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Geotechnical Characterization of Structurally Complex Formations: Advanced Laboratory Testing

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Geotechnical Characterization of Structurally Complex Formations: Advanced Laboratory Testing Geotechnical Characterization of Structurally Complex Formations: Advanced Laboratory Testing Mariacristina Bonini Research Assistant Politecnico di Torino (Italy) e-mail: [email protected] 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- - 3299 - 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. Vol. 17 [2012], Bund. W 3301 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. Vol. 17 [2012], Bund. W 3302 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.
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