Bearing Capacity of the Soil

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What is Bearing Capacity of the soil?

To explain the terms bearing capacity of the soil, first I will give a general example and then I will co-relate the example with actual terms.

Scenario 1

Consider a worker who lifts 100 kg weight on regular basis.
At some random day, he has to carry an extra weight of 25 kg.
The maximum weight that person can carry be 200 kg.

So weight carrying capacity of that person is 100 kg.
His gross carrying capacity is 100 + 25 = 125 kg.
His ultimate carrying capacity is 200 kg.
That persons net ultimate carrying capacity is 200 – 25 = 175 kg.

For his safety, he is asked to carry only 1/5th of his net ultimate carrying capacity
(here the factor of safety applied is 5).
Therefore his net safe carrying capacity is 35 kg of weight.

For gross safe carrying capacity, let him carry the extra weight of 25 kg.
Therefore his gross safe carrying capacity is 35 + 25 = 60 kg.

Scenario 2

Now, let us repeat the same example with terms related to Foundation soil and bearing capacity.

Consider a foundation soil which can bear a pressure of 100 kN/m2 on regular basis.
Sometimes, surrounding soil exerts a surcharge pressure of 25 kN/m2.
The maximum pressure that soil can carry before failing is 200 kN/m2.

So Bearing capacity of that soil is 100 kN/m2.
The gross carrying capacity is 100 + 25 = 125 kN/m2.
The ultimate carrying capacity is 200 kN/m2.
The net ultimate carrying capacity is 200 – 25 = 175 kN/m2.

For safety, apply only 1/5th of the net ultimate carrying capacity
(here the factor of safety applied is 5).
Therefore net safe carrying capacity is 35 kN/m2 of pressure.

For gross safe carrying capacity, let the soil carry the extra surcharge pressure of 25 kN/m2.
Therefore the gross safe bearing capacity is 35 + 25 = 60 kN/m2.

Terminologies:

Now as we understood the analogy between the two above mentioned examples, let us define the terms.

1. Bearing Capacity:- 

The load carrying capacity of foundation soil or rock which enables it to bear and transmit the load from the superstructure.

2. Gross Bearing Capacity:- 

It is the bearing capacity inclusive of the pressure exerted by the weight of soil standing on the foundation (surcharge pressure).

3. Ultimate Bearing Capacity (qu):- 

It is the gross pressure at the base of foundation at which soil fails in shear.

4. Net ultimate Bearing Capacity (qnu):-

Ultimate  Bearing Capacity – surcharge pressure at the base of footing.
It is the net increase in pressure at the base of footing that causes shear failure of soil.

qnu = qu - γ(Df)

5. Net safe Bearing Capacity (qns):- 

It is net soil pressure which can be safely applied to the soil considering only shear failure.

qns = qnu / F

where F is the factor of safety, which varies from 2.5 to 5 depending on the type of soil and structure.

6. Gross safe Bearing Capacity (qs):-

It is the maximum gross pressure which the soil can carry safely without shear failure.
The sum of net safe bearing capacity and surcharge pressure gives Gross Safe Bearing capacity. (also known as overburden pressure).

qs = qns + γ(Df)

7. Net safe settlement pressure (qnp):- 

It is the net pressure which the soil can carry without exceeding the allowable settlements.
Net safe settlement pressure is also known as unit soil pressure or safe bearing pressure.
Maximum allowable settlement varies between 25 mm to 40 mm for individual footing.

8. Net allowable settlement pressure (qna):- 

It is maximum allowable net loading intensity on the soil at which the soil neither fails in shear nor undergoes excessive settlement.
This is used for the design of foundation.

qna = smaller of qns or qnp

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