Let us go through the procedure to find out the SBC of soil at any construction area to design a footing.
Procedure:
1. Excavate a footing pit on the construction site of the required depth.
2. Take an iron or concrete cube & note down the dimensions of the cube.
3. Drop the cube from a known height over the bottom soil layer of the excavated pit. This process should be repeated several times at different locations of the pit. The measurement of free fall height should be kept equal for all the pits.
4. The free fall of the cube creates an impression over the soil surface. You have to note down all the readings of impression depth and the average should be calculated.
5. This process should be repeated for at least 3 different footing pits excavated at different locations of the site. Please remember that the excavated depth of all the footings should be the same.
Calculation:
Let us take a concrete cube of size 15cm. × 15cm. × 15cm.
The concrete cube is dropped from a freefall height of 1.5m. at different footing pits & the readings of impressions were noted down as shown below.
|
Sl. No. |
Footing pit |
Depth of impression in m. |
Average value of impression |
|||
|
Read-1 |
Read-2 |
Read-3 |
Read-4 |
|||
|
1. |
Pit-1 |
0.024 |
0.018 |
0.025 |
0.027 |
0.0235 |
|
2. |
Pit-2 |
0.019 |
0.028 |
0.023 |
0.024 |
0.0235 |
|
3. |
Pit-3 |
0.026 |
0.026 |
0.022 |
0.025 |
0.0248 |
|
|
Total average depth of impression |
0.0239 |
||||
1. Resistance of soil:
Resistance of soil
R = [( W × h ) ÷ d]
Where,
R = Resistance of soil
W = Weight of concrete cube.
h = Height of freefall of cube.
d = average depth of impression.
Here,
h = 1.5m.
d = 0.0239m. (As calculated in the above table.)
W = 8.1kg. ( Picked from below linked article.)
If you want to know how to calculate the weight of any concrete cube, Go through the article 👇
👀. How to calculate the volume & weight of a concrete cube?
Resistance of soil
R = [( W × h ) ÷ d]
= [( 8.1kg. × 1.5m. ) ÷ 0.0239m.]
= 508.37 kg.
The soil has a resistance of 508.37kg for the surface area of the given concrete cube.
Therefore,
Resistance of soil per unit area
= [R ÷ A]
Where,
A = surface area of the cube
= [ 0.15m. × 0.15m.]
= 0.0225 sqm.
Resistance of soil per unit area
= [508.37kg ÷ 0.0225 sqm.]
= 22,594 .20 kg/sqm.
2. SBC of soil :
SBC of soil
= [R ÷ (A × FOS ]
OR
= [ resistance of soil per unit area ÷ FOS]
Where,
SBC 👉 Safe bearing capacity.
FOS 👉 Factor of safety.
The factor of safety is taken between 2 to 3 depending upon the structural design.
Let us take FOS = 2 for the calculation purpose.
SBC of soil
= [ 22,594.2 kg/sqm. ÷ 2]
= 11,297 kg/sqm.
To go through all types of structural design articles, click here.
Thank you for going through these calculation steps❤. Have a good day 😄.
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