The Depth of Foundation is the most important part of a structure because the strength of the structure depends on the foundation.

We shall see some of the building was constructed by the Shallow foundation (Isolated footing, Combined footing or RR Masonry) & some other deep foundation (Mat foundation, Pile foundation) Why?.

The depth of the foundation was designed based on the following factor.

- Bearing Capacity of Soil (Load Resisting Capacity)
- The density of the Soil
- The water table of ground surface
- Self-weight of the Structure
- What would be the Live Load (Assumption)
- Wind Load & Seismic load (Earthquake)

The minimum depth of foundation formula has invented by Rankine.

### Rankine’s Formula

Df = P/ γ (1-SinႴ/1+SinႴ)^{2}

Df – Minimum depth of foundation in meters

P – the Bearing capacity of the soil in KN/m^{2}

γ – Density of soil or the unit weight of the soil in KN/m^{3}

Ⴔ – The angle of repose soil

Before to calculate the depth of shallow foundation we shall discuss the Bearing capacity of soil and Angle of Repose.

### What is Angle of Repose Soil?

A granular material is the steepest angle relative to the horizontal plane which a material can be piled without slumping (As shown in the below Picture). The Angle of repose shall range from 0° to 90°.

Here we listed some type of soil Angle of repose.

SI NO | Soil | Angle of Repose |
---|---|---|

1 | Dry Sand | 30-35 |

2 | Moist Sand & Consolidated Dry Sand | 35 |

3 | Wet Sand | 25-26 |

4 | Dry Earth | 20-30 |

5 | Moisture Earth | 45-50 |

6 | Wet Earth | 15-17 |

7 | Consolidated Dry Earth | 30 |

8 | Gravel | 40 |

9 | Rubble stone & Damp Clay | 45 |

10 | Dry Clay | 35 |

11 | Wet Clay | 15 |

12 | Mud | 0 |

13 | Coal Ashes | 40-45 |

### What is Bearing capacity of the Soil?

The capacity of the soil that supports the structural load to the ground without any shear failure or settlement is called safe bearing capacity of the soil.

Here we listed some Bearing capacity soils with Values

SI | Soil Type | Capacity KN/Sqm |
---|---|---|

1 | Hard rock - Granite,Dionite | 3236 |

2 | Sand Stone, Lime Stone | 1618 |

3 | Residual deposits of Shatters & broken bed rock | 883 |

4 | Soft Rock | 441 |

5 | Gravel Sand | 441 |

6 | Coarse Sand, Compact & Dry | 441 |

7 | Coarse Sand, Compact & Dry | 441 |

8 | Fine Sand & Silt | 245 |

9 | Loose Gravel | 98 |

10 | Fine Sand, Loose & Dry | 98 |

11 | Hard Clay | 451 |

12 | Medium Clay | 245 |

13 | Moist Clay | 147 |

The actual bearing capacity and other related detail of soil give in Soil investigation report.

The density of Soil list given below

SI | Soil | Density Kg/Cum |
---|---|---|

1 | Clay (Dry) | 1600 |

2 | Clay (Damp) | 1760 |

3 | Earth (Dry,Loose) | 1200 |

4 | Sand (Dry,Loose) | 1440-1700 |

5 | Gravel | 2000 |

6 | Silt | 2100 |

7 | Igneous rocks (Mafic) | 3000 |

8 | Igneous rocks (Felsic) | 2700 |

9 | Sedimentary rocks | 2600 |

10 | Metamorphic rocks | 2700 |

11 | Mud | 1600-1920 |

12 | Rubble Stone | 1600-1750 |

Let us see an example for Depth calculation for Hard Clay Soil

Df = P/ γ (1-SinႴ/1+SinႴ)^{2}

P -451 KN/Sqm

γ – 1600 Kg/m^{3}

Ⴔ – 35

Depth = (451×101/1600) x ((1-Sin35°)/(1+Sin35°))^{2}

= 2.10 m

So that we required 2.10m minimum depth for hard rock soil.

Kindly consider the following factor while designing the foundation depth.

- Dead load. Live load & other loads (Read about Types of Loads)
- If you need to extend the building in future, so design accordingly.
- Collect soil strata detail to do accuracy design.

We shall see the depth of deep foundation design in another article!.

If anything we missed to mention, let us know!.