1 Piled raft foundation for the W-TOWER Tel Aviv Prepared by A. Lehrer, S. Bar. 1. Introduction. Development of the world's largest cities dictated the need for high building housing in different soil conditions, as complicated as they are. In the 80's of the last century, due to the increasing load on the foundation piles and the need for planning groups of piles with a common head, began to develop a method of analysis of the combined action of the heads of piles or raft and piles. The combined action of piles and raft (Figure 1) allows the transfer of larger loads than on any individual system and reducing the settlement. Figure 1. Ongoing activities of the piles and the raft (Katzenbach, Arslan, Moormann, 1999).
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Piled raft foundation for the W-TOWER Tel Aviv
Prepared by A. Lehrer, S. Bar.
1. Introduction.
Development of the world's largest cities dictated the need for high building housing in
different soil conditions, as complicated as they are. In the 80's of the last century, due to
the increasing load on the foundation piles and the need for planning groups of piles with a
common head, began to develop a method of analysis of the combined action of the heads of
piles or raft and piles. The combined action of piles and raft (Figure 1) allows the transfer
of larger loads than on any individual system and reducing the settlement.
Figure 1. Ongoing activities of the piles and the raft (Katzenbach, Arslan, Moormann, 1999).
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Analysis of the interaction between piles and raft is a complex process:
- Loading of the foundations causes a settlement.
- During the settlement the loads transfer to the soil below the raft.
Piles below the raft are designed to:
- Reduce settlements.
- Increase the bearing capacity of foundation system.
Analysis of the abovementioned cases is slightly different. Following article focuses only on
the analysis of the system of piles that reduce settlement.
Piles – raft system behavior characterized by β coefficient, which describes the distribution of
load between piles and raft. Coefficient β is defined as follows:
∑=
=n
1i tot
i pile,
S
Rβ
For the same soil conditions and the area of the raft, the coefficient β is a function of number
of piles and their dimensions.
Figure 2. Settlement of the foundations as a function of the coefficient β (Katzenbach, Arslan,
Moormann, 1999).
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Since the piles under the raft intended only to reduce the settlement and are not required to
increase bearing capacity, it is clear that the piles can be used up to the loads approach their
bearing capacity.
2. Methods of analysis for the raft with piles designed to settlement
reduction.
2.1. Method for calculating the settlements of a raft on piles.
2.1.1. Equivalent Pile (Poulos and Davis, 1980).
A raft with piles can be represented by an equivalent pile, diameter de. For a raft with an area
Ar, de can be set as follows:
re A1.13d =
Settlement of equivalent pile, S, can be calculated by the formula for calculating the single
pile (Randolph and Wroth, 1974):
)d
2rLn(
G
)2(dτS
e
me0 ⋅⋅
=
0τ - shear stress acting along the pile
G - shear module of the soil
mr - pile effect radius.
2.1.2. Equivalent raft (Tomlinson, 1986).
The method is described in Figure 3. Settlement, S, of the equivalent raft is calculated as
follows:
s
0i
E
BqmmS
⋅⋅⋅=
4
mi, m0 - coefficients dependent on the foundation geometry, foundation depth and thickness
of compressive soil layer.
B - width of the raft.
Figure 3. Description of the method of equivalent raft.
2.2. An approximate method to evaluate the distribution of load between the raft and
piles.
Load distribution coefficient between the raft and piles, β, is a function of the number of piles,
n, and the stiffness of the raft and piles.
total
p
p
r
p
r
V
V
K
K
K
K0.81
0.21
1β =
⋅
⋅−
+
=
)V
VβR(1KpsnK
pu
totalfpip
⋅−⋅⋅=
)V
β)-(1VR(1KrK
ru
totalfrir −⋅=
pV - load transferred to piles
totalV - total load
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pK - piles group stiffness
iKps - initial stiffness of single pile
rK - raft stiffness
iKr - initial raft stiffness
puV - piles group bearing capacity
ruV - raft bearing capacity.
2.3. Accurate calculation method.
Accurate analysis of the foundation system – soil-pile-raft - is only possible by using
advanced computer programs, such as finite element software Plaxis 3D three-dimensional.
