Geotechnical Characterization of Structurally Complex Formations: Standard Laboratory Testing Mariacristina Bonini Research Assistant Politecnico di Torino (Italy) e-mail:
[email protected]
ABSTRACT The characterization of Structurally Complex Formations is a challenging task due to the number of issues which may be encountered, e.g. time-dependent behavior, swelling/squeezing conditions, complex structure, saturation, hydraulic properties. The excavation of the Bologna-Florence high speed railway line in Italy offered the opportunity to investigate the mechanical behavior of a Structurally Complex Formation both from standard and advanced laboratory testing point of view. The paper describes the relevant issues raised during the standard laboratory testing with the aim of being a useful guide for Geotechnical Engineers who approach first the study of structurally complex rocks.
KEYWORDS:
Structurally Complex Formations; Soft Rocks; Swelling Potential;
Triaxial Testing.
INTRODUCTION The Italian High Speed Railway project is intended to provide Italy with a network of lines able to achieve a higher operating speed. The 78.5 km-long Bologna-Florence line is located along the North-South alignment and is the focal point for railway traffic. The line involves nearly 80 km of underground works carried out partly in structurally complex formations. The presence of these formations, which often show high squeezing and swelling potentials, associated with the morphologic characteristics of the Apennine Chain leads to a number of difficulties in tunnel construction. The rock materials considered in the following pertain to the Raticosa tunnel and Osteria access adit. Fig. 1 shows a schematic longitudinal section of the Raticosa tunnel with the sites where cubic samples were cut at the tunnel face. More than half of the Raticosa tunnel was excavated through the Chaotic Complex Tectonised Clay Shales (CCTCS) formation, whereas the remaining length is in marly and arenaceous formations (sandstones, marl, silts and marl). - 2671 -
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The T laboratory y specimens obtained from m the cubic ssamples weree subjected too a number oof standaard laboratorry tests (e.g.. classificatio on, physical properties, m mineralogicall compositionn, swelliing behavior, hydraulic co onductivity, trriaxial testingg) devoted to give a first innterpretation oof the co omplex mech hanical behav vior of the CC CTCS Formattion, a soft roock with com mplex structurre subjeccted to time-d dependent beh havior. The T following paragraphs describe d the materials, m testting methods and results oobtained in thhe frameework of charracterization of o this compleex formation,, with the aim m of being a uuseful guide foor Geoteechnical Engiineers who approach a firstt the study off structurallyy complex roccks. The worrk provid des also com mparisons wiith other forrmations whiich share paarticular aspeects (structurre, swelliing or time--dependent behavior) b witth CCTCS, such as S. Donato, Terrravecchia annd Varicolori clay shaales (Barla et al. 1986), S. Barbara clayy shale (D’Elia, 1991), Opaalinus and Liaas Alphaa clay (Aristtorenas 1992), Caneva cllay (Barla 1 999), Frenchh marls (Bulltel 2001) annd Bisaccia (Picarelli et al. 2002).
Intersection w with Osteria accesss adit Chainage: ~6000 m Depth: ~1400 m
Raticosaa tunnel Chainage: 30+116 m Paleo-laandslide Depth:: 22 m
Figure 1: Raticosa R tun nnel longitud dinal section:: LC = Chaootic Complexx, OL = Olistostrromes; RMA A = marly an nd arenaceouus formationns (Bonini 20003).
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MATERIALS, METHODS AND RESULTS Structure, physical properties and mineralogical composition The samples considered (Fig. 1) pertain to two different sites, the paleo-landslide area and the remaining tunnel length. This implies that the samples of Raticosa tunnel (cubic sample 4) may have undergone some softening and/or weathering processes which may have modified both structure and properties of the rock material. How strong this influence may be is not easy to assess. The physical properties of the CCTCS are listed in Tab. 1 and 2. Comparing these data with those available for other Italian structurally complex formations shown in Tab. 2, allows one to draw the following conclusions: •
The index properties vary in a wide range, which underlines the great heterogeneity of the material, both at the sample and at the rock mass scale. The great variability of the index properties is typical of structurally complex formations. The natural water content and void index of sample 4 are higher than those obtained for the other samples, in accordance with the lower depth and the likely weathering (contact with fresh water) and softening processes undergone by the material of sample 4. The other index properties do not show a significant variation.
