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ASSESSING THE INTERACTION BETWEEN THE EXCAVATION OF A LARGE CAVERN AND EXISTING TUNNELS IN THE ALPSSYSTRA: SEMERARO Martino, MISANO Edoardo, SCHIVRE Magali, BOCHON Alain

ASSESSING THE INTERACTION BETWEEN THE EXCAVATION OF A LARGE CAVERN AND 

EXISTING TUNNELS IN THE ALPSSYSTRA: SEMERARO Martino, MISANO Edoardo, SCHIVRE Magali, BOCHON Alain

INTRODUCTIONThe 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

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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.

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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.

GEOLOGYDuring 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).

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IN-SITU INVESTIGATIONSAccording 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 liningIn 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]

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Convergence analysisThe 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.

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IMPACT OF THE NEW LSM ON EXISTING STRUCTURESConsidering 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 calibrationThe 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.

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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.

LithotypeE

[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.

SectionSigma 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

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Impact on existing structuresThe 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 effectsThe 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.

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MONITORING PLANThe 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.

SectionSigma 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).

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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

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