Systems Approaches to Urban Underground Space Planning and Management – A Review

3.4.1. Purpose definition

Planning for underground space should examine the visions, missions, and goals of the overall system under which it exists. This system can be the transport system, the water system, or the urban system as a whole. For a systems approach, it is important that the purpose of an element, task or problem dealt with is linked to its position in the overall system. Engineering tasks such as the design and implementation of a tunnel might have the purpose to improve the transport system whilst minimizing the impact on the existing built environment. The transport system in itself, in turn, might have the purpose to increase the ratio of public to private transport for environmental reasons, to boost the urban economy or to counteract inequality. Which of these is the main objective in a specific moment in time and consequently guides planning and design decisions is a fundamental systems choice, and it is important to keep that in mind. Predictions of demand often inform what capacity is planned for and where it is located and thus investment decisions are linked to this choice.

3.4.2. Integration and boundaries

The notion of integration across various boundaries goes hand in hand with the definition and analysis of these boundaries – in general as well as for a specific task. Boundaries are used to define which elements are internal or external to a system as well as differences between system elements (internal boundaries). To analyze a system, boundaries can be treated as temporarily stabilized, meaning that they were “created and agreed on by groups and individual actors over a long period of time” (Kerosuo, 2006 as quoted by van Broekhoven et al., 2015). In this context, it is important to recognize that apart from the constraining effects of boundaries that are the motive for the attempt of integration, boundaries can also have enabling effects because they reduce complexity, enable professional specialization, and in general provide structure (van Broekhoven et al., 2015). For example, the purpose definition as well as set goals can constitute enabling or constraining boundaries depending on the context. This recognition is helpful to accept that while aiming for a systems approach, it is not only impossible but also unnecessary to integrate everything.

In the context of the subsurface and the attempt to capture its role as well as the challenges and opportunities it provides, other boundaries that need careful consideration and definition are the actual spatial boundaries between different uses as well as the areas of responsibility of the involved authorities. This can be complicated as the uses are not necessarily exclusive and territorial boundaries can be fluid. For example, the same space can be used for bearing load and groundwater flow and the boundary for a catchment area might not be equivalent to that of the local boroughs in the city looked at. The boundary analysis provides the baseline for project evaluation and decision taking (see Section 3.2). This bridging from ‘soft’, holistic parameters and processes into ‘hard’, tangible projects that permanently change the built environment remains a major challenge. With regard to planning and management of underground functions, the local geological and geographical setting as well as the legacy of structures and human interventions in the ground and the legal and regulatory system constitute the major boundaries that are usually accepted as a starting point or baseline for planning specific interventions. They are also the starting point for the introduction of broader strategies or plans, in which currently the physical setting is often underrecognized, and the current legal and regulatory systems affecting or affected by subsurface use are found to be piecemeal (see 4.1 Boundaries: geological setting and physical legacy, 4.2 Boundaries: legal and institutional setting). Other aspects that require technical understanding as a basis for meaningful decisions are space requirements and compatibility of the different potential uses.

3.4.3. Emergence and continuous learning

Whilst it can be accepted that boundaries have to be analyzed as temporarily stabilized for specific tasks or purposes, the aspect of process integration is related to the notion that the behavior of the urban – and with it the underground – system is not fully predictable and the aim of systems thinking is to recognize and capture the emerging behaviors and situations in time to make meaningful adjustments. In other words, emergent and unpredicted systems behaviors should be met by an effort to continuously adapt and learn. The previously referred to first come – first served approach to allocation of space in the subsurface causes discontent because, looking back, it appears that with regards to the subsurface, this was not done, meaning subsurface use was not tackled systematically but piecemeal. On the other hand, was it possible to predict the increasing number of networks to be put into the subsurface over time? The problem was recognized by some at an early stage (e.g. Webster 1914) but this did not lead to any significant change in practice. Likewise, could planners and engineers have foreseen (when they planned the city layouts) that personal transport in cities would increase to the level it has and that it may now potentially decrease again due to climate and public health considerations?

There is a range of examples where how the subsurface or elements of it are managed today is clearly an effect of previous interventions or historical developments. This path-dependency becomes apparent in that any structure can create an impediment for future developments or impose increased management needs on a subsurface related sector. For instance, a lowered groundwater level in London were taken for granted when parts of the underground system were built. A decrease of groundwater use and a rising groundwater table later triggered concerns about the stability of existing constructions and water intrusions into service ducts (Dean and Sholley, 2006) with the result that the groundwater table is now managed carefully. The effect of ageing infrastructure is another example. Damaged sewage pipes can act as drains or recharge the groundwater table, depending on the hydraulic gradient (Boukhemacha et al., 2015). Re-sealing the pipes changes the groundwater levels again, which in turn can affect individual citizens for example when groundwater seeps into basements that had previously been considered as dry (Simicevic et al., 2005).

