BUILDING A STRONG BASE
Residential Foundations in the Brazos Valley
By DREW DUDLEY
DALLAS
WACO
AUSTIN
B/CS
HOUSTON
SAN ANTONIO
HIGH EXPANSIVE CLAY SOIL MAP FOR CENTRAL AND EAST TEXAS
T
hroughout most of the country,
residential foundations are given
little attention due to a long track record
of satisfactory performance and cost
effectiveness. However, in parts of Texas
and specifically in the Brazos Valley, the
soils tend to be highly expansive, and
foundations require much more attention.
Locally, clay soil is not the only factor
that plays a role in creating the shrink-
swell characteristics of expansive soil
that make foundation engineering the
best solution for long-term foundation
performance. The clay’s environment,
or climate cycle, contributes greatly.
When unsaturated clay is exposed to
moisture it will expand, and when the
clay begins to lose moisture it will shrink.
Using standardized climate measures,
the Brazos Valley climate cycle reflects
intense dry-moisture cycles. Coupled
with the high expansive clay that is
predominant throughout the Brazos
Valley, you have the perfect recipe for
substantial soil movement.
Foundation Options
The issues with residential foundations on
expansive soils have been well documented
since the 1930s. Due to the climate and
soil type that predominates in the Brazos
Valley, Type III foundations from the
Building Research Advisory Board Report
#33 are the best residential option. These
slabs are intended to behave similarly
to a rigid raft floating on water. With
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this concept, we are accepting the fact
that the soil is going to move. To design a
foundation for the best long term result, a
professional engineer should be consulted.
Standard Practice in the Brazos Valley
Based on my experience and
conversations with local builders,
approximately half of the residential
foundations in the Brazos Valley are
engineered. When not engineered, the
builders typically utilize a Type III slab
that is 4 inches thick with 36-inch deep
stiffening beams with reinforcement
in the top and bottom of each beam.
This exceeds the minimum foundation
standards that both Bryan and College
Station have added to the building code.
Applying a standard foundation design
to every house can lead to excessive
foundation movement with potential
undesirable consequences. It also
potentially leaves the owners paying for
material and labor that is not necessary.
Locally, the typical cost for foundations
is approximately $10 per square foot of
the total building cost. For an engineered
slab, the owner would need to pay for
a geotechnical report and a design fee
for the foundation design. Using local
cost estimates, in a $450,000 new home
construction, the approximate additional
cost of an engineered foundation would be
around $2,600.
Ultimately, the question becomes
whether or not the owner/builder
is willing to take the risk of a non-
engineered slab that represents
approximately 0.6 percent of the total
construction cost.
Factors Affecting All Slab-on-Grade
Foundations
Regardless of whether the foundation is
engineered, non-engineered, or which
design method is utilized, there are many
other factors that affect the performance
of slab-on-grade foundations.
Building Pad: Based on the site-specific
geotechnical report, the geotechnical
engineer may recommend removing a
certain depth of the native high expansive
material and replacing it with select fill to
create more favorable soil properties. The
building pad’s moisture content should
also be as close to the average long-term
moisture content for the soil in which it
is placed. If the building pad is saturated
when the foundation is poured, then over its
lifetime the potential to swell will be small
but the potential to shrink will be very large
and lead to greater foundation movement.
Drainage: When a house does not
adequately drain water away from the
building pad, the ponding of water is
generally concentrated on one section of
the house creating an imbalance in the
moisture content across the building pad,
which may cause some sections of the
foundation to rise significantly.
Moisture Maintenance: The key is to
maintain a uniform moisture level around