What is Bearing Capacity of Soil and Load Settlement Curve ?

Whenever any structure is constructed above the ground like building foundations, a crane, etc., the ground must have the capacity to support the structure without undergoing excessive settlement or failure. This capacity of soil to sustain loads coming over it is measured by the parameter named "bearing capacity".


What is Bearing Capacity of Soil and  Load Settlement Curve ?
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The bearing capacity of the soil is the capacity of the soil to support the loads that are applied to the ground. The bearing capacity of a particular soil is dependent on the:

  • Type of soil
  • The shear strength of the soil
  • The density of the soil


Features of Bearing Capacity of Soil

Any foundation for structure is constructed by identifying the type of soil and the bearing capacity of the soil, above which the foundation is constructed. 
When we determine that the soil owns poor bearing capacity, the ground is improved to increase the bearing capacity of the soil, or other alternative methods are employed to distribute the load fully to the soil.
The bearing capacity of soil is calculated from the maximum average contact pressure between the foundation and soil that would not create any type of shear failure in the soil. 
There are three modes of shear failure by which soil can fail under applied loads. They are:
  1. General failure
  2. Local failure
  3. Punching shear failure


Types of Bearing Capacity of Soil

The two main classifications of the bearing capacity of the soil are:

  1. Ultimate Bearing Capacity
  2. Allowable Bearing Capacity


1. Ultimate Bearing Capacity of the Soil (qu)

The ultimate bearing capacity of the soil is the minimum pressure that would cause the shear failure of the supporting soil immediately below and adjacent to the foundation. This is pressure corresponding to the point at which the shear failure mechanism develops in the supporting soil. 
As shown in the figure below, the depth of the foundation below the ground level (G.L) be Df, and the breadth is B. The vertical load acting on the footing is Q. With the increase in the load, the soil settlement, and the settlement of the foundation are measured as S. Based on the load and corresponding settlement, a load-settlement graph is obtained. 
Shallow Foundation with Breadth B subjected to a load Q
Shallow Foundation with Breadth B subjected to a load Q

The load-settlement graph gives an indication of the ultimate load that can be taken by the soil, from which the bearing capacity of the soil is determined.

The typical behavior of the load-settlement curve for dense and loose clay is explained below, along with the determination of the ultimate load (Qu).


Load-Settlement Curve for Different Soil

Load-Settlement Curve for Dense sand (C1) and loose soil (C2)
Fig.2. Load-Settlement Curve for Dense sand (C1) and loose soil (C2)


1. Dense Sand/Very Stiff Clay

The load-settlement curve for dense or very stiff clay sand is shown in fig.2 below as curve C1. For dense sand or very stiff clay, the curve C1 abruptly passes to a peak value and then drops down as shown. This peak point is considered as the peak load, i.e., Qu.


2. Loose/Soft Clay

For loose/soft clay, curve C2 continues to descend on a slope, which implies, the compression of soil is continuously taking place without giving a definite value for Qu. Here, Qu may be taken at a point beyond which there is a constant rate of penetration.

Once the ultimate load is determined, the ultimate bearing pressure or bearing capacity (qu) is calculated.


2. Allowable  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.

Equation for Bearing Capacity of Soil

The bearing capacity of the soil is given by the equation: 
qa = qu/FS

Where,
qa = Allowable Bearing Capacity ( in kN/m2 or lb/ft2)
qu = Ultimate bearing capacity in kN/m2 or lb/ft2
FS = Safety factor

The allowable bearing capacity of soil is considered for the design of the foundation. This value is calculated by determining either by practical test or empirical formula.


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