It
is generally recognized that the soils that underlay most houses
and buildings in Central Texas are, moisture-sensitive clays.
There are dozens of sub-categories of these clays but the major
characteristic all have in common is that they shrink when drying
out and swell when taking-on moisture. These clays behave this
way whether they are in the field or beneath buildings acting
as foundation support soils. Most problems with foundations
built on these soils are caused by non-uniform shrink/swell
of foundation soils, which causes non-uniform settling or up-heaving.
Foundation distress or damage from this natural soil behavior
can be controlled by managing the uniformity of foundation soil
moisture; in such cases, the goal is to maintain soil moisture
at a uniform and constant condition regardless of the weather.
This may mean re-grading yard surfaces near the foundation edges
to promote positive drainage away from the building. Such grading
can be accomplished by either removing soil to create a drainage
swale, and/or by adding fill dirt adjacent to the foundation
edge and sloping outward. Here, take care to not raise soil
level to approach the height of wood trim or masonry weep holes
(usually in the bottom course of stone or brick walls). Also
helpful is the use of rain gutters to collect and redistribute
rainwater to locations that dependably drain away from the foundation
edges.
During hot dry weather moisture needs to be added to replenish
evaporation losses and plant root removal of soil moisture.
This can be accomplished in many ways depending on available
moisture delivery systems; for example, yard sprinkler systems
can be adjusted to provide sprinkled water coverage for these
purposes, and the use of soaker hoses within 10-12 inches of
the foundation edge also can be effective. By whichever method
used, take care to add sufficient-but-not-too-much water; if
surface soil cracks become visible first near the southwest
foundation edges, then increase the amount of added moisture
in those areas.
In Central Texas, this usually amounts to operating soaker hoses
20-30 minutes daily during the hottest dry summer periods (shorter
periods at areas less likely to show soil cracks). Avoid watering
to the point of standing pools of water or creating perpetually
muddy surface conditions. Observe for soil cracks to learn how
long after rain showers to wait before resuming addition or
irrigation moisture. Where soaker hoses are used, these can
be placed on the soil surface or covered-over by soil added
to form a graded drainage surface sloped away from the foundation.
A desirable soil slope is 4-5 inches higher at the foundation
edge and sloping outward to zero at least 4 ft. away.
The soil-drying effect of trees and shrubs often goes underestimated;
the differences in moisture consumption by the many varieties
of trees and shrubs often are not recognized. It is well to
realize that the plants and trees noted for fast growth also
usually extract the most soil moisture (Arizona Ash, Tallow,
Mimosa, and Willow trees, and Photinia and Boxwood shrubs, to
name a few). Of course, the goal of many landscaping treatments
is a quick show of growth, but the faster-growing, more thirsty
plant species should not be used near enough to the building
to affect foundation soil moisture.
The presence of trees or shrubs carries the responsibility of
irrigating the soil so plant needs can be met without sending
out forager roots that affect sidewalks, driveways and building
foundations. Root growth control can be achieved by pruning
roots (much as pruning branches) and by various root shield
methods; there are the physical (plastic, metal or concrete)
barrier methods and there are chemically impregnated sheets
that time-release chemical substances that reportedly are environmentally
safe and act to discourage tree root cell growth beyond the
chemical barrier.
By recognizing that growing plants and hot dry weather deplete
soil moisture, and that replenishment moisture is available
by various man-controlled methods as well as rainfall, it is
feasible to control and reduce significant foundation movement
through enlightened soil moisture management
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