Question

Q2

A concrete strip footing with a width of B is founded at the depth of D, below the surface of a level site. The water table is at a depth of d below the base of footing. The soil at the site consists of clay having the following properties:

The unit weight = g_t,   the undrained cohesion = c_u, the undrained friction angle = f_u, the drained cohesion = c′ and the drained friction angle = f′

Use Hansen’s theory of bearing capacity to determine the allowable bearing pressure, which could be placed on the footing assuming a factor of safety of F for the following cases:

a) Immediately after loading the footing

b) Long time after loading the footing

Notes: The load is vertical. The ground and the base of footing are horizontal. D should be less than or equal to 2.5 B. The variable d can be positive (below the base), negative (above the base) or zero (at the base). Assume water level cannot be above the ground surface.

Answer 1

According to Hansen's theory

As D/B 1,  ,   

(a)

Immeadiately after loading the footing, the loading would be undrained, so undrained cohesion and undrained friction angle should be used.

q = (gt)D

  

where ,

(b)

Long time after loading the footing, the loading would become drained , so drained cohesion and drained friction angle should be used

where ,