Geotechnical Characterization of Structurally Complex Formations: Advanced Laboratory Testing

Mariacristina Bonini Research Assistant

Politecnico di Torino (Italy)

ABSTRACT This paper focuses on specific issues of advanced laboratory testing of Structurally Complex Formations. The former introductory paper on this subject showed that standard laboratory testing may clarify a few aspects related to swelling potential, water content and hydraulic conductivity and determination of the strength parameters by means of triaxial tests. It is was soon evident that further research was required to characterize the relationship between the behavior of the natural and reconstituted soil and to improve comprehension of the role which structure played in the setting up of the testing procedure and interpretation of results. The outcomes of this survey are described in the following. KEYWORDS: Structurally Complex Formations; Reconstituted Specimens; Advanced Laboratory Testing; Oedometer Tests; Triaxial Tests.

INTRODUCTION The rock materials considered in this study pertain to the Raticosa and Osteria tunnels, pertaining to the Italian High Speed Railway project between Bologna and Florence. The tunnels were partly excavated through the Chaotic Complex Tectonised Clay Shales (CCTCS) formation, soft rocks with complex structure subjected to time-dependent behavior. The laboratory specimens obtained from cubic samples taken at the tunnel face were subjected to a number of standard laboratory tests (e.g. classification, physical properties, mineralogical composition, swelling behavior, hydraulic conductivity, triaxial testing) devoted to interpret the complex mechanical behavior of the CCTCS Formation. An introductory paper (Bonini 2012) collected the results of the standard laboratory testing (Bonini 2003) which evidenced the presence of a structure made of clay fragments of various shapes and sizes, spaced by a net of fissures and planes of different roughness and orientation. Structure and index properties were found to vary from site to site and, locally, from sample to sample, due to the tectonic events occurred since deposition took place. The effects of a paleo-

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Vol. 17 [2012], Bund. W 3300 landslide, located at the north portal of Raticosa tunnel, contributed to soften and remold the original soil. A clear disagreement on the amount of clay minerals was detected through grain size distribution and X-ray diffraction analyses. This was reasonably explained by accounting for the effect of disaggregation on the size of aggregate particles and was interpreted as an indirect proof of the presence of a developed structure in CCTCS. X-ray diffraction analyses indicated also the presence of significant amounts of swelling minerals (e.g. smectite and illite). The quantitative swelling potential of CCTCS was studied through Huder-Amberg tests, which indicated a swelling coefficient similar to that of other Italian formations like S. Donato clay shales (Barla et al. 1986). The early testing evidences required further investigation on the role played by structure on mechanical behavior. Triaxial tests allowed for the determination of strength parameters in line with the characterization performed by means of shear test. However, difficulties met during the saturation stage, the presence of strain localization and significant time-dependent behavior required changes in the testing procedures and caution in the interpretation of results.

MATERIALS, METHODS AND RESULTS

Oedometer tests on natural soil The experimental program included 8 oedometer tests involving conventional tests on natural or reconstituted material as shown in Tab. 1. The Huder-Amberg tests were described in Bonini 2012. Due to the difficulties met during the cutting process, only one loading-unloading oedometer test (EDO1) on natural soil was performed. The high pressure oedometer equipment of the Soil Mechanics Laboratory of Enel-Hydro (Milan) was used. This is equipped with a Hottinger U1 50 kN load cell, two Hottinger W5TK axial displacement transducers and a prototype 250 mm internal diameter, 50 kN loading piston equipped with a very stiff loading frame with a maximum working pressure of 25 MPa (Fig. 1). This was required by the not usual stiffness of the soil tested. Due to the swelling potential of the soil, the conventional oedometer testing procedure was modified. After adding water, the specimen deformations due to swelling were prevented by the loading piston. Once an equilibrium state (stable loading pressure) was reached, the usual procedure was resumed by applying weights through arithmetic progression and by starting from a stabilized pressure of about 780 kPa. The loading sequence was the same as for conventional oedometer tests; loads were applied by weights through arithmetic progression (A.G.I., 1994). The void ratio-stress curve of the EDO1 test is plotted in Fig. 2. The approximately linear curve obtained does not allow one to determine the pre-consolidation pressure. This may be due to a disturbance of the sample or to the complex geological history of the CCTCS. The consolidation coefficient, determined through the Casagrande procedure gives a mean value of 3×10-8 m2/s. The average permeability is 7.4×10-12 m/s. Again, this low value is apparently influenced by the soil matrix more than by the presence of fissures. It is interesting to notice the very low void index exhibited by the CCTCS in natural conditions.

