1 Introduction
2 Green infrastructure
2.1 Classification
2.2 Horizontal greenery
2.3 Vertical greenery systems
Typologies | Plants | Characteristics |
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Green façade | Climbing plants | Plants rooted on the ground and climbing on the façades of the building by themselves or through the support of other structures |
Green wall | Modular plants or plants with hanging-down branches | Supporting structures such as steel nets, modular rails and planter boxes that should be built on façades for hanging and placing plants |
Green terraces | Short, medium and tall plants | Growing in the planting media which is located on horizontal terraces at different levels continuously along the façade |
Elevated forest | Trees | Trees growing in sheltered horizontal (open) spaces |
Vertical forest | Trees | Trees or groups of trees growing on cantilevering balconies |
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Green façade refers to vegetation rooted on the ground, which makes use of either the wall itself for climbing (traditional direct systems) or independent supporting systems, such as trellis, wires, cables or meshes (double-skin indirect system) affixed to walls (Fernández-Cañero et al. 2018).
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Green wall has been made using geotextile, pots, panels, boxes or modular nets, where pre-cultivated vegetation has been planted and subsequently suspended and fixed to a building structure (Bartesaghi Koc et al. 2017). Green walls demand more complex constructions and imply higher installation and maintenance costs in comparison to green façades (Dunnett and Kingsbury 2008).
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Green terraces are defined as plants growing on the horizontal terraces continuously along the façade, which are built on different heights and levels.
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Vertical forest uses cantilevered balconies around the building envelope, acting as an accommodation for trees to grow, where the group of trees is formed into vertical forests.
3 Benefits of vertical greenery
3.1 Environmental benefits
3.1.1 Shading effect
3.1.2 Thermal insulation
3.1.3 Other environmental benefits
3.2 Social benefits
3.3 Economic benefits
4 Planting strategies
Considerations | Parameters |
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Plant arrangement | Climatic consideration, orientation, wind effect, trees anchorage and fall off protection |
Plant species | Aesthetic appreciation, foliage, density, growth rate, tolerance of high height condition, maintenance and repair activities |
Planting media | Properties such as thickness, moisture content and density |
Maintenance and inspection | Pruning, weeding, watering, fertilization, plant replacement |
Irrigation and drainage system | Accessible and well equipped for regular watering, water consumption waterproofing work |
5 Vertical forests
5.1 Existing vertical forests
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It is observed that the orientation of branches and density of foliage are different between two sides of the tree, but also vary around four sides of the building due to the possibility of either phototropism and thigmomorphogenesis of the trees or human intervention.
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If trees are subjected to directional horizontal forces, they will grow in a bent shape. This means that trees that are under the effect of a prevailing wind direction, for instance and in particular at building corners or at higher elevations, will not grow straight.
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In the course of time, trees in containers grow in size. Tree stability is one of the main concerns for the safe development of vertical forest systems.
5.2 Stability of vertical forest
5.3 Natural mechanical fixation system
5.4 Preliminary research
Direction | Name (A tested; B calculated) | Force F (N) | Modulus of elasticity E (MPa) | Radius (mm) | Deflection \(\delta\) (mm) | Ratio between deflections of tree B and A | Type of connections (Fig. 8) |
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Radial | Tree A1 | 235 | 20,439 | 23 | 5 | 23.8 | Parallel-surface and crosswise |
Tree B1 | 119 | ||||||
Tangential | Tree A1 | 353 | 3 | 33 | |||
Tree B1 | 99 | ||||||
Radial | Tree A2 | 118 | 20,439 | 20 | 48 | 2.3 | Parallel-surface |
Tree B2 | 102 | ||||||
Tangential | Tree A2 | 24 | 4.3 | ||||
Tree B2 | 102 | ||||||
Radial | Tree A3 | 353 | 13,165 | 52 | 8 | 1.1 | Parallel-surface |
Tree B3 | 9 | ||||||
Tangential | Tree A3 | 2 | 4.5 | ||||
Tree B3 | 9 | ||||||
Radial | Tree A4 | 196 | 20,439 | 20 | 20 | 5 | Crosswise |
Tree B4 | 99 | ||||||
Tangential | Tree A4 | 235 | 2 | 107 | |||
Tree B4 | 214 |
6 Conclusion
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Classification and identification of green infrastructuresSix types of vertical and horizontal greenery have been identified and classified in this study. Particularly, when trees are integrated vertically into high-rise buildings using cantilevering balconies, this group of trees forms a vertical forest.
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Vertical forest and natural mechanical fixation systemsAn application concept of natural mechanical fixation system to the vertical forest is introduced to ensure the stability of trees in containers and along the façade of a building. The fixation system allows for load transfer between tree both in horizontal and vertical directions. The preliminary results show that self-growing connections clearly contribute to the stiffness of a tree network system. This natural fixation manner may replace parts of artificial measures in a vertical forest system in the future. The mechanical properties of self-growing connections require further analyses to understand how their mechanical properties develop over time. The properties will increase with the growth of the trees, but the self-weight and external loads will also rise with increasing tree size.
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Planting strategiesThere is still a lack of knowledge of how to deploy tall growing plants such as trees along high-rise building facades. From a botanical viewpoint, plant selection and planting strategies should be done with care, relating to the naturally available species in the region, but also with respect to the physiology and the root system. Considering plant characteristics and environmental conditions, a recommendation on planting strategies is presented for future development of vertical forest systems.
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Design considerations for VFEFor VFE, it is essential that design guidelines are developed including practical guidance for multi-criteria analyses. To design a vertical forest successfully, it is recommended to have a suitable planting strategy based on local climate and plant characteristics, to apply a natural mechanical fixation system and to consider both the structural and energy performance of buildings. Because the vertical forest is a dynamic growing system, it is necessary to introduce long term monitoring processes and reliability assessment tools with tree growth—time dependency. Part of the future work will focus on the natural mechanical fixation system regarding strength development and contribution to stability of vertical forests.