Barla, Giovanni Battista

Bonini, Mariacristina

Semeraro, Martino

English

PORTO Publications Open Repository TOrino

1 INTRODUCTION 
Tunnel construction in squeezing rock is very de-
manding due to the difficulty in making reliable pre-
dictions at the design stage. Squeezing conditions 
may vary over short distances due to rock heteroge-
neity and fluctuations in rock mass properties. In-
deed, the selection of the most appropriate excava-
tion-construction method to be adopted is highly 
problematic and uncertain. In deep tunnels, when-
ever squeezing conditions are anticipated, conven-
tional tunneling appears to be yet the method most 
often used. 
Conventional tunneling in squeezing rock gener-
ally takes place with a slow rate of advance. How-
ever, if the work at the face is well planned and ap-
propriate stabilization measures are implemented, 
excavation can proceed at an acceptable rate of ad-
vance. A clear need to develop appropriate techno-
logical systems that help increase such a rate of ad-
vance is to be recognized (Cantieni & Anagnostou, 
2009). It is the purpose of this paper to describe a 
recent innovative technological development and to 
present a case study to illustrate such a development. 
2 THE SAINT MARTIN LA PORTE ADIT 
The Saint Martin La Porte access adit is a vital part 
of the early works for the Lyon-Turin Base Tunnel, 
which is at the centre of the axes linking the North 
and South, and East and West Europe and is to be 
excavated between the portals in Italy and France. 
The tunnel is being excavated in a Carboniferous 
Formation, “Zone Houillère Briançonnaise-Unité 
des Encombres“ which is composed of black schists 
(45 to 55%), sandstones (40 to 50%), coal (5%), 
clay-like shales and cataclastic rocks. A characteris-
tic feature is the anisotropic, highly heterogeneous, 
disrupted and fractured conditions of the rock mass 
which exhibits a very severe squeezing behavior 
(Hoek, 2001; Barla, 2002). 
Excavation takes place in essentially dry condi-
tions. In order to assess the rock mass quality during 
excavation, detailed mapping of the geological and 
geomechanical conditions at the face was under-
taken systematically (Figure 1). 
Several support systems have been used in the 
Carboniferous Formation up to chainage 1400 m  in-
cluding either “stiff” or “soft” steel ribs (with sliding 
joints), rock  dowels  and  mesh reinforced shotcrete. 
 
F Π 1
hpg 
F Λ 1'
a
d/gps/q
a
a 
a-c
r 
gps
gps 
gps
gps
gps
gps
gps
gps 
l
a-c 
a-c 
a-c
c-a 
a-c 
l
c-a 
c-a 
c-a
c-a 
c-a
a
a-c
a a 
a
aq a
q
q
q
q 
gps
gps 
da-c
gps 
gps
a
a
a
a
gps
 
 
Figure 1. Geological and geomechanical conditions at the face 
at chainage 1443m (gps-sandstones, a-clay shales, c-coal, etc.). 
Performance monitoring and analysis of a yield-control support system in 
squeezing rock 
G. Barla & M. Bonini 
Politecnico di Torino – Department of Structural and Geotechnical Engineering, Torino, Italy 
M. Semeraro 
Systra, Paris, France 
 
 
 