Systems approaches to managing the subsurface should analyze the location specific past events, describe the according path-dependencies, and apply future methodologies to maximize the potential to recognize, change and adapt existing strategies and projects.

4.2.1. The new environment and planning law in the Netherlands

In the Netherlands, a new Environment and Planning Act (EPA) has been introduced and is expected come into force in 2021 (Ministerie van Binnenlandse Zaken en Koninkrijksrelaties, 2018). This Act combines and replaces 26 laws as well as a range of regulations and guidelines concerning the physical environment (Ministrie von Infrastructuur en Milieu, 2018b). It thereby overcomes the dichotomy between the built and the natural environment or more general between society/culture and nature that is still prevalent and integrates the legislation for various environmental sectors, which is currently perceived as scattered and fragmented (de Graef et al., 2018). Even though the term system is only used with regard to the legislative system in the Act and the according Explanatory Memorandum (Ministrie von Infrastructuur en Milieu, 2018a, Ministrie von Infrastructuur en Milieu, 2018b), and the act is not specifically about the subsurface, the subsurface is inherent in it as the EPA integrates amongst others the Earth Removal Act, the Water Act, the Mining Act and the Soil Protection Act with the Spatial Planning Act, and it comprises many of the elements of a system approach described above:

(1)

The EPA emphasizes the necessity to work across sectors, recognize the mutual relationships between different elements of the environment and formulate and achieve ‘interrelated objectives’. The Memorandum sets out that the role of government here is to link and monitor.

(2)

The EPA’s core element is a policy cycle of development of a vision, programming and rules design, implementation (permits and projects) and feed-back mechanisms (monitoring and evaluation). The Explanatory memorandum states: “the cyclical approach is a manifestation of a paradigm shift: from preservation and protection towards an active approach in order to continuously strive towards good quality of the physical environment.”

(3)

The EPA sets out an overarching purpose that is the quality of the physical environment as a whole. Thresholds for environmental values such as water or air quality stemming from European regulation are adopted as minimal parameters.

(4)

The EPA embraces emergence as a quality by aiming at encouraging and facilitating initiatives from members of the public or local authorities. It thus emphasizes a bottom up strategy of empowerment rather than control.

(5)

The EPA emphasizes stakeholder engagement, accountability and responsibilities.

Even if the subsurface is not separately addressed in the EPA, the definition of soil as the solid component of the earth, including the liquid and gaseous components and organisms contained (Ministrie von Infrastructuur en Milieu, 2018a) indicates a more holistic way of thinking about the subsurface as the separation between the soil as component of the earth which enables the growth of plants and deeper layers of the subsurface is abolished in this definition. The EPA will undoubtedly influence relevant decisions about interventions in and planning efforts that affect the subsurface.

4.2.2. The underground masterplan in Singapore

In 2007, the Singapore government set up an Underground Master Plan Task Force, with three key objectives: (a) to develop an underground master plan, (b) to identify imminent potential projects and resolve technical issues and funding mechanisms, and (c) to surface policy changes to facilitate underground development. In 2010, the Economic Strategies Committee (ESC) under the Prime Minister’s Office made specific recommendations on underground space use, and thus elevated the use of underground space to a strategic level (ESC, 2010). The ESC report made a key recommendation to invest in creating and using underground space, and that the government should take a lead in:

(1)

Creating basement spaces in conjunction with new underground infrastructural developments (e.g. rail) to add to its land bank;

(2)

Developing an underground master-plan to ensure that underground and aboveground spaces are synergized and better integrated with surrounding developments and infrastructure;

(3)

Establishing a national geology office to collate underground information that will benefit both private and public sector efforts in underground development;

(4)

Developing a subterranean land rights and valuation framework to facilitate underground development; and investing in underground development R&D and directly investing in cavern level.

