Page 43 - Unit2.docx

A = area of pile base
b

C = undrained strength of soil at base of pile

C
N = bearing capacity factor

………………………………………

Nevertheless, in practise, for a given pile at a given site, the undrained shear
strength C varies considerably with many factors, including, pile type, soil
a
type, and methods of installations.

Ideally, C should be determined from a pile-load test, but since this is not
a
always possible, C is correlated with the undrained cohesion C by empirical
a
u
adhesion factor α so that the general expression in e.q. (4-2) could be simplified
to the following expression:

© It's Engg. Life

…………………………………
…………4.2

Where: W s = weight of soil replaced by the pile

=average value of shear strength over the whole shaft length

4.3.2 The undrained load capacity (total stress approach)

For piles in clay, the undrained load capacity is generally taken to be the critical
value unless the clay is highly over consolidated. If the undrained or short-term
ultimate load capacity is to be computed, the soil parameters C, θ ,α , γ should be
appropriate to undrained conditions and σ and σ should be the total stresses.
v
vb
♦
If the clay is saturated , the undrained angle of friction φ is zero, and φ (angle
u
a
of friction between pile and soil) may also be taken as zero. In addition, N = 2, Nγ
q
= 2, so that the eq in(4-2) reduces to:

……………………………………………4.3

Where: N , N , Nγ ,= bearing capacity factors and are functions of the internal
c
q
angle of friction φ of the soil, the relative compressibility of the soil and the pile