Civil Engineering Reference
In-Depth Information
C
r
=
400
f
kPa
for driven piles in loose sand
s
C
r
=
150
f
kPa
for driven piles in non-plastic silts
s
Then Q
s
=
f
s
A
s
and, as before, Q
u
=
Q
b
+
Q
s
.
Example 10.3:
Allowable load from in situ testing results
A 5 m thick layer of medium sand overlies a deep deposit of dense gravel. A series of
standard penetration tests ca
rr
ied out through the depth of the sand has established
that the average blow count,
N
, is 22. Further tests show that the gravel has a standard
penetration value of N
=
40 in the region of the interface with the sand. A precast pile
of square section 0.25
×
0.25 m
2
is to be driven down through the sand and to penetrate
sufficiently into the gravel to give good end bearing.
Adopting a safety factor of 3.0 determine the allowable load that the pile will be able
to carry.
Solution:
Ultimate bearing capacity of the pile Q
=
=
Q Q
+
u
s
b
Q
b
: All end bearing effects will occur in the gravel. Now
N
D
q
b
≈
40
B
kPa or
400
×
N kPa whichever is the lesser
(
)
i.e.
D
q
b
= × ×
40 40
=
400 40
× =
16 000
kPa
0 25
.
16 000 0 25
40 40
×
×
.
Penetration into gravel D
,
=
=
2 5
.
m
and
Q
b
=
16 000 0 25
×
.
2
=
1000
kN
Q
s
in sand: Q
s
=
f
s
A
s
=
22
×
5
×
0.25
×
4
=
110 kN
Q
s
in gravel: Q
s
=
f
s
A
s
=
40
×
2.5
×
0.25
×
4
=
100 kN
i.e.
Q
u
=
210 1000
+
=
1210
kN
1210
3
Allowable load
=
=
400
kN
Example
10.3
illustrates that, as discussed earlier, the end bearing effects are much greater than
those due to side friction. It can be argued that, in order to develop side friction (shaft resistance) fully,
