HOW TO CALCULATE DEPTH OF FOUNDATION
Deciding the right depth of foundation for
a building structure is an important step in the process of building design.
Information given in this post will help you decide the proper depth of
foundation for a building.
After reading this post you will be able to
answer the following questions.
§ What is
a foundation?
§ What
are the factors affecting depth of foundation?
§ How to
calculate depth of foundation?
WHAT
IS FOUNDATION?
Foundation is that part of the structure
which receives load of the superstructure and then transmits that load to soil
underneath in such a manner so that the soil never fails in shear or never goes
through excessive settlement of differential settlement.
FACTORS
AFFECTING DEPTH OF FOUNDATION
Before calculating depth of shallow
foundation, the following factors have to be considered well in advance.
1.
Foundation should be placed at such a depth so that it is safe
against damages due to swelling, shrinkage or freezing of sub soil.
2.
Bearing capacity of soil beneath the foundation must be adequate
to support the load coming from foundation.
3.
If foundation has to be placed on cohesive soil then the
settlement due to consolidation should not be excessive.
4.
Never place foundation on loose or disturbed soils which have a
tendency to erode by wind or flood.
5.
If possible then foundation should be placed above ground water
table as this can avoid cost of pumping, and can prevent instability of soil
due to seepage of water into the bottom of an excavation.
6.
Make an investigation on foundation soil to know its physical
and chemical properties, because presence of sulphate can damage foundation.
CALCULATION
OF DEPTH OF FOUNDATION
The minimum depth of shallow foundation for
a soil can be calculated using the following formula as suggested by Rankine.
This is called Rankine’s
Formula.
Dmin =
(q/g) * [(1 – sinØ) / (1 + sinØ)]2
Where,
Dmin =
Minimum depth of foundation in m
g =
Density of unit weight of soil in kN/m3
Ø =
Angle of repose in Degrees
q =
Intensity of load or Safe bearing capacity of soil in kN/m2
EXAMPLE CALCULATION
Calculate the minimum depth required for a
foundation to transmit a pressure 55 kN/m2 in
a cohesionless soil having density 16 kN/m3 and
angle of repose 200?
Given Data
Intensity of pressure (q) = 55 kN/m2
Density of soil (g) = 16 kN/m3
Angle of repose (Ø) = 200
Calculation
Minimum depth of foundation, according to
Rankine,
Dmin =
(q/g) * [(1 – sinØ) / (1 + sinØ)]2
Dmin =
(55/16) * [(1 – sin200) / (1 + sin200)]2
Dmin =
0.82 m
HELPFUL
POINTS
For preliminary
calculation of depth of foundation, the values of density and angle of repose,
as given in the following table can be used.
|
Soil Type
|
Angle of Repose (in
Degree)
|
Unit weight (in kN/m3)
|
|
Dry sand
|
25 – 35
|
16.0
|
|
Moist sand
|
30 – 35
|
18.4
|
|
Wet sand
|
15 – 25
|
19.2
|
|
Dry & compacted sand
|
35
|
19.2
|
|
Clean gravel
|
30 – 40
|
17.9
|
|
Mixture of gravel & sand
|
25 – 40
|
19.2
|
|
Rubble stone
|
45
|
19.2
|
|
Dry clay
|
30
|
17.6
|
|
Wet clay
|
15
|
19.2
|
|
Ash
|
40
|
6.4
|
NOTE:
§ The values given the table above are
approximate value.
§ To know the density of soil on site you have
to test it onsite. There are two common methods which are widely used for
determination of density of soil on site. Click the following two links to read
the test procedure
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