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Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 1 1 INTRODUCTION The demand of energy in the world is growing continuously and is currently satisfied mainly by oil, natural gas and coal. As an example, Italy fulfills 43% of its demand by importing oil and is increasing the use of natural gas and solid fuels. At the same time high levels of air pollution and lack of green spaces affect the major cities of the world leading to an increasing use of the underground for transportation and utilities. Based on the above it becomes more and more necessary to develop local and low environmental impact energy resources, else than nuclear power (which is often associated to non-avoidable risks and dependent on the possibility of long term storage of radioactive waste) and hydroelectric energy (which has reached its maximum expansion). In this context Geothermal Energy may play an important role and this calls for the need to investigate the growth potential of its technology. Electric energy is currently produced by converting heat thanks to turbo generators. Heat is extracted at depths that are economically feasible. However, heat extracted may also be used for domestic heating or to produce hot water. This is currently done with closed circuit systems or by retrieving water from wells and re-injecting it in the aquifer after heat exchange. Underground civil infrastructures can be used as heat exchanger with little effort but with great economic and environmental benefit. The thermal activation can be obtained by installing absorber pipes in the geo structures in which the working fluid extracts or injects the heat from or into the ground. Most current practical applications are related to energy piles and retaining wall (Brandl 2006, Adam & Markiewicz 2009, Laloui & Di Donna 2011, Nicholson et al. 2013) but some examples of energy tunnels were recently proposed (Wilhelm & Rybach 2003, Brandl 2006, Markiewicz & Adam 2003, Schneider & Moormann 2010, Franzius & Pralle 2011, Lee et al. 2012, Nicholson et al. 2013, Zhang G. et al. 2013). With respect to building foundations, tunnels involve a larger volume of ground and surface for heat exchange. When mechanized tunneling is used, tunnel lining segments are precast in factory and then placed on site by the TBM. They can be therefore prepared and optimized for heat exchange by including Geothermal heat from the Turin metro south extension tunnels M. Barla and A. Perino Dept. of Structural, Geotechnical and Building Engineering, Politecnico di Torino, Torino, Italy. ABSTRACT: Underground geotechnical structures (piles, diaphragm walls, tunnel linings, anchors, etc.) can be instrumented to become energy geo-structures. This paper focuses on the use of energy tunnels, which are shown to have a number of advantages. An example of a possible application to the Turin Metro line 1 South Extension is discussed. Preliminary results of numerical analyses, performed to study the hydro-thermal interaction and the influence of the energy tunnel on the surroundings, will be described. These show that the energy stored in the ground can be exploited without generating relevant effects on the aquifer, highlighting the importance to improve the understanding of the geothermal process and to explore the potential of energy tunnels, allowing for great economic and environmental benefit. Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 2 hydraulic circuits or more advanced heat exchangers. The fluid may also allow cooling the tunnel from the heat produced internally. As a result, the geothermal potential is high also for shallow metro tunnels. A number of aspects still need to be enhanced in order to be able to fully exploit this resource and consider it as an alternative to traditional heating systems: 1) A first key aspect is connected to the design of a technological system that allow for a real application. New materials and techniques that can allow optimizing the process have to be taken into account together with commercially available ones. 2) A second aspect is the ability to quantify the heat that can be exploited from the ground as a function of the exchange surface available in order to verify if the process is sustainable from a technical and economic point of view. For this reason it is necessary to study the thermal interaction between the ground and the structural elements by advanced numerical modeling. 