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 is created by connecting a series of thermal-grade heat resistant plastic forms or Insulated Concrete Forms ICF's often made from Expanded or extruded polystyrene, set directly on grade, and then monolithically pouring a post tension, rebar or Fiber reinforced concrete slab (usually 4” – 8”). No footings are required. The monolithic pour creates concrete beams running throughout the footprint and perimeter in one operation. The completed slab then sits on the ground like a raft, the void areas underneath the slab, or the low pressure areas 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 little or no gravel, sand or moisture barrier.

IBC and an on-grade mat foundation

On-grade foundations adhere 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 4.5.2.1, 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.

Second, in section 4.5.2.3, the PTI Manual was revised to limit rib widths to a range of 6 to 14 inches (from the previous 8 to 14 inches).

Performance of an on-grade mat foundation

Structural considerations

An engineer designing an on-grade 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.

Geotechnical considerations

The voids or low pressure areas 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.

It should be noted, however, that if these beams or ribs are below the raft, the bottom of the excavation becomes extensively cut up with trenches, which destroys the bearing capacity of the soil due to its disturbance adjacent to the beams.[1]

Research considerations

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

Practical considerations

On-grade foundations have been installed in millions of sf of residential and light commercial space on low, moderate, and highly expansive soils in the USA and Mexico. There has never been a structural failure reported.

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:

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

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 an on-grade foundation system is made up of foundation-forming light weight material 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.

Now

Current design practice provides post-tensioned on-grade 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 On Grade Slab Forms in action.

References

  1. ↑ Reynolds, Henry Raymond. Practical Problems in Foundations. London: C. Lockwood, 1960. Print.

External links


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