1. Introduction: Mapping the Urban Underground
Cartographical representations of the city are typically founded on the ground plane. This has contributed to a dominance of the bird’s eye views in urban planning and to a general absence of the underground in theoretical models of “good” city form. These normative and prescriptive models, of which cosmological, mechanical and ecological are the most recurrent, tend to impose upon the landscape an idealized form that responds little to the local context [1
]. In the cosmological model, the underground often remains symbolically as a place into which one descends in death or in ritual acts of rebirth [2
]. At the turn of the twentieth century, the popularity of addressing the city as a machine inspired urban planners and engineers, such as Eugène Hénard [3
] and George Webster [4
], to call for a rational integration of the subsurface into aboveground urban systems of transport and logistics. In this same vein, Édouard Utudjian founded the GECUS (Groupe d’études et de Coordination de l’Urbanisme souterrain) in 1933, which campaigned well into the second half of the twentieth century for an “underground urbanism” [5
]. Within this normative model of the city as machine, geology and geography feature only as obstacles to be overcome through advances in technology.
The growing awareness of environmental degradation due to rapid urbanization gradually turned city planning and design to natural processes as an inspiration in thinking of the city as an organism [1
]. Landscape architect Ian McHarg’s work, including “Design with nature” [6
], called for geology to be a principle constraint in city design and established a tradition of overlaying geographical information in a single map in order to identify latent synergies and conflicts [7
]. The “biophysical world” becomes a source of a planning “ethic” [6
]. This version of the ecological model has been pursued to this day by a number of urbanists who seek to elaborate design responses to interactions between human settlements and ecological processes [8
]. In their work, however, the underground, understood as geology and hydrogeological processes, tends to be a point of departure for surface and near subsurface (within the first couple of meters) design and planning interventions. Only recently has someone in this intellectual community proposed to consider all “altitudes” of urbanization, from the deep subsurface to the sky [10
The approach to the underground within the norms of the mechanical model tends to be reductionist and to prove problematic in the long term. On the one hand, conflicts have occurred between competing uses. Mexico City is well known for its century-long problem of subsidence. Heavy pumping of the urban aquifer has led to a decrease in groundwater levels and a gradual drying up and sinking of the highly compressible lacustrine sediments underlying much of the urban area [11
]. In Paris, a similar problem led to a moratorium in the 1960s on overexploitation of the aquifer, which resulted in a rise of the water table and a flooding of buildings with basements built according to former groundwater levels [12
]. Paris, like New York and many other cities, suffers from a congestion of infrastructure beneath its streets. On the other hand, the rationalization of decision-making processes has led to disciplinary divisions in the collection, visualization and analysis of data [13
]. Within this paradigm, urbanization tends to first formulate needs and problems and only later examines the potential of resources to meet or solve them [14
Design practices operating within the ecological model tend towards the reverse logic, one in which the potentiality of resources is investigated prior to needs. Mapping and the map overlay tradition initiated by Ian McHarg seek to establish the “conditions for the emergence of new realities” [15
]. In interdisciplinary planning practices, however, the ecological model returns to a mechanical rationalization of stocks and flows. Borrowing from cybernetics and 19th century thermodynamics, the city is viewed as an organism whose metabolism drives towards “balanced flows of energy and materials between the human and natural subsystems of the material realm” [16
]. Within the imperatives of sustainability and resilience, this means, respectively, managing resources locally so that global stocks are not depleted and so that shocks to the (eco)system will not prove fatal or destructive. Achieving such a balance requires knowing in advance for which needs resources are entering and leaving urban systems [17
]. This returns the planning process to one oriented towards needs, which are met by carefully controlling the inputs and outputs of resources.