The importance of developing an underground master plan was recognized early in defining the Terms of Reference for the Task Force. However, the difficulties and challenges, especially the type of technical and policy input required, were probably underestimated. One major challenge was the lack of 3D geological information and accurate data on existing underground infrastructure (Zhou and Zhao, 2016). Other challenges included identifying the types of underground space applications and developing a vertical zoning framework to address the 3D nature of underground space use, coordination and integration among various government agencies, different applications, and integrating aboveground and underground space development (Zhao and Künzli, 2016).

To address these important information gaps, the government conducted a series of studies and set up a National Geology Office charged with investigating the bedrock geology with the main aim of providing 3D geological data for underground space planning and developing a cavern suitability map (Lim, 2009, Zhou and Cai, 2011, Lui et al., 2012, Zhou and Zhao, 2016). One of the more impactful studies is a benchmarking study, which explored international best practices and compared Singapore to several leading cities in the world in the areas of legislation, standards, policy, planning, and actual usage of underground space (URA, 2013, URA, 2018, Zhou and Zhao, 2016). The results of this study will no doubt provide critical input to the master planning activities in Singapore.

To address the issue of underground space ownership, the Singapore Parliament in 2015 passed two legislations addressing the issue of ownership and acquisition of underground space: the State Lands (Amendment) Act 2015, and the Land Acquisition (Amendment) Act 2015. The State Lands (Amendment) Act 2015 defines ownership of the subterranean space as land includes only as much of the subterranean space as is reasonably necessary for the use and enjoyment of the land. It further defines reasonable use as being: (a) such depth of subterranean space as stated in the State title for that land, or (b) if no such depth is specified, subterranean space to 30 m below the Singapore Height Datum. The Land Acquisition (Amendment) Act 2015 allows the acquisition of a specific stratum of underground space. However, some legal issues related to the use of underground space remain. These include the first rights of use, liability of one ownership to another, offset required for underground stability, movement of fluids underground and responsibility for flooding underground, as well as entrance for the construction and later use of the underground space. Efforts by the relevant government agencies were needed to address and clarify these issues.

This example shows the imperative and complexity involved in the establishment of a legal and technical baseline if the development of a holistic or systemic strategy for use allocation in the underground is sought.

4.3.1. Strategies: masterplans

Masterplans have been mentioned as a desirable tool or strategy for subsurface management by various authors. It was in particular promoted by Bobylev (2009) and authors often refer to the cases of Helsinki (Bartel and Janssen, 2016, Price et al., 2016, Sterling et al., 2012) or Montreal (Delmastro et al., 2016, Durmisevic, 1999) when suggesting that masterplans for underground space or including underground space are needed. The Helsinki underground masterplan sets out the allocation of underground space for a variety of public and private developments for the whole city and its establishment was practicable due to the fact that the bedrock under Helsinki is well suited for tunneling (Vähäaho, 2014). Montreal developed an extensive pedestrian network underground (Boivin, 1991) the main driver being described as the severely cold climate in winter as well as the strategic aim to create a compact city with combined transport systems (Durmisevic, 1999). Other cities mentioned frequently in this context are Singapore (Zhou and Zhao, 2016, see also Section 4.2.2) and Hong Kong, for both of which the scarcity of land is described as a main reason to explore and manage underground development opportunities (e.g. Delmastro et al., 2016, Price et al., 2016, Sterling et al., 2012). Zhao et al. (2016) list eleven Chinese cities that in some way integrate underground space in their masterplans.

In general, the term ‘masterplan’ can be associated with a variety of meanings; With reference to the example of Helsinki, Delmastro et al. (2016) describe masterplans as documents guiding allocation of space specifically for construction, integrating a map of existing and future facilities and safeguarded volumes and routes, as well as technical requirements. They emphasize that both, long term underground master plans as well as sectoral plans for transportation, leisure and commerce, and technical systems are needed. Similarly, Zhao et al. (2016) describe masterplans for underground space as planning for systematic development and utilization of subsurface space in urban areas with a focus on arrangement of underground structures. Underlying this idea are zoning plans which reflect a specific understanding of planning as present in some but not all national planning systems (see Newman and Thornley, 1996). Bobylev (2009) by contrast writes about masterplans that go beyond the allocation of engineered structural interventions only but are strategic documents that specify design principles and concepts to guide change and development in a whole city. These are different to zoning plans as described above as well as to site development masterplans that are dealing with a specific property development proposition (Bell, 2005). Bobylev in particular stresses the importance of sustainabilityconsiderations in these documents and describes how consideration of the subsurface including all its potential functions rather than only engineered structural interventions, can contribute to achieving these goals. The necessary actions to achieve an integration of the subsurface in these high-level planning documents are summarized in Bobylev (2009) as:

(1)

understanding the baseline (geological model, three-dimensional mapping),

(2)

prospective planning (establishing needs, risks and benefits for potential uses),

(3)

assessment and analysis (vulnerability, scenarios, weighing of different uses), and

(4)

decision-making (integrated assessment, analysis of potential conflicts, priority setting).