3) The third aspect is the sustainability of the system in the long term. Problem may arise both with long term behaviour of the tunnel lining, which may have important drawbacks on the lifetime of the infrastructure, as well as the influence on the surrounding ground in terms of increase of the temperature in the aquifer and subsidence and deformations in the surrounding buildings. This paper focuses on a possible application of energy tunnels to the Turin Metro line 1 South Extension, currently under construction, to exploit heat for the Regione Piemonte new headquarters skyscraper, which is also under construction in the near vicinity of the tunnel. Preliminary results of numerical analyses, performed to study the hydro-thermal interaction and the influence of the energy tunnel on the surroundings, will be described. 2 THE USE OF ENERGY GEOSTRUCTURES AS HEAT EXCHANGERS Thermo-active geotechnical systems or energy geo-structures are instrumented underground structures that allow exchanging heat with the ground. As described by Brandl (2006) and Adam & Markiewicz (2009), all geotechnical structures can be instrumented to become energy geo-structures (e.g. piles, diaphragm walls, basement slabs or walls, tunnel linings, anchors in tunnels or in retaining structures, etc.). In general, a geotechnical structure can be activated thermally by installing plastic pipes in the concrete. The fluid flowing in the pipes constitutes the means to transfer heat from the ground to the buildings or vice versa thanks to heat pumps. Like refrigerators, a heat pump operates on the basic principle that fluid absorbs heat when it evaporates into a gas, and likewise gives off heat when it condenses back into a liquid. Restricting the discussion to energy tunnels, the thermal activation of a concrete lining can be mainly done in two ways: 1) cast in-situ: the absorber pipes are attached to non-woven geosynthetics off site and then placed between the primary and secondary lining (Markiewicz & Adams, 2003); 2) precast: concrete elements of lining are instrumented in factory with absorber pipes and each segment is connected in-situ with particular sleeves (Pralle et al., 2009). The thermal activation of the lining may produce effects inside the tunnel and in the surrounding ground. Inside effects are essentially related to the air temperature and depend on whether the underground structure is a cold or a hot tunnel. A cold tunnel is characterized by air temperature close to the temperature of the external air while in a hot tunnel the air is heated from the surrounding ground and from humans and rail traffic. In hot tunnels the internal air temperature is higher than the ground temperature. After the thermal activation, cold tunnels can be used efficiently as exchangers, connected to a heat pump, for heating and cooling buildings. In this case, the heat is extracted from the ground. Conversely, the exchanger systems installed in a hot tunnel are efficient only for heating but allow to cool the internal air of the tunnel. Documented examples of thermal activation of the tunnel lining can be found in Markiewicz& Adams (2003) with reference to the Lainzer tunnel of the U2 extension of the Vienna metro, in Schneider & Moormann (2010) for the German Fasanenhof tunnel and in Franzius & Pralle (2011) for the Austrian Jenbach and the German Katzenberg high-speed railway tunnels (Table 1). Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 3 Table 1. Comparison among documented applications of thermal activation of tunnel lining* Tunnel name Lining type qG S QG QHP [W/m2] [m2] [kW] [kW] Katzenberg tunnel, Germany (Pralle et al., 2011) precast 17÷25 60 1.0÷1.5 1.4÷2.0 Jenbach tunnel, Austria (Franzius & Pralle, 2011) precast 10÷20 2036 20.4÷40.7 27.1÷54.3 Stuttgart- Fasanenhof tunnel, Germany (Schneider & Moormann, 2010) cast in- situ 3÷8 360 1.1÷2.9 1.4÷3.8 * qG = heat flux extracted, S = thermal activated area, QG = power extracted, QHP = power generated by a heat pump with COP (coefficient of performance) of 4. 