volume constant. There are too many variables.
This is because environmental and soil conditions vary from neighborhood to neighborhood. Also, the rigidity of one slab is typically different than another. The more rigid a slab is, the better able it is to overcome the movements of the soils caused by the variations in the seasonal moisture conditions (rainy season vs. dry season).
I am asked regularly: “can the builders build a slab that does not move”? The short answer is yes. The basic reason most slab foundations in North Texas move is that they are not founded in a stable bearing stratum which is below the moisture active zone. (There are other reasons a slab foundation move but moisture variation is the subject of this blog.)
So, to answer the primary question posed in the title of this blog: homeowners are told to add water to the soils supporting their foundation because most slab foundations are based in the active zone where the soil moisture content varies because these soils are susceptible to desiccation during the hot, dry summer months (and during other periods of drought).
I should say that I have observed a few homes with slab foundations where the homeowners appeared to be very diligent about applying water to the soils, but their foundations still moved and further (and more expensive) remedial efforts were required. Also, a word of caution: I have also seen homes where the homeowners started a vigorous watering program when the soils supporting his foundation were dry, expansive clays and the sudden increase in the moisture content of the clays caused the clay to expand and pushed (heaved) the slab up significantly. This upheaval is a problem.
The next blog will discuss the various methods commonly used to add water to the soils around the foundation.
GeoDynamics Engineering