 
ABSTRACT: Recent innovations in yield-control support systems allow to increase the rate of advance when 
tunneling in difficult conditions is associated with severely squeezing rock. Such systems which imply the in-
sertion in the lining of highly deformable concrete elements are being adopted successfully in tunneling pro-
jects using conventional excavation methods. The Saint Martin access adit excavated in a Carboniferous 
Formation along the Base Tunnel of the Lyon-Turin rail line is presented as a case study. Numerical analyses 
are discussed to compare the results of computed and measured performance of a typical  monitored section 
and to find out possible optimizations of the support system adopted. 
A horseshoe shape profile was adopted with full 
face excavation and fiber-glass dowels reinforce-
ment of the face. However, the stiff support experi-
enced significant overstressing due to the severely 
squeezing conditions encountered and the soft sup-
port underwent very large deformations with con-
vergences up to 2 m with the need of extensive re-
shaping of tunnel excavation. 
The design concept finally chosen to cope with 
the severely squeezing conditions encountered was 
based on allowing the support to yield in a con-
trolled manner, while using full-face excavation 
with reinforcement by fiber-glass dowels of the tun-
nel “core”. In order to improve the working condi-
tions and to control deformations, a novel “yield-
control” support system (DSM in the following) was 
adopted with a near circular cross section as shown 
in Figure 2. 
A detailed description of the construction stages 
can be found elsewhere (Barla et al., 2007; Barla, 
2009). The main innovation of the DSM section is 
the adoption of a full circular section in stage 2 at a 
distance of 20-30 m from the face (Figure 2), with 
application of a 20 cm reinforced shotcrete lining, 
yielding steel ribs with sliding joints (TH type), with 
longitudinal slots (one at the invert) fitted with 
Highly Deformable Concrete elements (HiDCon). 
The mechanical behavior of these elements is duc-
tile, nearly elastic perfectly plastic, and character-
ized by a constant limit stress of 8.5 MPa up to a 
maximum strain of about 50%. This allows the sup-
port system to undertake high deformations in a con-
trolled manner, while applying a constant radial 
stress to the ground surrounding the excavation. 
3 PERFORMANCE MONITORING 
High convergences developed between chainage 
1200 to 1550 m, with the DSM section applied sys-
tematically following chainage 1400m. The special  
monitoring section at chainage 1443 m is taken as 
representative. Figure 4 shows the displacement dis-
tribution given by the multi-position borehole exten-
someter along direction 4 (right side) versus time 
and the mobilized zone around the tunnel where the 
radial strain is greater than 1%, a strain limit taken 
as the onset of squeezing behavior. It is of interest to 
observe that the zone around the tunnel with strains 
greater than this value is non symmetrically distrib-
uted, ranging in extent from 10-11 m maximum 
(right) to 1-2 m minimum (left).  
It is clear that, where a more significant extent of 
the mobilized zone around the tunnel occurs in 
stage 1 (Figure 4), the support system in stage 2 un-
dergoes greater deformations. The non symmetric 
response of the tunnel is mostly due to the anisotropic 
features  of  the  rock  mass,  the  presence of “strong” 
C
Yielding steel ribs
with sliding joints
(TH type)
20 cm thick
shotcrete layer
High deformable
concrete elements
(HiDCon)
R6
.10
 
Figure 2. Detail of the yield-control support system (DSM Sec-
tion) in stage 2. 
 
 
 
Figure 3. Highly Deformable Concrete (HiDCon) elements 
during installation between the TH steel ribs and before shot-
crete placement.  
 
-600
-400
-200
0
0 4 8 12 16 20 24
Distance (m)
7 14 21 25
33 39 61 75
96 110 131 152
166
 
 
Figure 4. Displacement distribution along multi-position bore-
hole extensometer 4 with sketch of the zone around the tunnel 
where the radial strain is greater than 1%. 
 
