Many factors affects the depth of foundation. such as type of soil, ground water table, loads from structure, bearing capacity and density of soil and other factors. The minimum depth of foundation is calculated by Rankine’s formula when the bearing capacity of soil is known by soil investigation report.

How to Determine Depth of Foundation?

General factors to be considered for determining depth of foundation are:

  1. Load applied from structure to the foundation
  2. Bearing capacity of soil
  3. Depth of water level below the ground surface
  4. Types of soil and depth of layers in case of layered soil
  5. Depth of adjacent foundation

The minimum depth of foundation should be considered to ensure that the soil is having the required safe bearing capacity as assumed in the design. However, it is advised to carry out soil investigation before deciding on depth of foundation.

Soil investigation report will suggest the foundation depth based on the type of structure, soil properties, depth of water table, and all other variable that should be considered. Soil investigation report provides bearing capacity of soil at different levels and at different locations.

Depth of Foundation
Depth of Foundation

When the soil investigation report is not available, the depth of foundation should be selected such that it is not affected by swelling and shrinking of soil due to seasonal changes. Depth of foundation should also consider the depth of water table to prevent and scour below the ground.

For foundation near existing foundation, It must be ensured that pressure bulbs of foundations do not coincide if the depth of new foundation has to be taken below the depth of existing foundation.

The foundation should not be contracted at shallow depth considering the frost action in cold countries.

Rankine’s formula provides the guidance on minimum depth of foundation based on bearing capacity of soil.

Rankine's Formula for Depth of Foundation
Rankine’s Formula

Where, h = minimum depth of foundation

p= gross bearing capacity

density of soil

= density of soil

Angle of repose of soil

= angle of repose or internal friction of soil.

The above formula does not consider the factors discussed above and just provides the guidance on minimum foundation depth, assuming that the foundations are not affected by factors such as water table, frost action, types and properties of soil etc. as discussed above. This formula does not consider the loads from the structure on the foundation.

In the Rankine’s formula, it can be seen that foundation depth depends on the bearing capacity of soil, so, if the bearing capacity of soil increases, the depth of foundation also increases.

Foundation Depth Calculation

Gross bearing capacity of soil = 300 kN/m2

Density of Soil = 18 kN/m3

Angle of repose = 30 Degrees

Then minimum depth of Foundation

= 1.85m