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Table 1: Oedometer tests performed on the CCTCS1. Max. load K Test Soil Sample Type Soil Fluid (kPa) (%) EDO1 Osteria 6 OED-IL N D 25480 - EDO2 Raticosa 4 H&A N D 480 3.2 EDO3 Osteria 0 H&A N D 1570 7.5 EDO4 Osteria 5 H&A N D 1570 5.3 EDO5 Osteria 6 H&A N D 1570 9.9 EDO6 Osteria 5 OED-IL RD D 6276 - EDO7 Osteria 5 OED-IL RNaCl NaCl 6276 - EDO8 Osteria 5 OED-IL RD D-NaCl-D 6276 - 1 where OED-IL = incremental loading oedometer test; H&A = Huder-Amberg (1970) test; D = distilled water; K = axial swelling strain corresponding to 100 kPa; N = natural soil; NaCl = NaCl saturated solution; RNaCl = reconstituted in NaCl saturated solution; RD = reconstituted in distilled water.

Figure 1: The high pressure oedometer apparatus of Enel-Hydro (Milan): details and the specimen of EDO1 test.

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0.20

0.19 t100 t24h 0.18

0.17

e [ - ] 0.16

0.15

0.14

0.13 100 1000 σ 10000 100000 'v [kPa]

Figure 2: EDO1 test compression curve for CCTCS.

Oedometer tests on reconstituted specimens Three tests were carried out on reconstituted specimens by using the procedures described by Burland (1990). The natural material was worked by pestle and sieved trough the ASTM sieves nr. 10 and 40. Aggregates included in the matrix were thus separated from the rest. The materials were worked with water in the amount of 1.5 the liquid limit. Specimens of initial diameter 100 mm and height 40 mm were consolidated at 50 kPa. The intrinsic properties, usually denoted by an asterisk, are defined as (Tab. 2):

• The intrinsic void ratio e*100 corresponding to σ’v = 100 kPa; • The intrinsic void ratio e*1000 corresponding to σ’v = 1000 kPa; • The intrinsic compression index C*c defined as e*100 - e*1000.

Table 2: Compression and swelling indexes from oedometer tests2.

e*100 e*1000 C*c cc cs λ κ EDO1 - - - 0.033 0.021 0.014 0.009 EDO6 0.679 0.424 0.255 0.261 0.103 0.113 0.045 EDO7 0.578 0.350 0.228 0.212 0.056 0.092 0.024 EDO8 0.660 0.438 0.221 - 0.057-0.166 - 0.025-0.072 2usual parameters for Cam-Clay soil model.