The alternative paradigm, of resources to needs, which the Deep City project is advocating, does not presume a global balance as a point of departure or return. This precludes thinking in terms of the shortest and least path of resistance for underground resources in the evaluation of their potential in diagnostic phases. The underground is no longer, like for Hénard or Utudjian, simply a source of stabilization mechanisms for a system to be optimized, but rather a source of novelty, of evolution and change. Novelty and invention rely on the importing of information that is otherwise as yet undetermined. Information theory, taking thermodynamics beyond the 19th century understanding, refers to this information as negentropy. According to Léon Brillouin [18
], there are in fact two types of negentropy or information, one that participates in maintaining the current capacities of the system and the other that was considered “free” or of “no specified physical significance”. Both are of relevance to a sustainable mapping of the underground that embraces the information available already within the system and that which circulates “freely” like noise on a channel, information whose meanings have yet to be interpreted [19
Most maps that account for the underground are needs-driven and sectoral. The evaluation of potential and suitability is already oriented towards a particular resource. In response to land scarcities, Helsinki, Hong Kong and Singapore are investigating the potential for the local geology to accommodate underground space [20
]. Cities like Montreal and Toronto, which are often cited in the literature as exemplary cases of underground urbanism, remain space-oriented. Their expansion has been driven more by private investment than a masterplan or a diagnostic map of space potential [23
]; potential was simply locally intuited case by case. In China, eleven cities are in some way incorporating underground space in their masterplans [26
]. The Qingdao case study presented by Zhao and colleagues [26
] is exceptional in that the construction potential is geographically situated and multi-level. However, the geological formations and groundwater are only incorporated as constraints on the space resource, not as potential resources in themselves. What is the use value of excavated materials? Could groundwater be reused locally? What about thermal conductivity for geothermal cooling or heating systems? The message that urban underground space should find its way into masterplans has been heard, but the fact that this “space” comprises multiple resources has not [27
Despite being oriented towards underground space, in these examples, the existing urban form and distribution of activities is addressed as a limiting factor. The Helsinki plan only includes existing underground structures and potential cavern entry points [20
]. The Hong Kong cavern master plan identifies openings and land ownership status [21
]. The method used in the Qingdao study includes distributions of land uses, population densities, transit networks or property status, but these are understood as indicators of possible demand [26
]. Lu and colleagues [29
], in their rail station case study, only evaluate geological suitability, independently of the station’s urban context. The urban form is not only a passive source of demand, but also supplies a particular potential that is both latent and actual. The existing urban structure is the result of accumulated spatial practices over time periods of different scales. This structure is maintained not only materially, but also socially, by the continued presence of activities and people practicing the spaces of the city [30
]. They have both imported information that has contributed to their evolution and encode a certain amount of possibility that is latent, but that can be deciphered intuitively [32
] and mathematically [30
2. Deep City: Resources to Needs
The Deep City project at the Swiss Federal Institute of Technology (EPFL) has for almost ten years been working on cartographical, legal, political, social and economic strategies to better address the urban volume (potential space, groundwater, geothermal energy and geomaterials) in the maintenance and evolution of urban areas and their surroundings [14
]. The cartographical method is founded upon the principle that all resources must be taken into consideration and that no resource in its exploitation should be considered in isolation from the synergies and conflicts it may have with other resources. This is markedly different from existing mapping methods where conflicts tend to be minimized at the expense of synergies. It is also important for the mapping method to be able to make use of the data on hand, particularly in urban contexts where limited use of the underground has produced general or very project-specific information. Additionally, as the maps are intended for use by non-geologists, the geological formations are classified into geotypes according to their genealogy (Quaternary, Mesozoic, etc.) and sedimentology (petrographic/mineral qualities) [34
Without geographical information systems (GIS), map overlaying is limited to the superposition of multiple layers. The quantification of criteria facilitates the synthesis and analysis of map data. Where existing quantitative data on the geological properties is scarce, the tacit knowledge of local experts is translated into quantitative data situating elements on a relative scale using the analytic hierarchy process (AHP) [35
]. AHP places n
elements in an n
matrix and evaluates each element pairwise according to a particular criterion. For Deep City, these criteria are the potential of each geotype to be a better alternative for space construction (compactness, granularity and consistency), as geomaterials (water content, granularity, value in the local market), for the presence of groundwater (water content and permeability) and for geothermal systems (based on conductivity), as derived from either local experts (geologists, hydrogeologists, etc.) or geotechnical and geomorphological descriptions of the formations that were used to define the geotypes. AHP can also be used to situate each resource on a relative scale of priority when passing from overall multiple to single-use potential, allowing the Deep City method to be relevant for needs, as well as resource-based mapping.
