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Geotechnical Aspects for Design and Performance of Floating Foundations
S. Mohsenian1, A. Eslami2 and A. Kasaee3
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1
Geotechnical Engineer, Department of Civil Engineering, Guilan University, IRAN;
email: s_mha2004@yahoo.com
2
Associate professor, Department of civil Engineering, Amirkabir University of
Technology, Aut, Tehran, IRAN; PH (98) 21-64543057; email: afeslami@aut.ac.ir
3
Structural Engineer, Department of Civil Engineering, Guilan University, IRAN; email:
adelkasaee@yahoo.com
ABSTRACT
Floating foundations are often practical when foundations must be placed over
deposits of compressible soil and when deep foundations are expensive. The aim of the
present paper is to study parameters affecting the behavior of floating foundations,
including the kind of floating foundations, depth of excavation, and imposed pressure by
the superstructure. Herein, 3 buildings ranging from 15 to 25 stories with gross
foundation pressures from 195Kpa to 342Kpa with similar area plan, 40m × 50m, are
studied for 4 soil types ranging from very soft to strong type. Their foundations are
studied by using numerical method of finite element. Moreover five case histories have
been studied based on their records and analytical predictions. Results indicate that in
case of low bearing capacity and high settlement, the floating foundations can be chosen
as a reasonable alternative considering of technical and economical aspects.
INTRODUCTION
The building load can be carried on spread or strip footings, on a raft or mat
foundations, or on piles. If the soil is weak and for the given load there is a risk of shear
failure below the foundation, then the loads must be reduced, carried to a stronger
underlining layer by piles. Alternatively, the applied load can be reduced utilizing a
floating foundation, in which a total or partial superstructure load can be compensated by
excavating the soil.
The technique of reducing the total and differential settlement of a foundation by
decreasing the net applied load by excavation is called floatation. When the weight of a
structure is partly compensated by excavation, the building is said to be partially floating;
when it is entirely compensated the building is fully floating. Floatation schemes are
often practical when foundations must be placed over deep deposits of compressible soil
and when deep foundations are expensive. If basement space is required, floatation is
particularly attractive.
Although the idea of floating foundations was known in the early nineteenth
century, the principle was little used until the early twentieth century.
The first reference to the concept of a floating foundation in the technical
literature is Albion Mills built in 1783-86 in London. The building of Empress Hotel in
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Victoria, British Colombia is the first case in which the principle was consciously used,
although was not used in the original design (Crawford et al., 1971). The first built
example of a floating foundation in the literature is the Ohio Bell Telephone Company
Building constructed in 1925 in Cleveland. The Albany Telephone Building, which was
built in 1929, is the first fully documented case of a foundation in which the principle of
floatation was deliberately used in design (Glick, 1936). The most interesting cases come
from Mexico city, where the problem is complicated by the general area settlement of the
ground surface, such as Tower Latino Americana building (Zeevaert, 1957), where a
group of 8 buildings between 18 to 22 m high including basement and an 86 m high
tower were constructed over a 7 m deep partially floating box foundation (Rodriguezet et
al., 2009).
In this paper the parameters affecting the behavior of floating foundations are
studied by a numerical method, including the kind of floating foundations, depth of
excavation, and the pressure that is imposed by structures. Additionally, the records for
five case histories will be reviewed.
PERFORMANCE OF FLOATING FOUNDATION
The principle of a floating foundation involves balance of weights only, and the
foundation itself can consist of spread footings, a raft, a box, piles, or cylinders, or in
some cases a combination of these.
Problems of design and construction of floating foundation consist of excavation,
water lowering, bottom heave, settlement, and structure problems.
Excavation for a floating foundation involves the complete removal of the
pressure originally against the soil at the base level of the foundation. As a consequence,
the bottom of the excavation rises. During the subsequent period of construction, the
weight of the building becomes equal to and on occasion exceeds the original overburden
pressure; hence, the heave disappears, and the building settles. If the building has a
greater weight than the excavated soil, the settlement passes through two stages. The first
lasts until the load per unit of area at the base of foundation becomes equal to the original
overburden pressure, and the second begins when this pressure is exceeded, i.e.
recompression and virgin compression. The amount of heave and subsequent settlement
depend upon the nature of the subsoil and the depth of excavation.
