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Foundations Technical Guidance


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Foundations Technical Guidance

Please look at the information and related sources for Foundations in the publications or software links. Or, post a question in the Geotechnical Forum.



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.

Foundation Types and Selection

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

Subsoil Conditions

Light, Flexible Structure

Heavy, Rigid Structure

Deep compact or stiff soils

Shallow footing

Shallow footing, shallow mat

Deep compressible strata

Shallow footing overlying compacted granular fill, Shallow mat, Friction piles

Deep mat, Friction piles

Soft or Loose strata overlying firm strata

End bearing piles or piers, Shallow footing on compacted granular fill, Shallow mat

End bearing piles or piers, Deep mat

Compact or stiff layer overlying a soft or compressible strata

Shallow footing, Shallow mat

Deep mat (floating foundation), Piles

Alternating soft and stiff layers

Shallow footing, Shallow mat

Deep mat, Piles


General Foundation Considerations

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.


Bearing Capacity and Settlement Analysis

Click one of the following for technical guidance, theories, information, calculations, and example problems concerning:

Bearing Capacity
Settlement Analysis


Soil Properties

Typical soil and soil related properties used for foundation analysis are listed below:

Angle of Internal Friction
Bearing Capacity Factors
External Friction Angle
Factor of Safety
Lateral Earth Pressure Coefficients
Soil Unit Weights


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