The analytic hierarchy process is a widely-used method for the quantification of priorities and preferences for evaluating underground resource potential. The case study of the city of Qingdao mentioned earlier quantified the suitability of the geology at multiple depths for underground space construction. Criteria include geological conditions, construction status and development value (including population distribution, location, land use and economic conditions) [26
]. Pairwise comparisons by experts determined the relative importance of each criterion. Lu and colleagues [29
] also evaluated geological suitability for underground construction referring to expert opinion on geotechnical properties, geological structure, geomorphology, hydrogeology and adverse geological conditions. Their case study of the Wuchang Railway Station mapped suitability values according to the depth at which development was forecasted. Both studies account for uncertainty in the pairwise scores and the relative importance of each criterion in the evaluation using fuzzy sets, which permits either the preference information or the aggregation process to vary over a set of values [36
]. Zhao and colleagues used fuzzy set qualifiers [36
] in aggregating the decision criteria in response to different attitudes toward risk by decision-makers. Lu and colleagues incorporate a fuzzy set directly into the pairwise comparisons as a way to address dissensus in the expert opinions. Fuzzy sets provide an interesting way of addressing risk and uncertain in terms of group membership.
The Deep City mapping method has been tested on three cities, with a fourth and fifth currently underway. A first case study conducted on the city of Geneva tested the use of the AHP for evaluating the resources and fuzzy qualifiers for handling different degrees of risk [12
]. A more extensive study was later carried out in collaboration with Nanjing University on the Chinese city of Suzhou, where the potential for underground space development was investigated using geological characteristics as supply criteria and urban characteristics as indicators of demand [37
]. The AHP was used without fuzzy qualifiers as an aggregation method. San Antonio, Texas, was taken as a case study of the city where there is no explicit demand for underground space, but a complex relationship to an urban aquifer. Case studies that are still underway include an evaluation of the basement potential for Hong Kong as a complement to the cavern map [21
], including urban form as a supply source for commercial activities, and on the city of Dakar in Senegal where groundwater is a challenging source of synergy and conflict [38
One major addition in the recent work, and one that constitutes the originality of the Deep City method in its current state, is the integration of surface or near subsurface urban connectivity into the evaluation of underground potential. We propose to decipher this potential in terms of the urban structure’s current capacity to support different types of underground spaces. The interest in incorporating underground space into strategies to increase urban density or preserve public spaces means that we have focused on mostly those types of activities that find themselves in the urban subsurface, notably parking and commercial activities. Both types of places tend to benefit from high centrality. Being centrally located is often necessary to offset the additional construction and land acquisition costs that building underground entails [39
]. Rather than situate population or existing building density as possible indicators of demand, as with our Suzhou case study [37
], we take them as constitutive of potential, because we are looking at what the urban form and the distribution of activities affords in terms of potential scales and dynamics of movement (rather than the movement itself through origin-destination surveys or tracking data). Research using centrality metrics like closeness (proximity) and betweenness (being situated on shortest paths) suggests that centrality at multiple scales provides a certain sustainability and resiliency in the urban form [40
], and this has been found to be beneficial in particular for commercial activities [41
The research presented in this paper will return to the San Antonio case study, building on a previous article [43
]. That paper evaluated the separate resource potentials using the AHP and aggregated them according to a scenario of underground space development using fuzzy qualifiers to account for different attitudes towards risk, similar to the work by Zhao and colleagues [26
]. Urban potential was calculated using centrality metrics (gravity and betweenness) of non-residential property to commercial spaces and residential spaces at 10-minute walks of 800 m. The results of the evaluation of resource potential indicate that the inner 410 ring (Figure 1
