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Work ing Repor t 2003 -79

Natural Clays as Backfilling Materialsin Different Backfilling Concepts

Pau la Keto

March 2004

Pau la Keto

Saan io & R i ekko l a Oy

Working Reports contain information on work in progress

or pending completion.

The conclusions and viewpoints presented in the report

are those of author(s) and do not necessarily

coincide with those of Posiva.

Work ing Repor t 2003 -79

Natural Clays as Backfilling Materialsin Different Backfilling Concepts

NATURAL CLAYS AS BACKFILLING MATERIALS IN DIFFERENT BACKFILLING CONCEPTS ABSTRACT The applicability of bentonites, other smecite-rich clays, non-swelling clays and fine-rich tills as backfilling materials for six preliminary backfill concepts was evaluated in the first stage of a joint POSIVA-SKB backfilling project. The project aims at developing backfill concepts that fulfill the long-term performance requirements set for backfilling of a disposal tunnel in KBS-3V-type repository. Bentonite clay is a suitable material for concepts including pre-compacted blocks while it remains to be questioned whether the material can maintain its physical properties in the anticipated repository conditions when mixed with crushed rock (30:70). Smectite-rich clays, like the Friedland-clay, are most likely suitable for block production but the in situ compaction of these clays may not be technically feasible, especially when compacting the roof section of the tunnel. Non-smectitic materials need to be combined with bentonite in order to fulfill the performance requirements set for the backfilling concepts. Further studies are required with all of these materials to ensure their applicability for repository backfilling. Keywords: backfilling, applicability, bentonites, clays, tills, performance requirements, long-term safety.

LUONNONSAVET TÄYTTÖMATERIAALEINA ERI TÄYTTÖKONSEPTEISSA TIIVISTELMÄ Posivan ja SKB:n yhteisen täyttöprojektin ensimmäisessä vaiheessa arvioitiin bentoniit-tien, smektiittipitoisten seoshilasavien, paisumattomien savien ja moreenien soveltuvuus kuuteen erilaiseen alustavasti valittuun täyttökonseptiin. Projektin tavoitteena on kehittää ratkaisuja, jotka täyttävät KBS-3V-tyypin loppusijoitustilan sijoitustunnelien täytölle asetetut vaatimukset myös pitkällä aikavälillä. Bentoniittisavi soveltuu käytettäväksi konsepteissa, joissa käytetään esipuristettuja täyttölohkoja. Sen sijaan on kyseenalaista, pystyykö bentoniittisavi säilyttämään fysikaaliset ominaisuutensa odotetuissa loppusijoitusolosuhteissa, mikäli savi sekoitetaan murskeen kanssa (suhteessa 30:70) ja tiivistetään paikalleen. Smektiittipitoiset seoshilasavet, kuten Friedland-savi, sopivat todennäköisesti esipuristettujen täyttölohkojen raaka-aineeksi, mutta näiden savien paikalleen tiivistys on teknisesti vaikea toteuttaa, erityisesti tunnelin katon osalta. Jotta täyttökonsepteille asetetut vaatimukset täyttyisivät, paisumiskykyisiä mineraaleja sisältämättömiä materiaaleja voidaan käyttää täytössä vain mikäli niitä käytetään yhdessä bentoniitin kanssa. Lisätutkimuksia tarvitaan kaikkien edellä mainittujen materiaalien osalta, jotta voidaan varmistua niiden käyttökelpoisuudesta loppusijoitustilojen täytössä. Avainsanat: täyttö, soveltuvuus, bentoniitit, savet, moreenit, vaatimukset, pitkäaikais-turvallisuus.

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TABLE OF CONTENTS ABSTRACT

TIIVISTELMÄ TABLE OF CONTENTS.................................................................................................. 1

1 INTRODUCTION................................................................................................... 3

2 DESIRED BACKFILL MATERIAL PROPERTIES ................................................. 5

3 BENTONITES ....................................................................................................... 9 3.1 Bentonites in general .................................................................................. 9 3.2 Geotechnical properties ............................................................................. 11 3.3 Applicability for different concepts ............................................................. 14 3.4 Differences between commercial bentonites ............................................. 18 3.5 Need for further investigations ................................................................... 21 3.6 Summary.................................................................................................... 22

4 OTHER NATURAL SWELLING CLAYS.............................................................. 27 4.1 Friedland clay............................................................................................. 27

4.1.1 Friedland clay in general ................................................................ 27 4.1.2 Geotechnical properties ................................................................. 28 4.1.3 Applicability for different concepts.................................................. 30 4.1.4 Need for further investigations........................................................ 30

4.2 Other natural swelling clays ....................................................................... 35 4.3 Summary.................................................................................................... 36

5 NATURAL NON-SWELLING CLAYS AND OTHER POTENTIAL SOIL TYPES .... 37 5.1 Introduction ............................................................................................... 37 5.2 Geotechnical properties ............................................................................. 40 5.3 Applicability for different concepts.............................................................. 41 5.4 Need for further investigations ................................................................... 42

6 COSTS ................................................................................................................ 47

7 CONCLUSIONS & DISCUSSION ....................................................................... 49

REFERENCES ............................................................................................................. 55

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1 INTRODUCTION Within the project “Backfilling and closure of the deep repository” a desk study of suitable backfilling materials has been made. The aim of this particular report is to analyze whether certain natural clays and soil types are suitable materials for the six different backfilling concepts under evaluation by SKB and POSIVA Oy. The backfilling materials studied in this report include: - Bentonites - Other natural clays with swelling ability - Natural clays with no swelling ability - Other naturally occurring soils with high content of fineries. The advantages of using natural clays as a backfill material are due their naturally low hydraulic conductivity, high specific surface area and in some cases swelling ability. In addition, natural analogies can be used when assessing the long-term performance of these materials. The possible disadvantages of natural clays as backfilling material are linked e.g. to compressibility, sensitivity to water content during emplacement (liquification, dusting) and sensitivity to salt content of the groundwater in the expected repository conditions. Natural clays may be used solely or as a phase in a backfilling mixture. These materials can be processed into different forms (powder, pre-compacted blocks, pellets, etc.) depending on the backfill concept. The concepts under evaluation are: A In situ compacted mixture of crushed rock and bentonite B In situ compacted swelling clay (other than bentonite) C In situ compacted non-swelling clay/other potential soil type with bentonite

blocks at the roof section D Pre-compacted blocks of all materials E “Sandwich” concept (bentonite blocks & crushed rock) F Compartment concept (bentonite plugs along the tunnel & crushed rock). The SKB and Posiva requirements for the backfill are presented thoroughly in the concept report (Gunnarsson et al. 2003) analyzing the concepts in detail. The requirements for the concepts in general are summarized below: • The backfill shall be incompressible in order to prohibit upward expansion of the

buffer so that the density and other desired properties of the buffer remain unchanged.

