The Laboratoire Souterrain de Modane (LSM) is an underground research laboratory located in the Western Alps on the French – Italian border. It is located in the middle of the 13km long Fréjus highway tunnel that links Modane (France) to Bardonecchia (Italy) in correspondence of the highest overburden of 1800m. The LSM current activity is mainly based on the investigations about the dark matter and requires very sensitive instrumentation which shall be protected from cosmic rays. To comply with the new legislation about safety in tunnels, the highway tunnels 2 owners agreed in 2007 the construction of a parallel safety tunnel, at an average distance of 50m from the existing tunnel and the laboratory owner CNRS (Centre National de recherche scientifique) also decided to extend the existing laboratory with the construction of a new 17000m3 cavern allowing the installation of wider and more powerful instruments that could increase chances of success of research. Written by SYSTRA tunnel experts : SEMERARO Martino, MISANO Edoardo, SCHIVRE Magali, BOCHON Alain

1. www.systra.com
ASSESSING THE INTERACTION BETWEEN
THE EXCAVATION OF A LARGE CAVERN
AND EXISTING TUNNELS IN THE ALPS
SYSTRA: SEMERARO Martino, MISANO Edoardo, SCHIVRE Magali, BOCHON Alain

3. ASSESSING THE INTERACTION
BETWEEN THE EXCAVATION
OF A LARGE CAVERN AND
EXISTING TUNNELS IN THE ALPS
SYSTRA: SEMERARO Martino, MISANO Edoardo, SCHIVRE Magali, BOCHON Alain
INTRODUCTION
The Laboratoire Souterrain de Modane (LSM)
is an underground research laboratory located
in the Western Alps on the French – Italian
border. It is located in the middle of the
13km long Fréjus highway tunnel that links
Modane (France) to Bardonecchia (Italy) in
correspondence of the highest overburden
of 1800m. The LSM current activity is
mainly based on the investigations about
the dark matter and requires very sensitive
instrumentation which shall be protected from
cosmic rays. To comply with the new legislation
about safety in tunnels, the highway tunnels
2
owners agreed in 2007 the construction of
a parallel safety tunnel, at an average distance
of 50m from the existing tunnel and the
laboratory owner CNRS (Centre National de
recherche scientifique) also decided to extend
the existing laboratory with the construction of
a new 17000m3
cavern allowing the installation
of wider and more powerful instruments that
could increase chances of success of research.
Preliminary studies stated that, considering
both geological and functional issues, the most
appropriate location for the new laboratory shall
be in between the two tunnels and parallel
to the existing laboratory (see Figure 1).
Figure 1. Layout of the LSM extension and cross section of the main cavern.

4. 4
Hence, the CNRS asked SYSTRA, already in
charge of the works supervision of the safety
tunnel, to realize the design of the new
cavern. The laboratory extension includes
the construction of a 40m long cavern with
300m² cross section and two smaller tunnels
connecting the safety tunnel and the existing
laboratory to the new cavern. The minimum
distance between the front face of the new LSM
and the highway tunnel is about 25m.
The French and the Italian authorities,
responsible of the exploitation of the
Fréjus highway tunnel, asked the CNRS to
develop an accurate study indicating that the
construction of the laboratory extension would
not have any negative impact on the existing
structures of the highway tunnel. The main
results of the developed study by SYSTRA
are summarized hereafter. The construction
works should begin prior to the end of
the construction works of the Fréjus safety
tunnel in 2018.
GEOLOGY
During the construction of the highway tunnel,
three main Alpine lithotypes were encountered
as shown in Figure 2: the Briançon zone
(mainly consisting of Moraine, Waste slide
rocks and black and green schists), the Gypsum
zone (including Anhydrite and Chert) and
the Piemonte zone which are characterized
by calc-schist and represent most of the
geology of the highway tunnel. The LSM
extension cavern will be excavated in similar
geology condition.
Figure 2: Geological profile of the highway tunnel and location of LSM. [2]
The calc-schist presents different facies:
the phyllitic facies, characterized by significant
mica and graphite content affecting the
schistosity, which in turn influences the
mechanical behavior of the calc-schist
(significant decrease of the friction angle
along the schistosity planes) and generating
a weakness zone; the carbonate facies, more
compact and massive with reduced schistosity.
The schistosity planes strike are parallel to the
tunnel axis with an average dip of 45°.
The main water inflows were encountered in
the fractured rocks and close to the fault zones
in the Piemonte zone, generally characterized
by seepage and limited water inflow.
Nevertheless, the tunnel stretch excavated close
to the new laboratory was completely dry.
Anisotropic in-situ field stress condition has
been measured while the horizontal stress
was higher than the vertical stress by 1.2 to
1.4 ratio. The orientation of the field stress is
shown in Figure 3.
From a structural point of view the calc-
schist formation is marked by an intense
schistosity, with a typical direction of 315°/45°
(dip direction/dip angle). The structural surveys
have shown the presence of the following
fracturing system (as shown in Figure 3):
• schistosity in the direction of the Fréjus
highway tunnel excavation with 45° West
dip (system 1);
• discontinuities perpendicular to the tunnel
direction with East-West direction Strike
and North 45° dip (system 2);
• discontinuities with East-West direction
and South 45° dip (system 3);
• discontinuities sub parallel to the tunnel
axe and East dip (system 4).

