Geotec Hanoi 2011 October ISBN 978-604-82-000-8

Piled Raft - A New Foundation Design Philosophy for High-Rises

Phung Duc Long

WSP Vietnam, E-mail: phung.long@gmail.com

Keywords: Pile foundation, piled raft, soil-structure interaction, FEM, field model test, case history, simplified
design method.

ABSTRACT: Recently it is recognized that the use of piles only to reduce the foundation settlement and
differential settlement, not to carry the whole load from the superstructure can lead to considerable savings. Only a
limited number of piles, called settlement-reducers, may improve the ultimate load capacity, the settlement
performance, as well as the required thickness of the raft. This design philosophy has also been increasingly
applied for high-rise buildings. In this paper the result from the Author’s experimental study, which strongly
supports the concept of settlement-reducers are reviewed. The experimental results are surprisingly in good
agreement with case histories many years later. Applications of FEM in design of piled-raft foundations for high-
rises are also discussed.

1. INTRODUCTION diameter bored piles, barrettes or diaphragm wall,

which are in many cases driven as deep as 80-100 m
There are three principal foundation options for high-
into the ground to reach load-bearing layers.
rise buildings: 1) Raft foundations, where the loads are
The conventional design practice for pile foundations
transferred to the ground via a foundation raft; 2) Pile
is based on the assumption that the piles are free-
foundations, where the loads are transferred to a deeper
standing, and that the entire external load is carried by
load-bearing layers via piles or diaphragm wall
the piles, with any contribution of the footing being
elements; and 3) Pile and raft foundations (PRF),
ignored. This approach is over-conservative, since the
where the high-rise load is taken partly by the raft and
raft is actually in direct contact with the soil, and thus
partly by the piles or diaphragm wall.
can carry a significant fraction of the load. The
In subsoil with good load-bearing capacity, as dense
philosophy of design is recently changing. The concept
sand and gravel, un-piled raft foundation can be the
of piled-raft foundations (PRF), in which the load from
most economic option for high-rises. The Trianon
superstructure is partially taken by piles and the
tower, which is about 190m high, and Main Plaza
remaining taken by the raft is more and more accepted.
tower, 90m high, both in Frankfurt are good examples,
The piles are designed to reduce the settlement, not to
where the settlement remained under 100 mm and the
taken the total load from superstructure. This idea of
tilting less than 1:800.
using piles as settlement-reducers was started in the
Pile foundations are necessary for cases, where the
seventies (Hansbo et al., 1973; Burland et al., 1977). In
subsoil near the ground surface has low load-bearing
the case of piled raft in clay, this philosophy has been
capacity or heterogeneous conditions. The entire high-
developed into a refined design method in Sweden.
rise load is transferred to the firm layers only by piles or
According to this design method, the building load
diaphragm wall. In such a foundation, or so-called
inducing stresses in excess of the clay pre-consolidation
conventional pile foundation, the raft is designed not to
pressure is carried by the piles in a state of creep
take any load from the superstructure. According to
failure, while the remaining load is carried by the
most standards, the piles must be designed with a safety
contact pressure at the raft-soil interface (Hansbo, 1984;
factor of 2 to 3. This requirement results in a higher
Hansbo & Jendeby, 1998). A similar approach was
number and larger length of piles, and therefore the pile
introduced in the UK by Burland (1986). Enormous
foundation is considerably expensive. Conversely, the
contributions to the development of the piled-raft
settlement of the pile foundations is unnecessarily
foundation concept have been done in Germany during
small. The conventional pile foundation is the most
the 1980’s and 1990’s. Many piled raft foundations
common solution used for high-rises worldwide,
have been constructed in the Frankfurt clay using
especially in the US, South East Asia, and Vietnam.
settlement-reducing piles for heavy high-rises (Sommer
Foundations are predominantly founded on large-
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et al., 1985; Katzenbach et al., 2003). There are also requirements. The measured settlements of different