• The backfill shall have low hydraulic conductivity and there shall be no

boundary flow between the backfill and the rock surrounding the tunnel. This is important in order to restrict advection of water and to retard transportation of radionuclides along the tunnel.

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• The backfill shall contribute to keeping the tunnels mechanically stable. • The backfill shall not have any significantly harmful effect on the other barriers

in the repository. • The backfill shall maintain its performance under the expected repository

conditions for a time range determined in the safety analysis. The design basis & desired material properties determined according to the performance requirements will be described in chapter 2. Table 1-1 represents the natural clay materials that are discussed within this report. Table 1-1. Natural clay materials studied within the six alternative backfilling concepts.

Materials Concept Bentonite clay (consisting > 70% of smectite group minerals) A Natural clay with swelling ability (smectite bearing mixed-layer clays)

B

Non-swelling clay (e.g. illite, kaolinite) or other potential soil type & bentonite

C

Bentonites, natural swelling clays, non-swelling clays and other potential soil types

D

Bentonite E Bentonite F

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2 DESIRED BACKFILL MATERIAL PROPERTIES The design basis for using natural clays and other suitable soil types in different concept will be described in this chapter. Clays & other potential soil types can be used as a component in a backfilling mixture (concept A, C & D), as a raw material for pre-compacted blocks in order to limit the axial groundwater flow in heterogeneous concepts (C, E, F) or as the main backfill material (concept B & D). The following design bases should be used as guidelines for backfill design. Design base 1: Mechanical properties The compressibility should be low enough to restrain the upward expansion of the buffer so that the density of the buffer remains unchanged. The compressibility of clays depends on the density, porosity, water ratio and swelling pressure of the material. The compressibility of non-cohesive soil types is limited because the skeleton composing of mineral grains can resist the mechanical load (in this case swelling pressure induced by the buffer), assuming that the density of the backfill is near the maximum density determined in laboratory. The swelling ability, compression properties (deformation modulus, cohesion, angle of friction) and porosity need to be determined in the laboratory and verified with semi- or large-scale field tests for each backfill material (in 90-100% of the maximum proctor density). Calculations and modeling is required to evaluate the mechanical interaction between the buffer and the backfill according to the test results. The loss in the density of the bentonite buffer due to the expansion should be limited to minimum so that the performance of the buffer is not compromised. The dimensioning ground water salt content is 3.5% (CaCl2/NaCl, 50:50). The material should have suitable properties in order to resist erosion and piping in the expected repository conditions. Three different types of erosion can occur in the disposal tunnels: - Surface erosion: water flowing on the surface of the backfill will have the ability

to transport some of the material away with the flow (SYKE 2002) (see figure 2-1). This type of erosion is possible especially during the emplacement period, but it can be handled with certain technical measures.

- Piping (internal erosion): Repeated transportation of (usually finer) particles along the poresystem of the material leading to formation of channels (SYKE 2002) (see figure 2-1). Piping requires high hydraulic gradient. - Contact erosion: Transportation of materials in the contact zone between coarse and fine materials (SYKE 2002) (see figure 2-1). This kind of situation is possible in the contact zone between the buffer and the backfill and also in heterogeneous concepts between different backfilling materials, if the voids of the coarse backfill material are very much larger than the finest grains of the adjoining buffer/backfill material. It is very important to be able to prohibit contact erosion between the backfill and the buffer in order not to compromise the density of the bentonite buffer.

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Figure 2-1. Erosion types: A) surface erosion, B) piping & C) contact erosion (modified after GLR 1993 in SYKE 2002). Piping is not supposed to be a problem with smectitic clays due to the self-healing capacity of the material (assuming that the density of the clay is sufficient), while non-swelling clays have limited self-healing capacity. Erosion processes A & C (see figure 2-1) are possible with both clay types. The erosion of buffer & backfill clays can be restricted by: - Shielding the compacted layer from leakage water (surface erosion) during

installation pauses, limiting the ingress of water to the tunnel (e.g. with temporary drainage pipes).

- Effective compaction (density of the material needs to be at least 90% of Proctor maximum).

- Using filter materials and/or modifying the grain size distribution of the backfill material in order to prevent contact erosion between the buffer and the backfill and between clay and coarser backfill material in a heterogeneous concept. The filter material and/or the adjacent backfill material should consist of a material with a grain size that meets the certain requirements set for the filter material (Terzaghi & Peck 1967, Terzaghi et al. 1996). These requirements depend on the grain size distribution of the filter material and of the materials to be protected.

Piping of non- or semi-cohesive soil types is possible if the gradation curve of the material is discontinuous (meaning that the main bulk of the material consists of large grains of uniform size and fine particles between these grains start to erode leading to channeling). Therefore, such materials should have unsorted grain-size distribution and sufficient density (at least 90% of Proctor maximum). These properties would also inhibit the contact erosion between the buffer and the backfill, and also between different backfill materials (in heterogeneous concepts). Certain laboratory tests can be used to evaluate the sensitivity of the material to erosion.

A) B) C)

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Design base 2: Hydraulic conductivity & swelling pressure The hydraulic conductivity of the material should preferably be 10 E-10 or lower in order to limit advection of ground water through the material (so that the transportation of water and substances through the backfill would be a diffusion dominant process). In order to limit the advection in the contact zone between the backfill and the rock, the backfill should preferably have some swelling pressure. It has been estimated that the required swelling pressure should be approximately 100 kPa. These target values should be used as guidelines for the backfill design. However, it should be noted that the intention is to limit the advection in the backfilled tunnel and not to reach the estimated target values without conditions. The hydraulic conductivity and swelling ability of a backfill material should be studied in laboratory and verified with semi- or large-scale field tests (in 90-100% of the maximum proctor density) The dimensioning ground water salt content is 3.5% TDS (CaCl2/NaCl, 50:50), but the tests should also be made with fresh water, with 1% and even with 5% salinity in order to study the effect of salinity to the particular material. Design base 3: Long-term stability Long-time stability: the material should fulfill the performance requirements for a long time period. For example, the mineralogical changes due to groundwater chemistry shall not impair the mechanical properties of the material (design basis 1 & 2) remarkably in certain time range determined in the safety analysis. Design base 4: Interaction with other barriers The material should not have any significant harmful effect on other barriers. E.g. the chemical composition should be such that no remarkable amounts of corrosive substances (e.g. sulphur) can be leached from the material to the groundwater. The amount of organic substances should be limited and tested for each backfill material. The chemical effect of backfilling materials on bentonite buffer (e.g. illitisation and cementation) should be considered in further studies. Design base 5: Feasibility If the material is compacted at site, the material should have properties that make it easy to compact to densities that yield the properties stated under design base 1 and 2. The material should have the right set of characteristics so that it can be compacted to sufficient density also close to the roof. It is important that the material has optimal water content and the material is compacted with a technique suitable for the material. The water content of the material should be maintained close to the optimum during the compaction process. The optimal water content of the backfill material needs to be tested laboratory and verified with a field compaction test with the respective technique. In case of pre-compacted blocks, the raw-material for the blocks should have certain characteristics to enable successful pre-compaction and handling of the blocks without fear of degradation of the blocks. Only materials with sufficient plasticity and cohesion

