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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