Foundation Types
INCLUDES FOOTINGS, PIERS, P.T. SLABS, AND OVER-EXCAVATION
TYPES OF FOUNDATIONS
FOOTINGS
A spread footing is a type of shallow foundation that distributes concentrated loads over an area of soil so that soil pressure (the capacity of the soil to support a load) is not exceeded.
You can understand this principle by thinking about how a snowshoe works. If you try to walk over two feet of snow in your hiking boots, you will fall through. If you put snowshoes on, your weight is distributed over a larger area of the snowshoe, and hence, you do not fall through. Without footings, the foundation walls would sink into the ground.
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When engineers design the width of a footing, they make sure the weight distribution of the footing can be supported by the strength of the soil so that the house does not move vertically. Weak soil types do not have enough strength to support a spread footing, so concrete piers are typically implemented instead.
PIERS
A drilled concrete pier (or "caisson") is referred to as a "concrete pier" in residential applications. The word "caisson" typically applies only to large drilled concrete piers in the construction of bridges and large buildings.
The concrete pier is a deep foundation anchored into a denser soil at deeper depths, unlike the footing, which is supported only by soil below it.
A drilled concrete pier foundation is more costly to the homebuilder than a spread footing foundation.
A concrete drilled pier is constructed by drilling a hole with a drill rig to a depth specified by the engineer, and then the hole is filled with concrete.
POST-TENSION SLAB (P.T. SLAB)
IMPORTANT NOTE FROM ENGINEER MIKE: I do not recommend, ever, purchasing a single-family, townhome, or condominium constructed on a post-tensioned concrete slab-on-grade foundation (a PT slab). Builders try to sell this as a superior foundation for residential construction, which it definitely is not for many reasons.
OVER-EXCAVATION OF SOIL
Soil that does not have enough strength or is expansive can be deemed as "unsuitable" to accommodate a footing-type foundation. Piers, typically used to support structures on weak or expansive soils, are much more expensive than footings. Since builders want to control construction costs, geotechnical engineers are directed to design soil modifications so footings can be used instead of piers.
A foundation "over-excavation" is simply the replacement of unsuitable soil with suitable soil by the direction of the geotechnical engineer. Section 1808.6.3 of the IRC explicitly addresses the removal of expansive soil.
A geotechnical engineer determines the depth, consistency, and method of soil replacement. Then, the contractor replaces the soil in layers at a specified density and moisture content with appropriate means and methods. Testing of the compacted soil layers is conducted during the construction to verify compliance with the geotechnical recommendations.
BASEMENTS
Basement walls sit directly on top of spread footings or piers, and they are generally considered to be a part of the foundation system. Depending on what area of the country you live in, foundation walls are constructed of cast-in-place concrete or concrete blocks.
The four types of basements are none, full, garden-level, and walkout.
Basements typically have either a concrete slab-on-grade floor or a structural floor.
NO BASEMENT (CRAWLSPACE)
Homes can be constructed without a basement, either on a slab or footings. Most homes built without a basement have a crawlspace.
A crawlspace is a volume of space under the first floor that includes supporting columns, plumbing lines, and sump pumps (if required) and is enclosed by the foundation walls. A house with a crawlspace can be structurally supported by either footings or concrete piers.
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Crawlspaces typically have a dirt ground surface. In an area with excessive ground moisture, the floor of a crawlspace should be lined plastic liner to prohibit moisture from affecting the structure.
FULL BASEMENT
Full basements are completely underground and provide a full ceiling-height area for living and/or storage space. Full basements can be designed with standard egress window wells to comply with applicable Building Codes for bedrooms/sleeping areas.
A full basement can be constructed either with a concrete slab-on-grade or structural floors. The type of basement floor, either concrete slab-on-grade or structural floor, in many cases depends on the type of soil underneath the house.
GARDEN-LEVEL BASEMENT
Garden-level basements are partially underground and provide a full ceiling-height area for living and/or storage space. Garden-level basements allow more natural light into the basement level and can be designed with standard egress window wells to comply with Building Codes for bedrooms/sleeping areas.
A garden-level basement can be constructed either with a concrete slab-on-grade or structural floors. The type of basement floor, either concrete slab-on-grade or structural floor, in many cases depends on the type of soil underneath the house.
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WALKOUT BASEMENT
Walkout basements are partially underground and provide a full ceiling-height area for living and/or storage space with door access to the ground level (typically on one side of the home). Walkouts allow the most natural light out of the basement styles into the basement level and can be designed with standard egress window wells to comply with Building Codes for bedrooms/sleeping areas.
A garden-level basement can be constructed either with a concrete slab-on-grade or structural floors. The type of basement floor, either concrete slab-on-grade or structural floor, in many cases depends on the type of soil underneath the structure.
SINGLE-FAMILY
TYPES OF FOUNDATIONS
(FULL BASEMENT)
FOOTINGS
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(CRAWLSPACE)
FOOTINGS
(FULL BASEMENT)
PIERS
(CRAWLSPACE)
PIERS
(BASEMENT STRUCTURAL FLOOR)
PIERS
P.T. SLAB FOUNDATION
TOWNHOMES
TYPES OF FOUNDATIONS
(FULL BASEMENT)
FOOTINGS
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(CRAWLSPACE)
FOOTINGS
(FULL BASEMENT)
PIERS
(CRAWLSPACE)
PIERS
(BASEMENT STRUCTURAL FLOOR)
PIERS
P.T. SLAB FOUNDATION
CONDOMINIUMS
TYPES OF FOUNDATIONS
(FULL BASEMENT)
FOOTINGS
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(CRAWLSPACE)
FOOTINGS
(FULL BASEMENT)
PIERS
(CRAWLSPACE)
PIERS
P.T. SLAB FOUNDATION
FOUNDATION DRAIN COMPONENTS
Foundation drains are perforated that are installed around a building foundation or under the basement floor to collect water from the surrounding soil and discharge it at a safe distance away from the foundation. It prevents water from damaging the foundation and/or flooding the basement.
FOUNDATION DRAIN
FOOTING FOUNDATION DRAIN
PIER FOUNDATION DRAIN
SUMP PIT WITH PUMP
FOUNDATION DRAIN LAYOUTS
A foundation drain, consisting of perforated pipe, is an exterior drainage system installed on the outer face of the foundation wall and near the wall footing, covered with a layer of gravel, serving the purpose of draining out excess water seeping into the foundation through the outside soil (backfill).
Some foundations have a drain and some do not.
The water collected in the perforated pipe flows towards a sump pit, where it is pumped to the outside ground surface away from the foundation.
TYPICAL
SINGLE-FAMILY FOUNDATION DRAIN LAYOUT
TYPICAL
TOWNHOME FOUNDATION DRAIN LAYOUT