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can be pre-compacted to blocks. It is important that the water content of the pre-compacted material is optimal in order to reach maximum density for the blocks. It should be possible to backfill the tunnel within certain time constraint in order to limit the risk of buffer dislocation before the backfill is installed above the deposition hole. It is very important to be able to compact & install the backfill materials properly. Failing in this critical task will mean compromising the performance of the whole concept. The raw-material & installation costs of the backfill should be optimized without compromising the design basis 1-4.

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3 BENTONITES 3.1 Bentonites in general The term bentonite has many definitions and the same term is used for wide range of commercial clay materials (Bates & Jackson 1987). Common for all definitions is that bentonite clay composes dominantly of smectite clay minerals that give the clay its ability to swell when in contact with free water. Many of the bentonite clays have formed in the hydrothermal alteration of glassy, igneous material of volcanic origin (Bates & Jackson 1987). Bentonites can also represent hydrothermal alteration products of weathered sediments. The Wyoming bentonite clays formed during the Cretaceous (Knechtel & Patterson 1962), while most of the European bentonite formations are Tertiary of age (Pusch 2001b). The occurrences of bentonite differ from each other depending on the sedimentation environment and type of the alteration process. For example in Wyoming the bentonite deposits are relatively thin horizons between thicker formation of marine shale (see figure 3-1) (Grim & Güven 1978), while the bentonite deposits in Almeria, Spain occur in a more massive form (Keto 1999).

Figure 3-1. Example on how the bentonite deposits occur in Wyoming. Section of bentonite horizons (A-G) in New Castle, Mowry, and Belle sedimentary formations in the Black Hills area (Grim & Güven 1978).

GF

EDC

B

A

Clay Spur bed

Belle Fourche Formation (shale)

Hard Siliceous Shale

New Castle Formation(sandstone)

12 m

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The importance knowing the source of the clay should not be underestimated. Many of the properties determined in laboratory depend on the geologic environment and history of the source occurrence. By knowing these things, it is easier to interpret the laboratory results and to understand the special features of the material. In addition, source occurrences may be used in natural analogues and in estimating the homogeneity and reserves for the material. The crystal structure of smectite-group minerals composes of tetrahedral and octahedral sheets that are linked together so that one octahedral sheet is between two tetrahedral sheets thus forming a layer (see figure 3-2) (Brindley & Brown 1980). In the tetrahedral sheets the dominant cation is Si that can be replaced by Al or Fe. In the octahedral sheet the cations are usually Al, Mg, Fe. The smectite-group minerals are divided into two subgroups (dioctahedral & trioctahedral smectites) according to the type of predominant octahedral cation in their crystal structure (Güven 1988). For example montmorillonite, beidellite and nontronite are dioctahedral smectites, while saponite represent trioctahedral smectites (Güven 1988). Due to substitutions in the tetrahedral and/or octahedral sheets, the layers have negative charge, which is compensated by an interlayered hydrated cation. The interlayer cation can be replaced by other cations in aqueous solution (Brindley & Brown 1980). The swelling of smectite minerals is due to absorbtion of multiple layers of water molecules to the interlayer space.

OH�

�

OH�

�

OH�

�OH�

�

Exchangeable cations�� + n H O�� 2��

d-spacing��14-15 Å��

Oxygens��Hydroxyls��Aluminum, Iron, Magnesium��Silicon, occasionally aluminum��

Figure 3-2. Crystal structure of montmorillonite (Pusch & Karnland 1996).

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The physical properties of bentonite depend on the mineralogy and smectite content of the material and on the type of the interlayer cation (Na+, Ca2+ Mg2+, Fe2+). For example, there are some differences in the microstructure of Na- and Ca-smectites (Pusch 1994). These differences are probably the reason why Na-bentonites in general have lower hydraulic conductivity and higher swelling pressure than Ca-bentonites. Commercial calcium bentonites are often activated with sodium carbonate to enhance the physical properties of the product. If natural Ca-bentonite or activated Ca-bentonite will be used in backfilling, more studies are needed to investigate the differences between Ca- & Na-bentonites and the behaviour of these materials in the long-term (especially the exchange of cations in the repository environment). Na-bentonite from Wyoming (MX-80 or similar) should be used as a reference material. In addition to smectites, the main minerals usually present in bentonites include other clay minerals, mica, feldspars, calcite, colloidal silica & quartz depending on the geological history of the occurrence. For example, if the material is alteration product of volcanic ash, some of the minerals present were originally phenocryst in the volcanic parent rock (Bates & Jackson 1987). According to Rath (1986) the mineralogy of Wyoming bentonites consists 65-90% of smectite group minerals (montmorillonite & beidellite). The other minerals present include feldspars, quartz, biotite, cristobalite, kaolinite and minor amounts of minerals of igneous origin (Rath 1986). Carbonates, iron oxides, selenite and sulphates may also be present (Rath 1986). The chemical composition of bentonites depends on the type of the original material (igneous, sedimentary) and on the deposition and alteration environment. The SiO2 content is usually around 60 weight-% and the Al2O3 content around 20 weight-%. Table 3-1 presents the chemical composition of MX-80 bentonite provided by Volclay Ltd. Table 3-1. Chemical composition of MX-80 Na-bentonite.

