2748
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
3 PILE CONSTRUCTION AND TEST PROGRAMME
The two test piles, Piles TP01 and TP02 of 800 and 1,000 mm
diameter, were constructed using bucket drill technique with
bentonite slurry on 24 and 26 August 2012, to 34 m depth,
respectively. The production piles were designed with the
reinforcing cages of sixteen 20 and 22 mm bars to 11 m depth
and eight 20 and 22 mm bars below this depth. The O-cells
attached at 0.5 and 0.8 m above the pile toes, as shown in
Figure 2. However, to avoid damage to the instrumentation
during the lowering the reinforcing cages into piles the
reinforcing cages of the test piles were supplied with sixteen
bars.
Figure 2. Details of instrumented test piles
The test piles were instrumented with a pair of diametrically
opposed vibrating wire strain-gages at four levels and two pairs
of diametrically opposed vibrating wire strain-gages at levels
SG1 and SG4, respectively.
The test piles were constructed by first inserting 820 mm and
1,020 mm outer diameter temporary casings, respectively,
to 7.6 m depth. Thereafter, the test piles were drilled to 34 m
depth using a bucket drill with bentonite slurry. Before
concreting each shaft, the shafts were cleaned and a reinforcing
cage with the O-Cell assembly was lowered into the shafts. The
O-cell assemblies consisted of three O-cells; 200 mm diameter
in Pile TP01 and 220 mm in TP02.
On completion of the drilling, on August 24 and 26, 2012,
a 219 mm diameter tremie pipe was inserted to the bottom of
each shaft and tremie placing of the concrete was commenced,
displacing the bentonite slurry.
The concrete cube strength was determined 28 days after
casting to 40 MPa and 44 MPa for the concrete used in Piles
TP01 and TP02, respectively.
The bottom 0.5 and 0.8 m length of Piles TP01 and TP02,
respectively, was equipped with a 114 mm diameter coring tube
attached to the reinforcing cage. The coring was performed after
the concrete had cured and showed that an about 20 mm thick
layer of debris and slurry existed below the toe of Pile TP01.
Pile TP02 could not be cored because the coring tube was
obstructed by steel reinforcement bars..
The static loading tests were performed on September 16 and
17, 2012, 23 and 22 days after concreting.
4 TEST RESULTS AND ANALYSIS
1.1
Load – movement measurements
The internal bond of the O-cells was broken at loads of at 190
and 480 KN load for Piles TP01 and TP02, respectively.
Theoretically, the O-Cell does not impose an additional upward
load until its expansion force exceeds the bond breaking load,
the buoyant weight of the pile above the O-Cell and the residual
load, if any, acting at the O-cell level. For both test piles TP01
and TP02, the initial upward movement records were taken at
570 and 960 KN; that is, these recorded loads included bond
breaking load, the buoyant weight and residual load. The pile
buoyant weights above the O-Cell for Piles TP01 and TP02
were 163 KN and 255 KN, respectively. Therefore, the residual
loads determined at O-cell locations of Piles TP01 and TP02 are
217 and 225 KN, respectively.
Figure 3 shows the load-movement records of the two O-cell
tests. After subtracting the buoyant weight of the piles TP01 and
TP02 above the O-Cell, the maximum upward resistances were
3,636 and 4,544 KN, respectively. The maximum O-cell upward
movements were 3.3 mm and 7.4 mm, the maximum O-cell
downward movements were 28.0 and 49.3 mm, and the
maximum pile head movements (upward) were 0.2 and 2.5 mm,
respectively.
0
5
10
15
20
25
30
35
40
O-Cell
Pile TP01
D800
18.5 m
33 m
0.0 m
DEPTH (m)
Ground Surface
15 m
20 m
25 m
30 m
10 m
33.2 m
SILTY
SAND
FIRM
CLAY
SAND
STONE
SG1
SG2
SG3
SG4
SG5
SG1
SG2
SG3
SG4
SG5
Pile TP02
D1000
15 m
20 m
25 m
30 m
10 m
33.5 m
O-Cell
34 m
34 m
-60
-50
-40
-30
-20
-10
0
10
20
30
0
1000 2000 3000 4000 5000 6000
MOVEMENT (mm)
LOAD (KN)
Buoyant Weight
255 KN
UPWARD
DOWNWARD
Buoyant Weight
163 KN
TP02
TP01
Breaking Bond
TP01-190 KN
Breaking Bond
TP02-480 KN
Figure 3. Load-movement curves of the piles
1.2
Strain measurements and determining Axial Modulus
To determine the secant modulus of the pile material, the best
way is to use "tangent modulus" or "incremental stiffness" plot
which is the applied increment of load over the induced
increment of strain plotted versus the measured strain. The
tangent modulus is then converted to the secant modulus
(Fellenius 1989; 2011). The incremental stiffness plots for the
two tests are shown in Figures 4 and 5. The incremental
stiffness method assumes that for load increments applied after
the shaft resistance at the studied gage level has been fully
mobilized, the continued incremental stiffness values will plot
along a slightly sloping line, representing the tangent modulus
relation for the pile cross section. Because of the combined
effect of the strain-softening and the scatter of values, the
incremental stiffness method did not provide sufficiently precise
values for the tangent and secant stiffnesses. Therefore, for this
case, the authors have preferred to rely on the linear portions of
load-strain relations and convert the measured strains using
constant stiffnesses, AE, of 12.5 GN and 17.0 GN for Piles
TP01 and TP02, respectively. Correlated to the nominal cross
sectional areas, the values indicate that the E-modulus of Piles
TP01 and TP02 is about 25 and 22 GPa, respectively.
0
20
40
60
80
0
100
200
300
400
500
SG - 1A
SG - 1B
SG - 1C
SG - 1D
SG - 2A
SG - 2B
SG - 3A
SG - 3B
INCREMENTAL STIFFNESS AE
(GN)
STRAIN (μS)
AE = 12.5 (GN)
Figure 5. Increment stiffness plot of records from Levels 1 to 3,
Pile TP01