applications in non-cohesive soil, like the Berlin sand case histories of piled rafts in comparison with
(El-Mossallamy et al., 2006). Recently, super high-rises traditional raft, as well as piled foundation are shown in
in the Gulf have often been constructed upon piled Figure 1, in which factor αL is a load factor representing
rafts. The load of the buildings is shared between the the load taken by the piles relative to the total structural
piles in shaft friction and the raft in direct bearing, with load. This figure was originally made by El-
the pile system typically carrying about 80% of the total Mossallamy (2008) and modified by the Author by
load directly into the deeper strata (Davids et al., 2008). adding the cases in Table 1. Among the 20 cases shown
For piled footings in non-cohesive soil, a systematic in Figure 1, four cases were on raft foundation, four on
experimental study of the behavior of the piled footings pile foundations, and the remaining on piled raft
with the cap being in contact with the soil surface, has foundation.
been carried out by the Author, Phung (1993). The From Table 1 and Figure 1, a clear connection can be
study shows the footing, or pile cap, in contact with the seen between the settlement and the percentage of load
soil influences considerably over the bearing capacity carried by piles: the larger the load taken by piles, the
of piles and the load-settlement behavior of a piled smaller the settlement occurs. In fact the settlement
footing. The mechanism of load transfer in a piled (maximum value, differential settlement and its pattern)
footing involves a highly complex overall interaction can be control by changing the number of piles, their
between piles, pile cap and surrounding soil, which is length as well as their layout.
considerably changed due to pile installation and to the It can be also noted that some foundations were
contact pressure at the cap-soil interface. designed as a pile foundation, but they acted as a
combined piled-raft-foundation, i.e. the raft can take
2. CASE HISTORIES OF PILE AND PILED- some part of building load. Petronas Tower in Kuala
RAFT FOUNDATIONS Lampur is a good example. The foundation was
designed according to the conventional pile method.
During the last two decades, the quick growth of cities However, a certain part of the total load was still taken
all over the world led to a rapid increase in the number by the raft. According to the measurement, 15% of the
and height of high-rise and super high-rise buildings, dead load when the structure reached the height of 34
even in unfavourable subsoil conditions. Piled raft stories, or 40% of the total tower height. This
foundation concept has been successfully applied for percentage would have been smaller once the tower
many projects, some of which are summarised in Table reached its full height. Low percentage of load carried
1. by the raft seems to be due mainly to the presence of
Systematic monitoring the load transfer mechanism the soft soil near the ground surface. ICC Tower in
in piled raft foundations were performed to verify the Hong Kong is another example. The foundation was
design concept and to prove the serviceability designed as a conventional pile foundation; however the
requirements. The piled raft foundation has been widely Author’s analysis indicated that a major part, up to 30%
applied as suitable foundation technique for high-rise of the total load, could be carried by the raft, Phung
buildings in Frankfurt to achieve economic solutions (2002).
that fulfill both the stability and the serviceability

Table 1: Pile and Piled Raft Foundation - Case Histories

No Tower Structure Load share (%) Measurement Settlement
Height, m Stories Piles Raft smax (mm)
1 Messe-Torhaus, Frankfurt 130 30 75 25 Yes N.A.
2 Messeturn, Frankfurt 256 60 57 43 Yes 144
3 Westend 1, Frankfurt 208 53 49 51 Yes 120
4 Petronas, Kuala Lampur PF) 450 88 85 15 Yes 40
5 QV1, Perth, West Australia 163 42 70 30 N.A. 40
6 Treptower, Berlin 121 55 45 Yes 73
7 Sony Center, Berlin 103 N.A. N.A. Yes 30
8 ICC, Hong Kong PF) 490 118 70 cal) 30 cal) N.A. N.A.
9 Commerzbank, Frankfurt PF) 300 56 96 4 Yes 19
10 Skyper, Frankfurt 153 38 63 27 Yes 55
11 Dubai Tower in Qatar 400 84 67 23 N.A. 200 cal)
12 Incheon Tower PF) 601 151 98 2 N.A. 43 cal)
13 Emirates Twin Towers PF) 355 56 93 cal) 7 cal) N.A. 12
Note: PF) conventional pile foundations; cal) predicted load share by calculation; N.A.= not available info

