Bearing capacity is the ability of soil to safely carry the pressure from any engineered structure placed upon it without undergoing a shear failure and with the resulting settlements being tolerable for that structure.

• The foundation must be stable against shear failure of the supporting soil –Bearing capacity failure

• The foundation must not settle beyond a tolerable limit to avoid damage of the stucture

IntroductionBearing capacity and general requirements of foundation

Bearing capacity failures General shear failure

Bearing capacity failure is defined as a foundation failure that occurs when the shear stresses in the soil exceed the shear strength of the soil. Bearing capacity failures of foundations can be grouped into three categories, as follows:

1- General shear failureSeen in dense and stiff soilSudden or catastrophic failureThe failure surface in the soil will extend to the ground surfaceBulging on the ground surface adjacent to foundationThe length of disturbance beyond the edge of footing is large.

When the load is plotted versus settlement of the footing, there is a distinct load at which the foundation fails and this is designated Qu.

The value of Qu divided by the width B and length L of the footing is considered to be the ultimate bearing capacity (qu) of the footing.

The ultimate bearing capacity has been defined as the bearing stress that causes a sudden catastrophic failure of the foundation, it occurs when settlement of footing reaches 4 to 10% of B.

Q

Bearing capacity failures General shear failure

2- Local shear failureThis type of failure is seen in relatively loose and soft soil. A significant compression of soil below the footing and partial development of plastic equilibrium is observed.Failure is not sudden and there is no tilting of footing.Failure surface does not reach the ground surface and slight bulging of soil around the footing is observed.Failure surface is not well defined.Failure is characterized by considerable settlement.

Bearing capacity failures Local shear failure

3- Punching shear failure

This type of failure is seen in loose and soft soil and at deeper elevations.This type of failure occurs in a soil of very high compressibility.Failure pattern is not observed.Bulging of soil around the footing is absent.Failure is characterized by very large settlement.

Bearing capacity failures Punching shear failure

For nearly all practical shallow foundation design problems, it is only necessary to check the general shear case, and then conduct settlement analyses to verify that the foundation will not settle excessively. These settlement analyses implicitly protect against local and punching shear failures

Bearing capacity failures Summary

Bearing capacity formula General methodology – simple case

Let us assume a footing experiencing a bearing capacity failure, and that this failure occurs along a circular shear surface. The soil is an undrained clay with a shear strength Su. We will neglect the shear strength between the ground surface and a depth D.

Bearing capacity formula Terzaghi’s Bearing Capacity Equation – strip footing – general shear failures

With q=γ.Df

Strip footingValues of Nc, Nq, Nγ – Terzaghi’s bearing capacity factors

Square and circular footingsBasic bearing capacity equation - general shear failures

Square Foundations

Circular Foundations

Bearing capacity equation Purely cohesionless or purely cohesive Soils

Strip, square and circular footingsBasic bearing capacity equation – local shear failures

Square footing

Circular footing

Strip footing

Local shear failureValues of N’c, N’q, N’γ – Terzaghi’s modified bearing capacity factors

Shallow foundationAllowable bearing capacity

Allowable bearing capacity is lesser of:

Applied stress that will result in shear failure divided by FSUltimate limit criterion

OR

Applied stress that results in a specified amount of settlement of the structureServiceability criterion

Bearing capacity equation Factor of safety - gross allowable bearing capacity

The gross allowable bearing capacity can be calculated as:

qall is the allowable load per unit area to which the soil under the foundation should be subjected to avoid any chance of bearing capacity failure

A: area of the foundation.

Dead & live loadFoundation

Soil

Bearing capacity equation Factor of safety - net allowable bearing capacity

The net allowable bearing capacity can be calculated as:

qall(net) is the allowable load per unit area of the foundation in excess of the existing vertical effective stress at the level of the foundation

Square foundationExercise

Square foundationSolution

Square foundationSolution

B

Square foundationExercise

The total load of a footing near ground level is 4000 kN. The footing is to be located at a depth of 1.0 m below ground level. For a footing of size 3 x 3 m, determine the factor of safety by using Terzaghi’s bearing capacity factors and considering a general shear failure

Effect of Groundwater Table

In developing the bearing capacity equations given in the preceding section we assumed that the groundwater table is located at a depth much greater than the width, B of the footing. However, if the groundwater table is close to the footing, some changes in the bearing capacity equations are required.

Three different conditions:

Effect of Groundwater Table

Effect of Groundwater Table

Bearing capacity - Effect of Groundwater TableExample

Bearing capacity - Effect of Groundwater TableSolution

Bearing capacity - Effect of Groundwater TableSolution

If the soil fails by local shear failure, determine the net safe bearing pressure.

Bearing capacity - Effect of Groundwater TableSolution

If the soil fails by local shear failure, determine the net safe bearing pressure.

Bearing capacity - Effect of Groundwater TableExample

If the water table rises to the ground level, determine the net safe bearing pressure of the footing. All the other data remain the same. Assume the saturated unit weight of the soil γsat= 18.5 kN/m3. Use the general shear failure theory.

Bearing capacity - Effect of Groundwater TableSolution