• • •
Table 1: Physical and index properties of the CCTCS1. Site Raticosa Osteria Osteria Osteria
Cubic sample 4 0 5 6
Depth (m) 22 140 140 140
wn (%) 11.5 7.3 7.1 6.6
γu (kN/m3) 22.9 22.8 22.8 24.0
Gs (-) 2.72 2.72 2.68 2.66
e (-) 0.301 0.255 0.231 0.161
LL LP PI (%) (%) (%) 40 22 18 43 19 24 34 19 15 43 19 24
CaCO3 (%) 10 1 15
wn = natural water content, γu = specific gravity, Gs = grain density, e = void ratio, LL = liquid limit, LP = plastic limit, PI = plasticity index, CaCO3 = calcium carbonate content. 1
Table 2: Comparison of index properties of the CCTCS with other formations Index properties CCTCS Bisaccia Caneva S. Barbara S. Donato C. F. 2 (%) Liquid Limit (%) Plastic Limit (%) Plastic Index (%) 2
10÷34 28÷43 14÷22 12÷24
≈50 40÷50 100÷200 33÷64 60÷120 9÷23 12÷50
17÷25 28÷30 19÷21 9
15÷25 20÷30 13÷15 7÷15
Clay Fraction = % by weight of size smaller than 0.002 mm.
The grain size distributions evidenced a percentage of clay particles variable from 33% to 55%, with the presence of significant percentages of sand and gravel. The presence of gravel in CCTCS it is to be identified with quartz and albite inclusions. The sand percentage can be traced back to the presence of particle aggregates. According to the Plasticity Chart, the CCTCS can be classified as “inorganic clays of low to average plasticity” whereas the Unified Soil Classification System identifies the CCTCS as CL
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(inorganic clays). By comparing these values with those reported in the literature (Tab. 2) a certain similarity with both the S. Donato and S. Barbara clay shales is evidenced.
Swelling behavior The swelling potential of the CCTCS was studied in a qualitative way through a series of Xray diffraction mineralogical analyses. The results obtained are summarized in Tab. 3 and 4. The clay fraction represents more than half of the total. As the clay content evaluated from the grain size distribution is less than this quantity, one can infer that some of the larger particles (more than 0,002 mm in diameter) are made up of attached clay grains. As for similar soils, it is noted that the “effective grain size” may influence the mechanical properties which are directly related to the index properties (Picarelli and Olivares, 1998). The remaining minerals are quartz, with calcite and albite in traces. A comparison of the CCTCS with other similar rock materials shows that while the clay content is more or less the same, calcite is nearly absent and the quartz content is in appreciable quantity. The clayey minerals reported in Tab. 4 are listed by decreasing the cation exchange capacity (CEC), or rather by decreasing the swelling potential. The CCTCS contain expandable minerals in ratio of 20÷50 % of the total of the clay minerals. It is interesting to note that the cubic sample 4, which has been taken at the Northern portal of the Raticosa tunnel, in the paleolandslide area, which is quite far from the other sample locations, shows similar characteristics. The variability in mineral content is smoothing out the differences and makes it “homogeneous” the behavior of different sites. An interesting illustration of the swelling potential is given in Fig. 2 (left). The triangular plot gives the mineralogical components of a rock. Each point is defined by three percentages of clay minerals, quartz and carbonate content, respectively, as shown clockwise from 0 to 100%. The data confirm that the CCTCS exhibit medium to high swelling potentials. The triangular diagram based on the clay mineral content gives an indication of the tendency for a material to swell. However, it does not account for the effective amount of swelling shown by the different clay minerals (smectite, illite, etc.). A more effective classification for the CCTCS can be given by considering the percentages of different clay minerals, as illustrated in Fig. 2 (right). The diagram shown is similar to the previous one but the mineralogical components are now substituted by clay minerals having different activity and CEC. The simple principle which holds for this diagram is given by the variable activity and sensitivity of the minerals under consideration (smectite, illite and others) in relation to the physic-chemical reactions and the peculiar behavior which follows. Each zone in the diagram represents the reaction of clay mineral when exposed to physic-chemical reactions and the consequent effects. This classification, more than establishing the swelling potential of a given rock, determines the trend of swelling behavior. It is intended that a point in the central area of the diagram is indicative of the effects which may be superimposed in variable measure. According to Fig. 2 (right) the CCTCS are likely to be exposed to swelling due both to water adsorption and loss of strength as a consequence of bond weakening.