Solution takes into account the characteristics of all system elements, such as raft stiffness,
raft impact load under friction along the pile, nonlinear behavior of piles and so on.
Calculation process includes:
- Determination of the nonlinear model soil behavior.
- Imaging of single pile behavior.
- Calculation of a raft with piles behavior.
- Measuring the actual behavior of the raft-piles system during the construction process.
The software allows the combination of piles of various sizes under the same raft and
changing distance between the piles in different areas of the raft. The influence of the
distance between the axis of the pile on load distribution and settlements can also be
checked.
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3. W-TOWER Tower in Tel Aviv.
3.1. Project description.
W-TOWER Tower was built in the Bavli
district in Tel Aviv. The tower has 48 floors and
above 4 basement floors. Overall height of the
tower about 156 m.
Typical floor area is about 1100 square meters.
Project Architect - Yashar Architects Office.
Construction plans - Israel David Office.
Project Manager - Waxman Govrin Office.
Contractor - U. Dori.
Soil profile consists of layers clayey sand, sand with fines covered with kurkar (Figure 4).
Groundwater is at ±0.0 m (A.S.L).
0
5
10
15
20
25
30
35
40
0 25 50 75 100
Nspt
z, [
m]
Figure 4. Soil profile and a standard penetration test results for W-TOWER.
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3.2. Selection of foundation type.
During the designing were tested several methods of foundation.
Raft foundation.
Overall average effort under the tower is on the order of 95 tons/m .
Under the influence of forces of wind or earthquakes the raft effort will be, of course, larger.
Under elasticity sunsets above efforts are on the order of 50 m. To reduce sunsets permitted
values (3-4 cm) is required to increase significantly barge area.
Deep foundation.
Transferred loads are up to 3,500 tons per pile. Load on the piles that are in the building is
about 1000 tons per pile.
Assuming general effort allowed for the head foundational element is 600-650 tons/m,
obtained space required to transfer loads is about 165 square meters. Foundation elements
depth estimated at 20-25 m.
Raft foundation combined with piles to reduce the settlements.
According to preliminary simplified calculation, involving of 80-100 cm-diameter pile allow to
transfer to piles 50% -70% of total load. Settlement of all foundation system will be up to
about 5 cm. This method was found and selected as the most economical solution for pile
foundation system.
3.3. The analysis process with D3 PLAXIS software.
3.3.1. Choosing a soil model.
The Hardening Soil model (Schang, 1998) was chosen for calculations. The model is based
on elasticity theory, the theory of plasticity, the phenomenon of dilatation. The model
considers changes in soil stiffness as a function of strain. The main idea of the model is
hyperbolic relationship between the vertical strain, ε1, and deviatory stress, q, (see Fig. 5).
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The modulus value (limit), E50, depending on the initial elasticity module:
m
ref
'
3ref
5050sinφpcosφ c
sinφσcosφ cEE
+−
=
ref
50E - stiffness modulus referred to pressure pref=100 KPa,
c-cohesion,
φ - angle of internal friction,
0.5 <m <1 - coefficient depending on soil type.
For loading and unloading the following relation should be used:
m
ref
'
3ref
urursinφpcosφ c
sinφσcosφ cEE
+−
=
ref
urE - stiffness modulus for reloading and unloading relating to pressure pref=100 Kpa.
Figure 5. Relation between the vertical strain, ε1, and deviatoric stress, q.
Another parameter that defines the behavior of matter model is the angle dilatation, ψ. For
near-destruction conditions, the friction angle, φ, is a function of angle dilatation, ψ. In
practice it can be assumed.
ψ=φ−300
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The soil model parameter for foundation system calculation now specified as follows:
][ ψ, o m φ, [0] C,
[KPa]
ref
urE ,
[KPa]
50
refE ,
[KPa]
5 0.5 35 20 150,000 50,000
3.3.2. Single-pile model.
A simulation of the loading the pile with diameter 100 cm and a length 20 m was done with
the help of Plaxis 3D. Maximum load at the top of the pile is 1,500 tons.