Types of Bearing Capacities and Bearing Pressure in Foundation Design

Bearing capacity of a foundation is the maximum load the soil below the foundation can support without failure. In foundation engineering, we design a foundation based on two main criteria:

1. Shear failure criteria
2. Settlement failure criteria

During the design of foundation, we individually study the load or pressure values for both cases i.e. shear failure as well as the settlement failure. Out of the two values we take the least value and design the foundation based on it. 

When we study shear failure of foundation, we look into the Bearing Capacity and for settlement failure study, we study the Bearing Pressure. 

Let's learn in detail various bearing pressure and capacity studied in foundation engineering.

1. Based on Shear Failure Criteria of Foundation

Shear failure is a strength issue associated with the soil. This criterion checks whether the soil fail suddenly by shear when the load from the foundation spreads through the soil. 

Here we use Bearing capacity which is the maximum load per unit area the soil can take before the shear failure occurs. If the applied load exceed this value, the shear strength of the soil is not sufficient resulting in foundation rotation or punching. 

Read More On: Shear Failure in Soils

Under this criterion, we study several bearing capacities because soil can resist load in two ways:

• Natural soil pressure already acting due to its own weight
• Additional pressure applied from the structure

So the bearing capacity types help us separate:

• The total resistance of soil
• The extra resistance available after subtracting soil’s self-weight
• The maximum limit at failure
• The safe limit after applying factor of safety

Let's learn each type in detail:

1.1. Gross Bearing Capacity (qg)

Gross bearing capacity is the gross pressure that is generated under the footing due to  self weight of the footing, overburden pressure, and the load from the column (external load). 

This total load divided by the area of the footing gives the gross bearing capacity. 

1.2. Net Bearing Capacity (q_n)

Net Bearing Capacity = Gross Pressure (qg) - Overburden Pressure 

Eq.1 - Net Bearing Capacity (qn) 

γ = Unit Weight of the soil

γ (sub) = Fully Submerged Condition

So the net bearing capacity is the net safe load the soil can take after subtracting soil's existing pressure. This helps to study how much extra load the foundation can apply beyond the natural stress. 

It is the working allowable additional pressure, not the failure limit.

It tells how much load can I apply during the construction life or service life of the footing. 

1.3. Ultimate Bearing Capacity (qu)

Ultimate bearing capacity is the maximum pressure the soil can take at shear failure. It can be the raw capacity that is obtained from Terzaghi's equation before applying safety factors. It is the maximum value of gross bearing capacity (qg).

When a foundation is loaded, the gross bearing pressure (qg) at its base does not appear as a fixed number. It keeps increasing gradually as more load from the structure is applied. At first, the soil comfortably resists this pressure using its available shear strength. 

However, there comes a critical stage where the applied qg becomes so high that the soil can no longer mobilize additional shear resistance. At this exact point, the soil reaches shear failure. The particular value of qg at which this failure occurs, which is also the maximum value qg can ever attain, is called the ultimate bearing capacity (qᵤ). 

This value is obtained theoretically from equations such as Terzaghi’s bearing capacity equation, before applying any factor of safety, and it represents the raw failure pressure that includes the structural load, footing weight, and overburden pressure.

qu = maximum of (qg)

1.4. Net Ultimate Bearing Capacity (qnu)

Net ultimate bearing capacity (qnu) is the maximum additional pressure the soil can take at the point of shear failure, after deducting the natural soil pressure. 

The net ultimate bearing capacity (qₙᵤ) represents the maximum additional pressure that can be applied over and above this natural soil pressure before the soil reaches shear failure. In other words, it excludes the pressure that was already acting on the soil and considers only the extra load caused by the foundation and structure.

At the value of qₙᵤ, the soil has fully mobilized its shear strength, and any further increase in load will cause shear failure. Hence, qₙᵤ corresponds to the ultimate or failure condition and tells us how much additional load can be safely applied before shear failure occurs.