The EDO6 specimen was reconstituted in distilled water and tested in it. The EDO7 specimen was reconstituted and tested in a NaCl saturated solution, whereas the EDO8 test was performed on a specimen reconstituted in distilled water; during the test the cell fluid was changed from distilled water to NaCl saturated solution (when σ’v = 196 kPa) and vice-versa (when σ’v = 392 kPa) until the induced strains reached a constant level. The testing program was designed in order to obtain both the intrinsic properties and the sensibility of the double layer of clay minerals to the pore fluid composition. Index properties were determined for the reconstituted specimen by obtaining the following: LL = 35%; PL = 18%; PI = 17%; IC = 1.36. These values agree well with those of the natural material. In Vol. 17 [2012], Bund. W 3303 this case a negligible influence of the pore fluid composition on the index properties is shown, differently from what was found by Di Maio and Fenelli (1997) for the Bisaccia clay shales. Fig. 3 compares the oedometer tests performed on natural and reconstituted specimens on the void ratio-vertical stress plane. The close similarity of the curves pertaining to reconstituted specimens is observed; it is also observed that the curves maintain their shape, but the NaCl saturated specimen exhibits slightly lower void ratios. This may be due to an effective reduction of the double layer volume as a consequence of the high cation concentration. It is worth examining in detail the consolidation/swelling behavior induced during the EDO8 test. When the displacement reached a constant level for a 196 kPa applied stress, a saturated NaCl solution was substituted to distilled water. The displacement was measured during the following days until the osmotic process driven by the saline solution reached equilibrium. The remaining loading steps were applied regularly by maintaining the same pore fluid. During the unloading steps corresponding to 392 kPa the pore fluid was substituted again and frequently renewed in order to support the reverse osmotic process. Two clear effects of the substitution of the pore fluid are the swelling index increase shown by the EDO8 test after exposure to distilled water during unloading and the small, however not negligible, induced strains. The variation of the index values is confirmed as shown in Tab. 2. While the compression indexes of the EDO6 and EDO7 tests are more or less the same, the swelling indexes differ of nearly 40%. The EDO8 test shows that the swelling index of the material reconstituted in distilled water and exposed to NaCl solution changes from a value similar to the one determined for the material reconstituted in NaCl solution to a value which is greater than the one determined for distilled water. In other words the swelling index becomes three times the previous value when the pore fluid is changed from NaCl solution to distilled water. This is even more evident when represented on the intrinsic void index Iv - vertical stress plane (Fig. 4). The compression curves lay well on the intrinsic compression line, whereas the swelling curves have clearly different slopes.

σ 10 100 'v [kPa] 1000 10000 100000 0.8 EDO1 - natural 0.7 EDO6 - reconst. dist. water NaCl 0.6 EDO7 - reconst. NaCl solution EDO8 - reconst. mixed test 0.5

e [ - ] 0.4

0.3 Distilled water 0.2

0.1

Figure 3: Oedometer tests performed on natural and reconstituted soil: void index-vertical stress plane. Vol. 17 [2012], Bund. W 3304

σ 10 100'v [kPa] 1000 10000 100000 1.0 SCL ICL EDO1 - natural 0.5 EDO6 - reconst. distilled water EDO7 - reconst. NaCl solution 0.0 NaCl EDO8 - reconst. mixed test -0.5

-1.0 Iv [ - ]

-1.5 Distilled water -2.0

-2.5

Figure 4: Oedometer tests performed on natural and reconstituted material: intrinsic void index vs. vertical stress.