The function of a raft foundation is to spread the load over as wide as possible,
and to give a measure of rigidity to the sub-structure to enable it to bridge over local
areas of weaker or more compressible soil. The degree of rigidity given to the raft also
reduces differential settlement. Floating foundations are designed on the same principles,
but they have additional and important function in that they utilize the principle of
floating to reduce the net load on the soil. In this way the total settlement of the
foundation is reduced and follows that, differential settlements will also be reduced. In
view of spreading load of structure over the area, floating foundations include
establishing raft foundations in depth which can be supported by retaining walls and also
combination of raft and basement floor slab with walls (floating box foundation). In
floating box foundation the vertical load is transferred from the upper structure to the
basement through structural walls, and the maximum bending moment in the raft slab are
used to define the reinforcement of this slab, due to the contact pressures, as the same as
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design the slab of raft floating foundation. The upper slab is actually reinforced for loads
due to car parking or warehouse storage. Floating foundations allow the substructure to
be used for various purposes such as warehouse storage or underground car parks. This
requires reasonably large floor areas without close-spaced walls or columns, and the floor
generally consists of a slab or slab and beams of fairly heavy construction to give the
required degree of rigidity.
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MODELING APPROCH FOLLOWED
For studying parameters affecting the behavior of floating foundations, 3
buildings ranging from 15 to 25 stories with 40 m × 50 m in plan with 10 m column
spans are examined for 4 types of soil. The soil properties are shown in Table 1. The soil
thickness beneath the foundations was 80 m, and the groundwater level was at the ground
surface. The pressures at foundation level were 195, 264 and 342 Kpa for 3 buildings
with 15, 20 and 25 stories respectively. The overall thickness of floating box foundations,
were 3.6, 3.8 and 4 m with 0.4, 0.6 and 0.8 m thick raft slab and 0.3 m thick upper slab
for them. The thicknesses of raft of floating raft foundations for buildings with 15, 20 and
25 stories were 1.8, 2.1 and 2.3 m respectively.
Table 1. Soil properties used in numerical analysis
Soil type
1
2
3
4
Saturated density, KN/m3
20
20
18
18
19.5
19.5
17
17
Friction, degree
35
30
25
18
Drained shear strength, Kpa
50
30
15
15
Poison ratio
0.3
0.3
0.3
0.3
Elasticity index, Mpa
50
35
20
5
Unsaturated density, KN/m3
The foundations are studied by using numerical method of finite element. The
calculation stages of numerical method consist of initial conditions, excavation stage,
construction of the basement, and activation of loads.
EXCAVATION DEPTH AND KINDS OF FLOATING FOUNDATION
To assess the effect of excavation depth and the type of floating foundation on the
amount of settlement, the raft and box foundations were floated to various depths from
the surface of the ground, in 4 types of soil. Each case was analyzed for the maximum
settlement. The vertical displacements were used directly; with a downward vertical
displacement considered as positive. The retaining wall depth was 2 m greater than
excavation depth and the thickness of it was 1m. The diagrams of maximum settlements
of foundations with the retaining walls for 3 groups of construction with 15, 20 and 25
stories floated in 4 types of soil are shown in Figures 1-a to 1-c. At a superficial depth of
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ground, the amount of maximum settlement would be high, and by increasing the
excavation depth the settlement could be reduced to allowable limits.
The reasons for this case can be mentioned in this manner that after removal the
overburden pressure in the result of excavating and replacing with the imposed pressure
from the structure, the amount of remaining net pressure at the base of foundation will be
reduced. By decreasing is the weight (the reason for the settlement), the amount of
maximum settlement can be reduced. After all, by increasing the excavation depth, the
amount of settlement will reach to zero. This depth is equal to the full floating depth (i.e.
the imposed pressured from the structure has been equivalent with the equal pressure of
excavated overburden pressure and the amount of the remaining net pressure will be
negligible at the base of foundation and the amount of the settlement). Also, as shown in
Figure 1, after achieving the full floating depth in the case of foundation installing at the
lower depth, the bottom of the excavation rises, because of the reduced pressure imposed
from the structure with the pressure of excavated overburden pressure. This depth is not
suitable for foundation establishment. Moreover, it is notable that for each group of the
constructions floated in each type of soil, types 1 to 4, the type of raft and box
foundations was not very influential on the amount of total settlement and also in depth
of full floatation. Really, in the similar conditions in the manner of excavation depth, the
imposed pressure from the structure and conditions of subsoil, the amount of maximum
settlements are almost similar in two types of foundation.