) has a relatively high (within the 25th to 5th percentiles) underground construction and geomaterial excavation potential in the north (among the limestone and marl geotypes) with the southern portion characterized by groundwater, geothermal and space potentials that are all situated within a mid to low potential (60th to 90th percentiles) amidst the sand and clay geotypes. A good portion (77%) of the central downtown area is characterized by a medium to low potential (within the 50th percentile or higher) for all four resources. The overall potential within the 410 ring increased from 0.54 (between zero and one) to 0.66 in the risk-taking aggregation scenario and decreased to 0.36 in the risk-averse scenario. The resource potentials of two transit stops along projected surface light rail and bus rapid transit corridors were examined in more detail. The Woodlawn stop, situated along a commercial street in a neighborhood, had an overall high potential (within the top 20th to 10th percentiles) for groundwater, geomaterials and construction of underground space, as well as a relatively high destination potential to both commercial and residential (top 5%), revealing its potential for transit-oriented development. The Westside Multimodal Transit Center is situated in a neighborhood on the edge of the central business district where it is relatively disconnected (80th percentile) from residential areas. Its highest potential is for underground construction (25th percentile), but its geomaterials are of little reuse value due in part to their level of saturation (placing them in the 65th percentile). Geothermal and groundwater systems may be of greater interest (45th to 40th percentiles), but the interactions between them would have to be carefully monitored. If the Woodlawn station scores higher for combined resource interactions in the risk-taking and risk-neutral scenarios, both locations score almost equally for the risk-averse scenario.
Taking the San Antonio case study as a point of departure, while avoiding unnecessary repetition of information presented there [43
], this paper will present an analysis that tests an alternative aggregation procedure. Rather than aggregating the four resources with an emphasis on underground space construction and incorporate fuzzy qualifiers as capturing attitudes towards risk, the aggregation procedure will weight each resource with equal value, and fuzzy logic will be used to identify a series of interaction profiles to which each location will be assigned membership. This will allow for a non-deterministic, risk-neutral approach to evaluating potential resource interactions, leaving the decision of the best or most optimum use to a later interdisciplinary planning process accompanied, of course, by more detailed site investigations. In the Results section, the Westside Multimodal Transit Center will be examined again under this alternative mapping procedure, comparing with the results from the needs-oriented method adopted in the aforementioned article.
4. Conclusions: The Map as a Compass
The interaction maps presented in this paper are intended to inform the brainstorming or visioning process conducted by planners or urban designers prior to master planning. As such, they are most appropriate for the strategic phase. Like a compass, they can orient the planner without indicating a particular direction. They ensure that the urban volume is thought of in its multiple uses and affordances. The maps could provide criteria for multidisciplinary design charrettes or competitions where participants are asked to imagine multi-use scenarios that push the boundaries of the urban volume. This is the contribution of maps that seek to establish the “conditions for the emergence of new realities” [15
] and the strength of cartography in the map overlay tradition [7
]. The challenge is bridging the cartographical gap between urban planners and designers and the various disciplines whose expertise provides valuable information for the creative process. Following a visioning process, or multidisciplinary design competition, specific objectives can be formulated, missing data added or collected (where possible) or the existing data updated. Multicriteria decision-making can then begin to prescribe different directions for the design and planning process to take. For the public transit corridors in San Antonio, this would have meant strategically placing the surface lines so that they are already situated over areas of high underground potential. Even if the current financial means or political climate are not pursuing underground development, the conditions are set for future possibilities. This is also why resource potential needs to rely on as little (but relatively good) data as possible. Where investment has not occurred heavily in underground construction or resource management, data may be scarce and the motivation to collect it for a master planning process absent or of low priority.