Triaxial tests: specific issues Some standard aspects related to the triaxial tests were discussed in Bonini 2012. However specific issues due to the complex structure of the soil tested, its peculiar behavior with respect to the hydraulic conductivity and time-dependent behavior required further investigation. The triaxial tests were performed on the Soft Rocks Triaxial Apparatus (SRTA) as developed by the Rock Mechanics and Rock Engineering Research Group of the Department of Structural end Geotechnical Engineering at the Politecnico di Torino (Barla et al., 1999). The testing procedure involved closely controlled stress path conditions which reproduce the stress history around the tunnel through typical stress-paths for the rock mass surrounding the tunnel during excavation (Barla M. 1999; Barpi et al. 2011). The testing procedure adopted for triaxial testing of the CCTCS was the same as originally developed for the testing of swelling rocks in the SRTA: specimen set-up, flushing, saturation, consolidation, shearing, time-dependent strains development and swelling/consolidation. The particular aspects of each phase are described in the following. After the specimen set-up, the flushing phase was performed in order to saturate the dry back- pressure circuit. At the same time swelling was prevented applying proper vertical and cell pressures (Tab. 3). A certain variability at the end of the phase was observed with the stress deviator being alternatively positive or negative. This showed that the chaotic structure of the material tested does not exhibit a tendency to swell in one definite direction. The time needed for pressure stabilization during the flushing phase was relatively small compared to the time needed for primary consolidation in the oedometer tests. It is likely that the small gradient of water and the relative high pressures achieved succeeded in wetting only the exterior of the specimen. Therefore the swelling pressures may be due only to the swelling potential of the outer specimen zone, which makes it difficult to compare the flushing pressure with the swelling pressure from the oedometer tests. The saturation phase generally took place by a step by step increase of back-pressure. This technique is founded on the principle that air is soluble into water and that air solubility increases Vol. 17 [2012], Bund. W 3305 as air pressure increases was particularly difficult to perform as desired. The main problem encountered was due the very low permeability of the natural soil. This resulted in some changes in the features of the back pressure circuit and in the saturation procedure adopted, however this did not produce significant changes in the degree of saturation. With the low B values reached during the first series of tests several measures were implemented: • The duration of the saturation steps and B-checks were varied. Steps of 50 kPa were applied for a minimum of 1 to a maximum of 6 days, without appreciable influence. The B-check was increased from 10 minutes to more than 2 hours. This evidenced the tendency for the B value to increase up to an asymptotic value (Fig. 5). • The triaxial apparatus was modified by adding a by-pass in the back pressure circuit, in order to reduce the flow distance to half the specimen height (thus reducing the flow time four times). The diameter of the tubing was reduced to the minimum achievable and made the same for all the fittings and valves in order to increase the circuit stiffness and decrease the possibility for air to be still present. However these measures did not result in higher B values. • At the end of the saturation phase two more steps were performed by increasing and decreasing the back pressure of 50 kPa, thus coming back to the same value of final back pressure (Fig. 5). The resulting B value vas calculated as the mean value from the two last steps. The final B value was higher than the one attained at the end of the conventional saturation phase (0.77 > 0.68). The difficulties met during this phase were all connected to the very low permeability of the material. As mentioned before, it was demonstrated by oedometer tests on reconstituted specimens that permeability remained low even after the reconstitution treatment. It is worth noting that the B value calculated after applying the consolidation ramp was higher than the one measured at the end of saturation. This is due to the fact that between the last B check and the beginning of the consolidation phase further dissolution of air into the pore water occurs.

Table 3: Vertical, horizontal and deviator stresses attained at the end of the flushing phase. Test RTC1 RTC1a RTC1b RTC2 RTC3 RTC4 RTC5 Average

σ’vf (kPa) 177 303 272 278 239 220 227 245 σ’hf (kPa) 155 265 332 329 273 303 224 269 qf (kPa) 22 38 60 -51 -34 -83 4 -23 Time (h) 11 - - 9 5 6 9 8 The consolidation phase brings the specimen to its original stress conditions. Once the desired conditions are attained, stresses are maintained constant for the time required to reach a creep rate lower than 0.05 %/day. The specimens were loaded with a fixed loading rate of 0.5 kPa/s in order to reach the in situ state of stress in isotropic conditions. Tab. 4 gives the final volumetric strains as measured by means of the volume gage and of the external and local strain transducers. The volumetric strain at the volume gage was found to be intermediate between the values estimated on the basis of the external and the local strains, however closer to the value measured by the external transducers. This may be due to the shape of the deformed specimen, which definitely showed very small radial strains. Vol. 17 [2012], Bund. W 3306

800 1

700 Cell pressure 0.8 600 B.P. top 500 0.6 400 Vertical pressure [-] B 300 0.4 Pressure [kPa] B.P. base 200 0.2 100 B 0 0

Figure 5: B-check of RTC3 test (B.P. stands for back pressure at top and base pedestals). One point of interest was observed during the consolidation phase of the RTC5 specimen. The top drainage was left closed in order to measure the increase in pore pressure due to consolidation. The increment in pore pressure was 227 kPa, with a corresponding consolidation ramp of 275 kPa. The resulting B value is 0.83, which is higher than the value at the end of saturation (0.65).

