2899
Technical Committee 212 /
Comité technique 212
300
300
Figure 2 Design schematic diagrams of the test pile head for the
Shanghai Center Tower project
4.3 CONSTRUCTION AND MEASUREMENT
REQUIREMENTS
Practical construction conditions should be simulated in the
construction process of the test piles. Artificial drilling fluid and
desanding device should be used when the borehole is drilled
through the deep sand layers. Vertical deviation of the borehole
should be not more than 1/250. Thickness of the sediment at the
borehole tip should be less than 50mm after the secondary tip
cleaning. If post grouting technique is adopted for the test pile,
the grouting construction parameters should be determined. The
construction machineries, techniques and parameters are also
need to be determined to form a guideline for pile construction.
Measurement items of super-long bored pile load test are
illustrated as follows: (1) Drilling fluid density, viscosity, sand
content and other technical indexes in different depth of the
borehole. These parameters should be continuously monitored
for not less than 36 hours in the construction process. (2)
Concrete quality of the test pile, including pile shaft integrity
and concrete strength. (3) Sediment and grouting effect under
the test pile tip. (4) Ultimate bearing capacity of the test pile. (5)
Pile shaft axial force and pile shaft friction. (6) Pile shaft
deformations, including deformations of pile top, pile tip,
section at the rock surface, and other pile shaft sections under
each load level.
5 SINGLE PILE DESIGN
5.1 PILE ULTIMATE BEARING CAPACITY
The ultimate bearing capacity of super-long bored pile is
determined by filed load test. If the load versus settlement curve
of the test pile shows a slowly change in slope, the load
corresponding to the pile head settlement of 40mm~60mm or
5% of the pile diameter can be used as the ultimate bearing
capacity of the pile. For pile foundation under a condition of
deep excavation, some factors, such as the soils gravity and pile
shaft friction in pit excavation segment and unloading rebound
of the soil at the bottom of the pit, should be concerned to
determine the ultimate bearing capacity of engineering pile
(Wang et al, 2012).
Due to the problems of pile shaft mud and pile tip sediment,
measured values of the ultimate bearing capacities of the normal
super-long bored piles are often lower than the values estimated
by empirical methods. Test results of 10 field test piles from 5
different sites in Shanghai district were collected by the authors.
It illustrates that the ratios of the measured values of the
ultimate bearing capacities of the piles to the values estimated
by empirical method range from 0.5 to 0.97. The average ratio
is 0.69. The pile bearing capacity can be greatly improved by
post grouting technique. Measured data of 28 post grouted piles
from 9 Shanghai project sites indicate that the average ratio of
the measured values of the piles ultimate bearing capacities to
the values estimated by empirical method is 1.32. Therefore, the
post grouting technique should be adopted for super-long bored
piles in deep soft soils.
5.2 PILE SHAFT STRENGTH AND COMPRESSION
Due to the application of the post grouting technique, the
bearing capacities of the foundation soils around the super-long
pile are improved greatly. Therefore, the strength of pile shaft
should match well with the bearing capacities of the foundation
soils in the design of a single pile. The application of high-
strength concrete is helpful to achieve this object. As shown in
Table 2, in order to make the piles shaft strength meet the piles
bearing capacities requirements, Grade C45 and even Grade
C50 concrete were adopted for the foundation piles of several
super high-rise buildings in China. Meanwhile, concrete
strength can be enhanced by the effect of stirrup constraint.
Thus, the spacing of spiral stirrups at the pile top within a scope
of about 3
D
~5
D
(
D
is the pile diameter) should be appropriate
reduced to increase the bearing capacity of pile shaft.
Table 2 Pile shaft strength of several projects in China
Project name
Concrete
strength grade
UCS
(MPa)
Shanghai Center Tower
C50
40.0
Shanghai magnolia square
C45
44.3
Tianjin 117 Tower
C50
59.3
Wuhan Center Tower
C50
54.6
Note:
UCS
is the average unconfined compressive strength of the
concrete drilled from the shaft.
Pile shaft compression is a part of the pile top settlement
deformation. It is often estimated by the following empirical
formula:
0
0
s
e
0
0
p
p
1
L
z
Q L
S
Q d q z dz dz
AE
AE
(1
)
Where
Q
0
is the load applied at pile top;
L
is the pile length;
A
is
the pile section area;
E
p
is the elastic modulus of the pile shaft;
e
is the pile shaft compression coefficient. For friction pile,
e
=1/2~2/3.
According to measured data of nearly 40 super-long bored
test piles from 15 sites, diagram of the relationship between the
measured values of the pile shaft compression and the
calculated value of
Q
0
L
/
AE
p
was drawn, as shown in Figure 3.
As can be seen from the graph, under the working loads, the pile
shaft compression coefficients are less than 1/2. Therefore, the
value of
e
for calculating super-long bored pile shaft
compression by formula (1) should be not larger than 1/2.
0 10 20 30 40 50 60 70 80 90 100
0
10
20
30
40
50
60
70
80
90
100
e
=1/4
e
=1/2
Q
0
L
/
E
p
A
(mm)
Measured value of pile shaft compression (mm)
e
=1.0
Figure 3 Diagram of the relationship between the measured values of
the pile shaft compression and calculated values of
Q
0
L
/
AE
p
6 PILE FOUNDATION DESIGN
The synergism of the superstructure, foundation soils and pile
foundations should be considered in the design calculation of
pile foundations for super high-rise buildings. According to this,
a practical method for analysis and calculation of the pile
foundation is given in this paper. The theoretical framework and
procedures of this method are illustrated in Figure 4. The
general calculation process is shown in Figure 5.
Design calculation of the pile foundation consists of four
parts, including foundation settlement calculation, bearing
capacity calculation of the grouped piles, bending stress
