A multistory building is to be constructed in a stiff to very stiff clay. The soil is homogeneous to a great depth. The average value of undrained shear strength cu is 150 kN/m^2. It is proposed to use a drilled pier of length 25 m and diameter 1.5 m. Determine (a) the ultimate load capacity of the

Question

A multistory building is to be constructed in a stiff to very stiff clay. The soil is homogeneous to a great depth. The average value of undrained shear strength [latex]c_{u} ext { is } 150 kN / m ^{2}[/latex]. It is proposed to use a drilled pier of length 25 m and diameter 1.5 m. Determine (a) the ultimate load capacity of the pier, and (b) the allowable load on the pier with [latex]F_{s}=2.5[/latex]. (Fig. Ex. 17.1)

Accepted Answer

When [latex]c_{u} \geq 96 kPa \left(2000 lb / ft ^{2} ight)[/latex], use Eq. (17.8) for computing [latex]q_{b} . ext { In this case } c_{u}>96 kPa[/latex].

[latex]q_{b}=9 c_{u}[/latex] (17.8)

[latex]q_{b}=9 c_{u}=9 imes 150=1350 kN / m ^{2}[/latex]

Base load [latex]Q_{b}=A_{b} q_{b}=\frac{3.14 imes 1.5^{2}}{4} imes 1350=1.766 imes 1350=2384 kN[/latex]

Frictional load

The unit ultimate frictional resistance [latex]f_{s}[/latex] is determined using Eq. (17.24)

[latex]f_{s}=\alpha c_{u}[/latex] (17.24)

[latex]f_{s}=\alpha c_{u}[/latex]

From Fig. (17.15) [latex]\alpha=0.55 ext { for } c_{u} / p_{a}=150 / 101=1.5[/latex]

where [latex]p_{a}[/latex] is the atmospheric pressure =101 kPa

Therefore [latex]f_{s}=0.55 imes 150=82.5 kN / m ^{2}[/latex]

The effective length of the shaft for computing the frictional load (Fig. 17.13 a) is

[latex]L^{\prime}=[L-(d+1.5)] m =25-(1.5+1.5)=22 m[/latex]

The effective surface area [latex]A_{s}=\pi d L^{\prime}=3.14 imes 1.5 imes 22=103.62 m ^{2}[/latex]

Therefore [latex]Q_{f}=f_{s} A_{s}=82.5 imes 103.62=8,549 kN[/latex]

The total ultimate load is

[latex]Q_{u}=Q_{f}+Q_{b}=8,549+2,384=10,933 kN[/latex]

The allowable load may be determined by applying an overall factor of safety to [latex]Q_{u}[/latex].

Normally [latex]F_{s}=2.5[/latex] is sufficient.

[latex]Q_{a}=\frac{10,933}{2.5}=4,373 kN[/latex]

Question 17.1: A multistory building is to be constructed in a stiff to ver...

This Question has been Answered.

Already have an account?

Login

Related Answered Questions