Rule of Thumb for Foundation Depth

The purpose of the foundation of a building is to provide support to the structure that is situated over the ground. Foundations are always grooved into the soil, underneath the frost level, to make sure that no movement or shifting of a structure occurs.

Foundations should be level properly to bear the weight of a structure for a prolonged period. For this purpose, the larger bases known as footers are dug deep into the ground to give adequate support to the foundations. The depth and width of its foundation are dependent on the footers of a building.

While making the design of any building, the most vital factor is to retain the proper depth of foundation for a building structure.

In the following civil engineering tutorial, you will learn how to calculate the exact depth of the foundation for a building.

Prior to work out the depth of shallow foundation, the following factors should be taken into consideration :-

1. Foundation should be arranged at such a depth to keep it safe against damages caused by swelling, shrinkage or freezing of sub soil.
2. There should be sufficient bearing strength of soil underneath the foundation to bear the load generating from foundation.
3. In case, the foundation is provided on cohesive soil then the settlement as a result of consolidation should not be excessive.
4. The foundation should not be placed on loose or disturbed soils which may corrode by wind or flood.

5. If possible, the foundation should be arranged over the ground water table to avoid cost of pumping as well as instability of soil caused by leakage of water into the bottom of an excavation.
6. Examine the foundation soil properly to get details about its physical and chemical properties, since existence of sulphate can make the foundation weak.

The minimum depth of shallow foundation for a soil is measured with the following formula known as Rankine’s Formula.

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

Here,

Dmin denotes minimum depth of foundation in m

Also Read: Some useful guidelines on the construction of RCC foundation

g denotes density of unit weight of soil in kN/m³
Ø denotes angle of repose in degrees
q denotes intensity of load or safe bearing strength of soil in kN/m²

CALCULATION PROCESS

Work out the minimum depth necessary for a foundation to pass on a pressure 55 kN/m2 in a cohesionless soil with density 16 kN/m3 and angle of repose 200?

The calculation is done based on the following data:

Intensity of pressure (q) = 55 kN/m²
Density of soil (g) = 16 kN/m³
Angle of repose (Ø) = 20⁰

Calculation

Minimum depth of foundation, in accordance with Rankine,
Dmin = (q/g) * [(1 – sinØ) / (1 + sinØ)]²
Dmin = (55/16) * [(1 – sin20⁰) / (1 + sin20⁰)]²
Dmin = 0.82 m

In order to calculate the breadth of foundation, go through the following video tutorial presented by the renowned engineer S.L.Khan.

Video Source: SL Khan