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Table 3: X-ray X diffracttion analysess on the whoole sample: rresults and comparisons with available a datta Calcite C (%) Raticosa (cubiic sample 4) 10 Osteria (cubic sample 0) 1 Osteria (cubic sample 5) Osteria (cubic sample 6) 15 S. Donato 11÷13 1 Caneva 12÷25 1 French marls 25÷45 2 Soiil
Quartzz Clay mineraals Albite (%) (%) (%) 35 45 10 35 55÷60 5÷10 40 50 10 35 5 45÷50 12÷166 70÷77 10÷255 50÷65 20÷455 30÷45
Table 4: Comparisson of index x properties oof the CCTC CS with otheer clays (all av vailable dataa) Smectitee (%) Raticossa (cubic samplle 4) 5 Osteriaa (cubic samplee 0) 5 Osteriaa (cubic samplee 5) 5÷10 Osteriaa (cubic samplee 6) 5÷15 17 S. Donaato 28 13 Canevaa clay 40 25 French marls 60 45 Soil
Illite Illitee-Smectite Chhlorite Kaolinnite (%)) (%) (%) (%) 25÷50 10÷20 440÷50 20÷40 10÷25 330÷40 10÷25 25÷50 330÷40 5÷20 25÷50 220÷30 19 14 34 19 16 12 12 10 5 2 45 5 25 20 10 10 25 15 15
Figure 2: Swelling potentiaal of CCTCS S in term of pprincipal miineralogical componentss (left) and swelling pottential of CC CTCS in term m of swellinng minerals ((right).
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In situ stress conditions, water content and hydraulic conductivity The determination of the in situ state of stress is a difficult task due to the complex geologic history of the CCTCS formation. The in situ stresses are dependent not only on stress relief due to erosion but also on diagenesis, and additional stress conditions connected with tectonic movements and morphology. This let one infer the likely complex and variable characteristics of the in situ stresses in CCTCS. However, at present the experimental methods available in Rock Mechanics for determining the in situ state of stress cannot be used reliably. At the same time the indirect determination of the in situ state of stress based on the results of oedometer testing is nevertheless uncertain. Oedometer tests often give values for the preconsolidation pressure lower than the in situ stress. For this reason the results may not be significant. For pressures higher than the assumed in situ stress, inferred on the basis of the overburden value, the consolidation curves lead one to assume that the maximum preconsolidation pressure is higher than that reached during the test. The CCTCS are influenced by a two-fold aspect: on one end the clay matrix exhibits a very low permeability, on the other end the close network of fissures which are present in the CCTCS contributes significantly to the overall hydraulic conductivity. At shallow cover these fissures may have a considerable influence. The natural water content of the CCTCS samples ranges from 5 to 22%, owing to the high heterogeneity. It is worth observing that the natural water content (5÷15%), as well as the degree of saturation (80÷98%), decreases with the overburden (Fig. 3). The values of the primary consolidation coefficient cv and of the hydraulic conductivity k (no distinction between fissure or matrix conductivity has been introduced here), calculated from the oedometer tests, result to be equal to 1⋅10-7÷1⋅10-8 m2/s and 1⋅10-9÷1⋅10-12 m/s respectively. It is interesting to compare the results from the oedometer tests performed on natural and reconstituted materials as depicted in Fig. 4. It is shown that the hydraulic conductivity is strictly dependent on the void index, for both the natural and reconstituted conditions, even if the structure of the CCTCS is essentially different. This means that, at least at the sample scale, the fissures have a negligible influence on hydraulic conductivity. At the same time, the strong dependence of hydraulic conductivity on the void index for the natural material indicates, however, a likely influence of fissures. It is of interest to note that based on Lugeon tests, which were performed in situ at depth ranging from 30 to 270m, the resulting in situ hydraulic conductivity was estimated to be 1⋅10-7÷1⋅10-9 m/s.