Weight-% SiO2 63.0 Al2O3 21.2 Fe3O2 3.3 FeO 0.4 Na2O 2.6 MgO 2.7 CaO 0.7

3.2 Geotechnical properties The geotechnical properties of a bentonite depend on the mineralogy, smectite content and smectite type of the material together with the density and porosity of the material within the application. The most important geotechnical properties for the backfill are hydraulic conductivity, swelling ability and compressibility. In addition, the material should be easy to handle and compact. When comparing and evaluating test results

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made in different laboratories or different test series, the testing method should be considered as well as the scale effect and the amount of samples. In practice, the density achieved in the field is usually lower than the maximum density determined in the laboratory with the Proctor compaction test. Therefore, the laboratory tests should be done with samples at 90-100% of the maximum proctor density. According to several investigations and tests (e.g. Dixon 2000, Karnland 1998, Pusch 2001b), the swelling pressure and hydraulic conductivity of bentonites depend on the density of the sample and on the salinity of the percolating water. The compressibility of bentonite and other clays depend on the water content of the material and on the density obtained by compaction. In order to gain maximum density, the material should have optimum water content during compaction (this is true independent on the compaction technique). Water ratio of 17% was used when manufacturing bentonite buffer for the Prototype Repository (Johannesson 2002). The final density of these blocks at saturation was > 2 t/m3. If bentonite will be used as component in a mixture, the optimal water content will be determined for the whole mixture. No specific target values have been set for the strength of the backfilling materials. However, the issue has been studied in order to evaluate the mechanical interaction of the canister and the buffer in the long-term. The strength and deformation properties of bentonites & bentonite blocks have been discussed e.g. in Börgesson et al. (1995), in Börgesson (2001) & in Pusch (2001b). An example on the stress/strain behaviour of dense bentonite is shown in figure 3-3 (Pusch 2001b).

3

2,5

2

1,5

1

0,5

00 1 2 3 4 5

Strain (%)

Com

pres

sive

str

ess

(MP

a)

Figure 3-3. An example of uniaxial testing of saturated natural bentonite clay with density of 2 100 kg/m3 (Pusch 2001b).

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The strength of a sample consisting of expansive clay depends on various matters, e.g. the density, bentonite type and water content of the sample (Terzaghi et al. 1996). For example the samples with the highest water content give the lowest strength values see table 3-2 (Pusch 2001b). The shear strength parameters, angle of friction (Φ) and cohesion (c), evaluated according to triaxial tests performed for pre-compacted Na- and Ca-bentonite blocks are presented in table 3-3 (Börgesson et al. 1995). Triaxial tests on these materials were made both in undrained and drained conditions and with fresh water and salt water (3.5% NaCl). In these test it was found that the friction angle is higher for Ca-bentonite compared to Na-bentonite and actived Ca-bentonite. Salinity of the pore water also had an influence on the friction angle of bentonites. The stress-strain-strength behaviour and long-term creep of buffer bentonite blocks have been studied with several material models based on laboratory testing (Börgesson et al. 1995, Börgesson 2001, Pusch & Adey 1999 & Pusch 2001b). According to these studies it can be stated, that the strength and deformation properties of pre-compacted bentonite blocks are not supposed to affect the long-term safety of the disposal system. Bentonite clays have self-healing capacity that enables the curing of piping induced channels (assuming that density of the bentonite is high enough). In bentonite/aggregate mixtures, the sensitivity to piping depends on the grain size distribution of the aggregate. If the aggregate component has very sorted gradation, the clay component can be washed away with flow (SYKE 2002). Bentonite blocks are sensitive to contact erosion and therefore filter layers may be needed depending on the grain size distribution of the material placed adjacent to the blocks (Terzaghi et al. 1996). Table 3-2. Tensile strength of pre-compacted MX-80 bentonite blocks compated under 100 MPa pressure (Pusch 2001b).

Water content (%) Initial Final

Void ratio (e) Degree of water saturation (%)

Tensile strength(MPa)

10 10 0.51 55 2.8 18 17 0.60 79 1.9 26 24 0.76 88 1.3

Table 3-3. Results from triaxial tests performed by Börgesson et al. (1995).

Bentonite Test type Water Φ (º) c (KPa) MX-80 Na-bentonite Standard Distilled 9.9 56 MX-80 Na-bentonite Passive Distilled 8.7 87 MX-80 Na-bentonite Standard 3.5% NaCl 12.9 106 Na-activated Ca-bentonite Standard Distilled 9.1 104 Ca-bentonite Standard Distilled 13.5 124

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The consistency of bentonite products (powder or granules) may have some effect on the homogeneity and the compactibility of the backfill material. Bentonite powder may apply best in concepts where the bentonite is mixed with an aggregate and than for manufacturing of pre-compacted blocks. 3.3 Applicability for different concepts The aim of this chapter is to go through applicability of bentonite clay for different concepts. Differences between different bentonite products are discussed in chapter 3.4. In situ compacted mixture of crushed rock & bentonite The applicability of bentonite within this concept depends on the obtained density for the bulk material and for the bentonite phase. The bentonite should also be evenly distributed within the mixture. Based on the studies made by SKB (e.g. Johannesson et al. 1999, Pusch 1998 & 2001b) it is clear that it will be challenging to obtain sufficient bentonite density and the required properties within this concept at groundwater salinity of 3.5%. According to Dixon (2000 & 2002), the materials should have certain minimum Effective Clay/Montmorillonite Dry Density (at least 1000 kg/m3) in order to maintain the sufficiently low hydraulic conductivity and sufficient swelling pressure in groundwater environment of high salinity (35-60 g/l). SKB has made some hydraulic conductivity and swelling pressure tests on mixture of bentonite and crushed rock used in the Backfill and Plug test (Johannesson et al. 1999). The crushed rock was TBM-muck and the bentonite was commercial MX-80 Na-bentonite. Both distilled water and Äspö water (salinity 1.2% including both Na+ and Ca2+ ions) were used in the tests. The hydraulic conductivity of samples with 30% bentonite tested with Äspö water varied between 1E-9 m/s (dry density 1 700 kg/m3) and 4E-11 m/s (dry density 1 850 kg/m3) (Johannesson et al. 1999). The factors controlling the hydraulic conductivity in these tests were: dry density of the bulk material, initial water ratio at compaction, salinity of the percolating water, testing method and mixing & compaction technique. Most of the factors mentioned above controlled also the swelling pressure of the material. In order to reach the swelling pressure of > 100 kPa in Äspö conditions, the dry density of the bulk material should be > 1 600 kg/m3 (Johansson et al. 1999). The current status of this concept is that the in situ compacted mixture of crushed rock and bentonite (30/70) is assumed to perform in groundwater salinity up to 1% (Gunnarsson et al. 2003). In order for the material to perform in higher salinities, the density of the bentonite phase needs to be raised. The density of the bentonite phase within the mixture can be enhanced by using more efficient mixing and compaction techniques and by modifying the composition of the mixture. The gradation curve of the crushed rock component should be such that the material by itself has as low effective porosity as possible. In this case the hydraulic conductivity does not depend only on the bentonite component of the mixture and therefore the material is not as sensitive to the