Table 4: Volumetric strains during consolidation phase. Volumetric strain Test Volume gage (%) External (%) Local (%) RTC1 - 0.79 0.50 RTC2 0.67 0.71 0.58 RTC3 1.19 1.11 0.88 RTC4 0.90 1.22 0.99 RTC5 1.09 1.28 0.88 Mean value 0.96 1.02 0.77 The stress-path phase is intended to reproduce at laboratory scale the stress conditions typical of points located in the tunnel proximity. This phase is performed in undrained conditions, following the stress-path computed for a circular tunnel when Ko = 1, with s = (σv+σh)/2 = constant. The stress-path is carried out at constant strain rate (0.001 %/min to 0.01 %/min). As the vertical stress increases, the desired horizontal stress is adjusted to maintain s = constant within ± 0.4 kPa. Details on results and interpretation may be found in Bonini 2012. Following the stress-path phase, creep stage at constant deviator (the one attained at the end of the stress-path stage) was performed. Creep data in terms of axial and volumetric strains versus time, and axial and volumetric strain rates versus the applied stress deviator were obtained for the OST3, RTC3, RTC4 and RTC5 tests. The OST3 test, differently from RTC tests, which followed the procedure described at the beginning of this paragraph, was consolidated through 18 loading steps of 50 kPa to an isotropic stress of 900 kPa, without flushing and saturation phases. OST3 test was specifically designed to investigate the time-dependent behavior. Failure was reached by performing 10 loading steps, each of 100 kPa and maintaining the cell pressure constant. Each step was continued until the Vol. 17 [2012], Bund. W 3307 volumetric strain rate decreased to 0.05 %/d. It was therefore possible to obtain the axial strain curves in either isotropic (Fig. 6) or deviatoric conditions (Fig. 7).

1 900 kPa 850 kPa 800 kPa 750 kPa 700 kPa 0.8 650 kPa 600 kPa 550 kPa 500 kPa 450 kPa 0.6 400 kPa 350 kPa 300 kPa 0.4 250 kPa 200 kPa Axial strain [%] 150 kPa 0.2 100 kPa 50 kPa 0 0 250 500 750 1000 1250 1500 Time [min]

Figure 6: Axial strain curves versus time during the OST3 test: isotropic loading (numbers represent isotropic stress values).

3 1000 kPa RTC3 - 87% RTC4 - 54% 2.5 900 kPa RTC5 - 79% 800 kPa

2 700 kPa 600 kPa

500 kPa Axial strain [%] 1.5 400 kPa 300 kPa

200 kPa 100 kPa 1 0 500 1000 1500 2000 2500 3000 Time [min]

Figure 7: Axial strain curves versus times during OST3 test: anisotropic loading (numbers represent deviatoric stress values; boxed numbers show the percentage of the failure stress deviator). As a consequence of isotropic loading the sample showed an increase of stiffness and nearly constant axial strain rates with a mean value of 0.018 % /d approximately. Strain rate values were also recorded during the stress-path phase, showing a tendency to increase with the deviatoric stress according to an exponential law. At the same time the volumetric strain rate values are shown to be nearly constant with no influence due to an increase of the deviatoric stress. The creep behavior exhibited by the CCTCS was compared with the data available for similar complex formations (D’Elia et al. 1998). The two diagrams shown in Fig. 8 give the axial strain Vol. 17 [2012], Bund. W 3308 rate versus time for different values of the deviatoric stress. It is shown that tiime-dependent strains develop for a mobilized stress deviator nearly equal to 50% of the failure value; the strain rate is shown not to increase significantly as the stress inncrease, even if the failure stress deviator is attained. These results demonstrate that the CCTCS may exhibit a significant tiime dependent behavior even for a small deviatoric stress.

