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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. 
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