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

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

Typical Foundation Drawing

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 repose?
  • 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/m2

γ – Density of soil or the unit weight of the soil in KN/m3

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°.

bearing capacity of soil, depth of foundation

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

1 Mud
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

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

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/m3 ; 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 🙂


Bala is a Planning Engineer & he is the author and editor of Civil Planets.


  1. zakari ya`u balarabe Reply

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

  2. sir, for house plastering can we use psand, or riversand or psand mix with river san, advantages and disadvantages

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