(sandstones and schists) and “weak” (coal and clay-
like shales) layers which dip from the left to the 
right of the tunnel cross section  (Figure 1), and  the  
local variations of the shotcrete lining thickness, 
strongly related to the rock mass non homogeneity. 
The deformation (shortening) of the deformable 
elements installed in the primary shotcrete lining in 
the same cross section at chainage 1443 m were 
measured by using strain meters located across them 
R
ad
ia
l d
is
pl
ac
em
en
t (
m
m
) 
1 - 3.6m
2 - 1.6m
4 - 11.3m
5 - 7.2m
6 - 7.7m
3 - 11.5m
as depicted in Figure 5. It is shown that smaller dis-
placements occur in elements 4 and 5 (at the crown), 
whereas elements 6 and 7 (on the right sidewall) un-
dergo the most significant deformations attaining a 
25 % strain. 
4 ANALYSIS 
In order to simulate the response of the DSM yield-
control support system in stage 2, by accounting for 
the presence of the HiDCon elements incorporated 
in the shotcrete lining and the interaction with the 
surrounding rock mass, a numerical model was set 
up by using the Finite Difference Method (FDM) 
and the FLAC code (Itasca Consulting Group, Inc., 
USA).  
The 2D FDM grid was very finely discretised 
near the excavation perimeter and in the lining, 
where the typical size is 0.1m×0.1m. Since the target 
was the optimization of the support system in 
stage 2, the rock mass was modeled as a simple, iso-
tropic, linear elastic material and considered as a 
confining medium for the support, not subject to any 
initial state of stress. 
The lining and the HiDCon elements were mod-
eled as elastic perfectly plastic Mohr-Coulomb ma-
terials. The mechanical properties adopted are sum-
marized in Table 1. Loads were applied directly to 
the lining, according to a stress distribution which 
enabled one to reproduce the observed state of de-
formation of the composite lining. Therefore, load-
ing of the support system is not the result of stress 
relief as usually done in numerical modeling when 
excavation is being simulated. 
Interfaces were introduced between the support 
and the rock mass and between the deformable ele-
ments and the shotcrete lining, in order to prevent 
tensile stresses from developing in them. 
With the intent to reproduce the observed asym-
metric strain distribution in the lining and its de-
formed shape, the desired stress distribution as 
shown in Figure 6 was obtained. A comparison of 
the computed and measured displaced shape of the 
lining is shown in Figure 6, where illustrated is its 
initial geometry. 
The comparison of computed and measured radial 
displacements on the tunnel periphery is satisfactory 
(Figure 6). However, there are significant differ-
ences in the displacements across the HiDCon ele-
ments when compared to the measured values. This 
is believed to be due to the local change of thickness 
in the shotcrete lining in proximity of these ele-
ments, because of the over-excavation which took 
place on the tunnel periphery. This obviously could 
not be considered in the model. 
The computed thrust and bending moment in the 
lining are depicted in Figure 7. The bending moment 
distribution appears to be significantly influenced by  
-100
-80
-60
-40
-20
0
31/10/2006 30/12/2006 28/02/2007
2
3
4
5
6
7
 
 
Figure 5. Special monitoring section at chainage 1443 m. Dis-
placements in the elements versus time. 
 
Table 1. Material parameters used in the analyses. ______________________________________________ 
Setting        Rock  Shotcrete HiDCon 
          mass      elements ______________________________________________ 
Young Modulus E (MPa)  400  20000  4000 
Poisson’s ratio ν (-)    0.3  0.2   0.2 
UCS (MPa)       0   32.0   8.5 
Tensile strength σt (MPa)  0   3.2   0.85 
Cohesion c (MPa)     0   9.2   2.45 
Friction angle φ (°)     0   30    30    _____________________________________________ 
 
HiDCon
initial
mesh
extensometers
February 6, 2007
Mean orientation of
geological features
radial loading
= 10 MPa
deformed
mesh
 
 
Figure 6. Load distribution and computed displaced shape for 
cross section at chainage 1443 m (reference monitored sec-
tion). 
 
-4000
-3000
-2000
-1000
0
1000
2000
0 60 120 180 240 300 360
θ [°]
-200
-150
-100
-50
0
50
100
Thrust Sidewall Bending moment
 