5. 5
IN-SITU
INVESTIGATIONS
According to the as-built drawings, the
highway tunnel lining is composed of a horse
shape section with 5.30m radius and vertical
sidewalls. The span is about 11m and the
maximum height at the crown is 7.50m.
The highway tunnel lining is unreinforced
concrete and the average thickness close to
the LSM extension is about 0.80m. In order
to allow the tunnel smoke management,
a reinforced concrete slab 0.12m thick has been
casted and partially restrained into the lining
at 4.50m above the tunnel invert (respecting
to the roadway as shown in Figure 5).
All the available monitoring data have been
analyzed in order to understand better
the rock mass state of stress close to LSM
and predict the excavation condition of the
new cavern. The data were derived on one
hand from an investigation campaign executed
on the highway tunnel lining nearby the LSM
location and from measured convergences
already realized on the highway tunnel,
the safety tunnel and the existing LSM.
State of stress of
the tunnel lining
In order to evaluate the state of stress of the
highway tunnel lining and define the real lining
thickness, a complete in-situ and laboratory
investigation campaign was realized during
the design stages. The investigation program
includes three sections close to the cavern axis.
In particular, the planned tests were:
It is worth noting that both existing long
tunnels were excavated parallel to the
schistosity direction. During the construction,
the rock mass presented highly deformable and
anisotropic behavior resulting in asymmetric
tunnel convergence (from 15cm to 60cm
on diameter). The surveys highlighted the
anisotropic behavior of the calc-schist, the
influence of the mineral composition, the
tectonization degree and variations of the state
of stress. The recorded tunnel convergences
showed greater deformations on the West side
of the tunnel, perpendicular to the schistosity
planes, and particularly in the phyllitic facies
and in the high overburden zones (but not
necessarily corresponds to the maximum
overburden). The induced plastic zone around
the excavation was asymmetric with bigger
extension perpendicular to the schistosity
where a buckling phenomenon was developed.
Figure 4: Buckling and convergences phenomena
in the highway tunnel. [2]
Consequently the new laboratory will
be excavated preferably perpendicular to
the schistosity and the direction of the buckling
phenomenon in order to avoid asymmetric
convergence.
Figure 3: Fracturing system (on the left) and anisotropic in-site field stress (on the right). [2]

6. 6
Convergence analysis
The recorded convergences along the highway
tunnel during both the construction stages
and the tunnel setting up in the 1980s show
that the average measured value during
the tunnel exploitation was 20cm on the
tunnel diameter. The maximum convergence
during the excavation was recorded as 45cm
in the French sector and around the chainage
5+000 (see Figure 6).
At the existing LSM location, 12.5cm of
convergence was registered during the
excavation at the enlarged section. Additional
monitoring data in such sector are available
on the final lining (see Figure 7) 250m far
from the new LSM. The recorded final lining
displacements between 1980 and 1997
showed that no relevant displacement has been
detected confirming that the rock-mass does
not show any rheological behavior in this sector.
In the LSM sector, no additional convergence
has been detected during the safety tunnel
excavation by TBM in 2012.
Further information on the massif response can
be obtained from the monitoring data analysis
during the excavation of the first laboratory
perpendicular to the highway axis. These data
concern the convergences of the old cavern
lining during the 15 years monitoring period
from 1983 to 1995. Higher values reach 5-6mm
and confirm that the rock-mass behavior
is stable and realistic convergences are lower
than those registered in the other highway
tunnel stretches.
• 9 flat jacks located in the crown, sidewalls
and in to the slab in order to evaluate the
current state of stress of the highway tunnel;
• 3 surveys of the three dimensional state of
stress at the lining intrados and extrados by
means of CSIRO Hi-D cell;
• Several boreholes to investigate the lining
thickness and determine the lining mechanical
properties such as the ultimate strength
(UCS) and the deformation modulus.
The in-situ tests have pointed out that the
lining state of stress in the tunnel crown and
sidewalls is in the range of 2MPa to 5MPa
and the concrete slab is not loaded. The
analysis of the laboratory tests have shown
that the deformation modulus is 25000MPa
and the average concrete UCS is 22MPa.
The results of the investigation campaign have
been very helpful for the calibration of the
numerical models.
The lining state of stress is summarized in
the Figure 5.
Figure 6: Convergences of the Fréjus tunnel during the construction stages. [4]
Figure 5: Cross section of the highway Fréjus
tunnel and state of stress of the lining according
to the investigation results.

