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Underground structural design harmonized with high quality urban underground space

Underground structural design harmonized
with high quality urban underground space
Thiết kế kết cấu công trình ngầm hài hòa với không gian ngầm chất lượng cao
Shunji Ito(¹), Tran Kim Khoa(²), Tatsuo Yamada(3), Masayuki Muraki(4)

Tóm tắt
Hiện nay không gian ngầm cần phải càng ngày
càng hấp dẫn và thu hút người sử dụng hơn,
không gian ngầm cần đem lại sự thoải mái với
các mái lấy sáng và các khoảng trần mở nhằm
cải thiện chất lượng cuộc sống và phát triển đô
thị bền vững đặt biệt ở các đô thị châu Á. Thiết
kế kết cấu vì vậy cần phải đáp ứng được các
yêu cầu của không gian ngầm chất lượng cao.
Những vấn đề chính yếu của thiết kế kết cấu
cần được lưu ý như ổn định chống lại áp lực đẩy
nổi của nước ngầm hay gia cường hệ kết cấu
để chịu được áp lực ngang của đất và của nước
ngầm. Những vấn đề kỹ thuật này cùng các giải
pháp được nghiên cứu và đề xuất trong phạm
vi bài báo.

Từ khóa: ngầm, phát triển đô thị bền vững, không gian
rộng lớn, nước ngầm, ổn định kết cấu, lỗ mở sàn

Abstract
Currently, the urban underground space is required
to be more attractive and comfortable with top light
and open ceiling for the improvement of quality of life
and sustainability of urban development especially
in Asian cities. In this situation the underground
structure shall be harmonized with this underground
space planning. The major structural issues are
stability against uplift force by groundwater and
balance of lateral force by earth and water pressure.
These engineering issues are studied and their
solutions are proposed.
Keywords: underground, sustainable urban
development, soaring space, groundwater, structural
stability, slab opening

(1) Head of Industrial Facilities Division,
Nikken Sekkei Civil Engineering Ltd.,
Email:
(2)Urban Infra & Engineering Div., Nikken
Sekkei Civil Engineering Ltd.,
Email:
(3)Urban Infra & Engineering Div., Nikken
Sekkei Civil Engineering Ltd.,
Email:
(4)Urban Infra & Engineering Div., Nikken
Sekkei Civil Engineering Ltd.,Email:muraki.
masayuki@nikken.jp(1)

1. Introduction
In recent years, we can see rapid progress of urban development especially
in major cities of Asia along with economic growth. Development of infrastructure
such as subway/ highway, and construction of commercial/ office-use building
are prominent, which leads to high concentration and diversification of urban
functions. Those urban development provides improvement of quality of life and
makes the urban area more attractive and comfortable for the people living and
working there.


On the other hand, there are limited areas in the growing urban cities for
the development projects of private sectors, so that it is very important to utilize
public areas such as roads and parks more efficiently. Especially, the utilization
of underground space such as underground malls plays a key role in high quality
urban development, for it reduces traffic jam, and it also contributes convenient
linkage by formulating underground pedestrian network including the connection
to subway stations. Therefore, underground space in urban areas is extremely
valuable and effective for urban development to improve the quality of life, and
sustainable underground planning is desired to create more safe, attractive, and
comfortable underground space.
2. Current Urban Underground Space Planning
Underground development including underground shopping mall beneath
public space such as roads and parks is important and notable in the field of
urban development. More comfortable and attractive underground space
planning is needed for sustainable urban development. Moreover, since people
tend to get lost more likely than above ground due to less landmarks such as the
sun and high-rise buildings in underground space, easy-to-follow floor planning
and spacious underground space planning is needed. In addition, underground
space planning with high ceiling enables to store smokes in the event of fire
at the underground space where no windows are placed, so that people can
evacuate safely. Therefore, recent underground planning is likely to place large
space underground including soaring space and top-light, and it enables to
create attractive and comfortable urban space.
“Diamor Osaka” located in front of Osaka station is one of major underground
shopping malls in Japan which is famous with the spacious underground space
equipped with top light as shown in Figure 1. The top light standing on the grade
brings the sun light through its glass roof into the underground. In addition, when
the movable roof opens, the natural wind blows underground. These makes the
underground space bright, spacious, relaxed, and comfortable. Furthermore, in
the event of fire since the opened roof vents the smoke, the people can evacuate
safely, so that the relieved urban underground is created.
The underground shopping mall planning in Ho Chi Minh is shown in Figure
2. In this planning the bright and spacious open air with top light standing on
the grade creates the comfortable underground space. Figure 3 shows the
perspective view for Ben Thanh station of Ho Chi Minh Metro line. In this design
large soaring space connects the concourse on B1 floor with the platform on
B2 floor. This high quality underground space welcomes the passengers in the
centre of the city.
3. Structural Issue Harmonized with High Quality Underground Space
In order to create high quality urban underground spaces, large openings
and spacious open air are normally arranged in underground structures. Due
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KHOA H“C & C«NG NGHª

Figure 2. Underground Shopping Mall Planning in Ho Chi
Minh

Figure 1. Underground Shopping Mall
“Diamor Osaka”

