Systems approaches to the use of underground space in urban environments

Urban areas are coming under ever increasing strain.The population of urban areas has been steadily increasing over the last decades. For instance, as is shown in Figure 1, the proportion of urban dwellers has increased from 30% in 1950 to 55% in 2018 and is projected to rise to 68% by 2050. The rate of urbanisation is even more pronounced in less developed countries (United Nations, Department of Economic and Social Affairs, Population Division 2018a). Furthermore, the urban population is increasingly concentrating into so-called megacities, i.e., ‘urban agglomerations’ with 10 million or more residents; the proportion of urban residents residing in megacities has increased from 3% in 1950 to 12% in 2015 and is expected to grow to 16% in 2035 (United Nations, Department of Economic and Social Affairs, Population Division 2018b, 55, and 2018c). Because urban infrastructure is difficult to modify, as urban areas grow, new development must fit into the existing urban fabric. Moreover, as density increases, space will become more difficult to come by, requiring new ways of meeting the needs of urban dwellers. At a global scale, there is an increasing need to reduce resource use across all economic sectors, and disasters are increasing in frequency and their social and economic consequences. On top of this, the increased complexity and interdependency of infrastructure systems increases the chances of cascading infrastructure failures.