piles - 4b
TRANSCRIPT
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Case Studies of
Rock-socketed piles
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Sedimentary rocks Case studies (Leung, 1996) involving
a good number of load tests carried
out in instrumented piles socketed in
weak sedimentary rock mainly inAlexandra and Keppel areas
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0 4 8 12 16 20Load (MN)
0
4
8
12
16
Settlement(mm)
Load-settlement response is reasonably linear up to working load of 10 MN
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0 5 10 15 20
Load (MN)
0
4
8
12
16
Depthbelow
groundlevel(m
)
0 200 400 600
Unit shaft friction (kPa)
Firmsilty clay(N = 12)
V. denseclayey silt(N = 130)
Weaksiltstone(qu=6.5 MPa)
Fill
Marine clay
Weaksiltstone(q
u
=3.5 MPa)
(a) (b) (c)
1 2 3 4
1 2 3 4
Applied load
1: 5 MN2: 10 MN
3: 15 MN4: 20 MN
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Results show that a large percentage of shaft friction can
be mobilised even for piles not tested to ultimate failure.
Alexandra Distripark (after Radhakrishnan & Leung, 1989)
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Rock socket adhesion factor
Pile load test results reveal that
majority of socket shaft friction can be
mobilised even for piles not tested to
ultimate failure. Rock socket adhesion factor
= fs/qu
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Sedimentary rocks For qu < 5 MPa (i.e. very weak rock)
rock socket adhesion factor is
reasonably close to theoretical values.
For qu > 5 MPa (i.e. weak rock & above), value is considerably lower than the
theoretical values. This is caused by
heavy chiselling that had significantlyweakened the rock.
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1 10 100
Unconfined compressive strength qu(MPa)
0.0
0.2
0.4
0.6
Rocksocketadhesionfa
ctor
Williams and Pells
Rowe and Armitage
Rosenberg and Journeaux
Horvath and Kenny
Field data insedimentary rocks
All piles installed by chiselling.
Chiselling affects sockets with qu > 5 MPa
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Granite Load tests are mainly carried out in the
Woodlands area (Leung, 1996). Load-settlement curve
Load transfer
Unit shaft friction
t-z curves
Rock socket adhesion factor
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0 2 4 6 8 10Load (MN)
0
5
10
15
20
25
Settleme
nt(mm)
Load-settlement response is linear up to pile working load
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0 2 4 6 8 10
Load (MN)
0
10
20
30
Depthbelow
groundlevel(m)
0 200 400 600 800
Unit shaft friction (kPa)
Clayeysilt(N =14)
Clayey siltwith sand(N = 25)
Dense siltysand(N = 120)
Granitequ=12.5 MPa
(a) (b) (c)
1 2 3 4
1 2 3 4
Applied load
1: 3.6 MN2: 6.3 MN
3: 8.1 MN4: 9.0 MN
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0 4 8 12 16 20
Shaft movement (mm)
0
200
400
600
800
Unitshaftfriction(kPa)
Depth belowground level
6.5 m
9.5 m
12.5 m
15.5 m
18.5 m
21.5 m
24.5 m
27.5 m
29.5 m
t-z curves (much of the load transfer in the granite socket)
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Granite For all qu values, is considerably
lower than the theoretical values.
Heavy chiselling, that had significantly
weakened the rock, need to done onall classes of granite as they are
generally less fractured.
[Most of the load tests were
conducted in the Woodlands area.]
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1 10 100
Unconfined compressive strength qu(MPa)
0.0
0.2
0.4
0.6
Rocksocketadhesionfa
ctor Williams and Pells
Rowe and Armitage
Rosenberg and Journeaux
Horvath and Kenny
Field datain granite
Chiselling disturbs granite more than sedimentary rocks
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Construction concern As the studies revealed heavy
chiselling process can badly damagethe rock, it is believed that better
construction tool (e.g. down the holehammer) will result in less rock
damage and hence higher shaft
resistance. [Further studies are
required to confirm this.]
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Settlement of socketed piles
The load-settlement response under
working load is found to bereasonably linear in most cases
reported earlier. The load-settlement responses of all
the previous cases are back-analysed
using a FEM program and a Elastic
theory model (Leung & Chow, 1998).
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FEM
mesh
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Settlement of socketed piles
Granite
Es = 2N (unit in MPa)
Es = Em (need more data to confirm)
Sedimentary rocks Es = Em (needs more data to confirm)
Es < 2N (exact values hard todetermine as degree of rock fracture
highly variable)
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0 2 4 6 8Measured pile settlement (mm)
0
2
4
6
8
Predictedpilesettle
ment(mm) N value
Pressuremeter modulus
Figure 4 Comparison of predicted and measured pile settlements (Singapore granite)
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0 4 8 12
Measured pile settlement (mm)
0
4
8
12
Predict
edpilesettle
ment(mm)
Figure 5 Comparison of predicted and measured pile settlements (Singapore sedimentary rocks)
N value
Pressuremeter modulus
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Pile group, cap & creep Case studies:
PSA Building
Alexandra District Park
The presence of rigid pile cap, pile groupeffect and possibly rock creep will cause
the less load being transferred to the pile
shaft in the long term. I.e. more load is
transferred to the pile base.
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PSA
Building
(Case
study by
Leung etal., 1998)
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Foundation layout and instrument plan
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Alexandra Distripark (after Radhakrishnan & Leung, 1989)
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Load transfer under short term test load is different
form that under long term service loads condition.
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Summary of findings (1) Majority of shaft friction is almost
fully mobilised for all piles [See Table1 earlier]
(2) Observed fs is close to thosepredicted by Williams & Pells and
Horvath et al. But lower than those of
Rowe & Armitage and Rosenberg &
Journeaux [see Table 2]
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Summary of findings (3) More load transferred to the pile
base under long term service loadingcondition than short term load test
condition due to interaction between piles in a group
inetraction between piles and pile cap
pile and soil/rock creep
[See previous figure and Table 3]
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