New 'Texas Ribbed' Wafflemat Design
Texas and many Gulf states continue to have problems with failures of foundations
built on expansive soils. For proof, just look in the Yellow Pages under ‘Foundation
Repair Contractors’ and you’ll see the amount of foundation repair business being
done. Why aren’t builders able to consistently construct slabs that will last a
lifetime without need of additional work and re-work in these types of soils?
First, you have to look at how current foundations are designed for expansive soils.
The most common design used in Texas for residential and light commercial/industrial
buildings in expansive soils is the design method promoted by the Post-Tensioning
Institute in their publication titled ‘Design of Post-Tensioned Slabs-on-Ground.’
The method uses deepened concrete ribs under a thin concrete slab to provide the
flexural rigidity needed to resist the forces applied to the foundation by expansive
soil—whether those forces are due to expansion or shrinkage of the soil. See Figure
1 below for an example of this ‘ribbed slab’ design. Note that the thin slab also
rests directly on the soil.
Second, what’s the big deal here? Hasn’t the ribbed slab design been used for years?
Well, the answer lies in looking at what the soil does when it expands. For low
expansion soil, where the upward force of expansion never exceeds the downward gravity
loads of the foundation and structure, nothing happens. This is not where the problem
lies. The problem usually comes when a ribbed slab design is built on medium or
higher expansion soil, and the upward force of soil expansion exceeds the gravity
loads of the foundation and structure. The foundation will move upward. The result
of this movement can be cracking in the concrete foundation slab and structure.
Third, how can the detrimental effects of soil expansion on a foundation be stopped?
The short answer is by using a different approach to designing the foundation --
the Wafflemat™ foundation-forming system.
Wafflemat can be easily designed by a licensed civil or structural engineer using
the same structural design methods and computer-based tools currently used for designing
the ribbed slab (see ‘Wafflemat Design Guide’ on the left hand side of Home Page
under ‘Especially for Engineers’). Wafflemat can be designed with either post-tension
cable or rebar reinforcement, depending upon designer or builder preference, and
local code requirements.
Wafflemat can be used as infill with a typical ribbed slab design, or it can be
used by itself in its own design—depending on the degree of expansion in the soil.
Figure 2 below shows the same foundation as shown in Figure 1, but with Wafflemat
infill. The voids created by the System’s ‘Waffleboxes’ allow expansive soils to
move into the voids. The results of using Wafflemat are twofold: first, most of
the energy created by the expansive soil dissipates into the voids; and second,
the beams and overall deeper slab thickness created by the Waffleboxes increase
the rigidity of the entire foundation. This increased rigidity improves the resistance
of the foundation to forces caused by both expanding and shrinking soil. Alternatively,
Figure 3 shows the same foundation designed solely with Wafflemat.
(Remember all foundation designs, including those utilizing Wafflemat, must take
into account the following parameters: characteristics of the soil, weight of the
structure, type of structural design elements, architectural finishes used on the
structure, and environmentally imposed loads on the structure per local building
codes. For example, a house built on highly expansive soil will likely require a
more rigid foundation than the foundation for the same house built on medium expansion
Finally, the cost of the Wafflemat foundation will vary, depending upon the foundation
design parameters listed above. The cost can be the same as or less than other foundation
systems in more expansive soils, or it can be more in less expansive soil conditions.
The two real costs that always have to be kept in mind by the owner are the ongoing
cost of repairing cosmetic cracking in the foundation and the structure, and the
future cost of structurally repairing a foundation that has been built using a design
that does not work. Typical foundation structural repair costs can far exceed the
additional cost of using the Wafflemat when the foundation is built. That is why
using the Wafflemat is the best choice.
Now you know why Wafflemat is the ‘100 Year Foundation,’ and the reason you will
not have to find the ‘Foundation Repair Contractors’ section in the Yellow Pages
in the future, when you use Wafflemat.