On-Grade Mat Foundation for Expansive Soils

On-Grade Mat Foundation for Expansive Soils

An on-grade mat foundation is an above-ground type of foundation used to provide load-bearing capacity in expansive, rocky or hydro collapsible soils. The foundation, or “Wafflemat,” is created by connecting a series of 8½” or 12” high, 19” x 19” thermal-grade heat resistant plastic forms, “Waffleboxes,” set directly on grade, and then monolithically pouring a post tension or rebar re-enforced concrete slab (usually 4” – 8”). No footings are required. The monolithic pour creates concrete beams running throughout the footprint and perimeter. The completed slab then sits on the ground like a raft, the void areas underneath the slab formed by the placement of the forms allowing for expansive soil movement.

The on-grade mat foundation possesses great stiffness, with strength to resist differential swelling resulting from landscaping practices, surface drainage, or flooding from any source. It does not require presoaking underlying soil pads, and there is no need for footings, meaning no earth spoils. And, since the slab section is typically 13” – 14” above grade, it requires no gravel, sand or moisture barrier.



Post-tensioned slab-on-grade construction and rebar reinforced mats have been used for decades to provide adequate support for residential and light commercial construction. First generation, post-tensioned slabs were constructed by trenching to form in-ground beams (or “ribs”) to provide stiffness when combined with a relatively thin slab. Subsequently, uniform thickness post-tensioned slabs (or, “UTF” for “Uniform Thickness Foundation”) were utilized with or without perimeter in-ground beams. The uniform thickness slabs are much thicker than slabs of the in-ground beam system, and have gained some prominence in certain parts of the world.

Post-tensioned slabs provide a mat foundation which tolerates movement, but to an acceptable limit. The deflection limits must be compatible with the type of construction supported by the post-tensioned slab. Deflection arises from differential swelling or shrinking of the soil underneath the slab, expressed in terms of edge lift or center lift. This deflection is mitigated by the slab stiffness and strength.

The soil contact area of the slab in both the in-ground ribbed slabs and the uniform thickness slabs is equal to the total slab area. This means that when the slab is subjected to uplift forces from soil expansion, 100% of the soil expansion area below the slab exerts force on the slab. The upward force is resisted by the slab stiffness and weight of the slab and structure above.

The on-grade mat foundation, or “Wafflemat,” has emerged as a third-generation post-tensioned slab. The use of its hollow forms, or “Waffleboxes,” in a grid arrangement placed directly on grade to create voids in a waffle-like pattern, creates a series of ribs in both directions, with post-tensioning tendons located in the ribs. Concrete is then placed over the forms, creating the on grade mat foundation.

The Wafflemat system was patented on July 30, 1996.

IBC and an on-grade mat foundation

The on-grade mat foundation adheres to International Building Code requirements. By 2008, most states put into effect the changes adopted in the 2006 IBC and, in regards to foundations, the on-grade mat foundation has become a more attractive design because, as an engineered system, it already accommodates the 2008 design recommendations, and required no major modifications to bring it into compliance.

The Post-Tensioning Institute, in June 2008, approved two key specifications of an on-grade mat foundation and immediately incorporated them into the PTI Manual, the publication used by municipalities around the world to determine building code regarding specifications and requirements of post tensioned foundations.

First, in section, the PTI Manual was revised to permit rib spacing of less than 6 feet, and to clarify what spacing shall be used to compute moments and shears. The on-grade mat foundation spacing is 3' 8," now fully embraced by PTI.

Second, in section, the PTI Manual was revised to limit rib widths to a range of 6 to 14 inches (from the previous 8 to 14 inches). The beams of an on-grade mat foundation taper from 8" at the top of the plastic forms to 6" at the bottom—measurements now fully embraced by PTI.

Performance of an on-grade mat foundation

Structural considerations

An on-grade mat foundation contacts the ground only at the bottom of the ribs created by the plastic forms, a contact area much less than the total slab area. Reduction of the contact area is significant in that there's less surface area upon which swelling soils can exert force, and the superimposed bearing stress is increased, the on-grade mat foundation’s resulting higher bearing stress counteracting soil swelling pressures. In contrast, both in-ground ribbed slabs and uniform thickness slabs have lower bearing stress, and thus have a lower capacity to resist swelling pressures.

An engineer designing an on-grade mat slab makes the same calculations and follows the same requirements used in the design of traditional post-tensioned slabs on grade, and then applies them to the on-grade mat foundation ensuring the system possesses equal or greater stiffness.[citation needed]

Geotechnical considerations

The voids created by the individual plastic forms in the system allow unimpeded deformations of soil due to moisture variation, reducing the impact of the soil's volume change on the on-grade mat slabs.