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Figure 3: Natural watter content and a saturatioon degree forr CCTCS sam mples.
Figure F 4: Hy ydraulic con nductivity fro om oedometeer tests perfo formed on naatural and reconstituteed CCTCS saamples.
Huder-A Amberg tests The T swelling behavior b of th he CCTCS was w investigatted by meanss of 4 Huder--Amberg testts. The teesting proced dure followed d the I.S.R.M. recommendaations for dettermining the axial swellinng stress as a functio on of the ax xial swelling strain (Maddsen, 1999). Results from m the tests arre summ marized in Fig g. 5, compared with the Caaneva clay annd the S. Donnato clay shalles. The resullts are giiven by plottting the axiall strain versus the axial sttress in logarrithmic scale. The swellinng coeffiicient K variees in the rangee 3.2÷9.9%, with w a mean vvalue of 6.5%.
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Figure F 5: Hu uder-Amberg g tests: CCT TCS compareed with S. Donato clay sshales and Caaneva clay. A close study of o the plot sug ggests the folllowing remarrks: • • •
The CCTC CS of the Ostteria access ad dit show a treend of behavior similar too that exhibiteed by the S. Donato D clay-sshales, though the stress iimpairing sweelling is highher in the latteer case. Caneva claay shows a co omparable sw welling stress bbut moderate strains at low w stress. The specim men belongin ng to the Ratticosa tunnel (which is also the one w with the loweest expandablee mineral con ntent) exhibits lower strainns, in accordaance with its lower smectite content.
Triiaxial tessts A total of six triaxial tests were perform med as listedd in Tables 5 and 6. As shhown from thhe colum mn “Type of test” t the aim was w to simulaate at laboratoory scale the tunnel behavior in the shoort and lo ong term. Alll the tests, ex xcept RTC1 which w was carrried out up tto failure, weere subject to a s = (σ1 +σ3)/2 = co onstant stresss path, typical of a ppoint locatedd at the tuunnel sidewaall (Com mpression Loaading =CL). The T values of o the consoliidation isotroopic effectivee stress and oof back pressure p are all a the same and a are consiidered to be rrepresentativee of the site cconditions. Thhe pore pressure p in th he rock mass is not known. This is maainly due to ddifficulties off measuremennt due to o low permeab bility and com mplex structu ure of the soil and to the prresence of lanndslide. The T piezometeers in the zon ne revealed th he presence oof a water levvel ranging between 48 annd 64 m depth, leadin ng to an effecctive stress acting at the cconsidered deepth of aboutt 500 kPa. Thhe need of o saturating the sample reequires the application of a back pressurre of 400 kPaa.
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Table 5: Triaxial tests performed on CCTCS.
3
Test
Site
OST3 RTC1 RTC2 RTC3 RTC4 RTC5
Osteria Raticosa Raticosa Raticosa Raticosa Raticosa
Cubic Depth σ’c3 B.P.3 Triaxial wi3 Type of test sample (m) (kPa) (kPa) apparatus 2 142 7.9 CID + creep 900 0 SRTA 4 22 12.1 CIU 486 400 SRTA 4 22 12.3 CIU + creep 496 403 SRTA 4 22 11.9 CIU + creep 497 399 SRTA 4 22 11.6 CIU + creep 488 404 SRTA 4 22 12.4 CIU + creep + drained 501 396 SRTA
wi = initial water content, σ’c = consolidation effective stress, B.P. = back pressure.