3 METRO TORINO LINE 1 Construction of the completely automatic underground Metro Torino Line 1 represented a dramatic innovation of the city transportation system. The system started to play its role in the early 2006 just a week prior to the Winter Olympic Games that took place in the city. A second section of the line was constructed between 2006 and 2011 to connect Porta Nuova and Lingotto stations. The total length of the line in service is about 13.4 km and includes 21 stations. A new South extension of the line (1.9 km and 2 stations) towards Piazza Bengasi is currently under construction (Figure 1). Metro Torino uses the VAL 208 (Automated Light Vehicle) system, an excellent solution from the safety, reliability and cost standpoint. The main tunnel was excavated at an average depth of 15-20 m using four EPB Tunnel Boring Machines (Figure 2), 7.8 m in diameter and 80 m in length. The tunnel lining is formed with precast concrete segments (7 sections a day were completed for an average progress rate of 10 m/d). Major geotechnical issues for the project along the line were constituted by a number of manufacts to be under-passed: the underground rail link, with excavation below the water table, an underground car park, the historical Pietro Micca defensive tunnel system (dating back to the Great Siege to Torino in the 1700) and a number of buildings. Figure 1. The layout of Metro Torino Line 1, with the South Extension shown in green. Figure 2. The EPB TBM entering a station. Figure 3. The Metro Torino today. From the thermal point of view, the tunnel is of a cold type as ventilation is guaranteed by a Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 4 number of wells that inject external air in it. Moreover, the Val 208 is a four-car train that runs at a top speed of 80 km/h on rubber wheels that allow high acceleration and low vibrations (Figure 3). Traction and electrical brakes are provided on every wheel and the heat generated by these trains is very low. 4 SUBSOIL CONDITIONS Torino metropolitan area, located at the western edge of the Po valley, has an overall surface of about 130 km2, 80% of which are a level area enclosed by the rivers Stura di Lanzo, Po and Sangone, while the remaining 20% are made up by a hilly area connected to the low reliefs of Monferrato. The city area is situated on the end section of the great alluvium fan of the Dora Riparia river and, from the morphological point of view, appears to be almost flat with a weak dip starting from West and going towards East, with elevation ranging from 260-270 m a.s.l. to about 220 m a.s.l. (Bottino & Civita, 1986). The subsoil conditions in Torino are characterised by the presence of a sand and gravel deposit, ranging from medium to highly dense, down to a depth of 8 to 10 m; below this depth lenses of cemented soil (in cases a conglomerate) are often present. The ground water level along the tunnel axis is well known based on piezometric measurements and predictions on long term conditions have also been carried out. Table 2. Deformability and strength parameters assumed for design purposes (Barla & Barla, 2012)* Unit C% Dr γn E ν' φ' c' % % kN/m3 MPa - ° kPa GU1 50÷60 17÷19 10÷20 0.35 36÷37 0 GU2 0÷25 50÷70 18÷21 190÷240 0.30 37÷39 0÷30 GU3 25÷50 60÷80 19÷22 240÷300 0.30 37÷42 15÷80 GU4 50÷75 60÷80 19÷22 300÷340 0.30 39÷48 50÷200 * C% = percentage of cement, Dr = relative density, γn = natural unit weight, E = Young’s modulus, ν = Poisson’s ratio, φ’ = friction angle, c’ = cohesion With the available data from geotechnical investigations and numerical analysis performed in the past, a geotechnical model for the subsoil conditions has been derived. For the depth relevant to tunnelling, four soil layers (Geothecnical Units, GU) were defined (Barla & Vai 1999, Barla & Barla 2005, Barla & Barla 2012) with the parameters given in Table 2: - GU1: superficial layer (filling and clayey sandy silt); - GU2: sand and gravel from loose to slightly cemented; - GU3: sand and gravel from slightly to medi- um cemented; - GU4: sand and gravel from medium to heavi- ly cemented. 5 ENERGY TUNNELS FOR THE SOUTH EXTENSION The South extension of the Turin Metro line 1 would provide a good opportunity to test the technology in the Torino subsoil. The section of interest is located in the South East suburbs of the city and includes two stations “Italia ‘61” and “Bengasi”, two ventilation shafts (PB1 e PB2), the end shaft (PBT) which is located 200 m beyond the last station (Bengasi) and an intersection tunnels to allow for future distribution line to the Lingotto railway station. The 1.9 km long section is to be excavated by an approximately 8 m diameter (internal diameter 6.8 m and external diameter 7.4 m) shielded EPB TBM, with the exception of the section immediately after the Lingotto station (length: 125 m), already in operation. The average depth of the tunnel is 16.5 m and excavation takes place below the water table. The pre-cast concrete segmental lining is constituted by 7 elements (thickness 30 cm) mounted by the TBM itself. Cement foam is injected to guarantee full contact with the ground and the segments are appropriately sealed in order to avoid groundwater ingress. The tunnel runs very close to a new 220 m tower under construction to host the new headquarters and offices of the Piemonte Regional Government (Figure 4). The proposal considers the possible use of the tunnels to exploit heat for the Regione Piemonte new tower. The tunnel itself may constitute the geothermal heat exchanger where a thermo-fluid is conveyed through a net of pipes submerged in the concrete of the segments (Figure 5). Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 5 Figure 5. Sketch of the installation scheme of the geothermal pipes submerged into the concrete segmental lining of the Turin Metro Line 1 South Extension tunnel. The pipes for these applications are fabricated in reticulated polyethylene (Pe-Xa) and composed by three strata: the inner strata with high-density polyethylene, the intermediate strata in polymeric material and the outer strata that is formed by a barrier in ethylene vinyl alcohol (EVOH) which avoids permeability to oxygen. The pipes are able to withstand high pressures and temperatures, resistant to corrosion and guarantees high durability. Connections between segments are completed after the segmental lining is installed and are inspectable. The thermo-fluid is a glycol propylenemixed with water that can work down to a temperature of -20°C. In order to allow for easy inspection during the tunnel lifetime, with the metro system in service, the inflow pipe and the outflow pipe could be located in the sidewalls of the tunnel, below the security pedestrian footpath. The system described would allow to activate a total length of the tunnel of 1350 m. Assuming an extracted heat flux of 10 W/m2 production can reach 450 kW to cover at least half of the building heating/cooling demand year round. 6 PRELIMINARY NUMERICAL MODELLING With the aim to study the influence on the environment if the geothermal system is put into service, a number of coupled hydrothermal fluid flow analyses were executed with the finite element method (FEM) and the FEFLOW software (Diersch, 2009) within the framework of a research project at the Politecnico di Torino. At the site, the water table surface is about 12 m below the ground level and the thickness of the aquifer is estimated in 22-23 m. The ground around the tunnel was assumed isotropic and homogeneous. The hydraulic, hydro-dispersive and thermal parameters of the aquifer used in the numerical analyses, obtained from in situ pumping tests where the temperature and the hydraulic level were monitored for three months (Barla et al. 2013), are given in Table 3. The water in the aquifer has an average temperature of 14°C and flows towards the Po river (versus SE). Figure 4. View of the Turin Metro Line 1 South Extension tunnel alignment and the geothermal plant connected to the Regione Piemonte tower (vertical and horizontal scales are different). Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 6 Table 3. Hydraulic, hydro-dispersive and thermal parameters (Barla et al. 2013) Parameter Value Horizontal hydraulic conductivity Kh (m/s) ⋅10-3 3.8÷4.5 Hydraulic conductivity ratio Kv/Kh 0.05 Specific yield Sy (-)⋅10-2 4.0 Porosity n (%) 25 Heat capacity of the water cwρw (MJ/m3/K) 4.2 Heat capacity of the solid csρs (MJ/m3/K) 2.0 Heat conductivity of the water λw (J/m/s/K) 0.65 Heat conductivity of the solid λs (J/m/s/K) 2.80 Longitudinal dispersivity αL (m) 3.1 Transversal dispersivity αT (m) 0.3 Figure 6. FEM model used in hydrothermal flow analyses. A FEM model with 81449 elements and an extension of 10.34 km2 was built in FEFLOW to simulate the effect of the complete thermal activation of the South extension. A plot of the FEM model is shown in Figure 6. The first step was the calibration of the groundwater flow. The initial water table levels assumed in the numerical model were taken from the study of Civita & Pizzo (2001). Upstream and downstream boundaries were then modified in order to match available data of measured levels in the area by using the trial- and-error procedure (Spitz & Moreno 1996) to minimize the difference between measured and computed hydraulic levels of the water table in the aquifer. Numerical analyses were done for three working hypotheses of the geothermal system: 1) constant continuous heat extraction from the ground (H1); 2) variable heat extraction, as a function of the seasonal change in heating demand (H2); 3) extraction and injection of heat, for winter heating and summer cooling