Cities are intricate areas, where different elements interact. In the past, their expansion has generally occurred in the horizontal direction (urban sprawl) [1
]; despite this, underground urbanism was already conceived [4
]. According to previsions, 70% of the world’s population is expected to live in cities by 2050 [9
]. As a consequence of this rapid urbanization, space hunting is moving towards a three-dimensional trend [10
]: vertical urban development has thus been adopted to counteract urban sprawl [1
], thus increasing population density. This urban densification is leading to the building of deeper structures [12
], which increases the tendency to “go underground” [14
The increasing need for space in urban areas has recently enhanced urban underground consideration [8
]. Four subsurface resources are key to pursuing sustainable urban underground development: space for constructions, materials, water and energy [10
These resources interact with each other [22
]. In particular, a strong interaction between groundwater and underground infrastructures has been observed [21
]. In the last few decades, many cities around the world have faced a rising trend in groundwater levels, caused by the deindustrialization process. This produced some interferences between groundwater and underground infrastructures, such as subways, car parks and basements [25
]. The implementation of a geodatabase (GDB), including 3D locations and uses of the underground structures, could help to manage this issue [22
]. In this way, part of the large amount of data generally available in urban areas [33
], but not stored in a systematic way [34
], could be gathered in a unique structure. The GDB will contribute to process data to be used for groundwater management, thus enabling the definition of an urban conceptual model, a necessary step for 3D numerical groundwater flow modelling. For this reason, these data need to be integrated with geological, hydrological, geomorphological and other required information. Furthermore, the increasing interest in open data for urban management and groundwater issues is a topic to be considered [35
]. Indeed, the opening of data entails several barriers, related both to providers (i.e., incomplete or obsolete information) and users (i.e., complexity of using and interpreting data); however, a large number of benefits are related to open-data: among them, the improvement of policy-making processes, the creation of new information combining existing data, and avoiding repetitively collecting the same information are included [37
The city of Milan experienced a strong groundwater table rise in the last few decades [40
]. As numerical groundwater flow modelling is the primary tool for evaluating the interactions between groundwater and underground infrastructures [34
], different 3D models have been realized for the urban area of Milan [41
]. Among the underground infrastructures listed above, all these numerical models focused only on the subway lines: interactions between groundwater and car parks were not evaluated.
The aim of this work is to propose a methodology to estimate, on an urban scale, the volume of underground infrastructures lying below the groundwater table. This is the basis for a further evaluation of the impacts of the interaction between groundwater and infrastructures, such as the perturbation of groundwater flow by infrastructures or the groundwater flooding of non-waterproofed infrastructures. Three categories of infrastructures were considered in this study: (a) private car parks, (b) public car parks and (c) subway lines/stations and underground railway. To the best of our knowledge, this is the first time that car parks were considered in evaluating groundwater/infrastructure interactions in the city of Milan. On the contrary, car parks have been considered in numerical models in other towns [44
]. The last part of this work is devoted to the evaluation, within a pilot area, of the impact of groundwater (i.e., flooding) on non-waterproofed public car parks and subway lines and stations. The comparison between the results of this evaluation and actual flooding events, identified by local press reviews and photographic documentation reviews, helped to validate, qualitatively, the whole methodology. The methodology here proposed is developed for the case study of Milan—however, it could be applied to other cities worldwide with similar characteristics (i.e., municipalities characterized by a subsurface infrastructure development).
2. Study Area
The study area (Figure 1
) covers 440 km2
in the Milan metropolitan area, between longitudes 1,503,000 and 1,525,000 and latitudes 5,025,000 and 5,045,000 (Monte Mario Italy 1; ESPG: 3003). The city of Milan is inhabited by 1.4 million people [47
] and has had strong industrial and agricultural development [48
]. It is located in the Po Plain, which hosts a sedimentary aquifer system whose hydrogeological structure has been previously investigated in detail [49
]. Three main hydro structures can be identified: a shallow hydro structure (ISS), an intermediate hydro structure (ISI) and a deep hydro structure (ISP). ISS and a portion of ISI are visible in Figure 2
. ISS is mainly composed of gravels and sands and hosts a phreatic aquifer. In the study area, it has a medium thickness of 50 m and its bottom surface goes from 100 m a.s.l. (to the North) down to around 50 m a.s.l. (to the South). ISI hosts a semiconfined aquifer mainly composed of sand and gravels, with an increasing presence of silty and clayey layers compared to the upper hydro structure. Its bottom surface goes from 70 m a.s.l. (to the North) down to −50 m a.s.l. (to the South) for the area of interest, with an increasing thickness moving from N to S along the cross section (Figure 1
b, Figure 2
). ISP hosts a confined aquifer, but its composition is mainly uncertain due to a reduced number of available data.