Equation 2

Eq.2 - Net Ultimate Bearing Capacity (qnu)

This is the ultimate condition or the failure condition. 

It tells us how much load can I apply before shear failure happens.

1.5. Net Safe Bearing Capacity (qns)

After determining the net ultimate bearing capacity (qₙᵤ), which represents the failure condition, the foundation cannot be designed using this value directly. Designing at failure is unsafe. Therefore, a factor of safety (FOS) is introduced to obtain a safe working value.

The net safe bearing capacity (qₙₛ) is obtained by dividing the net ultimate bearing capacity by the factor of safety:

When it is net safe pressure, we must use factor of safety (FOS). 

Eq.3. Net Safe Bearing Capacity (qns)

From equation 2, we get the value of qnu.

Eq.4. Net Safe Bearing Capacity (qns)

Here, the factor of safety is applied only to the additional pressure caused by the foundation and structural load. The natural soil pressure (overburden pressure) is not reduced using FOS, because it already exists in the ground and is not responsible for inducing shear failure due to construction.

The value of qₙₛ therefore indicates the maximum additional pressure that can be safely applied to the soil without causing shear failure.

1.6. Safe Bearing Capacity (qs) or Gross Safe Bearing Capacity

After obtaining the net safe bearing capacity (qₙₛ), the next step is to determine the safe bearing capacity (qₛ), also called the gross safe bearing capacity.

The safe bearing capacity is defined as the total safe pressure that can be applied at the base of the foundation, including both the additional load from the structure and the existing overburden pressure. 

Safe bearing capacity is given by the sum of net safe bearing capacity and the overburden pressure. 

Eq.5. Safe Bearing Capacity (qns)

In bearing capacity calculations, the factor of safety is applied only to the external load contribution, which is represented by the net ultimate bearing capacity. The overburden pressure is not reduced using FOS because it is an already existing stress in the soil and does not arise from the foundation load.

2. Based on Settlement Criteria 

Under settlement criterion of foundation, we take into consideration the net safe settlement pressure (qnp). 

2.1. Net Safe Bearing Pressure or Net Safe Settlement Pressure (q_np)

It is the maximum net intensity of loading that you keep on applying such that the settlement occurred will be not greater than the permissible settlement. Here we do not consider any overburden pressure.  

Allowable Bearing Pressure or Net Allowable Bearing Pressure or Allowable Bearing Capacity (qna)

Out of all these bearing capacities and bearing pressure studied, we need to determine a single value based on which we can design a foundation. So, the net bearing pressure that is used in the design of foundations are called as Net Allowable Bearing Pressure or simple allowable bearing pressure. 

Now what is the value of allowable bearing pressure (q_na)?

As the design requirement is to design a foundation that would not undergo any settlement or shear failure, or they must be within the permissible limits,

Allowable bearing pressure is the smaller of the net bearing capacity (q_ns) and the net safe settlement pressure (q_np).

Allowable Bearing Capacity = Minimum of { Net Safe Bearing Capacity and Net Safe Bearing Pressure (Settlement)}

To find the Net Safe Load on a Foundation:

Net Safe Load = Allowable Bearing Pressure x Area of Footing

Key Take Aways

Bearing capacity and bearing pressure may sound similar, but they are not the same.

Bearing capacity refers to the maximum pressure the soil can resist before it fails in shear. It is a property of the soil, representing its strength or resistance. 

On the other hand, bearing pressure is the actual pressure applied on the soil by the building or structure. It comes from the load of the foundation, not from the soil itself. 

In design, we compare the applied bearing pressure with the safe bearing capacity to ensure the soil does not undergo shear failure. Even if the bearing pressure is lower than the bearing capacity, the soil may still compress too much, which is why we check settlement using allowable bearing pressure.

 In simple words, bearing pressure is the load we apply, and bearing capacity is the strength of the soil to resist that load. The structure is safe only when the applied pressure is less than both the soil’s shear strength and the pressure that will not cause excessive settlement.

 

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