Bearing capacity
Encyclopedia
In geotechnical engineering
, bearing capacity is the capacity of soil
to support the loads applied to the ground. The bearing capacity of soil is the maximum average contact pressure
between the foundation
and the soil which should not produce shear
failure in the soil. Ultimate bearing capacity is the theoretical maximum pressure which can be supported without failure; allowable bearing capacity is the ultimate bearing capacity divided by a factor of safety. Sometimes, on soft soil sites, large settlements may occur under loaded foundations without actual shear failure occurring; in such cases, the allowable bearing capacity is based on the maximum allowable settlement.
There are three modes of failure that limit bearing capacity: general shear failure, local shear failure, and punching shear failure.
was the first to present a comprehensive theory for the evaluation of the ultimate bearing capacity of rough shallow foundations. This theory states that a foundation is shallow if its depth is less than or equal to its width. Later investigations, however, have suggested that foundations with a depth, measured from the ground surface, equal to 3 to 4 times their width may be defined as shallow foundations(Das, 2007).
Terzaghi developed a method for determining bearing capacity for the general shear failure case in 1943. The equations are given below.
For square foundations:
For continuous foundations:
For circular foundations:
where
for φ' = 0
for φ' > 0
For foundations that exhibit the local shear failure mode in soils, Terzaghi suggested the following modifications to the previous equations. The equations are given below.
For square foundations:
For continuous foundations:
For circular foundations:
, the modified bearing capacity factors, can be calculated by using the bearing capacity factors equations(for 
, respectively) by replacing the effective internal angle of friction
by a value equal to 
Where:
= bearing capacity factors,
B = width of the foundation
Geotechnical engineering
Geotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials. Geotechnical engineering is important in civil engineering, but is also used by military, mining, petroleum, or any other engineering concerned with construction on or in the ground...
, bearing capacity is the capacity of soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
to support the loads applied to the ground. The bearing capacity of soil is the maximum average contact pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
between the foundation
Foundation (architecture)
A foundation is the lowest and supporting layer of a structure. Foundations are generally divided into two categories: shallow foundations and deep foundations.-Shallow foundations:...
and the soil which should not produce shear
Shear
Shear as a noun may refer to:*Bias , in clothing design, fabric may be cut on the shear*Cosmic shear, an effect of distortion of image of distant galaxies due to deflection of light by matter, as predicted by general relativity *Shear , a form of fault in rocks*Simple shear, a special case of...
failure in the soil. Ultimate bearing capacity is the theoretical maximum pressure which can be supported without failure; allowable bearing capacity is the ultimate bearing capacity divided by a factor of safety. Sometimes, on soft soil sites, large settlements may occur under loaded foundations without actual shear failure occurring; in such cases, the allowable bearing capacity is based on the maximum allowable settlement.
There are three modes of failure that limit bearing capacity: general shear failure, local shear failure, and punching shear failure.
Introduction
Spread footings and mat foundations are generally classified as shallow foundations. These foundations distribute the loads from the superstructures to the soil on which they are resting. Failure of a shallow foundation may occur in two ways: (a) by shear failure of the soil supporting the foundation, and (b) by excessive settlement of the soil supporting the foundation. The first type of failure is generally called bearing capacity failure.General shear failure
The general shear failure case is the one normally analyzed. Prevention against other failure modes is accounted for implicitly in settlement calculations. There are many different methods for computing when this failure will occur.Terzaghi's Bearing Capacity Theory
Karl von TerzaghiKarl von Terzaghi
Karl von Terzaghi was an Austrian civil engineer and geologist, called the father of soil mechanics.-Early life:...
was the first to present a comprehensive theory for the evaluation of the ultimate bearing capacity of rough shallow foundations. This theory states that a foundation is shallow if its depth is less than or equal to its width. Later investigations, however, have suggested that foundations with a depth, measured from the ground surface, equal to 3 to 4 times their width may be defined as shallow foundations(Das, 2007).
Terzaghi developed a method for determining bearing capacity for the general shear failure case in 1943. The equations are given below.
For square foundations:
For continuous foundations:
For circular foundations:
where
for φ' = 0 for φ' > 0
- c′ is the effective cohesion.
- σzD′ is the vertical effective stressEffective stressKarl von Terzaghi first proposed the relationship for effective stress in 1936. For him, the term ‘effective’ meant the calculated stress that was effective in moving soil, or causing displacements...
at the depth the foundation is laid. - γ′ is the effective unit weight when saturated or the total unit weight when not fully saturated.
- B is the width or the diameter of the foundation.
- φ′ is the effective internal angle of friction.
- Kpγ is obtained graphically. Simplifications have been made to eliminate the need for Kpγ. One such was done by Coduto, given below, and it is accurate to within 10%.
For foundations that exhibit the local shear failure mode in soils, Terzaghi suggested the following modifications to the previous equations. The equations are given below.
For square foundations:
For continuous foundations:
For circular foundations:
, the modified bearing capacity factors, can be calculated by using the bearing capacity factors equations(for , respectively) by replacing the effective internal angle of friction by a value equal to Factor of Safety
Calculating the gross allowable-load bearing capacity of shallow foundations requires the application of a factor of safety(FS) to the gross ultimate bearing capacity, or: Meyerhofs's Bearing Capacity theory
In 1951, Meyerhof published a bearing capacity theory which could be applied to rough shallow and deep foundations. The equation is given below:Where:
= bearing capacity factors,B = width of the foundation