1-a
1-b
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1-c
Figure 1. Effect of excavation depth and kind of floating foundation with retaining
wall a) 15 storey building b) 20 storey building c) 25 storey building.
The information related to the depth of fully floating and allowable settlement has
been shown in Table 2.
Table 2. Depth of fully floating and depth to reach to allowable settlement
Depth for, m
15 story
20 story
25 story
fully floating, m
Satisfy settlement, m
fully floating, m
Satisfy settlement, m
fully floating, m
Satisfy settlement, m
soil 1
plain box
11
11
6
5
14.5
15
8.5
7.5
18.5
19
12
11.5
soil 2
soil 3
soil 4
plain box plain box plain box
11
11 11.5
12
12.5
12
6.5
5.5
8.5
7
10.5
9.5
14.5 15 15.5 15.5 15.5 15.5
10
8.5 11.5 10.5 13.5 12.5
18.5 19
20
20
20
20.5
13
12 14.5
14
16.5
16
PRESSURE IMPOSED BY STRUCTURES
The diagrams of maximum settlements of the floating foundations with retaining
walls under the impression of pressure of construction with 15, 20 and 25 stories in four
types of soil have been shown in Figures 2-a to 2-d. As illustrated in Figures 2-a to 2-d,
considering that the excavation depth is fixed, by increasing the number of storey, the
amount of maximum settlement will be increased, and the foundation will get to full
floating in more depth. This is because of this case that by increasing the storey of the
building, the imposed pressure from them will be increased. Considering that the
excavation depth is fixed, by increasing the amount of imposed pressure from the
structure, the amount of the remaining of the net pressure on subsoil will be increased.
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7-a
7-b
2-a
2-b
2-c
2-d
Figure 2. Effect of pressure imposed by 15 to 25 storey buildings on different soils
a) Type 1 b) Type 2 c) Type 3 d) Type 4.
CASE STUDIES
Followings are the brief history of some typical cases of floating foundations to
confirm the results of finite element analysis to measured values. The predicted
settlement of the cases with finite element analysis, are shown in Table 4.
Case No. 1 [Somerville et al., 1972]
The sites proposed for six blocks of 15 storey flats in the Parkhead and Bridgeton
districts of Glasgow are underlain by deep alluvial deposits consisting of firm laminated
clays and silts, with deeper deposits of sands, gravels and glacial drift followed by the
productive coal measures. Accordingly, partially floating box foundations were adopted
to reduce the bearing pressure on the clays and silts. In this paper we studied one block of
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Bridgeton (BBS) of this Complex. The building, 27 m × 16 m in plan, is located 4.3 m
below the ground surface. The average gross unit load transmitted to the foundation is
135 KPa. The average net unit load transmitted to the foundation is 50 KPa. The
observed maximum settlement for the block at the Brigeton is 6 cm.
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Case No. 2 [Brown, 1972]
The Life Science building (LSA) is a reinforced concrete frame with curtain-wall
construction consisting of two basements, eight floors. The building is located on the east
end of the MIT campus. The structure measures 66.5 m × 18 m in plan, with gross weight
of the structure amounts to intensity of area load of 213 KPa. The subsoil consists of fill,
silt, sand, gravel, Boston blue clay and till. The foundation was designed as a fully
floating foundation, located in 10 m below ground surface. The observed maximum
settlement for the building is about 0.3 cm.
Case No. 3 [Brown, 1972]
The site of the Student center is located on the MIT campus (SCA). The center is
a 5-story reinforced concrete frame structure, with one basement, measuring about 43 m
× 71 m in plan. The gross unit weight of the structure distributed uniformly over the
foundation is about 107.5 KPa. The subsoil consists of fill, silt, sand, gravel, Boston blue
clay and till. The foundation was designed as a fully floating foundation, located in 5 m
below ground surface. The observed maximum settlement for the building is 0.