Whereas this list includes many aspects of systems approaches as discussed, review, monitoring or learning are not mentioned in Bobylev (2009). Scenarios are mentioned directed at specific selected solutions rather than for the development of the city as a whole and cost-benefit-analysis is listed as the prime tool in the context of identification of needs for underground structures and developments. Masterplans as a tool have been criticized in the 1970s as being too static and it was questioned if they can answer upcoming questions in time. Cooper et al. (1971) pointed out that a masterplan “can be regarded as one form of systems approach” but adds for consideration that it might “rest on a methodology and an associated point of view which are not adequate for dealing with an increasingly complex and dynamically changing urban scene.” However, since then views have changed and the term is now used for a variety of strategic documents. For example, Amirtahmasebi et al. (2016) emphasize that a masterplan has to be understood as “dynamic long-term planning document that provides a conceptual layout to guide future growth and development.” and that it is important to be able to change the plan based on changing conditions. Consequently, whether masterplanning can be referred to as a systems approach – with the masterplan as the according tool – cannot be answered generically but depends on the specific masterplan, how it is designed, established, and monitored (see for example the new masterplan for Singapore, Section 4.2.2).

4.3.2. Strategies: circular process approaches

Rather than focusing on the resulting plan and what it should entail, the Deep City Project as first described by Parriaux et al. (2007) and further elaborated on by Li et al., 2013a, Li et al., 2013b, Doyle, 2016 introduces a process for the development and ongoing improvement of a strategic plan for sustainable management of what they call underground resources. It emphasizes the role of the four resources groundwater, geothermal energy, geomaterials and space for urban development and stresses the idea of combined use of the same volume for various functions. Li et al. (2013a) describe a general process of plan-making in two strategic (policy making and criteria framing) and four operational steps (data collection, mapping of resource and development potentials, evaluation of projects and analysis of decisions), where the policy making is the last step and leads back to a revision of the criteria set to evaluate success of the overall process. Apart from the circularity, the approach also emphasizes the involvement of stakeholders in various steps of criteria framing and weighting of indicators. In that, the general vision of the particular city looked at is taken into account and it is accepted that not every city might need an underground specific plan. Li et al. (2013a) develop an applicability score – a method to assess whether a particular city requires management of the underground building on estimates of supply and demand of the four resources, driving forces, as well as a classification of the available information. In the Deep City method, the collection and analysis of the data above described as physical baseline is part of the circular process and thus it is accepted that technology and data needs might change. However, the categorization of the contribution of the subsurface to the urban physical environment in four resources remains unquestioned.

Asset Management of the Subsurface (AMS), a method still under development, described by Maring and Blauw (2018), also distinguishes between the strategic and operational level. Instead of focusing on the subsurface as a manageable space, and building on a pre-defined set of categories, Maring and Blauw (2018) suggest to understand all structures in the ground but also the ground itself and the services it provides as assets and to apply methods of asset management. The definition and importance of the assets can change with the challenge looked at. The strategic step in this method is described as evaluation as to how the subsurface can contribute to the achievement of the visions and objectives a city. By doing so, it emphasizes that how the subsurface might or might not be used best is not independent of overarching policy ambitions. The other three steps are: (1) preparation of an asset management plan, (2) implementation, and (3) maintenance and evaluation. Basing the approach in a framework that is already applied in practice (asset management), Maring and Blauw (2018) aim to reduce the threshold for acceptance of the need to integrate the subsurface in a variety of municipal considerations. However, they also point out that necessary adjustments of the standard asset management approach to enable consideration and maintenance of functions rather than objects, alongside the change in time-spans that would need to be considered, are challenging.