 
Figure 7. Bending moment and thrust distribution in the lining 
for the cross section at chainage 1443 m (θ increases clockwise 
from the left wall). 
2
3
4 5
6
7
D
is
pl
ac
em
en
t (
m
m
) 
T
hr
us
t (
kN
) 
B
en
di
ng
 m
om
en
t (
kN
m
/m
) 
the presence of the HiDCon elements incorporated 
in the lining, while the thrust is always compressive. 
It is noted that the loads imposed on the support, 
which is able to reproduce the observed deflected 
shape during excavation in a satisfactory manner, 
induce a distribution of stress which is far from be-
ing uniform as one would expect if the same support 
was to behave as a flexible support. 
To find out if any improvement could be obtained 
in the lining response, in order to avoid some of the 
deformable elements from being overstrained, other 
analyses were undertaken with the intent to model: 
1) the presence of less slender elements in the lining, 
with aspect ratio equal to 1 (case 1); 2) the increase 
in the number of elements, having the same shape 
with aspect ratio equal to 2, as in the actual tunnel at 
chainage 1443 m, incorporated in the lining on the 
right side, in the same sector where the most severe 
strains were observed to occur (case 2). 
It is apparent from the results reported in Table 2 
that the displacement across each deformable ele-
ment in case 1 is about one half of the displacement 
computed for the cross section at chainage 1443 m, 
with the strain level being however very similar. In 
case 2 the deformable elements 5 and 6 undergo a 
strain which is approximately twice that of the re-
maining elements, where the strain is approximately 
the same. 
Also, given that the deformable elements in 
case 1 experience a lower shortening, the shotcrete 
lining is to share a larger strain and stress level than 
in the previous case (Figure 8). On the contrary, the 
increase in the number of deformable elements on 
the right side results in a significantly lower stress in 
the lining. 
5 CONCLUSIONS 
It is concluded that the increase in the number of de-
formable elements placed in the lining where this is 
expected to be more strained is a reasonable option 
with respect to the corresponding symmetric distri-
bution.  
Therefore, in order to cope with non symmetric 
loading conditions of the lining, as expected when-
ever the rock mass exhibits an anisotropic behavior, 
it is suggested to distribute the deformable elements 
where more severe straining is expected to occur.  
In such a case, although the number of deformable 
elements in the lining remains the same, thus making 
it possible to cope with the same tunnel conver-
gence, a better behavior is expected in the tunnel in 
placing the deformable elements where the maxi-
mum strains occur. 
Table 2. Computed displacements and strains across the Hid-
Con elements. _______________________________________________ 
HiDCon  Chainage 1443 m  Case 1   Case 2      ______________  _______  _______  
     (mm) (%)   (mm) (%)  (mm) (%) _______________________________________________ 
2     22   6    13  7   40  10 
3     75   19    34  17   39  10 
4     57   15    30  15   51  13 
5     68   18    36  19   109 27 
6     84   22    40  22   93  23 
7     41   11    21  11   45  11 _______________________________________________ 
 
0
5
10
15
20
25
30
0 10 20 30
Length clockwise from left wall (m)
Reference case
Case 1
Case 2
w
a
l
l
 
 
Figure 8. Computed maximum principal stress in the lining for 
the monitored section at chainage 1443 m and in cases 1 and 2. 
ACKNOWLEDGEMENTS 
The studies described in this paper have been carried 
by Politecnico di Torino, DISTR, under a research 
contract with Egis, Antea, and Alpina, coordinated 
by Professor G. Barla. The authors would like to 
thank Egis, Antea, and Alpina for the permission to 
publish the results reported in this paper. Also to be 
thanked is LTF, Lyon-Turin Ferroviaire, Chambery, 
France. 
REFERENCES 
Barla, G. 2002. Tunnelling under squeezing rock conditions. In 
D. Kolymbas (ed.), Advances in Geotechnical Engineering 
and Tunnelling, 169-268. 
Barla G., Barla M., Bonini M., Debernardi D. 2007. Lessons 
learned during the excavation of the Saint Martin La Porte 
access gallery along the Lyon-Turin Base tunnel. In 
Schneider, John, Brandner (ed.), BBT 2007, Internationales 
Symposium Brenner Basistunnel uns Zulaufstrecken, 
Innsbruck University Press. 
Barla G. 2009. Innovative tunnelling construction method to 
cope with squeeezing at Saint Martin La Porte access adit 
(Lyon-Torino Base Tunnel). In Rock engineering in diffi-
cult conditions in soft rocks and karst. Proceedings of 
Eurock 2009. ISRM Regional Symposium. Dubrovnik, 
Croatia. 
Cantieni L., Anagnostou G. 2009. The interaction between 
yielding supports and squeezing ground. Tunnelling and 
Underground Space Technology 24, 309-322. 
Hoek E. 2001. Big tunnels in bad rock. 2000 Terzaghi Lecture. 
ASCE Journal of Geotechnical and Geoenvironmental En-
gineering 127(9), 762-740. 
σ 1 
(M
Pa
) 


Big tunnels in bad rock.

