Pile Foundations - Elements of Soil Mechanics - page 298

Table 10.1 Typical values for δ and K s suggested by Broms ( 1966) .

K s

Relative density of soil

Pile material δ Loose Dense

Steel 20° 0.5 1.0

Concrete 1.0 2.0

0.75 φ ′

Timber 1.5 4.0

0.67 φ ′

where

K s = the coefficient of lateral earth pressure
′

σ v = average effective overburden pressure acting along the embedded length of the pile shaft
δ = angle of friction between the pile and the soil.

Hence

′

Q = A K σ v tan δ s s s

and

= ′

′

Q σ N A + A K σ

v tan

δ

u v q b s s

Typical values for δ and K s were derived by Broms ( 1966) , and are listed in Table 10.1.
Vesic ( 1973) pointed out that the value of q b , i.e.

′

σ v N does not increase indefinitely but has a limiting
value at a depth of some 20 times the pile diameter. There is therefore a maximum value of

′

σ v N that can

be used in the calculations for Q b .
In a similar manner there is a limiting value that can be used for the average ultimate skin friction, f s .
This maximum value of f s occurs when the pile has an embedded length between 10 and 20 pile diameters.
Vesic ( 1970) suggested that the maximum value of the average ultimate skin resistance should be obtained
from the formula:

4

f s = 0 08 10 1 5 . ( )

. ( Dr )

where D r = the relative density of the cohesionless soil.
In practice if s is often taken as 100 kPa if the formula gives a greater value.
Unlike piles embedded in cohesive soils, the end resistances of piles in cohesionless soils are of con-
siderable significance and short piles are therefore more efficient in cohesionless soils.

Example 10.1: Undrained analysis

A pile of diameter 400 mm and length 6 m is to be installed into a deep deposit of clay.
The clay has an undrained shear strength of 180 kPa at a depth of 6 m and an average
undrained shear strength of 120 kPa over the depth 0-6 m.
Assuming N c = 9.0 and α = 0.6, determine the ultimate bearing capacity of the pile.