Groundwater has been extensively exploited for industrial use since the early 1960s. The maximum water depletion (i.e., minimum groundwater levels) was reached in the years from the 1960s until the early 1990s, with a groundwater table more than 30 m deep in the northern sector. During this period, some underground infrastructures (car parks, subway lines) were built, sometimes with no waterproofing works [40
], neglecting the possibility of any future groundwater level rise. Since the early 1990s, because of the decommissioning of many industrial sites, groundwater levels have started to rise (i.e., with a maximum rise of about 10–15 m in the northern area), generating many problems for underground infrastructures. Nowadays, the rising of groundwater is still causing severe problems, as occurs in other European urban areas, such as Paris, Barcelona and London [25
As an example of the infrastructure development of the subsurface, due to its widespread presence within the study area, the subway network (Figure 1
b) is described in detail below. Its construction began in the 1960s, focusing on the shallow portion of the unconfined aquifer. A top-down design mechanism was adopted, following a first-come-first-served basis approach [12
]. M1 and M2 lines were built at first, with a cut and cover method to avoid interrupting the traffic on the main roads [43
]. Built during the groundwater drawdown phase, they were not designed with waterproofing systems [41
]. M3 line and the underground railway were built in the 1990s: due to their greater depth, they were designed with waterproofing systems. Both these constraints and the diffusion of new excavation methods [21
] have led to the building of the most recent lines (M5 line completed in 2015; M4 line, still in construction) at greater depths; these lines have been designed to reach the most peripheral areas of the city.
Furthermore, Milan’s vertical development has increased in recent years, implying a deepest subsurface occupation from the underground infrastructures. At the beginning of 2019, the new Plan of Government of the Territory (PGT) [54
] was adopted. It aims at reducing soil consumption and developing new sections of subway lines. This will lead to a greater underground occupation, thus requiring a coordinated management of all the assets involved and reliable information on their location and properties.
An urban transformation, which also involves the underground aspects, is taking place for the city of Milan. A detailed inventory of all the underground infrastructures is thus required.
The GDB has allowed us to gather part of the wide array of urban data, usually coming from different sources (institutions, stakeholders, public and private owners) [24
], standardizing dissimilarities among data to properly settle them for groundwater management needs. Due to the database’s simple and updatable structure, data that with time could become available in the future will in fact be rapidly integrated with the already existing information. Its realization has been aided by Geographic Information Systems (GIS): their capacity for storing, analyzing and managing all types of geographical data [66
] has allowed us to easily collect information coming from different sources in a single structure; moreover, the underground infrastructures were accurately reconstructed according to their real depths and volumes.
The methodology applied to define the underground occupation of private buildings (private car parks) is, to the best of our knowledge, an element of novelty; it attempts to fill a lack of information through a spatial analysis procedure, exploiting all the cartographic content available in the DbT. However, it still requires a phase of refinement. Indeed, in some cases, the underground volumes may be overestimated, as for those ramps that have a superficial development but do not lead to underground car parks (instead leading into buildings). In other cases, the underground volumes may be underestimated: the methodology fails to highlight those access ramps that fall within the perimeter of the building and therefore are not visible in the creation phase of the DbT; however, this latter case is not a very common building typology for the study area considered. Future developments will concern the consequent elimination of the overestimated elements.
The application of the methodology for the city of Milan was possible due to the availability of the DbT data distributed by the “Decimetro” geoportal [55
]. The DbT is developed according to the European Standards (INSPIRE) [68
]: this contributes to the replicability of the procedure in other study areas. Other factors are needed to strengthen the application of this methodology elsewhere: the availability of the same typology of data, a strict collaboration among institutions, the presence of a policy aimed at stimulating the use of open-data, and the expertise of using and extracting valuable information from data [37
]. The association distance between the ramp and the building adopted for the city of Milan may not be suitable for other urban realities, thus making a previous site-specific calibration necessary.
The integration of the DbT with other supplementary sources brings out a lack of collaboration among institutions, typical of urban data management [24
]; a closer cooperation among institutions would contribute to easily managing data both for urban underground planning and groundwater management aspects.
The GDB application has thus allowed us to evaluate how the subsurface volumes lying below the groundwater table have changed among time.
In general, in the northern part of the study area, considering an assigned depth of five meters, and a higher depth of the groundwater table, private car parks do not present volumes below the groundwater table. However, in a few cases, volumes lying below the groundwater table were also identified in the northern sector during maximum groundwater levels. This was associated with problems related to the Digital Terrain Model, which can be not fully representative of the ground level at a given point. This can be considered as a limit of the methodology: however, this problem emerged only in a few isolated situations.