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groundwater salinity. One alternative currently studied by AECL is to use pre-compacted bentonite pellets to enhance the density of both bulk material and bentonite itself. There has been some discussion whether the proportion of bentonite in the mixture should be higher or lower than 30%. Relatively low proportion of bentonite (e.g. 5-10%) is commonly used in landfill lining structures composing mainly of graded crushed rock or till. The advantages of this kind of mixtures are good compaction properties and low hydraulic conductivity despite the low bentonite content. Adding more than 30% bentonite to the mixture (e.g. 50%) has also been discussed. Too high bentonite content may lead to a situation where rock grains float in the clay matrix having no contact with each other. This kind of mixture does not necessarily have very good compaction properties, at least not when in situ compaction is considered. The compressibility of the mixture of crushed rock and bentonite is controlled mainly by the crushed rock component. If the bulk density is near the maximum proctor density, the risk of unacceptable compressibility is very low. According to laboratory tests, a compression modulus of 10 MPa was considered sufficient for this type of material (Gunnarsson et al. 2003). The long-term performance of bentonite within this concept is linked to the obtained density of the bulk material and the bentonite phase. The less dense the bentonite phase is, the more sensitive it is to effects induced by groundwater (exchange of cations, effect of salinity on swelling pressure and hydraulic conductivity, piping, liquification by earthquakes etc.). In practice this means, that the bulk density has to be near the maximum proctor density and the aggregate (e.g. crushed rock) shall have certain type of gradation to enable compaction of the smectite particles between the aggregate grains. The possible mineralogical changes include at least illitisation of smectite and cementation* of the bentonite by silica, but the risk is relatively low for backfill materials compared to buffer clay near the heat source. The smectite-to-illite conversion is a very complex process and it requires at least source of K+-ions, heat (> 60C°) and time (Güven 1990, Pusch & Karnland 1996, Pusch 2001b). * Cementation: In the geologic sense cementation is determined as the nucleation and precipitation of dissolved silica that cements the soil/rock, in other words “binds together particles to a rigid structure (Bates & Jackson 1987).” The dissolved silica (in bentonites case) may originate from dissolution of cristobalite or from release of silica during the smectite-to-illite process. The solubility of silica increases with increasing pH, temperature & pressure. The processes controlling the transportation & precipitation of the dissolved silica are also various and not fully understood (SKB 1999). The cementing by silica can “lead to various changes in the rheological properties of the buffer material, for example increased mechanical strength, brittleness or reduced swelling capacity (SKB 1999).” Bentonite within the backfill is not supposed to have remarkable chemical effect on the other barriers. However, the content of sulphur and organics need to be surveyed in the quality assurance process. The effect of Ca-bentonite and Na-activated Ca-bentonite (as

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a backfill material) on the chemical stability of the buffer material (assuming that the buffer material is Na-bentonite) needs to be evaluated. Powdered bentonite may apply best to be used in mixtures. However, if the bentonite mixed with the crushed rock is in powder form, dusting needs to be taken into account when designing the mixing facility (sufficient ventilation). The material does not tolerate high water leakages during in situ compaction (surface erosion). The acceptable water leakage rate in to the deposition tunnel (from the backfilling point of view) has not been determined yet, but it will be determined with the help of laboratory and field tests. In order to limit surface erosion, bentonite/crushed rock mixture has been compacted to inclined layers in Äspö large-scale test. Horizontal compaction should be re-considered if the conditions in the deposition tunnel are dry enough to allow horizontal compaction with heavy equipments like compaction rollers used in normal road construction projects. Concepts including pre-compacted bentonite blocks According to previous experiences gained in buffer studies (e.g. Johannesson 2002, Pusch 2001b), bentonite can be pre-compacted to sufficient density, so that the blocks most likely fulfill the performance requirements set for the compressibility, hydraulic conductivity and swelling pressure even in high saline groundwater. According to tests made for buffer materials (Pusch 2001b), the hydraulic conductivity of pure bentonite (MX-80) at density of 1 600 kg/m3 at saturation was higher than 1E-10 m/s when salinity of the percolate was 3.5% (CaCl2). In the same test series the bentonite sample with density of 1 800 kg/m3 at saturation had hydraulic conductivity of only 3E-12 m/s. According to Pusch (2001b), the swelling pressure of bentonite is not significantly affected by the porewater chemistry when the density of the material at saturation is higher than 2 000 kg/m3. This should be considered as the target density also for pre-compacted bentonite blocks. Water ratio of the material at compaction should be optimal, in order to reach maximum density for the blocks. When bentonite blocks were manufactured for the Prototype repository, the desired water ratio for the compacted blocks was 17%. According to Pusch (2001b), pre-compacted blocks are sensitive to the moist in air (drying, degradation). Therefore pre-compacted blocks should be stored in a facility where the air humidity can be adjusted to a suitable level. The type of the bentonite may not have a very big role in the pre-compaction process. However, usage of granulated material instead of powder can be justified in order to avoid dusting and possible formation of air bubble- to the blocks. The long-term performance of bentonite as a material within this concept is linked to the final density in the backfilled tunnel. The less dense the bentonite phase is, the more sensitive it is to effects induced by groundwater (exchange of cations, effect of salinity to swelling pressure and hydraulic conductivity, erosion, piping etc.). Contact erosion is possible depending on grain size distribution of the adjacent material (Terzaghi & Peck 1967, Terzaghi et al. 1996). The possible mineralogical changes include at least

