Question

The undrained strength of a clay is 1.2 kips/ft2 , the water table in 4 ft below the ground surface, the total unit weight of the clay is 118 lb/ft3 , and a round shallow foundations with a diameter of 4.25 ft is placed at a depth of 10 ft supports a column load. a. Use a factor of safety of 2.8 and compute the safe column load on the foundation. b. An excavation near the drilled shaft removes the soil down to the water table. What is the effect on the factor of safety? (Hint consider what the physical meaning of the components in the Hansen equation. Which component is impacted by the excavation?)

Answer 1

where:

B = L = Diameter of circular footing = 4.25 ft

Part b) Due to excavation near the drilled shaft removing the soil down to the water table, the walls of drilled shaft will be unstable due to less drainage path between the excavation and drilled shaft, so that the water can easily erode the walls and higher seepage pressure causes the walls to fall down. According to Brinch Hansen the effect of excavation results in the higher safety factors than normal conditions in order to keep the footing safe to carry the load 48.62 kips safely.