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This page provides information relating to Foundation Types and Selection, information
pertaining to Foundation Considerations, links to
information and example problems for estimating Bearing Capacity and Settlement,
and links to typical Soil Properties and relationships that are used in foundation design.
Selection of an appropriate foundation system is dependent upon many factors. These factors may include:
- soil conditions
- groundwater conditions
- surface conditions
- structural loads
- structural function (i.e. basement, cold structure, etc)
- economy
Foundations are typically constructed utilizing either shallow or deep foundation structures. Foundations are sometimes considered shallow if the
footing depth divided by the footing width is less than 1. Shallow foundations include mat (raft, pad),
continuous footings and isolated (spread) footings. Isolated footings could be square or circular. Deep foundations include
piles, micropiles (minipiles, pin piles), mats and piers. Piles are designed to provide bearing either at the bottom tip of the pile,
friction between the pile sidewalls and subgrade, or a combination of both. Piles are typically driven (hammered) into the subgrade, or piles can be drilled or
jetted into the ground. In addition, piles may be wood, steel or concrete. Micropiles are usually used in lieu of piles in areas with difficult or restricted access.
Micropiles are usually drilled with a small machine, and grouted in place. The definition of piers varies greatly, and ranges from a prefabricated rebar cage
and wet concrete placed in a pre-bored hole to a larger diameter pile. Analysis of a pier is similar to a pile. Floating foundations are typically deep mats, in
which enough soil is excavated so that the weight of soil is equal to the weight of the structure.
Directly from a technical manual produced by the Department of Army (TM 5-818-1), the following foundation possibilities for various
subsurface conditions are feasible
Foundation Possibilities for Different Subsoil Conditions, TM 5-818-1
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Subsoil Conditions
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Light, Flexible Structure |
Heavy, Rigid Structure |
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Deep compact or stiff soils
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Shallow footing
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Shallow footing, shallow mat
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Deep compressible strata
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Shallow footing overlying compacted granular fill, Shallow mat, Friction piles
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Deep mat, Friction piles
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Soft or Loose strata overlying firm strata
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End bearing piles or piers, Shallow footing on compacted granular fill, Shallow mat
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End bearing piles or piers, Deep mat
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Compact or stiff layer overlying a soft or compressible strata
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Shallow footing, Shallow mat
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Deep mat (floating foundation), Piles
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Alternating soft and stiff layers
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Shallow footing, Shallow mat
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Deep mat, Piles
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1. The allowable soil pressure should not be exceeded.
2. Footings should be located below the frost line. Although you could analytically estimate the frost depth, your local City or
County Building Department provides these values.
3. The applied load should be within the middle third of the footing.
4. Foundations should have an adequate factor of safety against uplift, sliding and overturning.
Click one of the following for technical guidance, theories, information,
calculations, and example problems concerning:
Bearing Capacity
Settlement Analysis
Typical soil and soil related properties used for foundation analysis are listed below:
Angle of Internal Friction
Bearing Capacity Factors
Cohesion
External Friction Angle
Factor of Safety
Lateral Earth Pressure Coefficients
Soil Unit Weights
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