demands (H3). The simulations are based on a time scale of three years. The heating season was assumed between October 15th and April 15th in accordance with the Regione Piemonte energy savings regulations. Hence, for the working hypothesis H2, 1280 hours/year of heating were considered and 810 hours/year of cooling were added for H3. The hours of heating and cooling were considered with a monthly variation. The starting date for the computation is considered to be January the 1st. The extraction and the injection of heat is simulated by applying a positive or a negative heat flux along the tunnel. A heat flux per unit area of tunnel lining is conservatively assumed equal to 10 W/m2 in accordance with previous experimental experiences (see Table 1). A more reliable value of this parameter could be obtained by in-situ tests. Figure 7 shows the variation of the ground temperature versus time for the three working hypotheses as computed by the FEM analyses, with and without considering the groundwater flow (GWF). These results highlight the important influence of the groundwater flow in the process. If the groundwater flow is not included, the temperature in the ground close to the tunnel contour: - drops dramatically for the working hypothesis H1; - drops of about 3.2°C for the working hypothesis H2 and the annual recovery is not complete; - drops of about 2°C (heating period) and rises of 1°C (cooling period) for the working hypothesis H3. When the groundwater flow is considered, the impact of the thermal activation on the groundwater temperature is dramatically reduced. At the same time, a full recovery of the temperature occurs with time and there is no tendency to a decreasing trend over the years. Figure 8 shows the groundwater temperature in the aquifer versus the distance from the lining-ground interface for the three hypotheses. The range of variation of the temperature is very small (< 0.2°C) for all hypotheses. Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 7 Figure 7. Groundwater temperature versus time at the lining-ground interface for hypothesis H1, H2 and H3 (GWF = groundwater flow). Figure 8. Groundwater temperature in the aquifer versus the distance from the lining-ground interface for hypothesis H1, H2 and H3 considering groundwater flow (section A-A in Figure 6). A more detailed picture of the evolution of the groundwater temperature with the distance from the tunnel lining for the working hypothesis H3 is given in Figure 9. When the system works in cooling mode, the groundwater temperature, close to the tunnel lining, increases beyond the undisturbed value of 14°C. On the contrary, the temperature drops below 14°C when the system works in heating mode. The fact that the curves showing the temperatures computed after 1, 2 or 3 years are almost identical, is a clear indication that the process fully recovers year by year. Finally, the spatial evolution of the thermal plume is plotted in Figure 10 where the effect of the groundwater flow moving the thermal plume towards the Po river is clearly visible. Figure 9. Groundwater temperature in the aquifer versus the distance from the lining-ground interface for hypothesis H3 considering groundwater flow (section A- A in Figure 6). Figure 10. Thermal plume at the end of the third year of simulation for hypothesis H3. 7 CONCLUSIONS These preliminary results highlight that the activation of the tunnel lining of the Turin Metro line 1 South Extension allows to exploit the energy stored in the ground with great economic and environmental benefit and without generating relevant effects on the aquifer. These findings are in line with research results previously mentioned and call for the need to improve the understanding of the geothermal process (including site experiments) to enhance the potential use of underground infrastructures as effective and innovative heat exchangers for the future. In order to improve the prediction of the efficiency of the system, in-situ tests are Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 8 necessary as well as additional numerical analyses to evaluate the technical and economical feasibilityfrom the structural, geotechnical and installation point of view. ACKNOWLEDGEMENTS The Authors are thankful to InfraTo S.r.l. Torino, owner of the Turin metro, for the information provided on the South Extension project, which allowed performing the present study. REFERENCES Adam, D.; Markiewicz, R. 2009. Géotechnique 59 (3), pp. 229-236. 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