Innovative tunnelling construction method to cope with squeeezing at Saint Martin La Porte access adit (Lyon-Torino Base Tunnel). In Rock engineering in difficult conditions in soft rocks and karst.

Lessons learned during the excavation of the Saint Martin La Porte access gallery along the Lyon-Turin Base tunnel.

The interaction between yielding supports and squeezing ground.

Tunnelling under squeezing rock conditions.

Performance monitoring and analysis of a yield-control support system in squeezing rock

BARLA, Giovanni Battista

BONINI, MARIACRISTINA

SEMERARO, MARTINO

PORTO@iris (Publications Open Repository TOrino - Politecnico di Torino)

05 August 2020POLITECNICO DI TORINORepository ISTITUZIONALEPerformance monitoring and analysis of a yield-control support system in squeezing rock / Barla G.; Bonini M.; SemeraroM.. - STAMPA. - (2010), pp. 459-462. ((Intervento presentato al convegno Eurock 2010 tenutosi a Lausanne nel June15-18.OriginalPerformance monitoring and analysis of a yield-control support system in squeezing rockPublisher:PublishedDOI:Terms of use:openAccessPublisher copyright(Article begins on next page)This article is made available under terms and conditions as specified in the  corresponding bibliographic description inthe repositoryAvailability:This version is available at: 11583/2372234 since:CRC Press/Balkema1 INTRODUCTION Tunnel construction in squeezing rock is very de-manding due to the difficulty in making reliable pre-dictions at the design stage. Squeezing conditions may vary over short distances due to rock heteroge-neity and fluctuations in rock mass properties. In-deed, the selection of the most appropriate excava-tion-construction method to be adopted is highly problematic and uncertain. In deep tunnels, when-ever squeezing conditions are anticipated, conven-tional tunneling appears to be yet the method most often used. Conventional tunneling in squeezing rock gener-ally takes place with a slow rate of advance. How-ever, if the work at the face is well planned and ap-propriate stabilization measures are implemented, excavation can proceed at an acceptable rate of ad-vance. A clear need to develop appropriate techno-logical systems that help increase such a rate of ad-vance is to be recognized (Cantieni & Anagnostou, 2009). It is the purpose of this paper to describe a recent innovative technological development and to present a case study to illustrate such a development. 2 THE SAINT MARTIN LA PORTE ADIT The Saint Martin La Porte access adit is a vital part of the early works for the Lyon-Turin Base Tunnel, which is at the centre of the axes linking the North and South, and East and West Europe and is to be excavated between the portals in Italy and France. The tunnel is being excavated in a Carboniferous Formation, “Zone Houillère Briançonnaise-Unité des Encombres“ which is composed of black schists (45 to 55%), sandstones (40 to 50%), coal (5%), clay-like shales and cataclastic rocks. A characteris-tic feature is the anisotropic, highly heterogeneous, disrupted and fractured conditions of the rock mass which exhibits a very severe squeezing behavior (Hoek, 2001; Barla, 2002). Excavation takes place in essentially dry condi-tions. In order to assess the rock mass quality during excavation, detailed mapping of the geological and geomechanical conditions at the face was under-taken systematically (Figure 1). Several support systems have been used in the Carboniferous Formation up to chainage 1400 m  in-cluding either “stiff” or “soft” steel ribs (with sliding joints), rock  dowels  and  mesh reinforced shotcrete.  F Π 1hpg F Λ 1'ad/gps/qaa a-cr gpsgps gpsgpsgpsgpsgpsgps la-c a-c a-cc-a a-c lc-a c-a c-ac-a c-aaa-ca a aaq aqqqq gpsgps da-cgps gpsaaaagps  Figure 1. Geological and geomechanical conditions at the face at chainage 1443m (gps-sandstones, a-clay shales, c-coal, etc.). Performance monitoring and analysis of a yield-control support system in squeezing rock G. Barla & M. Bonini Politecnico di Torino – Department of Structural and Geotechnical Engineering, Torino, Italy M. Semeraro Systra, Paris, France     ABSTRACT: Recent innovations in yield-control support systems allow to increase the rate of advance when tunneling in difficult conditions is associated with severely squeezing rock. Such systems which imply