Standard Footing Size For Columns | Foundation Depth | Footing Foundation Details | Depth of Foundation Formula

What is Foundation

Foundation or Footing is underground structure usually below ground level that transmit load from super structure to sub structure. Thus foundation helps to transfer load form beam and column to soil.

Depth of foundation

Footing is made up of different material, like reinforced cement concrete, brickwork masonry. Column weight is transferred to the large soil area below ground.

Minimum Depth of Foundation

Depth of foundation depends upon several factors like,

  • Bearing capacity of soil.
  • Density of soil.
  • Dead load of the structure.
  • Live load on structure and possible wind load and seismic load on structure.
  • depth of shrinkage and swelling.
  • Possibility of excavation in nearby areas.
  • Level of ground water table.
  • In case of fine sand silt, depth of frost penetration is important factor.
  • Minimum depth of foundation should not be less then 50cm in any case.

Thus taking all these factors into account standard depth of foundation is 1 meter to 1.5 meter from the natural ground level.

The minimum depth of foundation based on the type of soil are as follows:

  • Depth of foundation on Sandy soil = 0.8 – 1 m
  • Depth of foundation on Rocky soil = 0.05 – 0.5 m
  • Depth of foundation on clayey soil = 0.9 – 1.6 m

The minimum depth of foundation for the load-bearing wall is 900 mm.

Generally speaking for small residential building minimum depth of foundation should be 1.5 times the width of footing.

In cold climatic condition, because of frost action minimum foundation depth should be 1.5 meters.

Shallow Foundation Depth

Foundation that are located not more then 6 feet below the ground level is called shallow foundation. Shallow foundation is also referred to as spread footing.

Depth of foundation for shallow foundation is less then 6 ft

Shallow foundation are used when the soil below the structure has sufficient strength to support the load acting on the structure. Small residential houses and wooden structure uses shallow foundation.

Minimum depth of footing for shallow foundation is 4 ft to 5 ft

Deep Foundation Depth

When shallow foundation are not able to support the load acting on the structure then deep foundations are use. In other word when bearing capacity of soil is not sufficient enough then deep foundation are used.

When the hard strata of soil is available at a greater depth then deep foundation becomes the ultimate option.

Deep foundation uses frictional force of the adjacent soil to distribute the load acting on the structure.

Foundation for G+4 Structure requires deep foundation and also if the soil above has poor bearing capacity then deep foundation is recommended.

Depth of deep foundation depends upon the applied load on the structure.

Applied LoadFoundation Depth for pile foundation
Load of Upto 500 KN3.5m to 4.5m
2000 KN to 3000 KN6 m to 20 m

Minimum Depth of Foundation According to IS Code 1080-1982

Depth of foundation depends on many factors. But no foundation should be less then 50 cm or 500 mm as per Indian standard code.

Minimum Depth of foundation = 50 cm

minimum depth of foundation is 1.5 times the width of the footing for a small residential building.

Rankine’s Formula For Depth Of Foundation

Minimum depth of foundation for building was invented by Rankine with great focus on soil characteristics.

Depth of foundation from Rankine’s formula is shown below;

Df = P/ w (1-SinႴ/1+SinႴ)2

Where,
Df = Minimum Depth of foundation in meter;
P = Gross bearing capacity of soil in KN/m2;
w = Density of soil KN/m3;
Ⴔ = Angle of repose of soil;

Bearing Capacity of Soil

Bearing capacity of soil is the amount of load that a soil can handle without getting failed in shear. Safe bearing capacity of soil is measured in KN/m2.

If the total load of the structure is greater then the bearing capacity of the soil then the structure will fail in shear.

Thus bearing capacity of soil is important for structure to be safe and sound. Bearing capacity of different soil is mentioned below:

Type of SoilBearing Capacity of Soil KN/m2
Gravel Sand441
Loose Gravel98
Sand Stone, Lime Stone1618
Fine Sand, Loose and dry98
Moist clay147
Medium clay245
Hard clay451
Fine sand and silt245
Soft rock441
Coarse sand, compact and dry441
Residual deposit of shatters and broken bed rock883
Hard rock – Granite, Dionite3236
Bearing capacity of soil for foundation depth

Density of Soil

Relation between mass and volume of a dry sample of soil specimen is called as density of soil. Normally density of soil is 2.65 gram/cm3

Density of different type of soil is shown in the below table:

Type of soilDensity of soil in Kg/m3
Gravel2000
Silt2100
Clay (Dry)1600
Clay (Damp)1760
Sand (Dry, Loose)1440 – 1700
Earth (Dry, Loose)1200
Metamorphic rocks2700
Sedimentary rocks2600
Igneous Rocks (Mafic)3000
Igneous Rocks (Felsic)2700
Rubble Stone1600 – 1750
Mud1600 – 1920
Bulk density of soil for footing depth

Angle of Repose

Steepest angle at which sand or any soil that can be piled without slumping or falling down.

Angle of repose for different soil is given below:

Type of soilAngle of Repose
Gravel40°
Dry sand30° – 35°
Wet sand25° – 26°
Dry clay35°
Wet clay15°
Moisture earth45° – 50°
Wet Earth15° – 17°
Dry Earth20° – 30°
Consolidated dry earth30°
Moist sand and
consolidated dry sand
35°
Rubble stone and damp clay45°
Coal Ashes40° – 45°
Angle of repose for depth of

Example Calculation for Foundation Depth

Example 1. Calculate minimum depth of foundation for a hard clay soil?

For Hard Clay Soil, Values for bearing capacity of soil, angle of repose and density of soil is taken from table.

P = 451 KN/m2 ,
w = 1600 Kg/m3
Ⴔ = 35°

Df = P/ w (1-SinႴ/1+SinႴ)2

Df = 451/1600 x {(1-sin 35°)/(1+sin35°)}

Df = 2.10 m

Thus minimum depth of foundation for hard clay soil is 2.10 meter.

Example 2. Calculate minimum depth of foundation for a soil with following properties
P = 370 KN/m2 ,
w = 1700 Kg/m3
Ⴔ = 35°

Df = P/ w (1-SinႴ/1+SinႴ)2

Df = 370/1700 x {(1-sin 35°)/(1+sin35°)}

Df = 1.60 m

Thus minimum depth of foundation is 1.60 meter.

Example 3. Calculate minimum depth of foundation for a soil with following properties
P = 150 KN/m2 ,
w = 1620 Kg/m3
Ⴔ = 28°

Df = P/ w (1-SinႴ/1+SinႴ)2

Df = 150/1620 x {(1-sin 28°)/(1+sin 28°)}

Df = 1.2 m

Thus minimum depth of foundation is 1.20 meter.

Factors Affecting Depth of Foundation

As discussed above depth of foundation depends on many factors and rankine’s formula can be used to find minimum depth of foundation.

Following factors affect the depth of foundation in a building or any structure:

  1. Structural load
  2. Density of soil
  3. Type of soil
  4. Water content of soil
  5. Refilled soil
  6. nearby excavation
  7. ground water table
  8. frost penetration depth
  9. Bearing capacity of soil

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