17

illitisation and cementation by silica, but the risk is relatively low for backfill materials compared to buffer clay near the heat source. The smectite-to-illite conversion is a very complex process and it requires at least source of K+-ions, heat (> 60C°) and time (Güven 1990, Pusch & Karnland 1996, Pusch 2001b). Bentonite blocks within the backfill are not supposed to have remarkable chemical effect on the other barriers. However, the content of sulphur and organics need to be surveyed in the quality assurance process. The effect of Ca-bentonite and Na-activated Ca-bentonite (as a backfill material) on the chemical stability of the buffer material (assuming that the buffer material is Na-bentonite) needs to be evaluated. The need for bentonite for pre-compaction concept (4) is considerable due to the high density of the blocks (approximately 2 t/m3). Concepts including pre-compacted bentonite pellets Bentonite clay is suitable material also for pellet production. Pre-compacted bentonite pellets may be used in different concepts, e.g. in concept A in order to enhance the density of the clay fraction in bentonite/crushed rock mixture. In this case the optimal proportioning of different components in the mixture is essential in order to gain homogeneity and suitable properties for the material after saturation. Another possible application is to fill the free volume left in the backfilled tunnel with pellets by grouting or blowing, if such undesirable situation occurs. In addition, bentonite pellets may possibly be used to replace the bentonite blocks in the roof section in concepts C & E. The advantage of this would be the ability to use a simple and effective emplacement technique. In addition, the requirements for the backfilled tunnel itself would not be as demanding as in the block case (smoothness of the surface etc.). The disadvantage of the pellets is clearly the relatively slow maturation of the material (Pusch & Johnson 2002). The homogeneity of a pellet-based backfill is another question under discussion. NAGRA has tested a groutable backfill material composing of bentonite pellets (50%) and bentonite powder (50%) (Fuentes-Cantillana & Huertas 2002). However, in this case the material is placed around a horizontal canister as a “buffer”, but the technique may possibly also be applied in KBS-3V tunnel backfilling. According to the previous studies by NAGRA (Naundorf & Wollenberg 1992) the highest possible dry density for pellet-based material is obtained if pellets of multiple size are combined with fine-grained bentonite granulate which act as a filling material. The pellets tested by NAGRA (Naundorf & Wollenberg 1992) had maximum grain sizes of 10-50 mm and the granulates of 0.63-3 mm. The pellets were compacted (in a roller press) under pressure of 150-200 MPa to a density of 2 100-2 200 kg/m3 from MX-80 bentonite with moisture content of approximately 2% (Pusch & Johnson 2002). According to NAGRA (Naundorf & Wollenberg 1992) the shape and the size of the pellets can easily be varied by changing the press segments. The average dry density of the mixture consisting of bentonite pellets and powder is not more than 1 500 kg/m3 at emplacement (Pusch & Johnson 2002). The emplacement will be done pneumatically or by using a conveyor. The feasibility of the concept is currently tested in the Mont Terri EB test. The special objective of the EB test is to study the achievable bulk dry density of the pellets-based backfill after the system has reached saturated state.

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3.4 Differences between commercial bentonites The smectite content of various commercial bentonites is presented in table 3-4. It should be noted when comparing data that the smectite content depends (to some extend) on the source of the sample, sampling, preparation of the sample and on the methods used for determination of smectite content in laboratory. For example, Herbert & Moog (1999) determined the smectite content of MX-80 to be 88%, while Pusch (1999) determined the smectite content of the same product to be 70-80%. In addition, the producer of the product (Volclay ltd) informs that the smectite content of MX-80 is above 90 %. It is clear that the smectite content may vary between different occurrences and also within a single occurrence, however, a homogenized product should have relatively constant smectite content. Most of the commercial Ca/Mg-bentonites are Na-activated and processed to powder or granules. Non-activated products are also available. There are no remarkable differences between the geochemical analysis results of different bentonites, excluding saponite of which Al2O3 content is lower and MgO content higher compared to other bentonites. Another exception is the Indian Na-bentonite that has relatively high iron (Fe2O3 0.5-10%) content for a bentonite. Cation exhange capacity and free swelling volume generally correlate with the smectite content of the sample. However, it should be noted that the CEC depend on the determination method, especially if the sample contains calcite that can be leached during the test procedure (Christidis & Scott 1993, Carlson 2004). In addition, some other minerals like zeolites also increase the CEC the sample (Christidis & Scott 1993). Table 3-5 gives some examples on CEC and swelling volume of the different commercial bentonites. Note the differences in the CEC results gained with two different methods. The free swelling volume (ml, saturated in fresh water) has been determined of a sample with a mass of 1 g. Table 3-4. Examples on the smectite content of different commercial bentonites.

Commercial name Bentonite type (initial)

Source area Smectite content (w-%)

Source of information

MX-80 Na-bentonite Wyoming/USA 70-80 Pusch (1999, 2001b) MX-80 Na-bentonite Wyoming/USA 80-85 Carlson (2004) BH-200 Na-bentonite Wyoming/USA > 90 SP-Minerals /BHB BH-200 Na-bentonite Wyoming/USA 75-80 Carlson (2004) Envirogel Na-bentonite Wyoming/USA > 75 Wyoming bentonite Inc Semi-dried crude Na-bentonite Wyoming/USA > 75 Wyoming bentonite Inc Volclay/Ashapura Na-bentonite Kutch/India > 90 Ashapura Minechem Ltd Volclay/Ashapura Na-bentonite Kutch/India 80-85 Carlson (2004) FEBEX Ca-bentonite Almeria/Spain 88-96 ENRESA (1998, 2002) AC-200 Ca-bentonite Milos/Greece 85-95 Decher et al. (1996) AC-200 unactivated Ca-bentonite Milos/Greece 75-80 Carlson (2004) IBECO Ca/Mg-bent. Greece 80 Pusch (2001b) Moosburg Ca-bentonite Germany 65 Pusch (2001b) Tixoton Ca/Mg/Na Various sources 90 Pusch (2001b) Saponite Mg-bentonite Spain 70 Pusch (1999, 2001b)

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Table 3-5. CEC and free swelling volume (ml/1 g) of different commercial bentonites. Commercial name

CEC (meq/100 g) Swelling volume (ml/1 g)

MX-80 102-140 (Pusch 1999) MX-80 84 (Carlson 2004)* MX-80 104 (Carlson 2004)** BH-200 84 (Carlson 2004)* 10-15 (SP-minerals) BH-200 102 (Carlson 2004)** FEBEX 80-119 (ENRESA 1998) AC-200 20-105 (Christidis & Scott 1993) 2.5-25 (Christidis & Scott 1993) AC-200 117 (Carlson 2004)* AC-200 122 (Carlson 2004)** Saponite 110 (Pusch 1999) Volclay/Ashapura 124 (Carlson 2004)* 14-18 (Ashapura Minechem) Volclay/Ashapura 120 (Carlson 2004)** Envirogel 9-13 (Wyoming bentonite Inc.) * Determined with BaCl2 -method. ** Determined with NH4-acetate -method. This method exaggerates the CEC, if the sample contains calcite (Ca will be leached in the test procedure). SKB (Pusch 1999 & 2001b) has presented hydraulic conductivity and swelling pressure results for various commercial bentonite products. ENRESA (1998, 2002) has also published corresponding results on FEBEX Ca-bentonite. Pusch (2001b) states that the hydraulic conductivity tests made with non-activated and Na-activated Ca-bentonites (ρsat 1 800 kg/m3) confirm the fact that generally the hydraulic conductivity of Ca-bentonites is higher than of Na-bentonites. The difference between the two materials evened out when the samples were compacted to higher density (ρsat 2 000 kg/m3). The same phenomenon was found in the results of swelling pressure tests. The difference between the physical properties of Ca- and Na-bentonites has been explained by differences in the microstructure (Pusch 1994). However, the Spanish non-activated (ENRESA 2002) Ca-bentonite seems to have comparable and even better physical properties than e.g. MX-80 Na-bentonite (ENRESA 2002). Preferably more data should be produced and analyzed in order to verify what are the factors within bentonite that really control the physical behaviour of the material in repository conditions. It seems probable that the geotechnical properties of bentonites are not solely controlled by the type of the smectite interlayer cation, but also by other factors, like the smectite content, amount of other clay minerals present and the chemical interaction between the percolating solution and interlayered cations. At this stage, there are no sufficient grounds to discard any bentonite due to the type of major exchangeable cation in the interlayer space of the smectite mineral within the bentonite. More statistically comparable data on the differences between various bentonites will be needed to draw this kind of conclusions. In addition, the groundwater composition will affect the chemistry of the bentonite in the long-term as long as the material reaches chemical equilibrium with the groundwater. E.g. the exchangeable cations within Na-bentonite may be replaced by calcium. This does not necessarily