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study the load distribution along the pile length. The

lateral earth pressure against pile shaft was measured
for the central pile, by twelve Glötzl total stress cells,
installed symmetrically on all the four sides of the pile.
Displacements were measured by electric resistance
transducers. All the instruments were monitored by a
data logger.
Comparison of the results from the tests on free-
standing pile groups with those on single pile shows the
pile-soil-pile interaction, while comparison of the
results on piled footings with those on free-standing
pile groups and on un-piled footings (cap alone) shows
the pile-soil-cap interaction.

Figure 1: Raft and piled-raft foundation-Case histories

(El-Mossallamy, 2008, modified by the Author by adding
cases • 4, 5, 6, 9, 10, 11, 12 and 13 shown in Table 1).

3. EXPERIMENTAL STUDY
Figure 2. Field large-model tests set up: a) Test on a
In order to clarify the overall cap-soil-pile interaction free-standing pile group; b) Test on a piled footing
and the load-settlement behaviour of a piled footing in with the cap in contact with soil.
non-cohesive soil, three extensive series of large-scale
field model tests were performed by the Author (Phung, Table 2. Summary of the large-scale field model tests
1993). Through the study, the Author has tried to create Test Pile Group and Cap Separate tests in one test
a better understanding of the load-transfer mechanism Series Geometry and Soil series
and of the load-settlement behaviour of a piled footing square group of 5 piles T1C, footing
in non-cohesive soil, as well as the overall interaction T1 pile spacing s= 4b T1S, single pile
between the piles, the cap and soil, especially the cap: 46cmx46cmx30cm T1G, pile group
settlement-reducing effect of the piles. sand ID = 38% T1F, piled footing
Three different series of large-scale model tests square group of 5 piles T2C, footing
(denoted as T1 T2 and T3) were performed. Each test T2 pile spacing s= 6b T2S, single pile
series consisted of four separate tests on a shallow cap: 63cmx63cmx35cm T2G, pile group
sand ID = 67% T2F, piled footing
footing/cap (denoted as C), a single pile (S), a free-
square group of 5 piles T3C, footing
standing pile group (G), and a piled footing (F) under
T3 pile spacing s= 8b T3S, single pile
equal soil conditions and with equal geometry, see cap: 80cmx80cmx60cm T3G, pile group
Table 2. As an example, T2G can be understood as the sand ID = 62% T3F, piled footing
test on a free-standing pile group in Test series T2, and
TG as the tests on a free-standing pile group in all the
The results from all the three test series, which were
three series. All the three test pile groups were square,
performed for different pile group and cap geometries
and consisted of five piles: one central and four corner
in soil with different relative densities, showed the same
piles. In these tests, the following measurements were
tendency. For illustration, only the comparison of the
made: individual pile loads, total applied load, lateral
results obtained from the separate tests in Test Series
earth pressure against the pile shaft and displacement of
T2 is shown in this paper, see Figures 3. Detailed test
the footing. Axial pile loads were measured by means
results for all three test series can be found elsewhere,
of load cells at the base and the top of each pile. A load
Phung (1993).
cell was also placed in the middle of a corner pile, to

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In the figure, we can see that the load taken by cap in • The load carried by the piles in a piled footings is
the piled footing test, the curve T2F-Cap, is very close much larger than that the load carried by a free-
to the load taken by cap in the test on footing alone, standing pile group, Fig. 3.
T2C-Cap. While the load taken by piles in the piled
footing, T2F-Piles, is much larger than the load taken 3.1 Bearing capacity
by piles in the free-standing pile groups, T2G-Piles.
From the test results, the Author suggested that the
Loads taken by the cap and the average load per pile
bearing capacity of a piled footing in non-cohesive soil
are shown against the total applied load in Figure 4.
Pft can be estimated as follows:

