Bearing Capacity of Shallow Foundations - Part 01

The foundation of a structure is defined as, that part of the structure in direct contact with the ground and which transmits the superstructure load to the ground.

Basically there are two types of foundations we can identify,

1). Shallow foundations
2). Deep foundations

Note : Generally depth the breath ration of shallow foundation is less than 2.5

 

Shallow foundation

Deep foundation

In this forum we will talk about shallow foundations only (deep later)

There are different types of shallow foundations in construction industry. such as,

Designing of shallow foundations 

Designing of shallow foundations are based on two factors,

1) Bearing capacity of the soil

2) Settlement of the structure

Only the bearing capacity will be discussed in this module.

Bearing capacity : 

 Bearing capacity is the maximum pressure that can be applied causing shear failure of the underlying soil.  

Bearing pressure : 

Bearing pressure is the force per unit area applied on the soil under the footing.

Ultimate bearing capacity : 

Ultimate pressure at which the soil beneath the foundation fails in shear. 

Load acting on a footing

where, 

Q col = load from super structure

W footing = weight of the footing

W soil = weight of the soil

Df = depth of the foundation

Note : B is the least dimension of the footing, if footing size is 2x3 B is 2, 6x8 B is 6, 2x2 B is 2.

Total load = Q col + W soil + Q footing

In our designs we need to design footing to bear this total load. 

There are many equations to calculate bearing capacity of a shallow foundation such as,

1) Terzaghi bearing capacity equation

2) Meyerhof's bearing capacity equation

3) Hansen's bearing capacity equation

4) Vesic's bearing capacity equation    

Terzaghi bearing capacity equation

By this equation we can only find the bearing capacity of direct loaded simple footings.

 

 

where, 

C = soil cohesion

gamma b = unit weight of the soil below the foundation

B = footing width (least dimension)

q = overburden vertical stress (q=Df*gamma soil)

Nc, Nq, N gamma = bearing capacity factors (depends on phi)

Sc, S gamma = shape factors 

Factor of safety 

Factor of safety = q ultimate / q applied

Meyerhof's bearing capacity equation

where,

C = soil cohesion

gamma b = unit weight of the soil below the foundation

B = footing width (least dimension)

q = overburden vertical stress (q=Df*gamma soil)

Nc, Nq, N gamma = bearing capacity factors (depends on phi)

Sc, S gamma = shape factors 

dc, dq, d gamma = depth factors

ic, iq, i gamma = load inclination factors

 If alpha (angle) is zero i factors will be equal to 1

Factor of safety 

Factor of safety = q ultimate / P

Factor of safety against sliding 

F.S = Resistance / T     where, T = Q sin alpha