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.
Trang 1S¬ 28 - 2017
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: <itous@nikken.jp>
(2)Urban Infra & Engineering Div., Nikken
Sekkei Civil Engineering Ltd.,
Email: <tran.kim.khoa@nikken.jp>
(3)Urban Infra & Engineering Div., Nikken
Sekkei Civil Engineering Ltd.,
Email: <yamadatat@nikken.jp>
(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
Trang 290 T„P CHŠ KHOA H“C KI¦N TR”C - XŸY D¼NG
KHOA H“C & C«NG NGHª
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
self-weight 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
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 re-written 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
Figure 1 Underground Shopping Mall
“Diamor Osaka”
Figure 2 Underground Shopping Mall Planning in Ho Chi Minh
Figure 3 Ben Thanh Station of Ho Chi Minh Metro Line
Trang 3S¬ 28 - 2017
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
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
Figure 5 Counter weight method to bottom slab
Table 1 Comparison between counter methods
Counterweight
method Earth Anchor Method
Advantage - Easy construction
- Cheap material
- Lower construction
cost
- No increasing load
to the underground structure
- Able to construct prior to underground structure to reduce construction time Disadvantage - Increasing loads
to the underground
structure
- Deeper excavation
- High technology construction method
is required
- Higher construction cost
γI: Structure factor;
Us: Hydrostatic uplift force on bottom of tunnel
WB: Self-weight of tunnel
Ws: Vertical loads of overburden
Qs: Resistance of cover soil
QB: Friction drag on tunnel side
Trang 492 T„P CHŠ KHOA H“C KI¦N TR”C - XŸY D¼NG
KHOA H“C & C«NG NGHª
- 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./
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.
Figure 7 Reinforcing structure due to opening