the in-sertion in the lining of highly deformable concrete elements are being adopted successfully in tunneling pro-jects using conventional excavation methods. The Saint Martin access adit excavated in a Carboniferous Formation along the Base Tunnel of the Lyon-Turin rail line is presented as a case study. Numerical analyses are discussed to compare the results of computed and measured performance of a typical  monitored section and to find out possible optimizations of the support system adopted. A horseshoe shape profile was adopted with full face excavation and fiber-glass dowels reinforce-ment of the face. However, the stiff support experi-enced significant overstressing due to the severely squeezing conditions encountered and the soft sup-port underwent very large deformations with con-vergences up to 2 m with the need of extensive re-shaping of tunnel excavation. The design concept finally chosen to cope with the severely squeezing conditions encountered was based on allowing the support to yield in a con-trolled manner, while using full-face excavation with reinforcement by fiber-glass dowels of the tun-nel “core”. In order to improve the working condi-tions and to control deformations, a novel “yield-control” support system (DSM in the following) was adopted with a near circular cross section as shown in Figure 2. A detailed description of the construction stages can be found elsewhere (Barla et al., 2007; Barla, 2009). The main innovation of the DSM section is the adoption of a full circular section in stage 2 at a distance of 20-30 m from the face (Figure 2), with application of a 20 cm reinforced shotcrete lining, yielding steel ribs with sliding joints (TH type), with longitudinal slots (one at the invert) fitted with Highly Deformable Concrete elements (HiDCon). The mechanical behavior of these elements is duc-tile, nearly elastic perfectly plastic, and character-ized by a constant limit stress of 8.5 MPa up to a maximum strain of about 50%. This allows the sup-port system to undertake high deformations in a con-trolled manner, while applying a constant radial stress to the ground surrounding the excavation. 3 PERFORMANCE MONITORING High convergences developed between chainage 1200 to 1550 m, with the DSM section applied sys-tematically following chainage 1400m. The special  monitoring section at chainage 1443 m is taken as representative. Figure 4 shows the displacement dis-tribution given by the multi-position borehole exten-someter along direction 4 (right side) versus time and the mobilized zone around the tunnel where the radial strain is greater than 1%, a strain limit taken as the onset of squeezing behavior. It is of interest to observe that the zone around the tunnel with strains greater than this value is non symmetrically distrib-uted, ranging in extent from 10-11 m maximum (right) to 1-2 m minimum (left).  It is clear that, where a more significant extent of the mobilized zone around the tunnel occurs in stage 1 (Figure 4), the support system in stage 2 un-dergoes greater deformations. The non symmetric response of the tunnel is mostly due to the anisotropic features  of  the  rock  mass,  the  presence of “strong” CYielding steel ribswith sliding joints(TH type)20 cm thickshotcrete layerHigh deformableconcrete elements(HiDCon)R6.10 Figure 2. Detail of the yield-control support system (DSM Sec-tion) in stage 2.    Figure 3. Highly Deformable Concrete (HiDCon) elements during installation between the TH steel ribs and before shot-crete placement.   -600-400-20000 4 8 12 16 20 24Distance (m)7 14 21 2533 39 61 7596 110 131 152166  Figure 4. Displacement distribution along multi-position bore-hole extensometer 4 with sketch of the zone around the tunnel where the radial strain is greater than 1%.  (sandstones and schists) and “weak” (coal and clay-like shales) layers which dip from the left to the right of the tunnel cross section  (Figure 1), and  the  local variations of the shotcrete lining thickness, strongly related to the rock mass non homogeneity. The deformation (shortening) of the deformable elements installed in the primary shotcrete lining in the same cross section at chainage 1443 m were measured by using strain meters located across them Radial displacement (mm) 1 - 3.6m2 - 1.6m4 - 11.3m5 - 7.2m6 - 7.7m3 - 11.5mas depicted in Figure 5. It is shown that smaller dis-placements occur in elements 4 and 5 (at the crown), whereas elements 6 and 7 (on the right sidewall) un-dergo the most significant deformations attaining a 25 % strain. 