Friction pile of a 24-in.² (154.8 cm²) reinforced concrete section are to be used with an embedded length of 40 ft (12.2 m) in a soft clay layer. The clay is known to have an unconfined compressive strength of 800 psf (38.3 kPa) and to be very uniform throughout the deep layer. An isolated footing

Question

Friction pile of a 24-in.² (154.8 cm²) reinforced concrete section are to be used with an embedded length of 40 ft (12.2 m) in a soft clay layer. The clay is known to have an unconfined compressive strength of 800 psf (38.3 kPa) and to be very uniform throughout the deep layer. An isolated footing load at this site will exert a concentric load to be required pile group of 250 tons (2224 kN).

a What is the design allowable bearing capacity per one pile using a factor of safety of 2?
b If the value obtained in (a) is used, how many piles will be required in the group in order to support the intended load?

Accepted Answer

a Allowable bearing capacity of a single pile

p = perimeter of simple pile = (4) (24 in./12) = 8 ft (2.44 m)

L = length of pile = 40 ft (12.2 m)

c = cohesion = 1/2 [latex]q_{u}[/latex] = 800/2 = 400 psf (19.2 kPa)

Allowable bearing capacity of a single pile = pLc/[latex]F_{s}[/latex]

[latex]=\frac{\left(8 ight)\left(40 ight)\left(400 ight) }{2} =\frac{128,000}{2}=64,000 lb \left(284.7 kN ight)[/latex]

b Number of piles required, N

[latex]N=\frac{250 imes 2000}{64,000}=7.8[/latex]

There are numerous formulas for estimation of pile capacity in both single and group piles as discussed by Fellenius (1991), US Army (1993) and many others.

Question 15.2: Friction pile of a 24-in.² (154.8 cm²) reinforced concrete s...

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