Bearing Capacity of Soil
Density of Soil
Water Table
Self weight of Structure
Live Load of Structure
Wind Load & Seismic Load (Earthquake)
Rankine's Formula
Df=Minimam Depth of Foundation
p=Bearing Capacity of soil
w=density of soil
θ=Angle of proposed Soil
Complex
Given data:
Load on the foundation ( p ) = 370KN/m2
Unit weight of soil (w) = 17KN/m3
The angle of repose of soil (θ )= 35°
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]
= 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 foundation falls between 1.2m. to 2.1m. ( i.e. 4ft. - 7ft.).
What are the factors that affect the depth of 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 vary, 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 upon 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.
Here is a list of various materials and their angle of repose.[3] All measurements are approximated.
