Geoscience Reference
In-Depth Information
•
Values shown are approximate only for estimation. Use charts for actual values
in a detailed analysis.
•
In layered soils and driven piles, the shaft capacity varies:
-
The adhesion decreases for soft clays over hard clays - due to smear effects
for drag down.
-
The adhesion increases for sands over clays.
-
Table in sands applies for driven displacement piles (eg concrete). For low
displacement (eg steel H piles) the values reduce by 50%.
21.15 Pile frictional values from sand
•
For sands, the frictional values after installation of piles is different than before
the installation (
φ
1
).
•
The in situ frictional value before installation is determined from correlations
provided in previous chapters.
Table 21.15
Change of frictional values with pile installation (Poulos, 1980).
Consideration
Design parameter
Value of
after installation
φ
Bored piles
Driven piles
Shaft friction
k
s
tan
¾
10
δ
φ
φ
+
1
1
End bearing
N
q
3
(
40)/2
φ
−
φ
+
1
1
21.16 End bearing of piles
•
The end bearing resistance (q
b
) of a pile depends on the cohesion (C
u
) for clays
and the effective overburden (
σ
v
) for sands.
•
There is currently an ongoing discussion in the literature on critical depths, ie
whether the maximum capacity is achieved at a certain depth.
•
N
q
values from Berezantsev et al. (1961).
•
The bearing capacity of bored piles in sands are ½ to 1/3 that of the bearing
capacity of a driven pile.
Table 21.16
End bearing of piles.
Soil type
Relationship
Values
Bored
Driven
Clay
q
b
N
c
C
u
N
c
9
N
c
9
=
ω
=
=
1.0 (Non fissured)
1.0
ω =
ω =
0.75 (Fissured)
ω =
Sands
q
b
N
q
σ
v
N
q
20 (Loose)
N
q
70 (Loose)
=
=
=
N
q
=
30 (Medium dense)
N
q
=
90 (Medium dense)
q
b
10 MPa maximum
N
q
60 (Dense)
N
q
150 (Dense)
=
=
=
N
q
100 (Very dense)
N
q
200 (Very dense)
=
=
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