Foundation depth is the most important calculation in the whole structure. Everyone knows there are two broad categories of foundation in construction.

- Shallow foundation
- Deep foundation

But do you know why the foundation method differs from others?

It all depends on various factors. Here we are going to see

- Factors affecting the depth of the foundation
- Minimum depth of foundation formula by Rankine’s
- What is the angle of response?
- What is bearing capacity of soil?

**Factors affecting the foundation depth**

While designing the foundation depth of a structure following factors plays a vital role.

- Bearing Capacity of Soil (Load Resisting Capacity)
- The density of the soil
- Water table level
- Self-weight of the structure (Dead Load)
- What would be the live load? (Assumption)
- Wind load & seismic load (Earthquake)

**Minimum Depth of Foundation by Rankine’s Formula**

The minimum depth of the foundation formula was invented by Rankine which focused on the soil characteristics.

**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 getting into example calculation. Let’s know about the bearing capacity of soil and Angle of Repose.

**What is Angle of Repose Soil?**

The steepest angle relative to the horizontal plane which a material can be piled without slumping (As shown in the below picture) is known as **Angle of Repose**. The Angle of response shall range from 0° to 90°.

Here we have listed different types of soil’s angle of repose.

# |
SOIL |
ANGLE OF REPOSE |

1 | Mud | 0° |

2 | Wet Clay | 15° |

3 | Wet Earth | 15°-17° |

4 | Dry Earth | 20°-30° |

5 | Wet Sand | 25°-26° |

6 | Consolidated Dry Earth | 30° |

7 | Dry Sand | 30°-35° |

8 | Dry Clay | 35° |

9 | Moist Sand & Consolidated Dry Sand | 35° |

10 | Gravel | 40° |

11 | Rubble stone & Damp Clay | 45° |

12 | Coal Ashes | 40°-45° |

13 | Moisture Earth | 45°-50° |

## What is the 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 the **safe bearing capacity of the soil**.

Here we have listed different types of soil’s bearing capacity with values

# |
SOIL TYPE |
CAPACITY kN/m² |

1 | Loose Gravel | 98 |

2 | Fine Sand, Loose & Dry | 98 |

3 | Moist Clay | 147 |

4 | Medium Clay | 245 |

5 | Fine Sand & Silt | 245 |

6 | Soft Rock | 441 |

7 | Gravel Sand | 441 |

8 | Coarse Sand, Compact & Dry | 441 |

9 | Hard Clay | 451 |

10 | Residual deposits of Shatters & broken bedrock | 883 |

11 | Sand Stone, Lime Stone | 1618 |

12 | Hard rock – Granite, Dionite | 3236 |

The actual bearing capacity and other soil-related details will be given in the soil investigation report.

The density of soil list has been given below

# |
SOIL |
DENSITY Kg/m³ |

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 |

Now let us see an example of depth calculation using the above table value.

**How to Calculate the minimum depth required for a foundation in hard clay soil?**

Df = P/γ (1-SinΦ/1+SinΦ)

P- 451 kN/Sq.m ; γ – 1600 Kg/m^{3}; Φ – 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.

The above is a simple example. Kindly consider the following factors while designing the foundation depth.

- The 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.

Happy Learning 🙂

## 3 Comments

What value mention 101

1 KN is equal to 101 Kg

Please, i need the article about the depth of foundation of 750mm base on the bearing capacity