7. 7
IMPACT OF THE
NEW LSM ON EXISTING
STRUCTURES
Considering the geometry of the study and
the structural connections, it is necessary to
take into account the tridimensional effects
during the study. Nevertheless, SYSTRA was
asked to quickly answer to the client who
had to present to the Security Committee
the proving results that the construction of
the new research site would have no impact
on the highway tunnel and on the lining of
the safety tunnel.
Therefore, a series of simplified two-dimensional
analyses have been developed to take into
account the tridimensional effects through
the application of plane strain and axisymmetric
models which permits a more flexible
calculation tool. All the presented numerical
analysis were developed using the PHASE²
calculation software (Rocscience).
Plane strain model
calibration
The first step consists in calibrating the
highway tunnel plane strain model according
to the investigation results on the final lining
stress state. Using the corrected parameters
shown in Table 1, the calibration includes
the study of the lithostatic field stress
which reproduces the observed stresses.
CONSTRUCTION
METHOD OF
LSM EXTENSION
The excavation of the main cavern (300m2
cross section) will follow the Drill b Blast
method starting from the safety tunnel. Firstly,
the tunnel crown will be excavated with the
maximum height of 8m. The crown final lining
includes 0.40m thick shotcrete layer combined
with two layers of wire mesh and will be
placed before the excavation of the invert.
The sidewalls and the invert will be excavated
in five 2m benched .
The temporary support consists of 10cm
of shotcrete combined with two layers
of Swellex type bolts 4m long as well as
self-drilling bolts 8m long. The final lining
consists of 40cm of shotcrete.
Figure 8: Temporary support of the main cavern.
Figure 7: Convergences of the final lining of highway tunnel at the LSM location.

8. 8
The solution has been studied by an iterative
process starting from the lithostatic field stress
and changing them until the correct stress state
inside the lining was reproduced. The rock-
mass behavior was modeled as elasto-plastic
using Mohr-Coulomb failure criterion, residual
resistance parameters are considered in plastic
deformation.
Lithotype
E
[GPa] [-]
c
[MPa]
ϕ
[°]
cr
[MPa]
ϕr
[°]
schists 15 0,25 5 40 1 35
Figure 9. Geometry of the interaction problem between the LSM and the highway tunnel.
Table 1. Geotechnical parameters.
Table 2. Calibrated lithostatic stress-field.
σx
[MPa] σy
[MPa] σz
[MPa]
Lithostatic field stress 53 38 38
Calibrated model 48 32 32
Figure 10. Principal stresses direction calculated into the lining in the calibration model.
Section
Sigma 1
[MPa]
Sigma 3
[MPa]
A-A 2.7 0.7
B-B 2.9 0.7
C-C 4.0 1.0
D-D 3.3 0.9
E-E 2.6 0.5

9. 9
Impact on existing structures
The results from the axisymmetric study
have been then extracted in the plane strain
model to reproduce the effects of the LSM
excavation in terms of field stress-deformation
perturbation.
To gain this goal, stresses have been modified
in area zone of the calculation mesh (red one
in Figure 9) which corresponds to the position
of the LSM with respect to the existing
structure. This zone has been softened by
decreasing its deformation module up to
zero and stress has been gradually decreased
to fit to those calculated values by the
axisymmetric model. In addition, reduced
resistance parameters of the rock mass have
been assigned to the zone ahead of the LSM
face that resulted to be at plastic state in
the axisymmetric model.
It has to be noted that the adopted calculation
procedure is more conservative than the 3D
model since the 2D model considers that the
extension of the perturbation in the direction
orthogonal to the problem plan is infinite,
while in reality such distance is comparable
with the cavern diameter (20m).
Finally, the results have been extracted to
evaluate the related excavation effect on the
highway tunnel in terms of highway tunnel
lining state of stress modification. The main
observed result in the numerical model is the
reduction of the normal effort at the crown
(as shown in the diagram at the left side of
the Figure 12) and the increasing of the induced
efforts in the sidewalls of highway tunnel
due to the LSM excavation (sections A-A, B-B,
D-D and E-E in the Figure 12); nevertheless,
the increment is not high enough to cause the
lining failure.
Study of the LSM effects
The second analysis concerns the evaluation
of the induced effects on the existing lining
in terms of stress and strain by the LSM face
during the excavation (§ Figure 11). Hence,
an axisymmetric model of the new LSM was
studied using the calibrated state of stress.
For the lithostatic field stress, the mean value
of the principal stresses has been considered.
The results showed that the mean displacement
of the cavern face (extrusion) is 3cm and that
the perturbation of the field stress ahead of
the cavern can be considered negligible at 15m
far from the face (as shown in the Figure 13).
The extension of the plasticized zone around
the cavern, evaluated on the basis of a 2D plane
strain model of the cavern, has been estimated
to be 6-7 meters. Then, the same extension of
the plastic zone has been evaluated ahead the
LSM face, according to the axisymmetric model.
Figure 11. Field stress perturbation produced
by LSM excavation head tunnel face.

