How much should be minimum depth of foundation- with Numerical

Hi, friends if you want to construct a house or building and you are not getting the idea that how much you should provide the minimum depth of foundation then read this article till the end.

Here I have described the depth of the foundation to be kept in the building in an easy way with numerical calculation.

Foundation is the lowest part of the building below ground level which transfers a load of superstructure to the ground through it. It plays a very important role to protect the building from overturning during an earthquake. It also provides strength to bear wind load to the building.

 minimum depth of foundation

Depth of foundation depends upon the following factors:

  1. Types of soil
  2. Groundwater table
  3. Load from structure
  4. Bearing capacity
  5. Density of soil

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Now, I have the formula to calculate the depth of the foundation called Rankine’s formula. Which is given below;

minimum depth of foundation


D = minimum depth of foundation in ( meter)

P = total load on foundation in (KN/m2)

W = Unit weight of soil

Φ = Angle of repose or internal friction of soil

Now, How can we calculate angle of repose ?

Here, I have given same example of soil type, angle of repose variation and according to angle of repose, unit weight of soil is given.

Soil typeAngle of reposeUnit weight
Dry sand250 – 30016 KN/m3
Moist soil300 – 35018 KN/m3
Wet soil150 – 25019 KN/m3

There are more types of soil or sand, but generally, these types of soil are found while testing.

Let us taken an example if we have moist sand then how much should be minimum depth of foundation?

Let us solve,

We have, load on structure(P) = 300 KN/m2

The density of soil = 18 KN/m3 ( Because we have moist soil )

Angle of repose (Φ) = 300


  Putting the values in Rankine’s formula,

minimum depth of foundation

By calculating above equation we got, the minimum depth of foundation = 1.85 m

Watch video to be more Clear

So, friends In this way, we can calculate the minimum depth of foundation for different buildings if we have a profile of soil and load acting on the foundation.

But, at last, I recommend you prefer, the soil test so that you will get actual density and angle of repose of soil for an accurate depth of foundation.

Generally, the minimum depth of foundation is kept 1.5 m to 2 m for general residential buildings and also for two to three-story compartments.

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"Structural Engineer" with over 5 years of experience in estimation, structural design, and surveying. I am passionate about using my skills to create safe and sustainable structures. I am also a keen writer, and I enjoy sharing my knowledge and experiences with others.

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