Types of Bearing Capacity of Soil- Design of Foundations

The different bearing pressures encountered while foundation design is discussed in this article, by considering the example of s shallow foundation that is subjected to various soil conditions. The total and effective overburden pressures that are possible to come over the foundation are studied to determine the bearing capacities of the soil.

Total and Effective Overburden Pressures on Foundation

As shown in fig.1, consider a shallow foundation, whose depth below ground is Df and depth B. The depths and unit weight at various levels of the soil are explained in the figure below.

1. Total Overburden Pressure (qo)

The total overburden pressure on the base level of the foundation is due to the weight of the soil above and the weight of the water.

2. Effective Overburden Pressure (q'o)

This is the effective overburden pressure at the base level of the foundation. Here we take the submerged soil unit weight.

Types of Bearing Capacity of Soil

Once we understand the total and effective overburden pressure, we will find the different bearing capacities of soil.

1. Ultimate Bearing Capacity of Soil (qu)

Bearing capacity of the soil at which the soil fails at shear. 

Read More on:  Ultimate Bearing Capacity

2. Net Ultimate Bearing Capacity, (qnu)

Bearing capacity of soil in excess of the effective overburden pressure (q'o), which is given by:

qnu = qu - q'o

3. Allowable or Gross Bearing Capacity of Soil (qa)

This is the maximum pressure that can be sustained by the soil from the foundation such that, it brings acceptable safety factors against shear failure and acceptable total and differential settlement. This is also called the Gross Allowable Bearing Pressure qa.

qa = qu/FOS = Ultimate bearing capacity / factor od saftey

where, FOS  is the factor of safety, qu = ultimate bearing capacity;

4. Net Allowable Bearing pressure, (qna)

qna = qnu / FOS;

qna = net ultimate bearing capacity / factor of safety

5.  Safety Bearing Pressure, qs

Net safety bearing pressure produces a settlement within the permissible limit. 

In the design of the foundation, between the net allowable bearing pressure and the safe bearing pressure, the least value of bearing pressure is taken into consideration.

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