4 ANALYSIS In order to simulate the response of the DSM yield-control support system in stage 2, by accounting for the presence of the HiDCon elements incorporated in the shotcrete lining and the interaction with the surrounding rock mass, a numerical model was set up by using the Finite Difference Method (FDM) and the FLAC code (Itasca Consulting Group, Inc., USA).  The 2D FDM grid was very finely discretised near the excavation perimeter and in the lining, where the typical size is 0.1m×0.1m. Since the target was the optimization of the support system in stage 2, the rock mass was modeled as a simple, iso-tropic, linear elastic material and considered as a confining medium for the support, not subject to any initial state of stress. The lining and the HiDCon elements were mod-eled as elastic perfectly plastic Mohr-Coulomb ma-terials. The mechanical properties adopted are sum-marized in Table 1. Loads were applied directly to the lining, according to a stress distribution which enabled one to reproduce the observed state of de-formation of the composite lining. Therefore, load-ing of the support system is not the result of stress relief as usually done in numerical modeling when excavation is being simulated. Interfaces were introduced between the support and the rock mass and between the deformable ele-ments and the shotcrete lining, in order to prevent tensile stresses from developing in them. With the intent to reproduce the observed asym-metric strain distribution in the lining and its de-formed shape, the desired stress distribution as shown in Figure 6 was obtained. A comparison of the computed and measured displaced shape of the lining is shown in Figure 6, where illustrated is its initial geometry. The comparison of computed and measured radial displacements on the tunnel periphery is satisfactory (Figure 6). However, there are significant differ-ences in the displacements across the HiDCon ele-ments when compared to the measured values. This is believed to be due to the local change of thickness in the shotcrete lining in proximity of these ele-ments, because of the over-excavation which took place on the tunnel periphery. This obviously could not be considered in the model. The computed thrust and bending moment in the lining are depicted in Figure 7. The bending moment distribution appears to be significantly influenced by  -100-80-60-40-20031/10/2006 30/12/2006 28/02/2007234567  Figure 5. Special monitoring section at chainage 1443 m. Dis-placements in the elements versus time.  Table 1. Material parameters used in the analyses. ______________________________________________ Setting        Rock  Shotcrete HiDCon           mass      elements ______________________________________________ Young Modulus E (MPa)  400  20000  4000 Poisson’s ratio ν (-)    0.3  0.2   0.2 UCS (MPa)       0   32.0   8.5 Tensile strength σt (MPa)  0   3.2   0.85 Cohesion c (MPa)     0   9.2   2.45 Friction angle φ (°)     0   30    30    _____________________________________________  HiDConinitialmeshextensometersFebruary 6, 2007Mean orientation ofgeological featuresradial loading= 10 MPadeformedmesh  Figure 6. Load distribution and computed displaced shape for cross section at chainage 1443 m (reference monitored sec-tion).  -4000-3000-2000-10000100020000 60 120 180 240 300 360θ [°]-200-150-100-50050100Thrust Sidewall Bending moment  Figure 7. Bending moment and thrust distribution in the lining for the cross section at chainage 1443 m (θ increases clockwise from the left wall). 