1.E-01 q/qf = 0.55 1.E-02 q/qf = 0.78 q/qf = 0.87 1.E-03 q/qf = 1.00

1.E-04

Axial strain rate [%/min] 1.E-05

1.E-06 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 Time [min] Figure 8: Axial strain rate versus time obtained for various stress levels during creep tests on structurally complex clays (D’Elia et al. 1998 and CCTCS). Due to the diffficulties associated with the creep phase, only the RTC5 specimen was subject to the drained phase. With a stress deviator of 268 kPa (ii.e. 80% of the stress deviator at failure) the stress path phase was stopped with an excess pore pressure equal to 9 kPa. During the creep phase, small creep strains were observed with the strain rate soon to stabilize to a limit value of 0.05%/d (Fig. 9). The volumetric strain at the end of the creep phase was nearly equal to zero. With the initiation of the creep phase the pore pressure was shown to decrease to a constant value of 305 kPa, which is equivalent to an excess poore pressure of approximately –95 kPa (Fig. 10). It is interesting to note that the excess pore presssure varied from 9 to –95 kPa during the creep phase, with a tendency for the volume to increase. The positive excess pore pressure at the end of the stress phase resulted in a negative pore pressuure following the development of a creep strain. This behavior may be explained as follows: • The specimen may be not well saturated, thus deecreasing its pore pressure in undrained conditions. • In spite of the low strain rate applied during the stress phase, the internal state of stress in the specimen may have been different from that measured at its ends. It was decided to wait for the stabilization of the pore pressure before opening the drainage. This operation was performed by connecting the bottom end of the specimen with the volume gage. This allowed for the contemporary measurement of the volumetric strain induced by the negative excess pore pressure and of the variation of thee pore pressure at the specimen top (Fig. 10). The volumetric strains measured by the displacement transducerrs and by the volume gage were quite different (Fig. 11). This may be due to the fact that the specimen does not assume the classical barrel shape and the theoretical value of the volume strain is far from the value given by the sum of the principal strains. Vol. 17 [2012], Bund. W 3309

0.50

Volume gage 0.25 Drainage ε11 + 2 ε33 opening Axial strain Radial strain 0.00 0 5 10 15 20 25 Strains [%]

-0.25

-0.50 Time [d]

Figure 9: Creep and drained phases of RTC5 test. 420

400

380

B.P. TOP 360 B. P. BASE 340 u, pore pressure [kPa] Δ 320 Drainage opening 300 0 5 10 15 20 25 Time [d]

Figure 10: Creep and drained phases of RTC5 test. It was evident that it is very difficult to interpret the results of the drained phase of the triaxial tests performed on the CCTCS. The difficulties lie mostly in the superimposition of various affects like mechanical swelling due to stabilization of pore pressure, to the beginning of chemical swelling as a consequence of water ingress and to unavoidable creep strains. Vol. 17 [2012], Bund. W 3310

0 400

-0.1 380

-0.2 Volume gage 360

-0.3 ε11 + 2 ε33 B.P. TOP 340 Strains [%] -0.4 Pore pressure [kPa] 320 -0.5

-0.6 300 0 5 10 15 Time [d]

Figure 11: Creep and drained phases of RTC5 test.

CONCLUSIONS Oedometer tests performed on natural and reconstituted materials evidenced the very low void index of the natural CCTCS and their sensitivity to the ionic concentration of the pore fluid. This indicates the possibility of a modification of structure for the CCTCS present in the superficial covers due to swelling induced by fresh water. The characteristics of the CCTCS reconstituted in NaCl saturated solution are more similar to those of the natural formation. The only oedometer test performed on natural soil did not allow one to determine the pre- consolidation pressure. The triaxial tests performed in closely controlled conditions allowed for the determination of the typical stress-strain behavior and strength parameters. Despite the efforts made to optimize procedures for specimen saturation, the results obtained suggest that wetting involves only the exterior of the specimen. Therefore the swelling pressures may be due only to the swelling potential of the outer specimen zone, which makes it difficult to compare the flushing pressure of triaxial tests with the swelling pressure from the oedometer tests. The drained creep phase, devoted to investigate the time-dependent behavior of the CCTCS after the stress-path phase, was affected by several difficulties due mostly to the superimposition of various effects like mechanical swelling due to stabilization of pore pressure, to the beginning of chemical swelling as a consequence of water ingress and to unavoidable creep strains. Anyhow significant time-dependent deformations were shown by the creep tests and the creep phases in the triaxial tests. Time-dependent deformations were shown to be present at low stress levels, generally with a rate decreasing with time. During the drained phases of the triaxial tests (simulating the behavior of the rock mass during a stand-still and/or at points located away from the tunnel contour) it was not possible to distinguish among mechanical swelling, chemical swelling and creep, as all the phenomena are shown to occur simultaneously. In conclusion, it was observed that, in this case, standard and advanced laboratory testing allow one to characterize the mechanical behavior of the CCTCS at the laboratory scale and Vol. 17 [2012], Bund. W 3311 determine the mechanical parameters of advanced constitutive laws for time-dependent behavior, as described in detail in Bonini et al. 2009. However, the availability of performance monitoring data proves to be crucial in the transition between the laboratory scale and the site scale (e.g. Bonini and Barla 2012, Bonini et al. 2012).