Case No. 4 [Lopes et al., 1994]
A reinforced concrete structure was built to house two large emergency dieselpower generators at the Angra dos Reis nuclear power plant in the state of Rio de Janeiro,
Brazil (PGB). The structure is rectangular in plan (16.6m × 27m) and is 9 m high with
three 0.5 m thick external walls. The total structural loading, including the power
generators and other installations, is 55 MN, Which corresponds to an average vertical
contact pressure of 123 KPa. The foundation was designed as a box foundation of overall
thickness 4.5 m, with a 1.3 m thick ground slab and a 0.4 m thick upper slab. The subsoil,
include medium-dense sand, was investigated by dynamic standard penetration tests,
cross-hole and laboratory tests. The subsoil profile is shown in Figure 3. The observed
settlement for the building after 3 years of construction was 2 cm.
Case No. 5 [Wong et al., 1995]
The Raffles City Complex, constructed in Singapore, is a comprehensive property
development comprising four high-rise structures and a 7-storey podium. There is a three
storey basement beneath the whole complex. The four towers are framed reinforced
concrete structures with internal concrete shear walls. The four towers in Raffles City
Complex are supported on individual floating raft foundations. In this paper we studied
Tower C (TCS) of this Complex. The subsoil consists of fine to coarse sand fill from the
ground surface down to depth 4 m depth and of the Boulder Clay down to a depth of
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more than 70 m. the engineering properties of the clay is given in Table 3. The water
table is located about 2 m below the ground surface. The 2.8 m thick raft for the tower
which is 57.7 m × 60.8 m, is located 17.5 m below the ground surface. The average gross
unite load transmitted to the foundation is 439 KPa. The average net unit load transmitted
to the foundation is 201 KPa. The settlement of the tower was monitored at regular
intervals during the construction. The maximum settlement of the tower is 4.8 cm.
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Table 3. Soil parameters for the Bouldery Clay (Wong et al., 1995)
project
Raffles City complex
Water content (percent)
10-20
Bulk density (KN/m3)
20-23
Young’s modulus (MPa)
Undrained shear strength
(KPa)
40-150 (95)
70-1150 (195)
Figure 3. Subsoil profile of reinforced concrete structure, case No. 4 (Lopes et al.,
1994)
Table 4. Cases built on floating foundation
foundation
NO.
Case
Location
type
depth
(m)
Area
(m2)
pressure
(Kpa)
number
of
stories
Soil
profile
Silt &
clay
Boston
blue clay
Boston
blue clay
observed
settlement
(mm)
Predicted
settlement
(mm)
60
67
3
7
0
7
1
BBS
Scotland
Box
4.3
27*16
135
15
2
LSA
America
Raft
10
66.5*18
213
10
3
SCA
America
Raft
5
43*71
107.5
6
4
PGB
Brazil
Box
4.5
16.6*27
123
-
Sand
20
26
76
Bouldery
clay
48
57
5
TCS
Singapore
Raft
17.5
57.7*60.8
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CONCLUSION
In this paper the parameters affecting the behavior of floating foundations have
been studied by numerical method. Also five case histories were compiled and the
measured settlement compared with predicted values. The following results have been
achieved:
(1) Increasing the excavation depth will be reduced the amount of maximum
settlement foundations, because of reducing the net pressure on the subsoil.
(2) Amount of maximum settlements will be high at the depth of ground surface
and the settlement will be reduced to allowable limits by increasing the
excavation depth. After all, by increasing more excavation depth, the fully
floating depth is reached.
(3) Increasing the number of storey, considering that the excavation depth is
fixed, amount of maximum settlement will be increased and the foundation
will reach to fully floating in more depth, because of increasing the net
pressure on subsoil.
(4) The predicted settlements of some typical case histories of floating foundation
with finite element method are compared with the observed settlements of the
cases and are within the observed settlement.
REFFERENCE
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FIStructE, 569p.
Wong, I.H., Ooi, I.K., and Broms, B.B. (1996). "Performance of raft foundations for
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