What are Common House Foundations?

All houses are constructed on foundations – we all know that!  But what type of foundation do you really have?  Most people readily recognize a concrete (often improperly referred to as cement) slab and foundation on their house.  But there are variances to house foundations.  Of course, each house should have its own foundation design to accommodate the load requirements and their application to the soil and ground conditions.  The following examples are traditional foundation types.  

Concrete Slab on Grade

By far, the concrete slab on grade foundation is the most common type of residential house foundation.  As the name implies, the concrete is placed on the ground.  Trenches are dug beneath the load bearing walls to develop thicker concrete (grade beams) to support the weight of the house.  Steel reinforcement bars, commonly called rebar, are strategically placed to strengthen the concrete foundation.  After the plumbing and electrical pipes are installed in the ground, the concrete slab and foundation are poured together to create a monolithic (all-in-one) slab and foundation.

Post-Tension, Concrete Slab 

A post-tension concrete slab on grade foundation is like a concrete slab on grade foundation in appearance and preparation.  However, the reinforcement of the foundation is different.  In a post-tension concrete slab and foundation, high strength steel cables are installed in lieu of the steel reinforcement.  After the concrete hardens, the post-tension cables are stretched and then clamped to the edges of the foundation with special anchors.  The cable extensions are then cut thereby introducing compression (squeezing) into the concrete creating a strong slab and foundation.  Post-tension concrete slab foundations must be certified (sealed) by a licensed engineer.

Pile Supported, Concrete Slab

Pile supported foundations often appear no different than the common, concrete slab foundation.  However, timber piles or concrete piles are driven deeply into the ground in a defined grid or pattern to support the slab and house.  Geotechnical engineering investigations are often performed to evaluate the soil in preparation of the pile supported foundation. Typically, a concrete slab foundation or a post-tension concrete slab is installed above the piles at the ground level.  Pile supported foundations must be certified (sealed) by a licensed engineer.

Elevated Pier and Beam

As the name suggests, this type of foundation has a floor system raised above the ground.  It is developed by having piers (concrete pedestal, CMU block, etc..) installed on spread footings (small slabs) or on concrete slab on grade foundations supporting wooden beams.  The piers and beams are generally oriented in a grid fashion to distribute the loads from the walls of the house into the ground.  Wooden floor joists are uniformly spaced and bear on the beams.  Above the floor joists, the floor deck (typically plywood) is laid uniformly and nailed or screwed to the joists.  The system of beams, joists, and the floor deck establishes a region below the floor system called the crawl space.  This type of foundation requires proper ventilation of the crawl space.  

Stem Wall 

Stem wall foundations are generally used to elevate the final elevation of the house above natural ground.  The stem wall foundation consists of spread footings (concrete slabs) installed below the ground and the actual wall itself.  The walls are commonly constructed with CMU blocks or cast-in-place concrete.  The area behind the stem wall and below the house is filled with select soil material.  Then, a concrete slab or post tension slab is installed above the stem wall foundation.  Stem wall foundations should be certified (sealed) by a licensed engineer.

Retaining Wall

Retaining wall foundations are generally used support a house in regions where the ground elevation varies greatly.  A retaining wall foundation is installed on a spread footing, a conventional concrete slab on grade, or a post-tension concrete slab.  In unique soil conditions, it may even be installed on a pile supported foundation.  This type of foundation is commonly associated with basements where the intention of the retaining wall foundation design is to keep exterior soil from entering the basement region.  Retaining wall foundations involve detailed structural and water management specifications.  Wood frame foundation features are usually installed above the retaining wall foundation.  Retaining wall foundations must be certified (sealed) by a licensed engineer.

Direct Buried Pile

Direct buried pile foundations are often used in low lying land where rising water may impact the house.  The purpose of a direct buried pile foundation is to raise the house and improvements above the water elevation during a flood event while structurally supporting the loads of the house.  Piles usually consist of treated timber, precast concrete, or cast-in-place concrete.  Treated timber piles and precast concrete piles are driven into the ground with a large hammer to deep depths and remain extending above the ground.  Cast-in-place concrete piles are installed by boring a hole in the ground and filling the hole with concrete.  Of course, forms above the bored holes are required above the ground for cast-in-place concrete piles.  Once in place, wood framed floor systems of the house are installed above the piles.  For proper function of a direct buried pile foundation, Geotechnical engineering investigations are performed to evaluate the soil and aid in the design of the foundation.  Direct buried pile foundations must be certified (sealed) by a licensed engineer.

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