4.3.3. Strategies: decision support system for social acceptance

Building on theory of decision making, in particular multi-criteria analysis and decision approaches, rather than planning theory, van Os et al., 2016, van Os et al., 2017 explicitly describe a decision support system for planning decisions regarding subsurface activities. The modular evaluation method for subsurface activities (MEMSA) is focused on social acceptance of the various activities possible and on the dimensions of a decision-making process with the aim to shift the focus away from pure profitability considerations to integration of the community through transparency and participation. It builds in a first step on an evaluation of potentials and their relation for concurrent or sequential use in a specific geological volume. In this, importantly, also the options to do nothing now or even do nothing forever are included. Consequently, acceptance of a project is scored separately in three classes: market acceptance (investment behavior, risk perception), social political acceptance(contribution to policy objectives) and community acceptance, and finally combined in a final ranking.

Being directed more at large scale, deep subsurface activities, van Os et al. (2017) emphasize that for a successful project implementation, policy goals need to be re-evaluated on a regular basis “to account for timing discrepancies between the realization of activities and policy deadlines, because this discrepancy can have a large impact on the necessity and therefore acceptance of subsurface activity”.

A similarly comprehensive approach to weigh different potential functions in a specific location could not be found in literature. However, multi-criteria decision-making approaches that rank possible alternatives by assessing a range of parameters including stakeholder views and cost-benefit considerations (Kabir et al., 2013), have been applied to a variety of subsurface related functions.

4.3.4. Tools: stakeholder engagement

In their method System Exploration of the Subsurface (SEES) Hooimeijer and Maring (2018) provide a method for knowledge exchange between practitioners focusing on a specific project area. Their aim is to unify the perception of the surface and the subsurface, and ultimately integrate the subsurface into established urban design processes. Four categories of subsurface use are distinguished – civil constructions, energy, water and soil. These shall integrate a large range of ecosystem services into a limited number of categories useful for the urban design process. Hooimeijer and Maring understand their approach as based in systems thinking and complexity theory, dealing with “inherent unexpected behaviour of agents”. The tool itself consists of a matrix with subsurface use categories on the X-axis and what they define as layers of planning on the Y-axis (people, metabolism, public space, infrastructure and subsurface). The tool is used in workshops to explore with groups of specialists influences and interdependencies of these categories in each of the planning layers. Even if emergent properties of the system itself are not studied, the method supports knowledge exchange and provides a thinking framework in which unknown synergies or problems can emerge and facilitates alignment of the overall project objectives and integration of further steps.

4.3.5. Tools: potential maps

For the second operational step of the Deep City approach (see above), Li et al., 2013a, Li et al., 2013b, Doyle, 2016 present maps of potentials specifically for construction (Li et al., 2013b) and the four resources (Doyle, 2016). Li et al. (2013b) develop evaluation criteria for different depths and explore their relative importance for evaluation of resource demand and supply in cooperation with local professionals. Doyle (2016)extends and refines the method for evaluation and mapping of potentials with the aim to shift the understanding of the subsurface from a resource or place that can satisfy urban needs to a potential that can be explored in the process of urban planning (Doyle, Thalmann and Parriaux, 2016). Doyle (2016) points out that the generation of these maps involves primary data gathering as well as assignment of resource related characteristics to the geological formations present. In a second step, surface data is included to assess the suitability of actual resource exploitation and inform the planning process.

Potential or suitability maps have also been used in other contexts. Hooimejer and Maring (2018) introduce a different kind of potential map: rather than showing what could be used in the area looked at, they overlay different information layers which illustrate the impact of subsurface assets on the surface. These maps are meant as an interactive tool or design guideline for an urban designer and focus on comparatively small areas and not on the city as a whole.

Rather than assessing the potentials of a site or area, Wassing and van der Krogt (2006) developed a set of suitability maps to assess the suitability of an area for building a specific kind of development. The maps are based on geotechnical, geochemical and geohydrological properties of the ground which are, in a second stage, weighted according to how they would influence future scenarios. The authors mention that the weighting is “somewhat arbitrary and subjective” and that the relevance of geological as well as socio-economic aspects will rely on the perception of the respective planner and project they have in mind.

Potential maps seem to be a valid tool for communication of information that is traditionally held in the technical disciplines to the planning and design disciplines. However, it is important to be aware that they are based on a previous definition of what is seen as potentials, i.e. a decision as to what is being mapped. Suitability maps for specific sites or areas are responding to specific demands or created as support tools for specific decisions. All of these tools produce an additional set of information to allow an intuitive use of technical subsurface information in the planning or design process.

4.3.6. Tools: scenarios

As can be seen with the potential and suitability maps described, there are attempts to look at what the ground could provide (supply) and those which focus/start from looking at the need (demand). In particular the latter relies on methodologies to predict or foresee the future, but also the former can change depending on the urban development and climate change and thus at least require constant updating. In planning strategies, both, supply and demand, need to be balanced and it has to be set out how these are determined.

