3D GIS Supporting Underground Urbanisation in the City of Turin (Italy)

Transcription

1 3D GIS Supporting Underground Urbanisation in the City of Turin (Italy) Francesca de Rienzo, Pierpaolo Oreste & Sebastiano Pelizza Dept. of Territorial, Environmental and Geotechnological Eng. Politecnico of Turin, Italy ABSTRACT This paper introduces a 3D geological and geotechnical model of the subsoil of the city of Turin managed by means of a Geographical Information System (GIS). The 3D GIS of the subsoil of Turin represents a useful decision-support tool in the underground management for engineering purposes and it s here proposed as base geological elaborate to support future underground work in the city. In the final part of the paper, an application of the information coming from the 3D model is shown to define the characteristics of the optimal excavation machines (the type and disposition of tools on the head and the necessary engine power) for the future developments of the Underground Metro System. 1 INTRODUCTION The subsoil of Turin is, at present, subject to the construction of two important rail transport infrastructures: Metro Line 1 of the Automatic Underground System and the Underground Railway Link, this later being part of the improvement works of the Turin Railway Link. In view of the development of the transport infrastructures, the performed geological and geotechnical 3D model of the subsoil of Turin represents a base elaborate in underground urbanisation. Any specific project in underground space development will start by the general task to find the suitable location, followed by the discussion of building alternatives. Both aspects are directly influenced by the geological setting and geotechnical aspects of the underground space. The geologic information, which consists of complex sequences of interrelated three-dimensional geologic surfaces, is often spatially represented in two dimensions. Nowadays 3D representation of such information is afforded with the rapid advances in computer technology enhancing the interpretability of the model. The model created by this method can grasp three-dimensional space more easily rather than it imagines from a geological cross section and a geological horizontal projection of two-dimensional. 2 GEOLOGICAL AND GEOTECHNICAL MODELLING The city of Turin (Northern Italy) is situated to the western side of the Po plain on the downstream side of the large alluvial fan of the Dora Riparia river. From a morphological point of view the studied area gently dips towards east passing from m a.s.l. to about 230 m a.s.l. close to the Po river. Existing studies (Civita & Pizzo, 2001) indicate, in the investigated area, the location of the water table between 235 m a.s.l. and 215 m a.s.l. with flow direction towards east where the Po river represents the base and subsurface water collection level. Official references (Bortolami et al., 1969; Bonsignore et al., 1969) geologically described this area as having the following series from the bottom upwards: - sandy sediments of the Marine Pliocene environment; - alternations of clayey and fluvial-lacustrine gravel layers and lenses (Villafranchian); 397

