Answers to Tutorial questions (Class 8)
2.1 As shown in the sketch, a group of seven piles is arranged in a circle.
All
the piles are floating concrete piles with a diameter of 1.2m and cast in place
with 12 meters imbedded into sand. No water exists within the layer. The sand
has a total weight of 20 kN/m
3
. Suppose the efficiency of the pile group is
75%, determine the bearing capacity of the pile group. The results of standard
penetration tests on the sand are listed below. Using the empirical expression
N
2
f
s
=
and
kPa
400N
D
L
0N
4
f
b
≤
⎟
⎠
⎞
⎜
⎝
⎛
=
for the estimation of the bearing
capacity of the pile group in sand.
D
g
= 8m
Results of standard penetration tests
Soil depth z (m)
0.2
3
6
8
11.5
N
Standard
penetration resist.
8
15
18
20
23
Answer
(Step 1)
The efficiency of a pile group (e
pg
) is given by
pile
single
a
of
capacity
bearing
pile
per
capacity
bearing
Average
e
pg
=
Therefore the bearing capacity of a pile group can be described
P
u,g
=n*e
pg
*P
u
(1)
(Step 2)
The bearing capacity of a single pile can be worked out by
P
u
=P
s
+P
b
W
Considering that the effect of pile weight and that of the overburden pressure for the base
resistance can be balanced, then
P
u
=f
s
A
s
+f
b
A
b
(Step 3)
The soil is sand and drained analysis is used. Parameter f
s
and f
b
are determined from standard
penetration resistance.
(Step 4)
Find out P
s
The average standard penetration resistance along
the pile shaft
N
ave
=(8+15+17+20+23)/5=17
f
s,ave
=2N
ave
= 34 kPa
P
s
=f
s,ave
π
DL
=34*3*1.2*12
=490 kN
(Step 5)
Find out f
b
A
b
The standard penetration resistance at the base
N=23 kPa, L/D=10
f
b
=400N
=400*23=9200 kN/m
2
f
b
A
b
=f
b
π
(D/2)
2
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View Full Document=9200*3*(1.2/2)
2
=9900 kN
(Step 6)
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 Spring '09
 ZheeenLing
 Geotechnical Engineering, 4 m, 1.2m, 3 kPa, 3kPa

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