Bearing capacity Notes | EduRev

: Bearing capacity Notes | EduRev

 Page 1


Module 4 : Design of Shallow Foundations
Lecture 17 : Bearing capacity [ Section17.1 : Introduction ]
   Objectives
   In this section you will learn the following
introduction
Basic definitions
Presumptive bearing capacity
 
 
Page 2


Module 4 : Design of Shallow Foundations
Lecture 17 : Bearing capacity [ Section17.1 : Introduction ]
   Objectives
   In this section you will learn the following
introduction
Basic definitions
Presumptive bearing capacity
 
 
Module 4 : Design of Shallow Foundations
Lecture 17 : Bearing capacity [ Section17.1 : Introduction ]
  
17. Bearing capacity : It is the load carrying capacity of the soil.
 Basic definitions
 
Ultimate bearing capacity or Gross bearing capacity ( ): It is the least gross pressure which will
cause shear failure of the supporting soil immediately below the footing.
 
Net ultimate bearing capacity ( ): It is the net pressure that can be applied to the footing by external
loads that will just initiate failure in the underlying soil. It is equal to ultimate bearing capacity minus the
stress due to the weight of the footing and any soil or surcharge directly above it. Assuming the density of
the footing (concrete) and soil ( ) are close enough to be considered equal, then 
where,
is the depth of the footing, Ref. fig. 4.7
Safe bearing capacity: It is the bearing capacity after applying the factor of safety (FS). These are of two
types,
Safe net bearing capacity ( ) : It is the net soil pressure which can be safety applied to the soil
considering only shear failure. It is given by,
 
Page 3


Module 4 : Design of Shallow Foundations
Lecture 17 : Bearing capacity [ Section17.1 : Introduction ]
   Objectives
   In this section you will learn the following
introduction
Basic definitions
Presumptive bearing capacity
 
 
Module 4 : Design of Shallow Foundations
Lecture 17 : Bearing capacity [ Section17.1 : Introduction ]
  
17. Bearing capacity : It is the load carrying capacity of the soil.
 Basic definitions
 
Ultimate bearing capacity or Gross bearing capacity ( ): It is the least gross pressure which will
cause shear failure of the supporting soil immediately below the footing.
 
Net ultimate bearing capacity ( ): It is the net pressure that can be applied to the footing by external
loads that will just initiate failure in the underlying soil. It is equal to ultimate bearing capacity minus the
stress due to the weight of the footing and any soil or surcharge directly above it. Assuming the density of
the footing (concrete) and soil ( ) are close enough to be considered equal, then 
where,
is the depth of the footing, Ref. fig. 4.7
Safe bearing capacity: It is the bearing capacity after applying the factor of safety (FS). These are of two
types,
Safe net bearing capacity ( ) : It is the net soil pressure which can be safety applied to the soil
considering only shear failure. It is given by,
 
Module 4 : Design of Shallow Foundations
Lecture 17 : Bearing capacity [ Section17.1 : Introduction ]
  
Safe gross bearing capacity ( ): It is the maximum gross pressure which the soil can carry safely without
shear failure. It is given by,
Allowable Bearing Pressure: It is the maximum soil pressure without any shear failure or settlement failure.
 
Fig. 4.7 Bearing capacity of footing
 
Page 4


Module 4 : Design of Shallow Foundations
Lecture 17 : Bearing capacity [ Section17.1 : Introduction ]
   Objectives
   In this section you will learn the following
introduction
Basic definitions
Presumptive bearing capacity
 
 
Module 4 : Design of Shallow Foundations
Lecture 17 : Bearing capacity [ Section17.1 : Introduction ]
  
17. Bearing capacity : It is the load carrying capacity of the soil.
 Basic definitions
 
Ultimate bearing capacity or Gross bearing capacity ( ): It is the least gross pressure which will
cause shear failure of the supporting soil immediately below the footing.
 
Net ultimate bearing capacity ( ): It is the net pressure that can be applied to the footing by external
loads that will just initiate failure in the underlying soil. It is equal to ultimate bearing capacity minus the
stress due to the weight of the footing and any soil or surcharge directly above it. Assuming the density of
the footing (concrete) and soil ( ) are close enough to be considered equal, then 
where,
is the depth of the footing, Ref. fig. 4.7
Safe bearing capacity: It is the bearing capacity after applying the factor of safety (FS). These are of two
types,
Safe net bearing capacity ( ) : It is the net soil pressure which can be safety applied to the soil
considering only shear failure. It is given by,
 
Module 4 : Design of Shallow Foundations
Lecture 17 : Bearing capacity [ Section17.1 : Introduction ]
  
Safe gross bearing capacity ( ): It is the maximum gross pressure which the soil can carry safely without
shear failure. It is given by,
Allowable Bearing Pressure: It is the maximum soil pressure without any shear failure or settlement failure.
 
