We knew that the main function of the foundation is transmitted the load to the soil which received from structure either building, bridges, dam, etc.,
A foundation will be design based on the soil stability and different types of load. The soil stability has calculated through the bearing capacity of the soil and it is the very important factor in designing the depth of foundation.
So in this post, we are going to discuss the bearing capacity of the soil and how it could be calculated in the field.
What is bearing capacity of Soil?
The capacity of the soil that supports the structural load to the ground without any shear failure or settlement is called safe bearing capacity of the soil.
The safe bearing capacity of soil formula
The safe bearing capacity of soil = Ultimate bearing capacity (UB)/Area x Factor of Safety
What is meant by the Ultimate bearing capacity of the soil
When the soil displacement starts at a point is called ultimate bearing capacity.
Ultimate bearing capacity of soil = Wxh/d
Let us explain in easiest way!…
A 1.5 Liter water bottle can accommodate 1-litre water it is called as Safe bearing capacity. At the same time it can able to store 1.5-litre water and can’t be able to store more than 1.5 litres is called Ultimate bearing capacity.
The safe bearing capacity of soil has calculated by Drop Weight method for the small structure at the field.
Drop Weight Method
- First, excavate the pit at the required depth.
- Take a solid square cube of know weight and dimension.
- Then drop the cube from known height for several times.
- Measure the average depth of impression made by the square.
The dimension of the Cube – 15cmx15cmx15cm Weight (W) – 1 Kg Area – 225 Sqcm h – 150cm
Average Depth – 0.6cm (d1+d2+d3)/3
The factor of Safety (FOS) – 2 (Generally it will be the range 2 to 3)
Ultimate bearing capacity = 1 x 150/0.6
= 250 Kg
Safe bearing capacity = 250/15 x 2
= 8.33 Kg/Sqcm
Here we listed some Bearing capacity soils with Values
| SI | Soil Type | Capacity KN/Sqm |
|---|---|---|
| 1 | Hard rock - Granite,Dionite | 3236 |
| 2 | Sand Stone, Lime Stone | 1618 |
| 3 | Residual deposits of Shatters & broken bed rock | 883 |
| 4 | Soft Rock | 441 |
| 5 | Gravel Sand | 441 |
| 6 | Coarse Sand, Compact & Dry | 441 |
| 7 | Coarse Sand, Compact & Dry | 441 |
| 8 | Fine Sand & Silt | 245 |
| 9 | Loose Gravel | 98 |
| 10 | Fine Sand, Loose & Dry | 98 |
| 11 | Hard Clay | 451 |
| 12 | Medium Clay | 245 |
| 13 | Moist Clay | 147 |
Here we illustrated the bearing capacity calculation just for your knowledge and the actual design should be calculated as per IS code specification. You may refer the calculation of minimum depth of shallow foundation.