A study completed in 1997 monitored 28 Wafflemat foundations under adverse conditions. The study showed little movement was measured over a 12-month period for the on-ground mat slabs, which were conventionally reinforced. The pad preparations varied greatly; some were heavily pre-soaked, others not at all. Level readings from the study showed mitigation of expansive soil effects for the 28 on-grade mat foundations on soil with PI’s ranging from 26 to 58.

Earlier research, noted in the 1997 study, showed that pressures caused by swelling soils were reduced by the voids in the on-grade mat foundations. The soil pressures were relieved by the soil expanding up to several vertical inches into the voids. Level readings taken on top of the on-grade mat foundation showed little movement in spite of having been built on moderately dry subgrade, and subsequently partially flooded on one side to simulate extreme conditions. Over time, the moisture under the Wafflemat foundation stabilized and became uniform. Throughout this process, the level readings varied little, and the performance of the on-grade mat foundations fell well within serviceability limits.

Current practice in the design of post-tensioned on-grade mat foundations utilizes the 3rd edition of the Post Tension Institute’s “Design of Post-Tensioned Slabs-on-Ground.”

Research considerations

Thorough research evaluations have been performed by geotechnical and structural engineers, and also by universities.

  • An on-grade mat foundation provides a more uniform distribution of stiffness than other ribbed slabs. The system readily eliminates the necessity of pads below interior columns, or beams below interior walls;
  • It provides a greater stiffness, and hence a greater resistance to induced soil displacement, than does a uniform slab with comparable material quantities;
  • It does not suffer from shear-lag-caused inefficiency in resisting flexural stresses, a condition inherent in the thin flange of other ribbed slabs—something overlooked by many SOG design engineers.
  • The contact area of an on-grade mat slab with the soil for the transfer of the loading from the superstructure and slab weight is less than other traditional slabs. An on-grade mat slab, therefore, develops a higher stress interface with the soil, and that higher stress dampens the swell of expansive soils. This results in a lesser imposed deformation of the on-grade mat, and leads to an improvement in performance.

Practical considerations

On-grade mat foundations have, since release in 1995, been installed in over 7,500,000 sf of residential living space on highly expansive soils in the US and Mexico. There has never been a structural failure reported.[citation needed]

Environmental considerations

Environmental engineers note reduced carbon emissions and air quality emissions are usually of the same magnitude as the reduction in concrete needed for any foundation project. Since use of an on-grade mat foundation typically results in a 20% - 30% reduction in concrete, a similar or even greater reduction in carbon emissions and air emissions will also occur. Reduced carbon emissions benefit the global carbon cycle, while reduced air quality emissions benefit the local environment of the job site and area where the cement is manufactured. And, because the basic nature of an on-grade mat is to utilize less raw materials (cement, iron, fuel, water, aggregate, and sand), the system is proven to provide a more sustainable environment in an era of resource limitations.

Green building

Administered by the US Green Building Council (USGBC), a non-profit organization committed to expanding sustainable building practices, LEED points are available to builders in six broad categories:

  • Sustainable Sites (14 possible points)
  • Water Efficiency (5 possible points)
  • Energy & Atmosphere (17 possible points)
  • Materials and Resources (13 possible points)
  • Indoor Environmental Quality (15 possible points)
  • Innovation and Design Process (5 possible points)

LEED certification occurs at four levels: “Certified” requires 26 – 32 points, “Silver” 33 – 38, “Gold” 39 – 51, and “Platinum” 52 – 69. Builders submit projects (and accompanying justification for receiving points) for review. Points for an on-grade mat foundation would likely be awarded in the categories Energy & Atmosphere, Materials and Resources, and Innovation and Design Process.

Environmental awards

In May 2007, the BREATHE CALIFORNIA organization selected the Wafflemat on-grade mat foundation as the recipient of their Clean Air Award in the category of “Green Building.”

A February, 2007, whitepaper provided analysis and concluded the manufacture and transportation of concrete produces significant amounts of carbon dioxide, which the main anthropogenic gas is contributing to the buildup of greenhouse gases in the earth’s atmosphere. Since a Wafflemat system is made up of foundation-forming Waffleboxes that displace concrete, less concrete is utilized in the foundation of a home when compared to conventional ribbed or uniform thickness foundation slabs, translating into several tons less CO2 released into the atmosphere per home.


Current design practice provides post-tensioned on-grade mat slabs with stiffness equal to or better than other post-tensioned slab types, but with less susceptibility to swell pressures exerted by expansive soils. An on-grade mat foundation provides all of the elements of the in-ground rib and uniform thickness slabs, but with greater performance provided by its geometry and smaller contact area.

Following is a gallery of images of the Wafflemat in action.


  • Ground Supported Post Tensioned Waffle Slab, Dr. Bijan Aaiami, San Francisco State University, 1995
  • Geotechnical Investigation, Richland Homes, Purcell, Rhoades, & Associates, 1997

External links

Wikimedia Foundation. 2010.

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