Determining the correct depth of foundation for a building structure is an important step in the process of building design. The information given in this post will help you to decide the proper depth of foundation of the building.

After reading this post you will be able to answer the following questions.

- What is a foundation?
- What are the factors affecting the depth of foundation?
- How to calculate foundation depth?

The foundation is that part of the structure that receives the load of the superstructure and then transfers that load to the soil below in such a way that the soil never fails to shear or never undergoes excessive settlement of differential settlement .

Before calculating shallow foundation depth, the following factors should be considered in advance.

- The foundation should be placed at such a depth that it is protected from damage caused by swelling, shrinkage or solidification of the sub-soil.
- The carrying capacity of the soil under the foundation should be sufficient to support the load coming from the foundation.
- If the foundation is to be laid on the connecting soil, the settlement should not be excessive due to consolidation.
- Never lay a foundation on loose or disturbed soil that has a tendency to be destroyed by wind or flood.
- If possible the foundation should be placed above groundwater level as this can avoid the cost of pumping, and prevent soil instability due to seepage of water into the bottom of the excavation.
- Examine the foundation soil to learn its physical and chemical properties, as the presence of sulfate can damage the foundation.

The minimum depth of a shallow foundation for a soil can be calculated using the following formula suggested by Rankin. it is called **Rankine’s formula**,

**D _{Minute} = (Q/**

**Yes) * [(1 – sin**

**Ø) / (1 + sin**

**Ø)]**

^{2}Where,

**D _{Minute}** = minimum depth of foundation in

**M**

**Yes** = Density of unit weight of soil in **kN / m ^{3}**

**I** = angle of stop **degree**

**Why** = load intensity or safe bearing capacity of soil in **kN / m ^{2}**

Table of Contents

## example count

55 kN/m . Calculate the minimum depth required for a foundation to transmit pressure^{2} 16 kN/m . in a compacted soil of density^{3} and stopping angle 20^{0},

__given data__

Pressure intensity (q) = 55 kN/m^{2}

Soil density (g) = 16 kN/m^{3}

Stopping angle (Ø) = 20^{0}

__calculation__

The minimum depth of foundation according to Rankin,

**D _{Minute} = (Q/**

**Yes) * [(1 – sin**

**Ø) / (1 + sin**

**Ø)]**

^{2}D_{Minute} = (55/16) * [(1 – sin20^{0}) / (1 + sin20^{0})]^{2}

D_{Minute} = 0.82 square meter

## useful points

For a preliminary calculation of the depth of the foundation, the values u200bu200bof the density and the angle of rest can be used as given in the following table.

Types of soil |
angle of rest (in degrees) |
Unit Weight (in kN/m.)^{3}, |

dry sand | 25 – 35 | 16.0 |

moist sand | 30 – 35 | 18.4 |

wet sand | 15 – 25 | 19.2 |

dry and compacted sand | 35 | 19.2 |

clear gravel | 30 – 40 | 17.9 |

gravel and sand mixture | 25 – 40 | 19.2 |

rubble stone | 45 | 19.2 |

dry soil | 30 | 17.6 |

Wet soil | 15 | 19.2 |

ash | 40 | 6.4 |

### pay attention:

- The values given in the table above are approximate values.
- To know the density of the soil on the site you have to test it on site. There are two common methods that are widely used for the determination of soil density at the site. Click on the following two links to read the test procedure.

**How to calculate onsite soil density using core cutter method?**

**How to calculate onsite soil density using sand replacement method?**

- To know the safe bearing capacity of soil, perform plate load test on site and calculate safe bearing capacity from plate load test data. Read the following two posts on PLT.

**How to do plate load test on site?**

**How to calculate safe bearing capacity of soil from PLT?**

- Read the following post for quick determination of safe carrying capacity of soil.

**How to determine the bearing capacity of the soil on the site?**

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