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mean that all of the Na+ ions within the bentonite will be eventually replaced by Ca2+ ions from the groundwater. The reaction depends on various factors like, the other cations within the groundwater (e.g. at Olkiluoto the groundwater at repository depth contains both Na+ & Ca2+ ions), proportion of different cations within the bentonite, other possible reactants within the bentonite, temperature and pressure. For example, diffusion of an ion through the clay material is due to the differences in the concentrations of this particular ion between the percolating groundwater and porewater of the clay. In other words, the final result depends on how the material eventually reaches the chemical equilibrium with the groundwater. In principal most of the commercial bentonites are suitable for different backfilling concepts, assuming that sufficient density can be obtained for the bentonite within the backfill concept. However, certain matters should be considered when choosing the material for backfilling. The material should be properly characterized and it should fulfill the quality requirements (to be) set for the material (e.g. minimum smectite content, suitable mineralogical and chemical composition, tolerable amount of organics and sulphur and suitable geotechnical properties tested in saline environment, TDS 35 g/l). These factors should be taken into account also in the quality assurance system. Materials with known source occurrences should be preferred rather than a bentonite product form unknown sources. If the occurrence is well characterized, natural analogues from the occurrences may be used in the performance assessment. It is also easier to interpret laboratory test results, if the geology of the occurrence is known. From the cost-optimization point of view, products with good quality and reasonable cost should be chosen for further studies. MX-80 Na-bentonite can be used as a reference when assessing the quality of the potential materials. In addition, it is not preferable to base everything on one particular product due to long time-span of the backfilling operation (resource & quality problems might appear in the long run, because the material is quarried from occurrences with limited volume and geological properties).

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3.5 Need for further investigations Investigations on bentonites in general: - Differences between Na-bentonite and Ca-bentonite (activated and non-activated)

need to be studied in detail in order to be able to assess the effect of Ca-bentonite based backfill on the Na-bentonite based buffer. This will require detailed studies on the mineralogy, chemistry and microstructure of these materials (with e.g. XRD, XRF, IR spectrometry & optical methods).

Investigations on potential commercial bentonite products: - Characterize a set of potential commercial bentonites with reasonable costs. The

characterisation should include determination of various material properties (mineralogy, chemistry, geotechnical properties) and studies on the geology of the source occurrence in order to evaluate the homogeneity and resources of the deposit. Special attention should be given to determine the amount of harmful substances (e.g. pyrite, unstable quartz etc.) within the bentonite. Some of this work has already been started in the ongoing mineralogical studies by SKB and Posiva (e.g. in Carlson 2004).

Investigations aiming at increasing the density of the bentonite phase in concept A (in situ compacted mixture of crushed rock and bentonite) so that the properties of the bulk material would remain acceptable in the dimensioning salinity of 3.5% TDS (NaCl/CaCl2): - Can the density of the bentonite phase be enhanced in the concept A by varying

and/or modifying the aggregate phase. This will require testing of different types of aggregates and mixtures. The tests include at least determination of optimal grain size distribution, maximum proctor density, optimal water content, porosity, compressibility, hydraulic conductivity & swelling pressure in fresh water and in salinities of 1%, 3.5% and 5% (TDS). Different types of aggregate that should be tested include: different rock types (e.g. the crushed rock from the possible repository area), refined crushed rock (obtained with optimal crushing process, sieving and separating different phases, proportioning, mixing and possibly adding fineries e.g. milled rock or silt size soil to the aggregate) and glacial till. The aim is to find an aggregate with optimal compaction properties, optimal grain size distribution and minimum porosity. The optimal grain size distribution curve should not necessarily correspond the Fueller-curve, because the Fueller-curve has been determined for spherical grains and the grains used in backfill material are rarely spherical in shape. Optimal aggregate is the key to reach high density also for the bentonite phase of the mixture.

- Can the material properties of the backfill mixture in concept A be enhanced by adding highly compacted bentonite pellets to the mixture? This would require mixing and testing of various samples with different composition. The tests should include at least determination of grain size distribution, maximum proctor density, optimal

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water content, porosity, compressibility, hydraulic conductivity & swelling pressure in fresh water and in salinities 1% and 3.5% (TDS).

- Can the salt resistance of the backfill mixture in concept A be enhanced with adding

non-swelling clay to the mixture (compare to the Canadian dense backfill by Dixon 2000, 2002)? This would require determination of the basic material properties of such mixture.

Investigations concerning all concepts including bentonite as a backfill material: - Acceptable water inflow to the backfilled tunnel. - Sensitivity to erosion (surface erosion, contact erosion & piping). Tests evaluating

the sensitivity of the backfill material to piping need to be performed in laboratory (e.g. with a “pin-hole test”) (SYKE 2002). The risk of contact erosion of bentonite blocks and coarser backfill materials (in heterogeneous concepts) need to be evaluated according to the porosities and grain size distribution of the adjacent materials with the method represented in Terzaghi & Peck (1967) and Terzaghi et al. (1996).

3.6 Summary Bentonite is proposed to be used in various backfilling concepts (see chapter 1). Some of the concepts include pre-compacted bentonite blocks, while in one concept bentonite is mixed with crushed rock and the mixture is installed into the tunnel by in situ compaction. The applicability of bentonite within the latter concept depends on whether high enough density can be obtained both for the bentonite phase and for the bulk material. High density contributes to maintain the physical and chemical properties of the backfill even in high salinity environment. Bentonite as material is suitable for concepts where the clay is used in a pre-compacted form. Potential commercial bentonites with high quality and moderate costs should be studied further in order to determine their suitability as backfilling materials. Summary on bentonites as backfill materials in the different concepts is presented table 3-6 in the following three pages.

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Table 3-6. Summary table/bentonites.

