Minimum Depth of Foundation

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 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°.

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/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 🙂