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
R pile, i
i=1
Stot
β=∑
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:
d e = 1.13 A r
Settlement of equivalent pile, S, can be calculated by the formula for calculating the single
pile (Randolph and Wroth, 1974):
S=
τ 0 ⋅ (d e 2)
2r
⋅ Ln( m )
G
de
τ 0 - shear stress acting along the pile
G - shear module of the soil
rm - 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=
mi ⋅ m0 ⋅ q ⋅ B
Es
3
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.
1
β=
1+
K 
0.2
⋅ r 
 K   K p 
1 − 0.8 ⋅  r  
K 
 p
K p = n ⋅ Kpsi ⋅ (1 − R fp
K r = Kri ⋅ (1 − R fr
Vp - load transferred to piles
Vtotal - total load
4
=
Vp
Vtotal
β ⋅ Vtotal
)
Vpu
Vtotal (1 - β)
)
Vru
K p - piles group stiffness
Kps i - initial stiffness of single pile
K r - raft stiffness
Kri - initial raft stiffness
Vpu - piles group bearing capacity
Vru - 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).
Nspt
0
25
50
75
100
0
5
10
z, [m ]
15
20
25
30
35
40
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:
 c cosφ − σ 3' sinφ 

E 50 = E 
ref
 c cosφ + p sinφ 
m
ref
50
ref
E 50
- 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:
 c cosφ − σ '3sinφ 

E ur = E 
ref
 c cosφ + p sinφ 
m
ref
ur
ref
E ref
ur - stiffness modulus for reloading and unloading relating to pressure p =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:
E 50
ref ,
E ref
ur ,
C,
[KPa]
[KPa]
[KPa]
50,000
150,000
20
φ, [0]
m
ψ, [ o ]
35
0.5
5
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.
Load, [ton]
0
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
1500
0.00
0.01
0.02
0.03
Settlement, [cm]
0.04
0.05
0.06
0.07
0.08
0.09
0.10
Figure 7. Calculation result of the settlement of a single pile.
The pile was modeled like a volumetric model, so you can get the load distribution along the
pile. Based on the above result, it was possible to simplify the system model by using the
foundation piles that are not volumetric, with known bearing capacity - embedded piles.
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3.3.3. The raft model and determination the number of piles.
For the
analysis,
parametric
raft model
was
built,
so
only
half a
barge
could
be calculated. Pile dimensions were selected as 100 cm diameter and 20 m length. These
piles were
implemented with continues
flying
auger (CFA), so
the
work
was
fast
and relatively low cost.
After checking a system that includes a different number of piles (different distances
between the axes), it was decided to plan a raft with 63 piles, i.e. the grid of 4.0/5.0 m.
0.70
0.66
0.65
β
0.60
0.55
0.55
0.50
0.45
0.41
0.40
45
50
55
60
65
70
75
n ‫מספר כלונסאות‬
Figure 8. Relative amount of load passed to the piles, β, as a function of the number of piles.
Figure 9. A raft with the piles.
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Figure 10. The computer model.
Development load on the piling during construction is interesting (Fig. 111. According to the
model, for low loads level (9 floors), about 85% of the load goes to piles. Later, with the
settlement development, part of the load passes to the raft, and on the final stage load goes
to piles is about 68% of total load.
1.00
0.95
0.90
0.85
β
0.83
0.80
0.79
0.75
0.75
0.70
0.69
0.68
0.65
0.60
0
5
10
15
20
25
30
35
40
45
50
55
Number of floors
Figure 11. Load distribution changes with the progress of construction of the building.
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4. Monitoring system behavior during construction.
4.1.
Loads on piles.
Strain gages were installed along the reinforcement of three piles. For each pile, the pair
of strain gages was installed at level close to connection with the raft. It allows measuring the
load delivered to the piles. The measured load versus the calculated load delivered to the
piling is described on graphs below.
Pile 39
700
600
556
537
593
570
Pile reaction, [ton]
500
442
400
343
383
368
300
241
200
118
100
0
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56
Calculadet load
Measured Load
Pile 44
700
600
585
604
550
Pile reaction, [ton]
500
484
412
400
350
300
250
240
200
190
121
100
0
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56
Calculadet load
Measured Load
12
Pile 48
700
600
567
515
500
Pile reaction, [ton]
583
580
400
400
396
300
288
236
239
200
118
100
0
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56
Calculadet load
Measured Load
Figure 12. The measured load versus the calculated load.
4.2.
Settlements.
The measured settlement of the tower at the end of the construction process is 22 mm.
As we see (Figure 13), there is a good match between predicted and measured values.
Number of floors
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52
0
5
Settlement, [mm]
10
15
20
25
30
Calculated
Measured
Figure 13. Calculated and predicted settlement of piled raft foundation.
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5. Comparing costs for different foundation solutions.
5.1.
Estimated cost of the raft foundation.
Raft area needed to reach the allowable stress of 40 ton/ m2 (no tensile stress in dynamic
calculation), is about 2,600 m2. Thickness of the raft is about 2.0 m. The estimated cost of the
raft as follows:
2,600 [m2] * 2.0 [m] * 250 [$/m3] = 1,300,000 [$]
5.2.
Estimated cost of deep foundation (piles).
Assuming the depth of drilled elements is about 25 m and the overall stress to head
foundation elements is about 600 t/m2, the obtained cost of piles foundation is:
2,640,000 [ton]
600 [ton/m2]
5.3.
* 25 [m] * 250 [$/m3]= 1,100,000 [$]
Estimated cost of piled raft foundation.
The piles are designed only to reduce the settlement, so that their security factor is minimal.
Raft cost
1100 [m2] * 2.0 [m] * 250 [$/m3]= 550,000 [$]
Piles cost
63 * 15.7 [m3] * 250 [$/m3]= 247,000 [$]
Total cost
797,000 [$]
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The costs for different foundation solutions is summarizes in the following table.
Foundation solution Raft cost Deep foundation cost Final cost
6.
Raft
1,300,000
1,300,000
Deep foundation
550,000
1,100,000
1,650,000
Raft and piles
550,000
247,000
797,000
Conclusions.
Foundation system of raft with piles for settlement reduction was found as economic,
especially for structures with high aspect ratio height/ floor area. For such structures, for
example W-TOWER in Tel Aviv, raft foundation with an area of the raft more than the floor
area or a deep foundation is requires.
Economic priority of a piled raft foundation can be easily calculated.
It is found that the match between the calculated and measured values for load delivered to
the piles and the settlements are good.
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