Pft = n ⋅ (η1s ⋅ η 4 s ⋅ Pss + η1b ⋅ η 4b ⋅ Psb ) + η6 ⋅ Pc (1)

where, n is the number of piles in the group; Pss and Psb

are the shaft and base capacities of a reference single
pile; Pc is the capacity of the cap; other load efficiency
factors are defined in Table 3, with indices "s" and "b"
indicating pile shaft and base.
Table 3. Definitions of load efficiency factors
Factor Definition Comparison
η1 Pgr/ nPs TG and TS
η4 Pfp/Pgr TF and TG
η6 Pfc/Pc TF and TC

The efficiencies η1s and η1b, which show the

Figure 3. Test series T2 - Comparison of separate tests. influence of the pile-soil-pile interaction on the pile
shaft and base capacities, can be estimated by
comparing the load per pile in a free-standing pile
group with that of a single pile at a certain settlement,
e.g. s = 10 mm. The efficiency η1b can be taken as unity
for medium dense to dense sand, and higher than unity
for loose sand. The efficiencies η4s and η4b, show the
influence of the pile-cap interaction on the pile shaft
and base capacities. For a pile long enough, with a pile
length lp>2.5Bc, in which Bc is the cap width, η4b can be
taken as unity. The efficiency η6 shows influence of the
pile-cap-soil contact on the cap capacity, and can be
taken as 1.0 for loose sand and 0.9 for medium dense to
dense sand.

3.2 Settlement Ratio

The traditional concept of settlement ratio ξ is used to
Figure 4. Test T2F – Loads taken by cap and piles compare the settlement of a free-standing pile group
versus total load with that of a reference single pile. However, as
discussed by the Author (Phung, 1992 and 1993), this
From the test results, very important remarks are
ratio ξ has little practical meaning and depends very
drawn:
much on the choice of failure criterion. The Author
• When the load is applied on the piled footing, the suggested different new settlement ratios, which were
piles at first take a major portion of the load; not obtained by comparison of the settlement of a single
until pile failure a considerable portion of load is pile, a free-standing pile group, a piled footing, and a
transferred to the cap, Fig. 4; shallow footing under equal conditions, see Table 4. In
• The load-settlement curve of the cap in a piled order to avoid the confusion caused by different failure
footing is very similar to that of a cap alone, Fig. 3; criterions, the comparison is done at the same load

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level, i.e. at the same load per pile, or at the same that of the footing without piles under the same load.
applied load on footings. From these conclusions, a simplified design procedure
for piled footing in sand can be carried out with the
steps below:

