How to Determine the Appropriate Foundation Depth

This civil engineering article focuses on how to determine standard depth of foundation from ground level in a building. The depth of foundation is settled on the basis of the following factors :-

1. Type of soil
2. Ground water table
3. Load enforced on structure
4. Bearing strength
5. Density of soil

The depth of foundation can be determined by applying the Rankine’s formula as follow :-

D = P /W x (1 – sinθ) / (1 + sinθ)²

Here, D stands for least depth of foundation in meter.

P stands for total load on foundation or load of structure in KN/m².

W stands for unit weight of soil or density of soil.

θ stands for Angle of repose or internal friction of soil.

Given below, the angle of repose and unit weight for different types of soils :-

Angle of repose - Unit Weight

Dry Sand = 25 to 35 degree - 16 KN/m³
Moist Sand = 30 to 35 degree - 18 KN/³

Also Read: Common types of foundations for buildings

Wet Sand = 15 to 25 degree - 19 KN/m³

As for instance, assume the load of structure, density of soil and angle of repose as follow :-

Load of structure (p) = 300 KN / m²
Density of soil = 18 KN/m³
Angle of repose = 30 degree

After putting all these data into the above mentioned formula, we get the following result :-

D = 300 / 18 (1 – sin30⁰) / (1 + sin30⁰)² = 1.85 meter (it is obtained through a calculator)

So, the standard depth of foundation = 1.85 meter

Video Source: Civil Engineer Deepak Kumar