10. 10
MONITORING PLAN
The results were finally exploited to set a
monitoring plan and the contingency measures
in the sections of the highway tunnel close
to the future LSM location. In order to fix the
displacement thresholds for the tunnel lining,
a 2D structural analysis with an embedded
model was performed. The tunnel lining was
loaded in different directions up to reach the
failure load and record the corresponding
convergence. The results shown that the tunnel
lining convergences lower than 5mm should
not cause the failure of structural elements.
Monitoring plan was then set considering the
following threshold for convergences:
• Vigilance threshold (4mm): The response of
the structure is safe, monitoring frequencies
shall be increased in order to activate a quick
action in case of necessity.
• Alert threshold(5mm): The structure is
reaching its resistance capacity, intervention
shall be foreseen on LSM construction (such
as increase of support at excavation stage)
in order to stop the convergence trend.
• Emergency threshold (7,5mm): The safety
of the structure is endangered, works
shall be stopped and a new construction
procedure shall be designed in order to
restart excavation operation.
Figure 13. Impact of LSM excavation
on the highway tunnel: σ3 distribution.
Section
Sigma 1
[MPa]
Sigma 3
[MPa]
A-A 7.7 1.9
B-B 6.9 2.1
C-C 0.8 0.2
D-D 9.1 2.6
E-E 8.9 2.3
Figure 12. Fréjus tunnel crown verification after the
excavation of LSM (N-M diagram at the left side of
the figure) and stresses in the main points of the tunnel
lining (sidewalls A-A and E-E, haunches B-B and D-D,
crown C-C) after the excavation of the cavern (table at
the right side of the figure).

11. 11
close to the LSM extension. The results
showed that the induced stresses inside the
tunnel lining were low and the ventilation
slab was not loaded at all. The results of the
investigations were used in order to calibrate
the 2D F.E.M. model.
A parallel axisymmetric F.E.M. model was
executed in order to study the induced effects
ahead of the face of the LSM excavation
in terms of field stress perturbation and
displacements. These results were then used
in the tunnel plane strain model to simulate the
excavation of the LSM and evaluate the related
effects on the lining of the highway tunnel.
The results of the analysis confirmed that the
related effects in terms of induced stresses
on the tunnel lining do not cause any structural
failure.
Results were finally combined in order
to establish a monitoring plan for tunnel
sections next to the future LSM and define
the thresholds.
Along the Fréjus highway tunnel alignment, at
1800m overburden, the LSM is in charge of
the research on the dark-matter and in 2007
the CNRS decided to extend the LSM dimension
with a new 17000m3
cavern which could give
an important contribution to the CNRS mission.
Preliminary studies stated that the best
location for the new laboratory should be
parallel to the existing cavern, in between
the Fréjus highway tunnel and the new safety
tunnel in order to excavate perpendicular
to the schistosity and avoid asymmetrical
convergences. SYSTRA was asked to prove
that the new excavation will not have endanger
the existing structures and the highway tunnel.
The first step includes analyzing the existing
recorded data during the highway tunnel
construction which proves that the LSM area
is particularity favorable in terms of rock-mass
response to the future excavation and that
design rock parameters are conservative.
Then, an investigation campaign was launched
to better define the lining state of stress
REFERENCES
[1]. Vinnac, A.; Marcucci, E.; Schivre, M.; Semeraro M. Ramond, P.; Chiriotti, E.; Fuoco. S.
“Back-analysis of hard rock TBM tunneling through deformable schistous rock mass:
the case of the Fréjus safety tunnel”. AITES WTC 2014.
[2]. Schivre, M.; Ramond, P.; Bochon, A.; Vinnac, A.; Bianchi, G.W.; Fuoco, S.
“TBM excavation of the Frejus safety tunnel through highly deformable schistous
rock mass under high cover”. AITES WTC 2014.
[3]. SETEC TP: Laboratoire Souterrain de Modane, Travaux de Génie civil
de deuxième phase
[4]. Fuoco et al. 2013. “Analisi delle problematiche connesse allo scavo di calcescisti
con sistema meccanizzato sotto grandi coperture: la galleria di sicurezza del Fréjus”.
Congresso società italiana gallerie, Bologna 2013.
CONCLUSIONS

12. © SYSTRA2016,© M.Kadri/CAPAPictures
72 rue Henry Farman
CS 41594
75513 Paris Cedex 15
+ 33 1 40 16 61 00