Depth Of Foundation

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.

Material (condition)	Angle of Repose (degrees)
Sand (water filled)	15–30°
Clay (wet excavated)	15°
Gravel (natural w/ sand)	25–30°
Clay (dry lump)	25–40°
Urea (Granular)	27° [5]
Wheat	27°
Clover seed	28°
Flour (corn)	30–40°
Asphalt (crushed)	30–45°
Bran	30–45°
Earth	30–45°
Malt	30–45°
Sand (dry)	34°
Granite	35–40°
Coffee bean (fresh)	35–45°
Snow	38°[4]
Ashes	40°
Bark (wood refuse)	45°
Chalk	45°
Coconut (shredded)	45°
Flour (wheat)	45°
Gravel (crushed stone)	45°
Sand (wet)	45°			https://civilplanets.com/types-of-loads/#Wind_Load