The congestion of public car parks in the downtown Milan area is related to a high demand for infrastructure [12
], due to socio-economic needs: the majority of the economic activities is located in the city center [47
]. The volumes of the deepest infrastructures were shown to lie below the groundwater table: therefore, future infrastructures in this area should be planned with adequate waterproofing techniques. The reduced subsurface volume in the peripheral areas is related to a decreased socio-economic demand: despite this, as for the private car parks, volumes lying below the groundwater table were identified, in particular when the hydraulic head was higher. This is due to hydrogeological reasons: in the southern portion of the study area, the groundwater table has always been historically close to the ground level due to the presence of fine deposits (i.e., silt and clay) with low values of hydraulic conductivity [40
], which force groundwater to reach the ground level; in the western area, the presence of clay lenses determines the existence of a perched aquifer located around 6–8 m below ground level, with strong seasonal oscillations [73
]. An overall reduced presence of subsurface volumes (Figure 5
) in these peripheral areas, compared to the downtown, is also amenable to these reasons.
The majority of the subsurface volumes lying below the groundwater table for the subway line M1 is in the northern stretch, between Rho Fiera and Pero stations (M1-b): their construction method differs from that used for the rest of the line; these two stations were built at greater depths. For the same reason, Sant’Agostino station was revealed as the most recurring area below groundwater level for M2 line: its two rails were built as overlapping pipes, thus determining a major depth of subsurface occupation. As determined by the focus on the pilot area, in Dec02 and Dec14, the considered stretch of gallery from Porta Genova to Sant’Agostino (M2-a) was completely submerged, with the groundwater level above the top of the gallery. The stretch between Loreto and Udine stations (M2-b) was revealed as another critical area. In particular, the section between Piola and Lambrate stations (M2-c) was subjected to waterproofing works during the summer of 2019 to overcome flooding problems. Since these lines were built without any impermeabilization, the increase in stretches lying below the groundwater table due to groundwater rising, both for the M1 and M2 lines, should be monitored by the subway managing company. Due to their depth, M3 line and the underground railway revealed a high percentage of subsurface volumes below the groundwater level: to overcome this problem, they were designed with waterproofing systems; M3’s interaction with groundwater in the southern sector of the domain is amenable both to a deeper development of the line and a closer elevation of the groundwater table to the ground level.
As reported in Section 4.3
, the methodology allowed us to verify what was already described in a previous work [65
], where M1-a, M1-c to M1-g, M2-a and M2-c to M2-e areas were already pointed out as the most critical concerning groundwater/infrastructure interactions. At the same time, as reported in Section 4.4
, flooding evidence, also reported by local press reviews, occurred where the oldest underground infrastructures, showing volumes below the water table, were designed without waterproofing techniques. This acts as a qualitative validation both of the methodology used to implement the GDB and its usefulness in groundwater management. In the future, citizen science approaches [74
] or social media (i.e., tweets of metro passengers) could be exploited to validate the methodology, thus enlisting the public in organized scientific research. Both the city administrations and private companies could benefit from the implementation of this methodology, identifying the main critical areas of interaction, thus properly planning future underground development or adopting remediation strategies if necessary, especially focusing on the oldest non-waterproofed infrastructures. The GDB has in fact allowed us to analyze the interaction between groundwater and underground infrastructures both at a city scale and at a more detailed level.
The integration of the GDB with numerical groundwater flow models will make it possible to define future scenarios of interaction according to the trend of the piezometric levels. The infrastructural elements have both an active and passive effect on groundwater [41
]. This contributes to characterizing urban modelling as a specific branch of hydrogeology, with its own time, scales, and dynamics of the hydrogeological processes [85
]. Thus, this information needs to be analyzed and combined together with the large set of geological, hydrological, geomorphological and other features [86
] necessary to detail a complete urban conceptual model for the domain: this is an important step, as the conceptual model is the basis of an appropriate groundwater management plan. Using a standardized 3D GDB, the urban conceptual model would not need to be frequently revised, a both time- and cost-consuming activity [85
The implementation of a 3D vision of the volumes below the groundwater table over time (Figure 11
) was revealed to be a comprehensible tool to evaluate this phenomenon: an increased use of these instruments will both guarantee a complete 3D vision of the subsurface and a proper 3D urban planning. The use of the GDB in a wider coupled 3D GIS–groundwater model (such as MODFLOW [87
] or FEFLOW [88
]) system will be thus efficient to plan sustainable and integrated groundwater management, helping local stakeholders and regulators to manage not only groundwater, but all the underground resources in a more efficient and sustainable way. To this aim, the use of tools as WebGIS services could guarantee an effective way of spreading the existing information.
Moreover, an easy identification of the main underground infrastructures will help to overcome the lack of coordination, lack of planning and lack of understanding of the other domains among the different stakeholders [19
], thus avoiding jeopardizing the potential of the resources below the city [19
]. Considering the urban development declared in the Plan of Government of the Territory, the GDB would contribute to maintaining the underground potential, guaranteeing a long-term management of the urban underground space.