Foundation Design Considerations
Foundation Design Considerations
When designing a structure there’s more to consider than the aesthetics and materials.
It’s important to evaluate the soil bearing capacity of where the structure will rest and the weight of the materials within the building, and the local weather.
These factors can play a role in the structure’s stability and lifespan.
When building a structure, the soil’s bearing capacity should be taken into account. The bearing capacity is the maximum average contact pressure between the soil and the foundation which shouldn’t result in shear failure.
Shear Failure is when the shearing stress in the soil compromises the strength of the soil and its ability to support a structure.
It’s important to consider the density of the local soil. The International Building Code provides a list of soil bearing strengths. Different soils have a different bearing capacity. For example, very fine soils, like clay or silt, typically have lower bearing capacities than coarse soils like sands and gravel.
|Soil Class of Materials||Load-Bearing Pressure
(pounds per square foot)
|Sandy gravel or gravel||3,000|
|Sand, silty-sand, clay-sand, silty-gravel, clay-gravel||3,000|
|Clay, sandy clay, silty-clay, clay-silt||2,000|
A great resource is a hand penetrometer which provides you with a good idea of the soil bearing capacity. This hand-held device can estimate the pressure that the soil can resist. This tool isn’t 100% failsafe so if you’re still unsure, it’s best to consult with a geotechnical engineer.
Foundation Types and Soil
Concrete footings, spread footing and mat-slab foundations are classified as shallow foundations. A shallow foundation transfers the building load to the soil near the surface. Unlike a deep foundation, shallow foundations can be prone to settlement when the soil capacity isn’t taken into consideration.
Deep foundations typically utilize pier systems like push piers or helical piers. These systems are installed into load bearing rock or stratum. This installation removes the likelihood of settlement due to changes in the soil or bearing capacity.
Beautiful design can sometimes result in a heavier structure. Stone fireplaces, marble countertops or travertine flooring can add a lot of weight. When you’re designing the building’s foundation and footing, take into account these additional design weights to ensure that the structure is designed to support these elements.
Local weather is a factor in the building’s stability. If the structure is situated on expansive soil and is in a location that has a lot of rain, or a high water table, then hydrostatic pressure could play a role in the foundation’s health.
Hydrostatic pressure is the force of water at rest. If the structure is located on clay soil (expansive soil) it’s more likely to have problems with hydrostatic pressure since clay dramatically expands when wet and contracts when dry.
For example: If adequate drainage isn’t provided for around the foundation, water from rain, or an underground source, will start to build up around the foundation – since it has no where else to go. The pressure, a.k.a. hydrostatic pressure, will push against the foundation walls. Leaks will start to appear then foundation cracks and eventually bowed walls.
External waterproofing measures like footing drains, internal draining system, and extended downspouts can help protect the safety and stability of the foundation.