Shallow foundation | Spread, Combined, Strap and Raft foundation

Friday, November 18, 2016 10:43
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Shallow foundation

Shallow foundation may be defined as:

Foundation whose depth is equal to or less than its width.

Types of shallow foundation

1.Spread footing

Spread footings as the name suggest, spread the super imposed loads of the structure over a large area.

Spread footing may be of many types such as

  • Single footing for a column.
  • Stepped footing for a column.
  • Sloped footing for a column.

The base for these types of footings is made of concrete.

  • Wall footing without steps and with steps.
  • Grillage foundation

When heavy structural loads of a column are required to be transferred to the soil of low bearing capacity, the most economical foundation is grillage foundation. Depth of such a foundation is limited to 0.9 to 1.6 meter.

2.Combined footing

The footing which is constructed for two or more columns is called combined footing.

Shape of combined footing is so proportioned that the center of gravity of the supporting area is in line with the center of gravity of the two column loads. Its shape is either rectangular or trapezoidal.

A combined rectangular footing is provided where both the columns carries equal load or interior column carries greater load. A combined trapezoidal footing is provided under any conditions of loading.

3.Strap footing

When two or more footings are connected by a beam, it is known as strap footing.

It is provided if the distance between the columns is so great that a combined trapezoidal footing becomes quite narrow, with high bending moments.

4.Mat or Raft foundation

A thick reinforced concrete slab covering the complete area of the bottom of the structure is known as mat or raft foundation. 

Mat or raft foundation is provided where

  • When the soil underneath is of low bearing capacity and the building loads are heavy.
  • When the combined area of individual footing is more than half of the total area of the structure, then it is economical to use mat or raft foundation.
Reference: Civil Engineering (Conventional and objective type) by R.S. Khurmi and J.K Gupta
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