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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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.

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500

Load, [ton]

Sett

lem

en

t, [

cm

]

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.

0.41

0.55

0.66

0.40

0.45

0.50

0.55

0.60

0.65

0.70

45 50 55 60 65 70 75

n מספר כלונסאות

β

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.

0.83

0.79

0.75

0.690.68

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

0 5 10 15 20 25 30 35 40 45 50 55

Number of floors

β

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

118

241

383

556

593

343368

442

537

570

0

100

200

300

400

500

600

700

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

Pile

re

actio

n, [t

on

]

Calculadet load Measured Load

Pile 44

121

240

412

585604

190

250

350

484

550

0

100

200

300

400

500

600

700

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

Pile

re

action

, [t

on

]

Calculadet load Measured Load

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Pile 48

118

239

400

567583

236

288

396

515

580

0

100

200

300

400

500

600

700

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

Pile

re

actio

n, [t

on

]

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.

0

5

10

15

20

25

30

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

Number of floors

Se

ttle

me

nt,

[m

m]

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] * 25 [m] * 250 [$/m3]= 1,100,000 [$]

600 [ton/m2]

5.3. 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.

Final cost Deep foundation cost Raft cost Foundation solution

1,300,000 1,300,000 Raft

1,650,000 1,100,000 550,000 Deep foundation

797,000 247,000 550,000 Raft and piles

6. 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.