Refer to Problem 15.8. If an additional requirement is that the factor of safety against uplift is at least 4 with the dead load on (live load = 0), what should be the diameter of the bell?

Problem 15.8.

Refer to Figure 15.26b. For the drilled shaft with bell, given:

Thickness of active zone, Z = 9 m

Dead load = 1500 kN

Live load = 300 kN

Diameter of the shaft, D_s = 1 m

Zero swell pressure for the clay in the active zone = 600 kN/m²

Average angle of plinth–soil friction, ϕ'_ps = 20°

Average undrained cohesion of the clay around the bell = 150 kN/m²

Determine the diameter of the bell, D_b. A factor of safety of 3 against uplift is required with the assumption that dead load plus live load is equal to zero.

Figure 15.26

Dead load, D Grade beam Ground surface +D,> U Active zone, Z Drilled shafts with bells (a) (b)