Figure 3. Ben Thanh Station of Ho Chi Minh Metro Line
to these space plannings integrated into underground
structures, some structural issues have to be dealt with. In
this section, some major issues and solutions are described
such as stability of underground structures against uplift
forces and reinforcing the structures against lateral forces.
3.1. Stability of Underground Structure
In general the underground structure weight is lighter
than removed soil, so that occurrence of settlement has less
possibility. Contrarily, in the high groundwater level area
such as Ho Chi Minh City, stability against the uplift force
by groundwater becomes much more critical. According to
Japanese Design Standards for Railway Structures and
Commentary (Cut and Cover Tunnel: 03/2001) (DSRSC-CT),
for cut-and-cover tunnels constructed below the groundwater
level, an examination of uplift is needed. Equilibrium can be
examined as indicated below by using a sum of tunnel selfweight and vertical load, and the uplift pressure caused by
hydrostatic pressure.
(1) Counterweight Method
In cases of large openings and spacious open air
integrated into underground structures, WB in equation (1) is
significantly reduced, and the top light makes vertical loads
of overburden Ws much smaller. In order to overcome this
issue, additional weight may be added into the structure
such as adding concrete to bottom slab. Consequently, WB
can be increased to achieve the stability check shown in
Equation (1). Although this counterweight method makes the

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excavation deeper for adding concrete, since this is the most
simple and easy way with the weight balance, the structural
reliability is higher.
(2) Earth Anchor Method
Recently, earth anchor method as shown in Figure 6
is also used to resist underground structures against uplift
forces. In this method, earth anchors are installed into hard
soil layer and connected to the structures. If RE is sum of
anchoring forces by earth anchors, Equation (1) can be rewritten as shown in Equation (2). Earth anchor system can
be designed to provide enough anchoring force RE to satisfy
the check in (2). In this method there are some engineering
issues, which are the acquirement of stable tension strength,
the anticorrosion treatment of tendons, and the maintenance.
In addition, as the connection detail of anchor and concrete
slab is complicated, the higher construction skill and quality
control are required.
(3) Application for Ho Chi Minh Project
Table 1 shows advantages and disadvantages of the two
counter methods and depending on projects, appropriate
method would be applied. In case of “Diamor Osaka” project
shown in Figure 1, from not only structural point of view but
also construction cost view point, the counterweight method
was adopted.
In Ho Chi Minh City project, since hard clay layer is
located beneath approximately GL-35m, in case of earth

T„P CHŠ KHOA H“C KI¦N TR”C - XŸY D¼NG


γI: Structure factor;
Us: Hydrostatic uplift force on bottom of tunnel

Figure 5. Counter weight method to bottom slab

WB: Self-weight of tunnel
Ws: Vertical loads of overburden
Qs: Resistance of cover soil
QB: Friction drag on tunnel side

Figure 4. Examination of stability of tunnel
against uplift force (DSRSC-CT)

Figure 6. Earth Anchor Method
anchor method, the anchor length becomes extremely long,
so that construction cost is expected increasing significantly.
In case of counterweight method, due to the weight balance,
cost increase is assumed to be not much high. In addition,
construction method is slightly easy and reliability is high with
counterweight method. Consequently, counterweight method
is evaluated as more adoptable and suitable for Ho Chi Minh
City projects, where and if necessary. Here, one engineering
point to be paid attention that flood water level needs to be
taken into account for the uplift force since flood by heavy
rain often happens in Ho Chi Minh City.
Table 1. Comparison between counter methods

Advantage

Counterweight
method

Earth Anchor Method

- Easy construction

- No increasing load
to the underground
structure.

- Cheap material

- Lower construction
- Able to construct
cost
prior to underground
structure to reduce
construction time
Disadvantage - Increasing loads
to the underground
structure

- High technology
construction method
is required

- Deeper excavation - Higher construction
cost

3.2. Reinforcing for Large Slab Opening to Lateral Force
Lateral forces such as earth pressure, water pressure
and polarized earth lateral pressure of building load are major
loads acting on the side walls of underground structures.
With the large openings to provide spacious area, stiffness
of underground structures resisting the lateral forces is
significantly reduced. Therefore, it is necessary to reinforce
the structure stiffness against the lateral forces. To model
the openings, horizontal virtual beam members are modeled
in structure frame as shown in Figure 7. Moment of inertia
of horizontal virtual beam section is adequately calculated
depending on opening size, slab thickness, etc. In this
modeling, main horizontal beam size is increased to reinforce
the stiffness of structural frame.
In case of high groundwater level such as in Ho Chi Minh
City, lateral forces acting on side walls become much larger,
so that much stiffer structure is required. In this case, several
additional countermeasures are necessary such as making
slab thickness (integrated with horizontal beam) thicker,
adding supporting beams inside of slab opening, and/or
installing structural walls in cross sectional direction. These
countermeasures also reinforce the underground structure
against seismic loads.
4. Summary
In order to create the spacious urban public space for the
sustainable urban development, the following major structural
issues and the solutions are clarified for the underground
structure design.
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Figure 7. Reinforcing structure due to opening
- The stability shall be ensured against the uplift force by
groundwater due to large underground open space. Through
the comparative study between the counterweight method
and earth anchor method from not only structural viewpoint
but also economical point of view, the solution shall be
determined.
- In order to make the large slab opening for soaring
space the structural countermeasure such as a horizontal
beam shall be studied. Since this reduces the underground
structural stiffness, in the structural analysis the stiffness
shall be evaluated correctly considering the comprehensive
underground structure.
The underground space beneath the public space such as
roads and parks will be developed much more in the coming
years. It is expected that the underground structural design
harmonized with high quality urban underground space will
be advanced more effectively./.

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T„P CHŠ KHOA H“C KI¦N TR”C - XŸY D¼NG

Tài liệu tham khảo
1. Design Standards for Railway Structures and Commentary
(Cut and Cover Tunnel: 03/2001); DSRSC-CT
2. Nikken Sekkei Civil Engineering Ltd, Preparatory survey
of Ben Thanh Central Station, Architectural Magazine of
Vietnam Association of Architects, 215-03-2013.
3. Nikken Sekkei Civil Engineering Ltd, Urban Development
Centering on Station Squares, Architectural Magazine of
Vietnam Association of Architects, 260-12-2016.



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