Table 4. Definitions of settlement ratio factors 1) To estimate the load taken by the cap/raft without
causing excessive settlement. This load is equal to
Factor Definition Comparison that can be taken in the cap in the piled footing Pcap;
ξ1 sgr /ss TG and TS 2) To estimate the load taken by the piles:
ξ3 sf /ss TF and TS
Ppiles = Ptotal − Pcap (2)
ξ5 sf /sgr TF and TG
ξ7 sf /sc TF and TC where, Ptotal is the total applied load;
3) To determine the number of piles: As the piles are
In Table 4, ss is the settlement of a single pile, and very close to failure state, the number of piles can be
sgr, sc, and sf are the average settlement of a free- calculated as:
standing pile group, a shallow footing and a piled n = Ppiles / Ps (3)
footing under equal conditions. The ratios ξ1 and ξ3,
estimated by comparing the settlement of a pile group where, Ps is ultimate capacity of a single pile.
or a piled footing with that of a single pile, are similar In Step 1, any method for estimating settlement for
to the conventional settlement ratio ξ. These ratios have shallow footings can be used. As an example, the
little practical meaning in estimating settlement of piled following equation for square footings can be used :
footings, and are not discussed here.
Comparison of settlement of a piled footing with that 0.815 ⋅ q ⋅ B ⋅ (1 − ν 2 ) 0.815 ⋅ P ⋅ (1 − ν 2 )
s= = (4)
of a free-standing pile group leads to the ratio ξ5. The Ei Ei ⋅ B
test results show that this ratio at the same applied load
is always much less than unity. This means the fact that where, B is the width of the footing; Ei is the soil initial
due to the contribution of the cap, the increase in Young's modulus; ν is the soil Poisson's ratio; q is the
stiffness of the piles footing, as compared with the applied uniformly distributed load; and P is the total
corresponding free-standing pile groups, is concentrated load P= q·B2.
considerable. This conclusion is contrary to that drawn As a result, with a chosen (allowable) settlement s,
in most of the theoretical studies basing on the theory of the load taken by the footing can be estimated as:
elasticity (Butterfield & Banerjee, 1971; Poulos &
s ⋅ Ei ⋅ B
Davis, 1980; and Randolph, 1983). P= (5)
The ratioξ7, which is defined by comparing the 0.815 ⋅ (1 − ν 2 )
settlement of a piled footing and that of a corresponding
In Step 2, the remaining load will be taken by the
shallow footing at the same applied load, seems to be
piles. In Step 3, if we do not know about the pile-soil-
the most useful settlement ratio. This ratio means the
pile interaction factor η1 and the pile-cap interaction
reduction in settlement of a piled footing as compared
with that of a shallow footing under equal conditions. In factor η4, both the factors can be taken as unity. And
other words, this ratio shows the settlement-reducing the number of piles can be estimated by dividing the
effect due to the presence of the piles. As expected, the load taken by pile to the failure or creep load, of a
single pile. This is on the safe side because under the
ξ7-value, obtained from the tests is always lower than
cap-soil contact pressures the pile shaft resistance
unity. The ratio is smaller in looser sand than in dense
increase considerably.
sand. This ratio will be further discussed in Section 3.4.
The proposed method was exemplified for all the
three test series, and the estimated settlements were
3.3 Simplified design method
quite comparative with the measured results, (Phung,
From the test results we see that when the load is 1993). Poulos & Makarchian (1996) also used this
applied on a piled footing, the piles first take a major method to estimate the settlement of the model footing
portion of the load, and only after pile failure, the load in their study and found a fair agreement with the test
is considerably transferred to the cap. This means that results.
the piles are close to failure (with a safety factor close
to unity). We can also see that the load taken by cap in Example: To determine the number of piles to
the piled footing is very close to the load taken by cap control the settlement for a square raft footing with a
alone. This means that the load-settlement relationship width B= 40m, in a soil condition with Ei = 30MPa, ν =
of the footing in a piled footing can then be estimated as 0.3 under an uniformly distributed load q = 50kPa, i.e.

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a total load of Ptotal = 40m*40m*50kPa = 80,000 kN or

80MN. The ultimate/failure load of a single pile is
assumed Ps= 1,500 kN.

Choosing the design settlement s= 40 mm, using Eq.

