Bearing Capacity of Shallow Foundation

The foundation should be designed such that:

a. The soil below does not fail in shear

b. Settlement is within the safe limits

It is the gross pressure at the base of the foundation at which soil fails in shear.

Ultimate Bearing Capacity (q_u)

It is the net increase in pressure at the base of foundation that cause shear failure of the soil.

Net ultimate Bearing Capacity (q_nu)

It is the net soil pressure which can be safely applied to the soil considering only shear failure.

Net Safe Bearing Capacity (q_net)

It is the maximum pressure which the soil can carry safely without shear failure.

Gross Safe Bearing Capacity (q_s)

It is the net pressure which the soil can carry without exceeding allowable settlement

Net Safe Settlement Pressure (q_np)

It is the net bearing pressure which can be used for design of foundation.

Net Allowable Bearing Pressure (q_na)

Net Allowable Bearing Pressure is also known as ———

Allowable Soil Pressure (ASP)

He classified shear failure of soil under a foundation base into three categories depending on the type of soil & location of foundation.

Vesic (1973)

Modes of Shear Failure:

1. General Shear failure

2. Local Shear failure

3. Punching Shear failure

The failure occurs at very small vertical strains accompanied by large lateral strains.

General Shear Failure

The shearing strength is fully mobilized failure planes are well defined.

General Shear Failure

The failure occurs at large vertical strain & very small lateral strains.

Local Shear Failure

The shear strength of soil is not fully mobilized along planes & hence failure planes are not defined clearly.

Local Shear Failure

Failure planes are difficult to locate.

Punching Shear Failure

(T/F) The failure zones extend above the horizontal plane passing through base of footing.

False - do not extend

(T/F) The failure occurs when the down ward pressure exerted by loads on the soil adjoining the inclined surfaces on soil wedge is equal to upward pressure.

True

Downward forces are due to:

load and weight of soil wedge

Upward forces are the vertical components of:

resultant passive pressure and cohesion acting along the inclined surfaces

(T/F) The equation for ultimate bearing capacity by Terzaghi has been developed based on assumption that water table is located at a great depth.

True

(T/F) If the water table is located far to foundation; the equation needs modification

False - close to foundation

It takes place during or immediately after the construction of the structure.

Elastic Settlement (Immediate Settlement)

It is also known as the distortion settlement as it is due to distortions within foundation soil.

Elastic Settlement (Immediate Settlement)

It occurs due to gradual expulsion of water from the voids at the soil.

Consolidation Settlement

It is determined using Terzaghi's theory of consolidation

Consolidation Settlement

It occurs after completion of the primary consolidation.

Secondary Consolidation Settlement

It is non significant for inorganic soils.

Secondary Consolidation Settlement

Settlement due to other causes:

• Structural collapse of soil

• Underground erosion

• Lowering of water table

• Thermal changes

• Subsidence etc.