REFERENCES 1. A.G.I. (1994) “Raccomandazioni sulle prove geotecniche di laboratorio”. Italian Geotechnical Society. 2. Barla G., Pazzagli G., Rabagliati U. (1986) “The San Donato tunnel (Florence)”, Proc. Int. Congress on Large Caverns. Florence, Italy, 61-69. 3. Barla M. (1999) “Tunnels in swelling ground – Simulation of 3D stress paths by triaxial laboratory testing”, Ph. D. Thesis, Politecnico di Torino, Italy. 4. Barla M., Barla G., Lo Presti D.C.F., Pallara O., Vandenbussche N. (1999) “Stiffness of soft rocks from laboratory tests”, Proceedings of the IS Torino ’99, Second International Symposium on Pre-failure deformation characteristics of geomaterials. Torino, Italy. 5. Barpi F., Barbero M., Peila D. (2011) “Numerical modelling of ground-tunnel support interaction using bedded-beam-spring model with fuzzy parameters”. In: Gospodarka Surowcami Mineralnymi 27( 4), 71-87. ISSN 0860-0953. 6. Bonini M. (2003) “Mechanical behaviour of Clay-Shales (Argille Scagliose) and implications on the design of tunnels”. Ph. D. Thesis, Politecnico di Torino. 7. Bonini M., Debernardi D., Barla M., Barla G. 2009, “The Mechanical Behaviour of Clay Shales and Implications on the Design of Tunnels”, Rock Mechanics and Rock Engineering, 42(2), 361-388. DOI: 10.1007/s00603-007-0147-6. 8. Bonini M. (2012) “Geotechnical Characterization of Structurally Complex Formations: Standard Laboratory Testing”, Electronic Journal of Geotechnical Engineering 19(T), 2671-2683. ISSN 1089-3032, DOI Ppr12.250. 9. Bonini M., Barla G. “The Saint Martin La Porte Access Adit (Lyon - Turin Base Tunnel) Revisited”. Tunnelling and Underground Space Technology 30, 38-54. 10. Bonini M., Lancellotta G., Barla G. (2012) “State of stress in tunnel lining in squeezing rock conditions”, Rock Mechanics and Rock Engineering, On-Line First. DOI 10.1007/s00603-012-0326-y. 11. Burland J.B. (1990), “On the compressibility and shear strength of natural clays.” Géotechnique, 40(3), 329-378. 12. D’elia B., Picarelli L., Leroueil S., Vaunat J. (1998), “Geotechnical characterization of slope movements in structurally complex clay soils and stiff jointed clays”, Italian Geotechnical Journal, 3, 5-32. 13. Di Maio C., Fenelli G.B. (1997) “Clayey soil deformability: the influence of physico- chemical interactions”, Italian Geotechnical Journal 1, 43-54. Vol. 17 [2012], Bund. W 3312

14. Huder J., Amberg G. (1970) “Soft Quellung in mergel, Opalinuston und anhydrit”, Schweizerische Bauzeitung, 88(43), 975-980.