Different to forecasting, scenario approaches aim to provide a set of possible futures which can be compared and assessed to inform decisions. They can be applied at different scales and with different focuses. In the context of subsurface planning, as described above, Wassing and van der Krogt (2006) use scenarios to assess the relative importance of different geological parameters for specific developments. Hooimejier et al. (2017) design subsurface related provoking scenarios – extreme design solutions to current planning tasks – and challenge groups of practitioners in workshops to concretize these scenarios in an explorative manner to create a feasible vision for a city area. Rather than a concrete solution for a specific task, here possibilities and relationships are explored and cross-disciplinary conversations are fostered. Rogers (2018) presents an assessment approach for engineering interventions in cities in that (a) the aspirations the city and citizens associate with the intervention are tested through the development and contrasting of future visions, (b) interventions are tested in the current situation as well as in the context of four extreme future scenarios, and (c) assessing alternative business models for implementation. The approach is not focused on subsurface interventions but is particularly relevant in view of the longevity of these interventions.

4.3.7. Valuation

Commonly, engineering interventions in the subsurface are assessed with cost-benefit-analyses (CBA). In these analyses, particularly for underground infrastructure it has proven difficult to equally account for the initial capital cost and the long term social and environmental benefits (ITA, 2004). No explicit market for underground space exists and consequently other ways to assess its value are needed (Pasqal and Riera, 2005).

The problem of value capture for projects or services whose values cannot simply be translated into monetary units is not unique to subsurface space management. De Groot (2006), for example, developed a method for comparative analysis and valuation of different land use functions, and new terms like social value (e.g. Frischmann, 2012) or social return on investment (e.g. Lingane and Olsen, 2004) gain importance for a variety of decisions in the built environment.

Related to the urban subsurface, Coogan (1979) developed a valuation scheme for subsurface developments including nine parameters: need, scarcity, substitutability, duration of change resulting from the use, rate of change once the use has begun, primary and secondary impact on the surrounding area, revocability of the decision for a particular use once the commitment is made, and need for an orderly decision on the use before the commitment. For more specific functions, Lim et al. (2016)evaluate the public value of soil remediation in Korea, and Matthews et al. (2015) assess the social cost of pipeline infrastructure. Maring and Blauw (2018) suggest to refer to methods that have been applied for ecosystem service valuation and provide an overview over these methods.

Instead of assigning monetary value equivalents, multi-criteria decision frameworks aim at the integration of CBA with other relevant criteria for project or intervention decisions (Kabir et al., 2013). Whilst these approaches support specific project decisions, a more general understanding of values of the subsurface for various aspects of urban life, including for example precautionary measures for natural disasters, and systemic approaches that are able to assist with the evaluation of different options for specific projects or locations as well as overarching planning objectives still need to evolve (see also Section 4.3.4).

4.3.8. Benchmarking and comparison

It has been mentioned in the introduction to this section that there appears to be an underlying assumption in the literature about integrating the underground or subsurface into urban planning strategies. These strategies are often informed by Urban Indicators that can be used for comparison between cities as well as for longitudinal studies by measuring the development of indicators over time. The subsurface is not currently covered by the established indicator schemes (Bobylev, 2016). Bobylev (2016) proposes a list of underground space related indicators for inclusion, including if regional planning is taking into account the geological and hydrogeological setting and quantitative measures of underground space use. Admiraal and Cornaro (2018)emphasize that underground space functions contribute or can contribute to seven of the 16 sustainable development goals set by the United Nations (United Nations, 2015). However, correlation of these indicators with other indicators for overarching objectives like sustainability or resilience could not be shown and needs further investigation.

Indicators are one way to benchmark the development of a city in a specific topic area and comparison with other cities can provide valuable insights for policy makers. For the development of the masterplan for Singapore, the Urban Redevelopment Authority of Singapore commissioned a benchmark study about underground developments (URA, 2018, see Section 4.2.2) to learn about underground planning efforts worldwide. The already mentioned COST action (sub-urban.squarespace.com) supported short-term missions through which two cities could create direct exchange about specific subsurface related topics. However, whereas in several publications specific aspects of underground related aspects of the planning regime (e.g. Li et al., 2013a) or specific parameters (e.g. Bobylev, 2016) are listed for several cities, in depth comparisons between two or more cities are lacking in the literature.