Table 6: Results of the triaxial tests performed on CCTCS. ε a 4 wi4 wf4 σ’vf ;σ’hf; qf 4 B4 σ’c 4 B.P. 4 tmax s’max tmax/tfail Δu Test (kPa) (kPa) (kPa) (kPa) (%) (%) (%) (-)
(kPa) (mm/min) OST3 7.9 7.1 900 0 0.0005 538 1438 RTC1 12.1 13.1 0.81 486 400 0.001 170 555 -65 RTC2 12.3 13.7 0.75 496 403 0.01 181 524 106 -28 RTC3 11.9 13.7 0.77 497 399 0.005 148 480 87 18 RTC4 11.6 12.6 0.80 488 404 0.005 94 453 54 31 RTC5 12.4 13.4 0.65 501 396 0.001 134 491 79 9++ 4 wi/f = water content at beginning/end of the test, σ’vf ;σ’hf; qf = vertical, cell pressure and deviator (kPa)
177;155 ;22 278;329;-51 239;273;-34 220;303;-83 227;224;4
at the end of flushing, σ’c = consolidation effective stress, B.P. = back pressure, ε a = rate of vertical displacement in stress path phase. ++ Value recorded at the end of the stress path phase.
The stress-strain curves illustrated in Fig. 6 show an elasto-plastic type hyperbolic behavior. It is noted that the tests were performed with different values for the rate of axial displacement during the shearing phase, in order to outline the influence of this parameter on both the soil stiffness and the effective stress path. The five RTC tests were carried out according to a CL stress path, with s’o = 500 kPa. The resulting effective stress paths are shown in Fig. 7, together with the Mohr-Coulomb failure envelope holding true for the CCTCS (c’=20.3 kPa, φ′ = 16.6°). It is of interest to compare in the following the stress-strain behavior of the CCTCS with that observed for similar complex formations, as illustrated in Fig. 8, where also shown are the typical results of a triaxial test performed on the Bisaccia clay shale (Olivares et al., 1997). In both cases an elasto-plastic type hyperbolic stress strain behavior is observed and failure of the specimens takes place concurrent with the development of shearing bands. As the stiffness is observed to decrease abruptly, in both the CCTCS and the Bisaccia clay shales the pore pressure decreases. In particular, in the latter case the pore pressure measured at the base and at mid height of the specimen show a different response.
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Figu ure 6: Stress-strain behav vior for RTC C tests, show wing differennt axial strainn rate valuess.
ve stress path hs of RTC teests, showinng the B valuue. Figurre 7: Effectiv
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Fiigure 8: Streess-strain an nd excess porre pressure ccurves for thhe Bisaccia cclay shales (Olivarees et al. 1997 7) compared d with the Raaticosa CCTCS (RTC1 ttest).
Shear stre ength pa arameterrs The T results off the OST3 and RTC1 tessts are shownn in Fig. 9, w where also reeported are thhe failuree envelopes obtained for the CCTCS on the basis of isotropic consolidatedd drained (TX XCID) and undraineed triaxial tests (TX-CIU), as carried out on sampples taken froom the Osteria access adit. Also illustrated i aree the results of o drained dirrect shear testts (DS-CID). It is observeed that th he peak and residual streng gth envelopess pertaining too the direct shhear tests lie w well within thhe range of shear stren ngth values reesulting from m the triaxial ttests. If attenttion is paid too the OST3 annd RTC1 1 tests as reprresentative off the CCTCS S in the Raticcosa tunnel, iin the paleo-llandslide zonne, the faailure envelop pe would be represented r by b the followiing shear streength parameeters: c’ = 20.3 kPa an nd φ’ = 16.6°°.
