For elevated guideways (structures that support tracks in the air), the main disruption is the construction of the support piers. The guideway is usually constructed using precast segments that are lifted into posi- tion using a crane mounted on the piers and then post-tensioned to form the span. This has a distinct advantage over more traditional methods of con- struction such as casting concrete in situ, which relies on extensive support scaffolding to support the concrete deck during construction. In elevated sections, the station is also constructed above ground. In many cases, station construction can be achieved without significant disturbance to the local environment. Environmental Impact No significant construction project, such as an urban metro, can pro- ceed without a thorough evaluation of its impact on the environment. An Environmental and Social Impact Assessment (ESIA) is a formal process used to predict the environmental consequences of any devel- opment project. It ensures that the potential problems are foreseen and addressed at an early stage in the projects planning and design. The main purpose of the ESIA is to inform decision makers of the likely impacts of a proposal before a decision is made. ESIA provides an opportunity to identify key issues and stakeholders early in the life of a proposal so that potentially adverse impacts can be addressed be- fore final approval decisions are made. The ESIA should be prepared initially by the project promoter and built upon by the designers and contractors as the project progresses. The Goals of an ESIA are: • to predict environmental, social, economic, and cultural consequences of a proposed activity. • to assess and review plans to mitigate any adverse impacts resulting from the proposed activity. • to support the goals of environmental protection and sustainable de- velopment. • to integrate environmental protection and economic decisions at the earliest stages of planning and activity. A Risk Worth Taking Traffic congestion is a serious consequence of urbanization in any country, with significant negative effects on both the quality of life and the economy. In a rapidly urbanizing world, the urban rail system is an efficient way of reducing traffic congestion, reducing emissions, and decreasing pollution. The high capacity offered by rail systems can serve the high transport requirements generated within and between cities. The construction of rail in an urban environment will entail numerous environmental and social impacts that require careful management and monitoring, particularly during the construction phase. It is imperative that they are understood, managed, and monitored extensively through a variety different means.
Construction methods will vary from location to location, but their overall aim must be to minimise disruption to the urban environment during construction where possible. Deep excavations in densely pop- ulated urban areas impose specific challenges, especially the potential impact on adjacent structures from induced ground and structural movement. Tunnelling is a good option for constructing metros in older and larger cities. It is the least disruptive construction activity in most ground conditions. And, depending on the ground conditions and depth of excavation, a range of construction methods exist. But it can be ex- pensive. Care must be taken to select the right tunnelling option for a particular project to optimize cost and minimize impact. In considering any project and in particular one where there is inevi- tably significant disruption to daily life during the construction phase, conducting an ESIA and engaging early with stakeholders is essen- tial. Lines of communication with the local community must be open throughout, as well. When the construction starts, the promoter will keep track of the imple- mentation of the management plans addressing social risk and identify any further issues through establishing a grievance mechanism and robust stakeholder engagement. After the work is finished, the final im- pact on the surrounding area is minimised and often greatly improved by the creation of passenger transit and access areas, in-station retail outlets, and infrastructure improvements to the local road network. Ultimately, the disturbance to the daily routine of the local inhabit- ants during the construction phase is worth tolerating, especially when it will be these local inhabitants who reap the ultimate benefits of a modern public transport system that reduces congestion and impact on the environment. References 1 UN Habitat (2016). World Cities Report; 1.2 World Cities: A Gathering Force, available at http://wcr.unhabitat.org/wp-content/uploads/sites/16/2016/05/ Chapter-1-WCR-2016.pdf 2 United Nations Department of Economic and Social Affairs (UNDESA), (2018) Revision of World Urbanisation Prospects, available at https:// www.un.org/development/desa/publications/2018-revision-of-world- urbanization-prospects.html 3 World Bank (2018). Understanding Poverty, available at https://www. worldbank.org/en/topic/transport/overview 4 Ritchie, H. and Roser, M. (2019) - "Urbanization". Published online at OurWorldInData.org. Retrieved from: 'https://ourworldindata.org/urbanization' 5 Chappell, M. and D. Parkin (2004). 8 - Tunnel construction. Sewers. G. F. Read. Oxford, Butterworth-Heinemann: 150-192. 6 Maidl, B., M. Thewes, U. Maidl, B. Maidl, M. Thewes and U. Maidl (2014). DAUB Recommendations for the Selection of Tunnelling Machines. Handbook of Tunnel Engineering II, Wiley-VCH Verlag GmbH: 363-409 7 Wilton, J. L. (1996). Cut-and-Cover Tunnel Structures. Tunnel Engineering Handbook, Springer: 320-359 8 Rudolf, J., V. Gall and A. Nitschke (2008). Selection of alignment and tunneling methods in urban settings. Proceedings of the World Tunnel Congress 2008—Underground Facilities for Better Environment and Safety 9 Kolymbas, D. (2005). "Heading." Tunnelling and Tunnel Mechanics: A Rational Approach to Tunnelling: 75-129 10 Lavon, E. V. (1980). Method and device for breaking a hard compact material, Google Patents found at https://patents.google.com/patent/ US4204715A/en 11 Arshad and R. A. Abdullah: “A Review on Selection of Tunneling Method and Parameters Effecting Ground Settlements” Electronic Journal of Geotechnical Engineering, 2016 (21.14), pp 4459-4475
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