(5), Pcap = 64,720 kN or 64.72 MN. The load taken by
piles is estimated using Eq. (2), Ppiles = Ptotal – Pcap= 80
– 64.72 = 15.3 MN. The number of piles needed is
calculated according to Eq. (3): n= Ppiles/Ps = 15,300kN
/1,500kN = 10 piles.
If the design settlement s= 20 mm, Pcap = 32,360
kN= 32.36 MN. The load taken by piles will be Ppiles=
47.6 MN. The number of piles needed is n= Ppiles/Ps=
47,600 kN /1,500 kN= 32 piles.
If the conventional pile design approach is used,
with a safety factor Fs=3, the number of piles needed
can be estimated: n= Ptotal/(Ps /Fs)= 80000/(1500/3)= Figure 5. Settlement ratio ξ7 versus αc , (Phung, 1993)
160 piles. The relative cap capacity αc shows the relative
contribution of a cap to the total bearing capacity of a
The above simple example illustrated that using the piled footing. With a chosen settlement of 5 mm, the αc
piled raft concept with settlement-reducing piles; the value is 0.27, 0.48 and 0.55 for Tests T1, T2 and T3,
number of piles needed to control settlement is much respectively. We can accept two extreme cases: 1) too
smaller than that needed in the conventional pile many piles (pile foundation); αc= 0, and the settlement
footing design. Moreover with a bigger settlement is close to zero, or ξ7= 0; and 2) no pile (raft
allowed, the number of piles can be reduced foundation), αc= 1, and ξ7= 1. The ratio ξ7 can then be
considerably. Poulos & Davids (2005) suggested an plotted versus the relative cap capacity αc for different
allowable settlement of 150 mm for high-rise load levels between 60% and 120% of the failure load
foundations. of the cap alone, Pcf , see Figure 5.
The simplified design method also implies that if we
know the load-settlement curve of a shallow footing and This figure shows a clear tendency that when αc is
the failure load of a single pile we can predict the load- smaller than approximately 0.5, the settlement ratio ξ7
settlement curve of a piled footing quite well. This decreases slowly with a decreasing αc value. In other
simplified design method is good enough for the words, with αc less than 0.5, a considerable increase in
concept design phase. pile capacity (induced by increasing the number of piles
or the pile length) will not lead to a significant further
3.4 Settlement-reducing effect reduction in the settlement of the footing. However,
with αc higher than 0.5, i.e. when the cap contributes a
Let us define the so-called relative cap capacity αc as major part to the capacity of a piled footing, the
the ratio between the load taken by the footing/cap to presence of piles has a clear effect in reducing the
the total applied load on the piled footing at a certain settlement of piled footings. This can also be illustrated
settlement, αc= Pc/Ptotal. Basing on the results from the by the example in Section 3.3.
three test series, the Author also tried to make a relation Figure 5 can also be used for a quick estimation of
between αc and the settlement ratio ξ7. the settlement-reducing effect. As an example, let us
assume that the cap has a capacity of 20 MN, the
settlement-reducing piles have a total capacity of
10MN. The relative cap capacity αc is therefore 2/3.
From Fig. 8, the settlement ratio ξ7 is about 0.5, which
means a settlement reduction of 50%.
It is very interesting that many years later a similar
relationship was made from case histories in Germany
(Katzenbach et al. 2003 and El-Mossallamy et al.
2006), see Fig. 6.

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must in high-rise buildings especially when they