2 - fluvial and fluvial-glacial sediments (Quaternary) which made up the bedrock of the city of Turin (from 30 m to 70 m in thickness) and are characterised by a conglomeratic horizon with a variable degree of cementation; - Clayed löess wich cover the alluvial horizon. The alluvial sediments, making up the substratum of the city of Turin and in general of the plain sector between the Alpine Arc (N-W side) and the Turin Hill (E side), are considered to be the results of several depositional events of the wide alluvial fans of the Dora Riparia and the Stura of Lanzo. Lucchesi (2001) in a study concerning the central Piedimont plain reconstruct the feature of the lower bounding surface of the Quaternary cover. In the present study about 400 boreholes, 70 DAC-test (continuous automatic diagraphy) and 120 SPT (standard penetration test) were found. The collected data have been located on the Technical Map of the Piedmont Region, georeferenced and marked with an identification number. The collected data has been analysed using litho-stratigraphic criteria thus to identify the relationships between the geological bodies. The sediments founded in the first 40 m below the surface are from the bottom towards the high: - yellow fine sands with marine fossil remains ascribed to the marine Pliocene stratigraphical succession the bottom of which have not founded in the analyzed data; - clayey sediments with lenses of gravels and organic remains were reached with the deepest boreholes until 80 m in depth, these sediments have been attributed to the fluvio-lacustrine stratigraphical succession known as Villafranchiano (Upper Pliocene- Lower Pleistocene); - a surface alluvial horizon made up of gravel and pebbles in sandy matrixes with silty and sandy layer are widely diffused throughout the area under examination these sediments have been ascribed to the Pleistocene fluvio-glacial succession. Basing on these criteria the boreholes data have been analysing through about 100 geological sections and fence diagram to lead up a geological 3D model of the studied area. The geological model shows that the alluvial Quaternary horizon mainly constitute the subsurface of Turin with a thickness variable from more than 70 m in the western portion of the studied area, to 30 m in the eastern sector where is placed the historical centre of the city of Turin. Besides the lithology, the aspect manly come out from the boreholes interpretation is the cementation variability of the fluvio-glacial deposits described as: a) poorly cemented sediments, b) cemented sediments, c) highly cemented sediments. The existing references (Bonsignore et al., 1969) points out the presence of a conglomerate horizon with different degrees of cementation (called ceppo ), which is attributed to the Mindel-Gunz Interglacial stage. From the analysis performed in this study, no lateral continuity of the most cemented sediments can be deduced that would allow them to be defined as stratigraphic units and the cementation appears variable both longitudinally and vertically with sometimes complex geometry. Given the great conditionings of this property on the geotechnical parameters and on the scavability of the alluvial terrain, as the aim of the performed geological model to furnish a base elaborate for future underground opera realization in the Turin subsoil, the cementation degree has been identified as a unit and its variability spatially described in the investigated area. As one single cemented horizon was not encountered but rather discontinuous decimetric levels, to describe the cementation variation three cementation classes were distinguished on the basis of the stratigraphic report. The presence and consistency of the cemented levels as encountered in the stratigraphic descriptions have allowed to identify as C1 class loose deposits with local weakly cemented lenses, C2 class loose deposits with frequent highly cemented levels and C3 class deposits ranging from cemented o very cemented (pudding-stone) with local alterations of loose levels. The existing collected data (in situ test) and new data coming from laboratory test compared with the stratigraphical description of the boreholes have shown different geotechnical parameters according to the boreholes description, that has allowed to both testify the cementation variability (poorly cemented, cemented and highly cemented sediment) recorded in the boreholes and to assign different geotechnical parameters to each cementation class. The new performed laboratory test, executed on conglomerate samples belong to the alluvial horizon, have involved: uniaxial compression test, OECH shear test and 398

3 point load test which determine a point load index that thorough a conversion factor allow to valuate the compressive uniaxial strength of the core. The geotechnical results for the C2 and C3 cementation classes, also compared with precedent study (Barla & Vai 1999) are shown in the table 1. Table 1. Geotechnical parameters for each cementation class. E (MPa) Φ( ) c (kpa) C C C To determine the elastic module (E) of the C1 class, which refers to poorly cemented gravels in sandy matrix with pebbles and silt levels, a back analysis has been performed starting from monitoring data measured during the excavation of a metro station in Turin. The reconstruction of the spatial distribution of the cementation in the subsoil of Turin show that the C3 class which refers to highly cemented gravel and sands is major concentrated in the ancient centre of the city between 20 and 30 m below the surface, and in general, instead of what known in literature as a conglomeratic tabular horizon, the reconstruction of the cementation variability has shown a marked spatially discontinuity. 3 THE 3D GIS MODEL OF TURIN SUBSOIL AND ITS APPLICATION Geological data, especially boreholes, can provide useful information about both surface and underground conditions of the earth. Thus, they have been frequently used in a number of fields such as civil construction, natural resource exploration, environmental problem, transportation and so on. Especially in urban area the geological data from many construction projects should be standardized, structured, archived and properly used through suitable system and applications for efficient management Applications using Geographic Information System (GIS) are very essential to maximize the sharing of geological information and to solve problems related to geotechnical engineering. One of the most commonly applications of the GIS in the field of geology is the geological mapping: with the help of GIS, maps of any scale can be scanned, georeferenced and reproduced in any desired scale thus bringing all old maps to one scale, at which more information can be collected (either by field investigation or by remote sensing techniques) to prepare a final updated geological map. In the ambit of the underground city area urbanisation the base elaborate to support the project design have to represent a spatial variability of the involved sediments which mean to represent a volume. Points, lines, polygons, surfaces, and images comprise the list of geographically located entities in a GIS. 3D geology requires only one more entity, a volume. Tanks to the advances in digital visualisation and modelling techniques nowadays is possible to express geological knowledge of subsurface in easy-touse digital 3D models. The dynamic digital 3D models are built by using existing data sets, as example by using drilling down hole and outcrop information the GIS model allow to build up a net of consistent cross-sections and to define the spatial distribution as well as top and base of each geological unit. These 3D models can be supplemented by artificial ground distributions and manmade modifications of the subsurface, as infrastructure lines, tunnels, building foundations. Further processing of the 3D structure-models allow an attributing of each individual geological unit, sub-unit or lenses in terms of engineering conditions or hydro-geological parameters, enabling any user to customize his visualization and analysation of the subsurface for his individual tasks in the fields of mineral resources exploration, engineering-geology, hydro-geology, or environmental-geology. As example, many mining activities (from exploration and production to mine rehabilitation) with the advent of the GIS, evolved from pure luck to science. In the mining exploration phase mapping of mineral potential using GIS is conducted to delineate areas with different probabilities of hosting 399

