Example 6.7

An offshore pole is going to be founded on a pile group consisting of 4 cylindrical piles, with the geometry shown in the figure below. Determine the short-term design bearing capacity of the pile group, according to AS 2159 provisions, For the specific project, assume a basic geotechnical strength reduction factor φ_gb equal to φ_gb = 0.60.

Answer:

In the Example 6.1 we have determined the ultimate geotechnical strength of each single pile with the α-Method as:

[latex]{Q_f} = {Q_{sf}} + {Q_b} = 1059{\rm{ \:kN}}[/latex]

1. Ultimate geotechnical strength of pile group considering efficiency, n_g:

According to FHWA (2006), pile group efficiency n_g for cases where the pile cap is not in firm contact with the ground and s/D = 4 m/1 m =4 (Eq. 6.88) is:

[latex]{n_g} = 0.1\left( {\dfrac{s}{D}} \right) + 0.4 = 0.8[/latex]

Therefore the ultimate geotechnical strength of pile group considering is (Eq. 6.87):

[latex]{Q_{f,group}} = {n_g}\left( {n{Q_f}} \right) = 0.8 \times 4 \times 1059 = 3389{\rm{ \:kN}}[/latex]

2. Ultimate geotechnical strength of pile group for block-type failure:

We will apply Eq. 6.89, considering B = 4 m and Z = 4 m i.e., the dimensions of the pile group, not the pile cap (Figure 6.65b). Also, as for the calculation of the skin friction resistance of a single pile, the first 1.5D meters of the pile shaft will not contribute to the friction resistance:

[latex]{Q_{f,group}} = {S_{u,ave}}\left[ {2L\left( {B + Z} \right)} \right] + {N_{cp}}{S_{u,b}}\left( {BZ} \right) = 40\left[ {2\left( {8 - 1.5} \right)\left( {4 + 4} \right)} \right] + 40 \times 9 \times \left( {4 \times 4} \right) = 9920{\rm{\: kN}}[/latex]

Observe that the ultimate geotechnical strength for block-type failure is grossly larger than the ultimate geotechnical strength considering failure of individual piles, even when pile group efficiency n_g < 1 is applied to the latter. This does not come as surprise: Block-type failure can, by definition, be critical only for very dense pile spacings, when the area of the pile group (B ×Z) + 2(B ×L)+ 2(Z ×L) is smaller than the surface area of the individual piles.

3. Design bearing capacity of pile group

The design bearing capacity φ_gbQ_f,group is:

[latex]{\varphi _{gb}}{Q_{f,group}} = 0.60 \times 3389 = 2033{\rm{ \:kN}}[/latex]