Figure 9: Shear failure envellopes obtain ned for the C CCTCS in diffferent testinng conditionss.
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CONCLUSIONS The laboratory testing program described in this paper was intended to point out the main characteristics of the CCTCS. Although several points have yet to be cleared, it is possible to draw in the following a few conclusions. The CCTCS show 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 are variable form site to site and, locally, from sample to sample, due to the tectonic events occurred since deposition took place. The samples coming from the Raticosa tunnel belong to a paleo-landslide, whose effects contributed to soften and remold the original soil. Tests on reconstituted samples indicate that the original soil has a high stiffness, lower void index and lower hydraulic properties. Triaxial tests evidence strain localization, although the data available so far are not sufficient to describe the phenomenon. A clear disagreement on the amount of clay minerals was detected through grain size distribution and X-ray diffraction analyses. This may be explained by accounting for the effect of disaggregation on the size of aggregate particles. This is another indirect proof of the presence of a developed structure in CCTCS. Grain size distribution shows the presence of hard aggregates of several millimeters in size. The X-ray diffraction analyses indicate also the presence of significant amounts of swelling minerals, which are smectite and illite. These clay minerals behave differently. Smectite increases its volume in presence of water while illite undergoes a process of degradation of strength at particle contacts. The quantitative swelling potential of CCTCS was studied through HuderAmberg tests, which indicate a swelling coefficient similar to that of S. Donato clay shales. Triaxial tests allowed for the determination of several factors. The CCTCS show an isotropic behavior, at least in the little strain regime. In undrained conditions, the stress-strain behavior of the CCTCS could be studied to give shear strength parameters which are in the range of other structurally complex formations. However, it was not yet possible to determine an effective stress path for the performed tests. The effective stress paths proved to be dependent on the saturation degree and on the axial strain rate. The standard laboratory testing program performed on CCTCS points out that it is necessary to better investigate the origin of the swelling behavior and of the time-dependent deformations shown by the constant loading phases in the triaxial tests.
REFERENCES 1. Aristorenas G.V. (1992) “Time-dependent behaviour of tunnels excavated in shale”, Ph. D. Thesis, Massachusetts Institute of Technology. Boston, USA. 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. Bonini M. (2003) “Mechanical behaviour of Clay-Shales (Argille Scagliose) and implications on the design of tunnels”. Ph. D. Thesis, Politecnico di Torino.
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5. Bultel F. (2001) “Prise en compte du gonflement des terrains pour le dimensionnement des revêtements des tunnels”, Ph. D. Thesis. Ecole Nationale des Ponts et Chaussées. Paris. France. 6. D’Elia B. (1991) “Deformation problems in the Italian structurally complex clay soils”, Proc. 10th Europ. Conf. On Soil Mech. and Found. Eng., Florence, 4, 1159-1170. 7. Madsen F.T. (1999) “Suggested methods for laboratory testing of swelling rocks”, Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., 26, No.3, 211-225. 8. Olivares L., Urciuoli G., Picarelli L. (1997) “Mechanisms of rupture of reconstituted and natural fissured clay shales in undrained triaxial tests”, Proc. Int. Symp. On Deformation and Progressive failure in Geomechanics, Nagoya, 229-234. 9. Picarelli L., Olivares L. (1998) “Ingredients for modelling the mechanical behaviour of intensely fissured clay shales”, The Geotechnics of Hard Soils – Soft Rocks, Balkema, Rotterdam, 771-770. 10. Picarelli L., Olivares L., Di Maio C., Silvestri F., Di Nocera S., Urciuoli G, (2002) “Structure, properties and mechanical behaviour of the highly plastic intensely fissured Bisaccia Clay Shale”, Int. Workshop on Characterisation and Engineering Properties of Natural Soils, Singapore, Centre For Soft Ground Engineering, NUS.
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