Environmental Engineering Reference
In-Depth Information
COV = 0.4 gives ξ = 1.31 along the shaft of the pile. Hence, the characteristic undrained
shear strength along the pile shaft is
c
u;k
= c
u;mean
/ξ = 40/1.31 = 30.5 kPa
This value is a little more conservative than the c
u;k
=30.9 kPa obtained above using the
Vanmarcke (1977)
(
Equation 10.16
).
The vertical extent of the failure mechanism at the pile base is much smaller in
extent than along the pile shaft. Assuming the failure mechanism beneath the pile base
consists of a 45° cone and a strain fan, the vertical extent of the failure mechanism
L
=
/
2082 057
=
.
/
=
.
m
. Again ignoring the horizontal extent of the failure mecha-
z
2
2
==., and adopting Schneider and Schneider's (2013)
Equation 10.20
since L
z
< δ
z
as L
z
=0.57 m and δ
z
=2 m gives Γ
2
nism, so that ΓΓ
x
10
y
2 2 2
= in
Equation 10.22
and with COV=0.4 gives ξ equal to 1.93 so that the characteristic undrained
shear strength for failure at the base of the pile is
= .
000
95 Substituting for ΓΓΓΓ
S
.
x
yz
c
u;k
= c
u;mean
/ξ = 40/1.93 = 20.7 kPa.
This characteristic undrained shear strength value is much less than the value obtained for
c
u;k
along the pile shaft and demonstrates that a more cautious characteristic value should
be selected when the extent of the failure mechanism is smaller, corresponding to a local
failure, for example, at the base of a pile. A less conservative c
u;k
value would be obtained if
the lateral extent of the failure mechanism were also taken into account.
10.4.3.5 Characteristic pile compressive resistances
Four methods are presented in Eurocode 7 for calculating the compressive resistance of pile
foundations from test results, which are the methods using:
• Static pile load tests
• Proiles of test results (referred to as the “model pile procedure” by Frank et al., 2007)
• Soil parameter values (referred to as the “alternative procedure” in Eurocode 7)
• Dynamic pile tests
Profiles of test results are obtained using tests such as CPT, PMT, and SPT, and these give
individual test profiles that are used to calculate the pile compressive resistance at the loca-
tions of the test profiles.
The compressive resistance of a pile varies over a site due to variations in the ground prop-
erties and also due to stratification. This spatial variability needs to be taken into account
when selecting the characteristic pile compressive resistance, R
c;k
. Eurocode 7 provides a
simple procedure involving the use of two sets of correlation factors ξ to take spatial vari-
ability into account and determine R
c;k
when designing piles from static pile load tests or
from profiles of test results. For piles designed from static pile load tests, the R
c;k
value is
obtained by dividing the mean and lowest measured pile resistances, (R
c;m
)
mean
and (R
c;m
)
min
by correlation factors ξ
1
and ξ
2
, with the recommended values given in Eurocode 7 shown in
(
R
)
;
(
R
)
Š
'
'
cm
;
mean
cm
;
min
R
=
in
(10.24)
ck
;
ξ
ξ
1
2
Similarly, for piles designed from profiles of test results, the R
c;k
value is obtained by divid-
ing the mean and lowest calculated pile compressive resistances by correlation factors ξ
3
and
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