1 Introduction
2 Brief on the Tested Rocks and Test Methods
Methods | XRD analysis | Swelling tests | Thin-section analysis | |
---|---|---|---|---|
Powder | Intact | |||
Samples tested | 7 flysch samples 7 volcanic samples | 7 flysch samples 7 volcanic samples | 3 flysch samples 7 volcanic samples | 3 flysch samples 7 volcanic samples |
Aim of test | Mineralogical composition | Maximum swelling pressure | Maximum swelling pressure and pressure evolution at cycles with controlled deformation | Structural and mineralogical characterization |
2.1 Tested Rocks
Rock type | Specimen name | Description | Preparation/testing condition |
---|---|---|---|
Flysch | Flysch A | Clay-/siltstone, highly disturbed, fractured | Powder |
Flysch B | Claystone-/siltstone, highly disturbed, crumbly | Powder | |
Flysch C | Alternating claystone/siltstone | Powder and intact | |
Flysch D | Intact claystone | Powder and intact | |
Flysch E | Siltstone, intact | Powder and intact | |
Flysch F | Claystone, disturbed | Powder | |
Flysch G | Claystone-/siltstone, highly disturbed | Powder | |
Volcanic | Volcanic A | Altered volcanic rock, intact, strong | Powder and intact |
Volcanic B | Basaltic rock, intact, strong | Powder and intact | |
Volcanic C | Altered volcanic rock, intact, weak | Powder and intact | |
Volcanic D | Altered volcanic rock, intact, strong | Powder and intact | |
Volcanic E | Volcanic breccia, intact, weak | Powder and intact | |
Volcanic F | Volcanic breccia, intact, weak | Powder and intact | |
Volcanic G | Altered volcanic rock, intact, weak | Powder and intact |
2.2 Mineralogical Investigation by XRD
2.3 Structural and Textural Assessment by Thin-Section Analysis
2.4 Oedometer Swelling Tests
3 Achieved Results
3.1 XRD- and Thin-Section Analysis Results
3.2 Flysch Rocks
Sample | Texture | Estimated clay content | Fracture characteristics* | Estimated porosity | Estimated swelling potential |
---|---|---|---|---|---|
Flysch C | Finely laminated micrite with dispersed calcite microspar | 15–20% | Mainly calcite cemented Some very thin clay-filled microfractures Low permeability | < 0.5% | Medium |
Flysch D | Laminated mudstone with sandy layers. Concentrated, lenticular clay aggregates within the laminations | ~ 40% | Open microfractures with parallel cleavage defined by clay aggregates, chlorite and mica. Some calcite as intergranular cement Low–medium permeability | ~ 1% | High |
Flysch E | Distinct turbidite lamination included fine lamina of micrite, clay aggregates and sand | ~ 40% | Carbonate cemented fractures transecting matrix. Thin, open fractures and voids in cemented fractures Medium permeability | ~ 2% | Medium–high |
3.3 Volcanic Rocks
Sample | Texture | Clay content* (%) | Fracture characteristics | Visible porosity (%) | Estimated swelling potential |
---|---|---|---|---|---|
Volcanic A | Random oriented laths and needles. Laumontite pseudomorphs after feldspar. Interstitial clay minerals (chlorite and corrensite) | 5–10% | A few crosscutting veins consisting of silt-sized fibrous mica/clay aggregate Low permeability | < 1% | Medium |
Volcanic B | Random oriented laths and needles. Network of wedge-like needles and opaque rods with interstitial quartz and chalcedony. Green clay mineral rims and patches | 5–10% | 0.1 mm thick open fracture ~ 1 mm thick quartz cemented vein Low permeability | < 0.5% | Low |
Volcanic C | Curly texture with sparce microphenecrysts of feldspar, clinopyroxene, quartz, and spherulitic textures | 10–15% | Quartz veins, and thin crosscutting green veins of chlorite corrensite Low permeability | ~ 1% | Medium |
Volcanic D | Volcanic texture, random oriented 0.5 mm albite and laumontite pseudomorphs, patches of fibrous aggregates1-2 mm, pyroxene altered to calcite and prehnite | 10–15% | Quartz and calcite cemented fractures, local thin porous central parts. Minor leaching of matrix along ~ 1 mm thick veins. Low permeability | < 0.5% | Low |
Volcanic E | Volcanic breccia, fragments from sand size to several cm. Volcanic glass with very thin feldspar laths. Fragments rich in speherulites with clay | ~ 20% | Open fractures around fragments (possibly from sampling) High permeability | 1–2% | Very high |
Volcanic F | Volcanic breccia. Green fibrous clay as coronas and in pseudomorphs | ~ 10% | Thin fractures partly cemented by calcite Medium permeability | 1–2% | High |
Volcanic G | Volcanic breccia/weathered rock. Fibrous quartz aggregates, oxidised matrix. Irregular calcite patches and fracture fill | ~ 10% | Dark irregular fractures. Some open fractures. Green clay minerals filling thin fractures in quartz and spheroids Medium permeability | < 1% | Medium |