become higher and heavier, and more complex in
configurations. There are number of commercial codes
available, both in 2D and 3D versions. The most
common softwares are:
• PLAXIS 2D and 3D, Finite Element Code for Soil
and Rock Analyses;
• FLAC 2D and 3D, Fast Lagrangian Analysis of
Continua;
• ABAQUS 2D and 3D, general-purpose nonlinear
finite element software;
• DIANNA & Midas GTS.
Numerical analysis is very effective tool for
analysing any foundation and structure system.
However, it is too complicated and time consuming to
simulate a complicated soil-structure interaction
Figure 6. Ratio sCPRF/sRF versus αCPRF problem as piled-raft foundation. There are a number of
(Katzenbach, R., Schmitt, A., Turek, J., 2003) approaches that numerical analyses can be carried out:
• Full three-dimensional (3D) analysis;
In this figure, the settlement ratio sCPRF /sRF is the ratio • Equivalent two-dimensional (2D) plain strain model;
between the settlement of a combined piled raft • Equivalent axi-symmetrical model.
foundation (CPRF) and that of a raft foundation (RF),
Full 3D numerical analyses were almost impossible
which is exactly the same definition of ξ7; and αCPRF is
for complicated foundation configurations until this
the ratio between the pile load share and that the total
decade when the softwares could be developed due to
load on a piled footing, αCPRF = Ppiles/Ptotal. Because Ptotal
faster computers. It is only recently that this technology
= Ppiles + Pc, it can be easily seen the relation between
has become a viable option to the engineers in the
αCPRF and αc, the above-defined relative cap capacity: design office. This evolution may be explained by
αCPRF = 1-αc. It is easy to get the two graphs having the several factors. Pile groups and piled rafts are
same co-ordinates by turning 180o Fig. 6. The two challenging design problems in the sense that they are
graphs are surprisingly almost identical. 3D by nature and that soil-structure interaction is
central to the behaviour of deep foundations. Although
4. DESIGN APPROACHES the background theory and the numerical tools
In the last decades, there has been considerable necessary to model such deep foundation systems have
development of methods of calculating settlement for been available for years, it is only in the last few years
(free-standing) pile groups and piled footings, several that available commercial softwares have reached a
of which are suggested to be used for piled-raft degree of maturity and user friendliness necessary to
foundations. However, most of the methods are based meet the needs of the design office.
on the theory of elasticity and are therefore unsuitable
for piled footings with settlement-reducing piles, 4.1. Equivalent axi-symmetrical modelling-Analysis
especially in non-cohesive soil. of Piled Raft Foundation of ICC Tower
Piled raft foundation is a complicated soil-structure This is an example of simulating a piled-raft foundation
interaction problem. Many methods of analyzing piled using the simplified numerical approach: equivalent
rafts have been developed, and can be classified to four axi-symmetrical model, performed by the Author,
broad groups: 1) Simplified calculation methods; 2) (Phung, 2002). ICC Tower in Hong Kong is nowadays
Approximate computer-based methods; 3) More the fourth tallest building in the world with a height of
rigorous computer-based methods; and 4) Accurate 484m and 118 stories. The foundation for the tower has
numerical methods, as FEM. The methods were a circular plan, and consists of 240 shaft-grouted
reviewed and discussed elsewhere (Phung, 1993; and barrettes (2.8m x 1.5m or 2.8m x 1.0m) within a
Poulos, 2001). circular perimeter shaft-grouted diaphragm wall (DW),
For practical design, a conceptual design must be see Figure 7.
done first using simplified and less time-consuming
methods, especially for feasible foundation option
study. Detailed design of piled raft foundation for high-
rises should however be done by numerical analyses
using FEM or explicit finite difference codes. This is a
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were modelled as a linear elastic material with a long-

term elastic modulus E for concrete. Soils were
modelled as elasto-plastic materials with Mohr-
Coulomb failure criterion, see Figure 8.

Figure 8. ICC Tower-Axisymetrical modelling

foundation using Plaxis 7, Phung (2002)

