Ultimate bearing capacity of shallow foundations

Question 1

We have 2 limit states in considering the foundation design, Both of them can be the controlling agent in design

Answer 1

First we have the bearing limit state which mean we dont want the bearing capacity of the soil to fail against the structure.
Second we dont want excessive settlement of the structure.

Question 2

Three modes of failure in shallow foundations

Answer 2

General shear failure
local shear failure
punching

Question 3

General shear failure

Answer 3

The failure starts showing relatively small stains, then suddenly the foundation tilts into the ground showing a surface heave of the adjoining soil. the failure planes are easly defined because the shearing strength along the surface of failure are totally mobilized.

Question 4

Local shear failure

Answer 4

The failure starts at localized spots beneath the foundation then migrates leading to ultimate failure however without showing a large surface heave. Also the failure planes are not defined clearly because the shear strength of soil is not totally mobilised

Question 5

Punching failure

Answer 5

The foundation sinks without showing any heaves, also failure planes are difficult to determine.