3.4 Oedometer swelling test results
3.4.1 Maximum Swelling Pressure
Rock type | Specimen | Initial moisture content, intact samples (%) | Density (g/cm3) | Maximum Swelling Pressure (MPa) | ||
---|---|---|---|---|---|---|
Compacted powder | Intact rock | Compacted powder | Intact rock | |||
Flysch | Flysch A | – | 2.11 | – | 0.53 | – |
Flysch B | – | 2.15 | – | 0.29 | – | |
Flysch C | 2.6 | 2.16 | 2.58 | 0.66 | 0.46 | |
Flysch D | 7.2 | 2.32 | 2.50 | 4.13 | 0.91 | |
Flysch E | 3.8 | 2.18 | 2.47 | 1.50 | 3.79 | |
Flysch F | – | 2.15 | – | 0.61 | – | |
Flysch G | – | 2.06 | – | 0.18 | – | |
Volcanic | Volcanic A | 1.2 | 1.87 | 2.60 | 4.9 | 2.08 |
Volcanic B | 1.9 | 1.98 | 2.76 | 0.4 | 0.05 | |
Volcanic C | 1.4 | 1.99 | 2.66 | 2.4 | 0.13 | |
Volcanic D | 0.6 | 1.94 | 2.82 | 0.4 | 0.04 | |
Volcanic E | 5.0 | 1.95 | 2.32 | 3.0 | 0.38 | |
Volcanic F | 5.5 | 1.69 | 2.26 | 2.9 | 0.17 | |
Volcanic G | 1.9 | 1.92 | 2.50 | 0.8 | 0.49 |
3.4.2 Cyclic Tests with Controlled Deformation
4 Analysis of the Results
4.1 Relationship Between Swelling Pressure, Critical Minerals and Petrographic Data
4.1.1 The Flysch Rocks
Sample | Swelling potential | Critical minerals from XRD (% mass) | |||||
---|---|---|---|---|---|---|---|
Estimated from thin section | Lab tested pressure for intact rock (MPa) | Lab tested pressure, for rock powder (MPa) | Amorphous | Chlorite | Swelling clay | Calcite | |
Flysch A | – | – | 0.53 | 19 | 22 | 24 | 9 |
Flysch B | – | – | 0.29 | 14 | 22 | 9 | 19 |
Flysch C | Medium | 0.46 | 0.66 | 9 | 11 | 5 | 41 |
Flysch D | High | 0.91 | 4.13 | 22 | 3 | 32 | 19 |
Flysch E | Medium–high | 3.79 | 1.50 | 22 | 18 | 26 | 13 |
Flysch F | – | – | 0.61 | 18 | 19 | 20 | 19 |
Flysch G | – | – | 0.18 | 10 | 8 | 8 | 53 |
4.1.2 The Volcanic Rocks
Sample | Swelling potential | Critical minerals from XRD (%) | ||||||
---|---|---|---|---|---|---|---|---|
Estimated from thin section | Lab. tested pressure for intact rock (MPa) | Lab. tested pressure for powder (MPa) | Amorph | Chlorite | Swell. clay* | Calcite | Laumontite | |
Volcanic A | Medium | 2.08 | 4.9 | 45 | 2 | – | – | 31 |
Volcanic B | Low | 0.05 | 0.04 | 44 | 5 | – | – | 2 |
Volcanic C | Medium | 0.13 | 2.4 | 46 | 12 | u.a | – | 8 |
Volcanic D | Low | 0.04 | 0.4 | 29 | 9 | − | 4 | 1 |
Volcanic E | Very high | 0.38 | 3.0 | 53 | − | u.a | 8 | 3 |
Volcanic F | High | 0.17 | 2.9 | 58 | − | u.a | 1 | − |
Volcanic G | Medium | 0.49 | 0.8 | 40 | 5 | u.a | 4 | 4 |
4.2 Analysis of Cyclic Swelling Tests
5 Discussion
5.1 XRD- and Thin-Section Analyses in Swelling Rock Assessments
5.2 Comments on the Oedometer Swelling Tests
6 Conclusions
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The XRD analysis is not a proper method for describing weathered and swelling rock compositions unless it is complemented with thin-section analysis, which considers other features of the minerals to describe their origin and their characteristics.
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Heterogeneous rocks with uneven distribution of swelling clay minerals and open and/or clay-filled microfractures may hold a higher swelling potential compared to rock types with a homogeneous texture.
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Structural breakdown and disaggregation of the intact rock may occur during cyclic swelling, allowing a greater volume of swelling minerals to adsorb water in the next wetting cycle. The location of the swelling minerals within the rock texture seem to play an important role and underlines the importance of performing swelling tests on intact rock and to analyze the structure of the rock to assess the swelling characteristics of rocks containing even low amounts of swelling minerals.
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Significant physical swelling can develop even for small concentrations of swelling clay. Induced and/or advancing microfractures in the rock due to allowed deformation during cyclic wetting may lead to alteration of the rock properties. The relation between swelling strain and stress proposed by Grob (1972) is therefore not valid in the case of swelling affected by cyclic wetting.
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None of the applied methods, when applied isolated, can assess the swelling potential of rocks, but the combination of the different methods gives a fair estimation on the swelling behavior.
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The cyclic swelling test carried out on intact rock represent a condition closer to the in-situ condition of a water tunnel when compared to the standard swelling test and is recommended to be adopted for swelling tests in future hydropower projects.