The settlement at the raft bottom level is about 40mm at

the centre and 9mm at the DW edge. This compares
quite well with the project engineer’s settlement
estimation. The loads at the head of the pile rings were
calculated and the results show that the central piles
carry higher loads than the boundary piles. The
foundation was designed as a conventional pile
Figure 7. ICC Tower-Foundation plan foundation, but the Author’s analysis indicates a major
part, up to 30% of the total load, can be carried by the
Below the raft, the soil profile consists of alluvium and raft. It is quite common that the foundation is designed
CDG overlying rock. Within the basement area, as a pile foundation, but acting as a combined piled-
rockhead level varies between -61mPD and -106mPD raft-foundation.
under ground surface. In order to minimise differential
settlement, the barrettes and DW panels are generally 4.2 3D Modelling
placed at a depth of about 2m above rockhead. The
barrettes have thus a length varying between 35m and 3D numerical analysis is nowadays commonly applied
70m. An 8m-thick base raft connects the barrettes and for modelling high-rise and super high-rise foundations.
the DW. The excavation, 26m deep, is required for the 3D analysis with appropriate soil constitutive laws is a
construction of the 4-level basement and the pile cap. powerful tool to model complex piled-raft foundation
The foundation was designed by the project problems. However, the main disadvantage with
engineers, as a conventional pile foundation, using the applying the 3D analyses is the need of a huge number
finite element program SAFE. Their design is not of volume elements which can exceed the available
discussed here. The Author, as the independent verifier, computer capacities. To cover this problem, a new
re-simulated the foundation using the FEM code technique combined the so called embedded pile model
PLAXIS Version 7.2, Phung (2002). The analysis is with the 3D finite element model was developed by
based on an axi-symmetric model with the barrettes and Plaxis. Figure 9 shows an example of FE model for a
DW simulated as equivalent concentric rings. The piled raft with more than 600 piles using Plaxis 3D
objective of the analysis is to study the settlement Foundation Version 2 (Schweiger, 2008; Brikgreve,
behaviour of the foundation system, the load sharing 2008).
between the foundation components, the barrettes, the 3D modelling is also needed to simulate foundations
DW panels and the raft. The 240 barrettes were subjected to a combination of vertical, lateral and
modelled as 8 circular concentric rings representing the overturning forces, which are real 3D problems.
same surface areas of the barrettes. The barrette rings
were modelled as a linear elastic material with an
equivalent Young’s modulus for bending E1, and an
equivalent Young’s modulus for axial loading E2. The
DW was also included in the model as a ring. This
allows the DW to carry a part of the load as a
component of the pile group. The DW and the raft

Figure 9. 3D-Modelling a piled-raft using Plaxis 3D

Foundation (Schweiger, 2008; Brikgreve, 2008).
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foundations. Proc. 10th Int. Conference on Piling and

Deep Foundations, 31 May-2 June, Amsterdam
5. CONCLUSIONS
El-Mossallamy, Y. (2008). Modeling the behaviour of
Piled-raft foundations, in which piles are designed to piled raft applying Plaxis 3D Foundation Version 2.
reduce the settlement, not to taken the total load from Plaxis Bulletin, Issue 23, March, Delf.
superstructure, have been increasingly used for high- Hansbo, S., Hofmannn, E., Mosesson, J. (1973). Östra
rises. However, predicting the settlement for piled-rafts Nordstaden, Gothenburg. Experience concerning a
is a difficult task for geotechnical engineers due to the difficult foundation problem and its unorthodox
complex pile-cap-soil interaction. The available solution. Proc. 8th ICSMFE, Moscow, Vol. 2, 105-
prediction methods, which are based on the theory of 110.
elasticity, are not suitable for piled-raft foundations Hansbo, S. (1984). Foundations on friction creep piles
with settlement-reducing piles, especially in non- in soft clay. Proc. Int. Conf. on Case Histories in
cohesive soil. Results of the experimental study, Geotechnical Engineering, St. Louis, Vol. 2, pp. 913
performed by the Author, have created a better -922.
understanding about the load-transfer mechanism of Hansbo, S. (1993). Interaction problems related to the
piled footings in sand, as well as the load-settlement installation of pile groups. Proc. 2nd Int. Geotech.
behaviour. The study strongly supports the idea of Seminar on Deep Foundations on Bored and Auger
settlement-reducing piles. The simplified methods Piles, Ghent, Belgium, 59-66
suggested in this paper can be used as a practical design Hansbo, S. & Jendeby, L. (1998). A follow-up of two
procedure, especially in the foundation option study different foundation principles. Proc. 4th Int. Conf.
phase. Detailed design for foundation of high-rises must on Case Histories in Geotech. Engng, March, St.
include 3D modelling, which can be realised by Louis, Missouri, 259-264.
different commercially available computer codes. Katzenbach, R., Moormann, Ch. (2003). Instrumen-
tation and monitoring of combined pile rafts (CPRF):
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