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. firstname.lastname@example.org(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 S¬ 28 - 2017
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Figure 2. Underground Shopping Mall Planning in Ho Chi Minh
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
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
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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
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. S¬ 28 - 2017
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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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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.