Let us find out the minimum depth of foundation for the following given data.
Given data:
Load on the foundation ( p ) = 370KN/m2
Unit weight of soil (w) = 17KN/m3
The angle of repose of soil (θ )= 35°
Calculation:
Now, we will calculate the minimum depth of foundation (Df )by using Rankine's formula.
Where,
Df = [( p ÷ w ) × ( 1- sinθ ÷ 1+sinθ )2]
= [( 370 ÷ 17 ) × ( 1- sin35° ÷ 1+sin35° )2]
= [(21.764 ) × ( 1- 0.5735 ÷ 1+ 0.5735 )2]
= [ 21.764 × ( 0.4265 ÷ 1.5735 )2]
= [ 21.764 × (0.271)2]
= [21.764 × 0.07346]
Df = 1.598m.
The minimum depth of foundation for the above-given data is calculated as 1.598 by using Rankine's formula.
In the following table, I have given the angle of repose & the unit wt. of different types of soil.
|
Sl. No. |
Type of soil |
Unit wt. of soil in KN/m3 |
Angle of repose |
|
1 |
Moist soil |
16.5-17.5 |
40°-45° |
|
2 |
Wet soil |
17-18 |
15°-20° |
|
3 |
Dry soil |
16-17 |
20°-30° |
|
4 |
Wet sand |
18-19 |
15° - 25° |
|
5 |
Dry sand |
15 -16 |
25° - 35° |
|
6 |
Gravel |
17-18 |
40°-45° |
|
7 |
Gravel sand |
18-19 |
25°-35° |
|
8 |
Wet clay |
18.5-19 |
15° |
|
9 |
Moist clay |
17.5-18.5 |
35°-40° |
|
10 |
Dry clay |
17-17.5 |
30°-35° |
|
11 |
Mud |
16-18.5 |
0° |
Note: The above-given data are the average values for the general considerations.
I recommend the soil test of the foundation, to get the accurate values of the angle of repose & unit wt. of that particular soil.
Usually, for residential buildings, the depth of the foundation falls between 1.2m. to 2.1m. ( i.e. 4ft. - 7ft.).
What are the factors that affect the depth of the foundation?
The factors that affect the foundation depth are,
1. Structural load:
If the foundation has to withstand more loads, the depth of the foundation increases. More the load the foundation has to carry, the more will be the foundation depth.
2. Density of soil :
The higher the density or unit wt. of soil, the less will be the foundation depth. In other words, foundation depth is inversely proportional to the density of soil for a given load.
3. Type of soil:
The angle of repose of different types of soil varies, which affects the depth of the foundation.
4. Water content of soil:
The water content in the soil decides the density & angle of repose of the soil. So the depth of the foundation changes depending on the water table of the foundation site & the amount of moisture content in the soil.
5. Refilled soil:
If the foundation base is prepared by refilling the soil over the existing ground surface, the foundation may take more depth, depending upon the degree of compaction & final density of the refilled material.
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