4 certain types of mineralization. In production planning, GIS can help to site and query the location of service facilities relative to the main production centre. The three-dimensional GIS model specifically developed to manage the geological-geotechnical model of the subsoil of Turin is based on the Intergraph Corporation MGE software. The GIS model was built in different stages: first, the stratigraphic data were analysed according to the aforementioned criteria and then organised in the three stratigraphic successions that were identified. A relational-type data bank was then compiled which was based on a series of interconnected modules that could be used together. The database containing information concerning the positioning and type of data, the lithological characteristics and the geotechnical parameters, allowed all the available data to be accessed, consulted and analysed according to specific modules for the querying of the database. The data base structure also allows an easy integration to be made of the existing data and other information that could become available in the future. Beyond to the stratigraphical units the database contains the three cementation classes identified in the alluvial horizon. By using the Erma (Environmental Resource Management Application) Data Manager and Erma Site geologist module the input data have been managed from the database to generate 3D geological profiles (Figure 1) and DTM (Digital terrain model) of the geological boundary surface recognised in the studied area. the DTM allow to visualise the spatial distribution pattern of each horizon and the variation of their thickness, observe the presence or absence of a given horizon at any particular point and compare the spatial distribution patterns of the various horizons. Fig. 1. 3D geological profile generated by the GIS showing the subsoil of Turin. White, clear grey and dark grey indicated the cementation degree (C1, C2, C3) of the alluvial sediments, the pointined area is the fluvial lacustrine deposits and the stripped indicates marine pliocenic sands. At last by means of the software Voxel Analyst the geological data have been interpolated to achieve a spatial representation both of the sediments and of the cementation variability in the subsoil of Turin. Being each cementation class identified also with geotechnical parameters the model represents also the geotechnical spatial distribution in the alluvial horizon. The 3D representation shows the soil as a volume in space and conserves the advantages of a crosssectional representation. It contains the same information but it presents them in a volumetric view and allows several cuts, following different perspectives, to be made simultaneously. This provides a greatly increased clarity and simplicity of view, thus constituting a support tool in the design of underground projects. The performed model has been applied to several technical studies concerning the realisation of the Metro line at present in progress in the subsoil of Turin. Basing on the mechanical characteristics of the soils as identified in the 3D geological-geotechnical model these studies achieve a forecasting analyses of the subsidence induced on the ground surface by the excavations for the metro line and the evaluation of the reciprocal influence between the metro line and the existing Underground Railway Link in the centre of the city. 400

