Bearing Capacity of Different type of soil

2019-04-28 civilsitevisit

The bearing capacity of soil is defined as the capacity of the soil to bear the loads coming from the foundation. The pressure which the soil can easily withstand against load is called allowable bearing pressure.

Bearing failure occurs when the load on the footing causes large movement of the soil on a shear failure surface which extends away from the footing and up to the soil surface. Calculation of the capacity of the footing in general bearing is based on the size of the footing and the soil properties.

Types of Bearing Capacity of Soil

Gross safe bearing capacity
Net ultimate bearing capacity
Ultimate bearing capacity
Net allowable bearing pressure

Types of Soil	Bearing Capacity
Soft rocks	450 ( kN/m ²)
Black cotton soil	150 ( kN/m ²)
Fine, loose and dry sand	100 ( kN/m ²)
Soft, wet clay or muddy clay	50 ( kN/m ²)
Loose gravel	250 ( kN/m ²)
Compact sand	450 ( kN/m ²)
Compact gravel	450 ( kN/m ²)
Hard rocks such as granite,	3300 ( kN/m ²)
Moist clay and sand clay Mixture	150 ( kN/m ²)

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:

Dividing the ultimate soil bearing capacity by a safety factor we get the maximum safe bearing capacity of soil for design of foundations. Value varies from 1.2 to 3

Terzaghi Bearing Capacity

The basic method was developed by Terzaghi, with modifications and additional factors by Meyerhof and Vesić. The general shear failure case is the one normally analyzed. Prevention against other failure modes is accounted for implicitly in settlement calculations.

He 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.^[

For square foundations:

q_{ult} = 1.3 c' N_c + \sigma '_{zD} N_q + 0.4 \gamma ' B N_\gamma \

For continuous foundations:

q_{ult} = c' N_c + \sigma '_{zD} N_q + 0.5 \gamma ' B N_\gamma \

For circular foundations:

q_{ult} = 1.3 c' N_c + \sigma '_{zD} N_q + 0.3 \gamma ' B N_\gamma \

then,

N_q = \frac{ e ^{ 2 \pi \left( 0.75 - \phi '/360 \right) \tan \phi ' } }{2 \cos ^2 \left( 45 + \phi '/2 \right) }

for φ’ = 0

for φ’ > 0

N_\gamma = \frac{ \tan \phi ' }{2} \left( \frac{ K_{p \gamma} }{ \cos ^2 \phi ' } - 1 \right)

c′ is the effective cohesion.

σ_zD′ is the vertical effective stress 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

.K_pγ is obtained graphically. Simplifications have been made to eliminate the need for K_pγ. One such was done by Coduto, given below, and it is accurate to within 10

N_\gamma = \frac{ 2 \left( N_q + 1 \right) \tan \phi ' }{1 + 0.4 \sin 4 \phi ' }

Local shear failure

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: