Table of Contents

## 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.

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.
- The minimum depth of the foundation should not be less than 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 Load | Foundation Depth for pile foundation |

Load of Upto 500 KN | 3.5m to 4.5m |

2000 KN to 3000 KN | 6 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/m^{2};

w = Density of soil KN/m^{3};

Ⴔ = Angle of repose of soil;

### Bearing Capacity of Soil

The bearing capacity of soil is the amount of load that a soil can handle without getting failed in shear. The safe bearing capacity of soil is measured in KN/m^{2}.

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

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

Type of Soil | Bearing Capacity of Soil KN/m^{2} |

Gravel Sand | 441 |

Loose Gravel | 98 |

Sand Stone, Lime Stone | 1618 |

Fine Sand, Loose and dry | 98 |

Moist clay | 147 |

Medium clay | 245 |

Hard clay | 451 |

Fine sand and silt | 245 |

Soft rock | 441 |

Coarse sand, compact and dry | 441 |

Residual deposit of shatters and broken bed rock | 883 |

Hard rock – Granite, Dionite | 3236 |

### Density of Soil

The relation between mass and volume of a dry sample of a soil specimen is called as the density of soil. Normally density of soil is 2.65 grams/cm^{3}

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

Type of soil | Density of soil in Kg/m^{3} |

Gravel | 2000 |

Silt | 2100 |

Clay (Dry) | 1600 |

Clay (Damp) | 1760 |

Sand (Dry, Loose) | 1440 – 1700 |

Earth (Dry, Loose) | 1200 |

Metamorphic rocks | 2700 |

Sedimentary rocks | 2600 |

Igneous Rocks (Mafic) | 3000 |

Igneous Rocks (Felsic) | 2700 |

Rubble Stone | 1600 – 1750 |

Mud | 1600 – 1920 |

### Angle of Repose

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

The angle of repose for different soil is given below:

Type of soil | Angle of Repose |

Gravel | 40° |

Dry sand | 30° – 35° |

Wet sand | 25° – 26° |

Dry clay | 35° |

Wet clay | 15° |

Moisture earth | 45° – 50° |

Wet Earth | 15° – 17° |

Dry Earth | 20° – 30° |

Consolidated dry earth | 30° |

Moist sand and consolidated dry sand | 35° |

Rubble stone and damp clay | 45° |

Coal Ashes | 40° – 45° |

### 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/m^{2} ,

w = 1600 Kg/m^{3}

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 the minimum depth of foundation for soil with the following properties

P = 370 KN/m^{2} ,

w = 1700 Kg/m^{3}

Ⴔ = 35°

Df = P/ w (1-SinႴ/1+SinႴ)^{2}

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

Df = 1.60 m

Thus the minimum depth of the foundation is 1.60 meters.

Example 3. Calculate the minimum depth of foundation for soil with following properties

P = 150 KN/m^{2} ,

w = 1620 Kg/m^{3}

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:

- Structural load
- Density of soil
- Type of soil
- Water content of soil
- Refilled soil
- nearby excavation
- ground water table
- frost penetration depth
- Bearing capacity of soil

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