Fig. 4.7 Bearing capacity of footing
 
Module 4 : Design of Shallow Foundations
Lecture 17 : Bearing capacity [ Section17.1 : Introduction ]
  
 
Presumptive bearing capacity : Building codes of various organizations in different countries gives the
allowable bearing capacity that can be used for proportioning footings. These are “Presumptive bearing
capacity values based on experience with other structures already built. As presumptive values are based only
on visual classification of surface soils, they are not reliable. These values don't consider important factors
affecting the bearing capacity such as the shape, width, depth of footing, location of water table, strength
and compressibility of the soil. Generally these values are conservative and can be used for preliminary
design or even for final design of small unimportant structure. IS1904-1978 recommends that the safe
bearing capacity should be calculated on the basis of the soil test data. But, in absence of such data, the
values of safe bearing capacity can be taken equal to the presumptive bearing capacity values given in table
4.1, for different types of soils and rocks. It is further recommended that for non-cohesive soils, the values
should be reduced by 50% if the water table is above or near base of footing.
Table 4.1 Presumptive bearing capacity values as per IS1904-1978.
 
Type of soil/rock Safe/allowable bearing
capacity (KN/ m
2
)
Rock 3240
Soft rock 440
Coarse sand 440
Medium sand 245
Fine sand 440
Soft shell / stiff clay 100
Soft clay 100
Very soft caly 50
 
Page 5


Module 4 : Design of Shallow Foundations
Lecture 17 : Bearing capacity [ Section17.1 : Introduction ]
   Objectives
   In this section you will learn the following
introduction
Basic definitions
Presumptive bearing capacity
 
 
Module 4 : Design of Shallow Foundations
Lecture 17 : Bearing capacity [ Section17.1 : Introduction ]
  
17. Bearing capacity : It is the load carrying capacity of the soil.
 Basic definitions
 
Ultimate bearing capacity or Gross bearing capacity ( ): It is the least gross pressure which will
cause shear failure of the supporting soil immediately below the footing.
 
Net ultimate bearing capacity ( ): It is the net pressure that can be applied to the footing by external
loads that will just initiate failure in the underlying soil. It is equal to ultimate bearing capacity minus the
stress due to the weight of the footing and any soil or surcharge directly above it. Assuming the density of
the footing (concrete) and soil ( ) are close enough to be considered equal, then 
where,
is the depth of the footing, Ref. fig. 4.7
Safe bearing capacity: It is the bearing capacity after applying the factor of safety (FS). These are of two
types,
Safe net bearing capacity ( ) : It is the net soil pressure which can be safety applied to the soil
considering only shear failure. It is given by,
 
Module 4 : Design of Shallow Foundations
Lecture 17 : Bearing capacity [ Section17.1 : Introduction ]
  
Safe gross bearing capacity ( ): It is the maximum gross pressure which the soil can carry safely without
shear failure. It is given by,
Allowable Bearing Pressure: It is the maximum soil pressure without any shear failure or settlement failure.
 
Fig. 4.7 Bearing capacity of footing
 
Module 4 : Design of Shallow Foundations
Lecture 17 : Bearing capacity [ Section17.1 : Introduction ]
  
 
Presumptive bearing capacity : Building codes of various organizations in different countries gives the
allowable bearing capacity that can be used for proportioning footings. These are “Presumptive bearing
capacity values based on experience with other structures already built. As presumptive values are based only
on visual classification of surface soils, they are not reliable. These values don't consider important factors
affecting the bearing capacity such as the shape, width, depth of footing, location of water table, strength
and compressibility of the soil. Generally these values are conservative and can be used for preliminary
design or even for final design of small unimportant structure. IS1904-1978 recommends that the safe
bearing capacity should be calculated on the basis of the soil test data. But, in absence of such data, the
values of safe bearing capacity can be taken equal to the presumptive bearing capacity values given in table
4.1, for different types of soils and rocks. It is further recommended that for non-cohesive soils, the values
should be reduced by 50% if the water table is above or near base of footing.
Table 4.1 Presumptive bearing capacity values as per IS1904-1978.
 
Type of soil/rock Safe/allowable bearing
capacity (KN/ m
2
)
Rock 3240
Soft rock 440
Coarse sand 440
Medium sand 245
Fine sand 440
Soft shell / stiff clay 100
Soft clay 100
Very soft caly 50
 
Module 4 : Design of Shallow Foundations
Lecture 17 : Bearing capacity [ Section17.1 : Introduction ]
   Recap
   In this section you have learnt the following
Introduction
Basic definitions
Presumptive bearing capacity
 
 
 
 
 
 
 
 
Read More
Offer running on EduRev: Apply code STAYHOME200 to get INR 200 off on our premium plan EduRev Infinity!