How to ascertain depth of foundation

There are various factors for ascertaining the depth of foundation. Estimation for foundation depth is accomplished on the basis of the type of soil, ground water table, loads from structure, bearing capacity of soil and other factors.

Factors liable for ascertaining the depth of foundation:

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

The lowest depth of foundation is taken into consideration to make sure that the soil contains the necessary safe bearing capacity as supposed in the design. However, it is recommended to accomplish soil investigation prior to settle on depth of foundation. Soil investigation report will provide the foundation depth on the basis of the type of structure, soil properties, depth of water table, and all other variable that should be taken into considered. Soil investigation report delivers bearing capacity of soil at diverse levels and locations.

If the soil investigation report is not obtainable, the depth of foundation has to be chosen so that it is not influenced by swelling and shrinking of soil because of seasonal changes. Depth of foundation should also concern the depth of water table to avoid and clean underneath the ground.

For foundation adjacent to current foundation, make sure that pressure bulbs of foundations do not overlap when the depth of new foundation should be selected underneath the depth of current foundation.

The foundation should not be compacted at shallow depth based on the frost action in cold countries.

Rankine’s formula offers the guidance on lowest depth of foundation on the basis of bearing capacity of soil.

Where h denotes lowest depth of foundation

p denotes gross bearing capacity

gama density of soil

phai denotes angle of repose or internal friction of soil.

The above formula only offers the guidance on lowest foundation depth, keeping in mind that the foundations are not influenced with the factors like water table, frost action, types and properties of soil etc. This formula does not include the loads from the structure on the foundation.

In the Rankine’s formula, it is found that foundation depth is dependent on the bearing capacity of soil, so, with the rise in bearing capacity of soil, the depth of foundation is also enhanced.

To view the foundation depth calculator, go through following link


How to ascertain depth of foundation