234 567Displacement (mm) Thrust (kN) Bending moment (kNm/m) the presence of the HiDCon elements incorporated in the lining, while the thrust is always compressive. It is noted that the loads imposed on the support, which is able to reproduce the observed deflected shape during excavation in a satisfactory manner, induce a distribution of stress which is far from be-ing uniform as one would expect if the same support was to behave as a flexible support. To find out if any improvement could be obtained in the lining response, in order to avoid some of the deformable elements from being overstrained, other analyses were undertaken with the intent to model: 1) the presence of less slender elements in the lining, with aspect ratio equal to 1 (case 1); 2) the increase in the number of elements, having the same shape with aspect ratio equal to 2, as in the actual tunnel at chainage 1443 m, incorporated in the lining on the right side, in the same sector where the most severe strains were observed to occur (case 2). It is apparent from the results reported in Table 2 that the displacement across each deformable ele-ment in case 1 is about one half of the displacement computed for the cross section at chainage 1443 m, with the strain level being however very similar. In case 2 the deformable elements 5 and 6 undergo a strain which is approximately twice that of the re-maining elements, where the strain is approximately the same. Also, given that the deformable elements in case 1 experience a lower shortening, the shotcrete lining is to share a larger strain and stress level than in the previous case (Figure 8). On the contrary, the increase in the number of deformable elements on the right side results in a significantly lower stress in the lining. 5 CONCLUSIONS It is concluded that the increase in the number of de-formable elements placed in the lining where this is expected to be more strained is a reasonable option with respect to the corresponding symmetric distri-bution.  Therefore, in order to cope with non symmetric loading conditions of the lining, as expected when-ever the rock mass exhibits an anisotropic behavior, it is suggested to distribute the deformable elements where more severe straining is expected to occur.  In such a case, although the number of deformable elements in the lining remains the same, thus making it possible to cope with the same tunnel conver-gence, a better behavior is expected in the tunnel in placing the deformable elements where the maxi-mum strains occur. Table 2. Computed displacements and strains across the Hid-Con elements. _______________________________________________ HiDCon  Chainage 1443 m  Case 1   Case 2      ______________  _______  _______       (mm) (%)   (mm) (%)  (mm) (%) _______________________________________________ 2     22   6    13  7   40  10 3     75   19    34  17   39  10 4     57   15    30  15   51  13 5     68   18    36  19   109 27 6     84   22    40  22   93  23 7     41   11    21  11   45  11 _______________________________________________  0510152025300 10 20 30Length clockwise from left wall (m)Reference caseCase 1Case 2wall  Figure 8. Computed maximum principal stress in the lining for the monitored section at chainage 1443 m and in cases 1 and 2. ACKNOWLEDGEMENTS The studies described in this paper have been carried by Politecnico di Torino, DISTR, under a research contract with Egis, Antea, and Alpina, coordinated by Professor G. Barla. The authors would like to thank Egis, Antea, and Alpina for the permission to publish the results reported in this paper. Also to be thanked is LTF, Lyon-Turin Ferroviaire, Chambery, France. REFERENCES Barla, G. 2002. Tunnelling under squeezing rock conditions. In D. Kolymbas (ed.), Advances in Geotechnical Engineering and Tunnelling, 169-268. Barla G., Barla M., Bonini M., Debernardi D. 2007. Lessons learned during the excavation of the Saint Martin La Porte access gallery along the Lyon-Turin Base tunnel. In Schneider, John, Brandner (ed.), BBT 2007, Internationales Symposium Brenner Basistunnel uns Zulaufstrecken, Innsbruck University Press. Barla G. 2009. Innovative tunnelling construction method to cope with squeeezing at Saint Martin La Porte access adit (Lyon-Torino Base Tunnel). In Rock engineering in diffi-cult conditions in soft rocks and karst. Proceedings of Eurock 2009. ISRM Regional Symposium. Dubrovnik, Croatia. Cantieni L., Anagnostou G. 2009. The interaction between yielding supports and squeezing ground. Tunnelling and Underground Space Technology 24, 309-322. Hoek E. 2001. Big tunnels in bad rock. 2000 Terzaghi Lecture. ASCE Journal of Geotechnical and Geoenvironmental En-gineering 127(9), 762-740. σ 1 (MPa) 

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Performance monitoring and analysis of a yield-control support system in squeezing rock

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