How to Calculate Depth of shallow Foundation ?

Hello friends in this article I will discuss about shallow foundation , that how to calculate depth of shallow foundation so let’s get start.

What is Shallow Foundation: 

shallow foundation is a type of building foundation that transfers building loads to the earth very near to the surface. and shallow foundation depth is less then its width..

Footings are basically shallow foundations. Types include : Strap footing, Combined footing, Stepped footing, Raft footing etc.

read more: Remember 10 Points before Starting Quantity survey of Building

Factors Affecting depth of Foundation:

Before calculating depth of shallow foundation, the following factors have to be considered well in advance.

  1. Foundation should be placed at such a depth so that it is safe against damages due to swelling, shrinkage or freezing of sub soil.
  2. Bearing capacity of soil beneath the foundation must be adequate to support the load coming from foundation.
  3. If foundation has to be placed on cohesive soil then the settlement due to consolidation should not be excessive.
  4. Never place foundation on loose or disturbed soils which have a tendency to erode by wind or flood.
  5. If possible then foundation should be placed above ground water table as this can avoid cost of pumping, and can prevent instability of soil due to seepage of water into the bottom of an excavation.
  6. Make an investigation on foundation soil to know its physical and chemical properties, because presence of sulphate can damage foundation.

The minimum depth of shallow foundation for a soil can be calculated using the following formula as suggested by Rankine. This is called Rankine’s Formula.

Dmin = (q/g) * [(1 – sinØ) / (1 + sinØ)]2


Dmin = Minimum depth of foundation in m

g = Density of unit weight of soil in kN/m3

Ø = Angle of repose in Degrees

q = Intensity of load or Safe bearing capacity of soil in kN/m2

Calculate the minimum depth required for a foundation to transmit a pressure 55 kN/m2 in a cohesionless soil having density 16 kN/m3 and angle of repose 200?

Given Data

Intensity of pressure (q) = 55 kN/m2

Density of soil (g) = 16 kN/m3

Angle of repose (Ø) = 200


Minimum depth of foundation, according to Rankine,

Dmin = (q/g) * [(1 – sinØ) / (1 + sinØ)]2

Dmin = (55/16) * [(1 – sin200) / (1 + sin200)]2

Dmin = 0.82 m

more read: What is 1.54 in Concrete ?

written by: Engr Sami Ullah Stanikzai

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