The Constructor

Contact Pressure Distribution Under Foundations

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The stability of structure is majorly depends upon soil – foundation interaction. Even though they are of different physical nature, they both must be act together to get required stability. So, It is important to know about the contact pressure developed between soil and foundation and its distribution in different conditions which is briefly explained below.

What is Foundation Contact Pressure?

Generally loads from the structure are transferred to the soil through footing. A reaction to this load, soil exerts an upward pressure on the bottom surface of the footing which is termed as contact pressure.

Contact Pressure Distribution under Footings

The distribution of contact pressure under different types of footings on different types of soils are explained below.

  1. Under Flexible Footing
  2. Under Rigid Footing

1. Contact Pressure Distribution under Flexible Footing

Fig 1: C.P Distribution - Flexible Footing - Cohesive Soil
Fig 2: C.P Distribution - Flexible Footing - Cohesionless Soil

2. Contact Pressure Distribution under Rigid Footing

Fig 3: C.P Distribution - Rigid Footing - Cohesive Soil
Fig 4: C.P Distribution - Rigid Footing - Cohesionless Soil

Factors Effecting Contact Pressure Distribution

Following are the factors effecting contact pressure distribution

  1. Stiffness of Footing
  2. Compressibility of soil
  3. Type of loading

1. Stiffness of Footing

Contact pressure is uniform in case of flexible footings such as earth embankments. Contact pressure varies in case of rigid foundations such as R.C.C pad foundations etc. If the footing is partly flexible and partly rigid like raft foundation, contact pressure slightly varies.

Fig 5: Flexible and Rigid Footings of a Structure

2. Compressibility of soil

Compressibility or stiffness of soil also plays a role in contact pressure distribution. If the soil is coarse grained, contact pressure is more at the center of foundation than edges where as in case of clayey soils contact pressure is uniform.

3. Type of loading

Concentrated Loading

Uniform Loading

General Assumption of Contact Pressure Distribution

Fig 6: Pad Footings


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