Although each city, in its geology, its decision-making structure and procedures and its political and economic climate, is different, there are several common conditions that will facilitate the operationalization of the interaction maps in the planning process. First, data collection should be cross-departmental yet centrally controlled. Each project or site investigation should be submitted to the city GIS database. This includes drawing up standards for data collection. We showed in this article that the maps can be produced without recourse to onerous quantities of data. However, more and more rigorously-compiled data would improve the maps. Second, if the data are centralized, the underground potential and interaction maps should not be. The process of the evaluation of geological potential is certainly not the sole responsibility of geologists, and geologists have the responsibility of becoming the stewards of the maps.
Third, a sustainable management of underground resources means considering all four of them where they exist and regardless of their current significance. In San Antonio, discussions with local decision-makers suggested that the public sector did not want to be responsible for managing geomaterial use, even if its use could be something that public policy encouraged. Perhaps more private sector involvement will be necessary with public sector encouragement. In Switzerland, the company Terrabloc (www.terrabloc.ch
) has been working with construction sites to use excavated materials for the production of terra cotta blocks, reducing the amount of material to be placed in landfills. The underground potential maps would help in identifying the sites where contractors would have the greatest interest in contacting companies like Terrabloc. The potential maps indicate areas where there are likely local aquifers or where the water table is high. In certain situations, it may be of interest to coordinate with the local water management system and permit local extraction, for example for irrigation. Geothermal energy is often given too little attention because fossil fuels remain inexpensive, although geothermal is only second after photovoltaic in being a renewable resource harvesting solar energy indirectly through the Earth’s crust. When placing resources before needs, geothermal energy will increasingly be harvested as a competitive (and cleaner) alternative to petroleum and natural gas.
Finally, the interaction and potential maps would need to be integrated into any form of territorial diagnostic conducted prior to or during the planning process. Part of centralizing the data collection is making the maps easier to produce using richer datasets, but the maps themselves need to be readily available for use by interested parties. In a city like San Antonio, the underground would likely only be considered long term. The message would not be to develop the underground at all costs, but to consider it where density is being increased and as an alternative to the seas of surface parking scattered throughout the downtown. The potential maps would serve to identify the areas of greatest interest, and then, the interaction maps would show which other resources are present. In some cases, the underground would be off-limits. Like aquifer recharge protection zones, it may be a question of a potential of high value. However, it can also be a question of protecting a resource, like geothermal in many places, for which there is more potential than current value. In zones of high underground construction and geothermal potential, any excavation project may be required to not only ensure that the excavated materials can somehow be used on site (or elsewhere), but also to make use of the local geothermal potential and reduce reliance on non-renewable sources.
Further work will fine-tune the methodology and test it in other contexts. Deep City is already conducting case studies on Hong Kong and on the city of Dakar in Senegal. One of the objectives is to test the pairwise comparison exercise with local experts, addressing specifically possible dissensus in the evaluation of geotypes. Where the comparison matrix incorporates judgments from multiple participants, sensitivity analysis should be conducted as a complement to the consistency index. A sensitivity analysis identifies the stability of the priority vector and of the compared alternative elements. The legibility of the information contained in the interaction and resource potential maps would be improved by developing a series of sectional drawings and by reminding people working with them that multiple depths are of concern. Future work should also test the maps in a design or planning setting. Where this may not immediately occur in practice (and where the status quo often prevails), the academic environment could test the interaction maps within a design studio and present its proposals to local actors and stakeholders for feedback.
Reversing the needs to resources paradigm in addressing underground resources means exposing first the latent potentials, before prescribing particular outcomes. As mentioned in the introduction, it takes this information as a source of uncertainty. While the prevailing ecological model of the city, in practice, treats this uncertainty as a problem to be eliminated, we propose here to first preserve it as a source of unforeseen opportunity. Following this, the advances made in resource management can contribute to dealing with the complex reality of resources flowing out of project sites or being better used, existing groundwater flow being harnessed or protected, as well as the concurrent development of geothermal energy systems. For the city to maintain its delicate imbalance, its maintenance practices must be able to evolve and to incorporate novelty and the unknown. The underground has always been a sometimes silent, sometimes noisy contributor to these practices. It is not a question of condemning it to silence, but rather of finding unheard opportunities in the noise.