5 A 3D geological-geotechnical model, such as the one set up for the subsoil of Turin, is also able to supply very useful information for the definition of the head characteristics of excavation machines for metro tunnels. It can in particular be a base in the choice of the number of disks and rippers that must be positioned on the excavation head and in the definition of the necessary engine power of the head. More compacted or cemented soils require a greater number of disks and a lower number of rippers, while the opposite is true for softer and less cemented soils. The identification of the changes in the cementation classes along a tunnel section therefore allows the specific characteristics of the excavation head to be chosen for each single homogeneous section. In the case under examination, for example, the 3D geological-geotechnical model pointed out the crossing from a C2 cementation class to a C3 class in correspondence to Piazza Statuto (Figure 2). The functioning parameters of the excavation machine (rotation speed, advancement speed, total thrust and torque applied to the head) monitored during the Metro line realization have allowed to obtain a trend of the excavation specific energy E as the energy spent to drill a unit volume of soil. The excavation specific energy, in correspondence to Piazza Statuto (Figure 3), increased from a mean value of about 30 MJ/m 3 (kept almost constant in the C2 cementation class) to values that, at a local level, were higher than 45 MJ/m 3 (values that can be associated to a C3 cementation class). As shown the comparison between the data monitored and the performed model allowed the forecasts to be confirmed. Fig.2. Cementation classes at 20 m below the ground surface in the zone of Piazza Statuto. It can be noted a crossing between the C2 and C3 class. Fig. 3. Excavation specific energy (E) measured starting from the functioning parameters of the machine along the Metro route in the zone of Piazza Statuto. 401

6 4. CONCLUSION The development of the urbanisation of large cities underground requires a base elaborate to support the plan of the opera by the way of a detailed geological and geotechnical knowledge of the involved terrain. The forecasting of the spatial distribution of the geological bodies is fundamental to optimize some technical choose as for instance the equipment excavation machine in a tunnel realisation. Basing on a great amount of subsoil data a three-dimensional model of the subsoil of Turin have achieved in view of future underground project in the city. The model shows a spatial distribution of the geological bodies and the geotechnical variability of the sediment which made up the bedrock of the city. One of the most interesting results of the study regards the description and 3D visualization of the degree of cementation at various depths from the surface. The nature and intensity of the cementation of the soils have in fact important consequences on any possible static problems connected to the construction of the underground works and on the excavation techniques that result to be necessary to adopt. Thanks to the management by mean of a GIS the achieved model allow to storage a great amount of data, previously analysed and organised in units (stratigraphic, lithological, geotechnical etc.), and rapidly construct geological or geotechnical 2D and 3D views. This allow several multidisciplinary application for instance the model has been used as base elaborate in technical studies concerning the evaluation of the subsidence induced by the excavations of the Metro line. The easily visualisation of the geological setting and the spatial variation of the geotechnical parameters along the planning axis of an underground work allow to optimize the technical and geometric characteristics of the project. The application reported in the final part of the present paper regard the comparison between the forecasts of the degree of cementation supplied by the 3D model along a metro tunnel tract and the specific excavation energy effectively absorbed by the head of the EPB machine that was used. From this comparison it was possible to note the good reliability of the model and also its potentiality in the defining the characteristics of the optimal excavation machines (the type and disposition of tools on the head and the necessary engine power) for the future developments of the Underground Metro System. REFERENCEES Barla, G., and Vai, L., Geotechnical investigation to characterize the subsoil of Turin alung the line of the Underground Railway Link,, XX Italian National Congress of Geotechnic Parma, Sept., (in Italian). Bonsignore, G., Bortolami, G.C., Elter, G., Montrasio, A., Petrucci, F., Ragni, U., Sacchi, R., Sturani, C., Zanella, E., Illustrative notes to the Geological Map of Italy 1: : Sheets (Turin Vercelli), Serv. Geol. It. Roma (in Italian). Bortolami, G.C., Crema, G.C., Petrucci, F., Sacchi, R., Sturani, C., Zanella, E., Sheet 56 Turin, of the Geological Map of Italy 1: II Ed., Serv. Geol. It., Roma. Civita, M., and Pizzo, S., The spatial temporal evolution of the piezometric level in the aquifer of Turin subsoil, GEAM Geoingegneria Ambientale e Mineraria, n 4, 104 (in Italian). Lucchesi, S., 2001, Preliminary synthesis of the subsoil data of the Central Piemontese Plain GEAM Geoingegneria Ambientale e Mineraria, n 2-3, 103,