Concept & compaction technique

Requirements/design basis for the material

Fulfillment of requirements for the material

Needed research & development work in order to fulfill the requirements

Design base 1. Low compressibility in order to restrain the upward expansion of the buffer. Resistance against erosion.

The compressibility of the mixed material is mainly controlled by the crushed rock phase (the clay is supposed to fill the voids between the grains). It is assumed that the compressibility for the mixture is low enough to restrain the upward expansion of the buffer (the tested compression modulus has been over 10 MPa). In addition, the gradation curve of the mixture is such that the buffer bentonite cannot intrude to the poresystem of the backfill material (the risk of contact erosion is low). Despite the self-healing capacity, piping may occur in the mixture if the gradation curve of the aggregate is too sorted or the density is too low.

Development work for in situ compaction techniques is needed to ensure high density for the bulk material. The gradation curve and water ratio for the mixture should be optimal in order to reach maximum density and minimum porosity. These properties depend on the materials used in the mixture (e.g. rock types etc.) and they should be determined for set of potential materials (see section 3.5). The erosion resistance of the mixture needs to be evaluated (see section 3.5). Binding agent (e.g. slag) as an additive has also been proposed in order to enhance the strength of the backfill. The long-term safety aspects of such additive are under debate. There is also opposite opinions, whether very high strength is needed for the backfill because it turns the backfill into a rigid structure sensitive to fissuring in the long-term.

Concept A. Mixture of bentonite & crushed rock / in situ compaction (Reference bentonite MX-80 Na-bentonite)

Design base 2. Low hydraulic conductivity (1E-10 m/s) and swelling pressure (100 kPa) in order to prevent the advection of groundwater in the tunnel. Dimensioning water salt content 3.5% (CaCl2/NaCl, 50:50).

The current status for the concept is, that the in situ compacted mixture of bentonite and crushed rock is assumed to perform in salinity only up to 1% without any further R&D work aiming at enhancing the salt resistance of the material. In order to enhance physical properties of the material in saline conditions, the density of both bulk material and bentonite phase need to be enhanced. According to the tests made by SKB (Johannesson et al. 1999), the dry density for the bulk material should preferably be > 1800 kg/m3, but the key issue is the obtained effective clay/ montmorillonite dry density (mass of bentonite/ montmorillonite divided by the volume of occupied by bentonite/montmorillonite and voids) (Dixon 2000).

Higher densities for the system can be obtained e.g. by: using more efficient mixing & compaction techniques, optimizing the gradation curve of the aggregate and by adding pre-compacted bentonite pellets to the mixture (see section 3.5). The salt resistance of the mixture may be enhanced by adding (or replacing some of the bentonite with) non-swelling clay to the mixture (see the Canadian lower backfill by Dixon 2000). Basic geotechnical properties of such material need to be tested in laboratory (see section 3.5).

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Design base 3. Long-time stability/long-term performance

The long-term performance of the material depends on the obtained density for the clay phase. The geological history of bentonite clays speaks in favor of good long time stability (Smellie 2001). Some mineralogical changes are possible due to cation exchange and cementation by silica, but their effect to the long-term performance may be unremarkable.

If the requirements and design basis 2 (see the box above) can be fulfilled, the long-term performance of the material is supposed to be acceptable. Therefore, the development work should be aimed at enhancing the bulk and clay density of backfill mixture (see section 3.5). The mineralogical stability of bentonites will be further investigated within the buffer projects (SKB 2001).

Design base 4. No harmful effect on other barriers.

The material is not supposed to have any significant harmful effects on the other barriers.

The content of contaminants (e.g. sulphur and organics) need to be characterized and surveyed in the quality assurance system.

Design base 5. Suitable compaction properties in order to gain sufficient density also close to the roof.

The compaction properties of the mixture are not solely controlled by the bentonite phase, but also by other factors like gradation curve of the mixture etc.

The composition of the mixture needs to be developed in order (see the previous boxes) to enhance the physical properties of the material, including the compaction properties. The optimal water content of the mixture need to be tested in laboratory and verified with field-compaction tests with the respective compaction method.





All concepts (C, D, E, F) including pre-compacted bentonite blocks. (Reference bentonite MX-80 Na-bentonite)


Bentonite blocks can be pre-compacted to high densities (Pusch 2001b)(ρsat > 2 000 kg/m3)

compressibility is supposed to be very low (assuming that the conditions are confining, i.e. there is limited volume of free space available in the tunnel, where the material of the blocks could intrude leading to lower density for the block). The self-healing capacity of the blocks is supposed to be high enough to be able to cure the channels induced by piping. Contact erosion between the blocks and other backfill materials need to be taken into consideration.

The compressiblity and swelling pressure of the backfilling blocks need to be tested in laboratory and compare the results to the corresponding buffer block results in order to evaluate risk of buffer dislocation. The risk of contact erosion needs to be evaluated with the Terzaghi mehtod (Terzaghi & Peck 1967, Terzaghi et al. 1996). The risk can be reduced by usage of filter materials and by development of the material properties of the adjacent backfill material.

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Design base 2. Low hydraulic conductivity (1E-10 m/s) and swelling pressure (100 kPa) in order to prevent the advection of groundwater between adjacent blocks, in the roof section of the tunnel and along the tunnel axis. Dimensioning water salt content 3.5% (CaCl2/NaCl, 50:50)

According to the buffer studies (Pusch 2001b), bentonite blocks can be pre-compacted to high enough density (ρsat > 2000 kg/m3) requirements set for the hydraulic conductivity and swelling pressure can be achieved, assuming that limited space is left between the blocks and the tunnel surface after installation.

The hydraulic conductivity and swelling pressure of the blocks need to be tested in laboratory in the dimensioning water salt content. Successful installation is essential in order to fulfill the design basis and therefore the installation technique needs to be developed and tested. In order to reach tight contact between the backfill and the tunnel face, the tunnel may have to be excavated with tunnel boring machine for smooth and even surface.

Design base 3. Long-time stability/Long-term performance

The long-term performance of the pre-compacted blocks is assumed to be acceptable depending on the obtained density. The geological history of bentonite clays speaks in favor of good long time stability. Some mineralogical changes are possible due to cation exchange, but their effect to the long-term performance may be unremarkable.

The mineralogical stability of bentonites will be further investigated within the buffer projects (SKB 2001).


The material is not supposed to have any remarkable harmful effects on the other barriers.

The content of contaminants such as sulphur and organics need to characterized and surveyed in the quality assurance system.


According to buffer studies (Pusch 2001b), bentonite has suitable properties for pre-compaction.

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4 OTHER NATURAL SWELLING CLAYS Other natural swelling clays include clays that contain smectitic components and therefore have swelling capacity. These clays differ from bentonites due to their lower smectite content (usually < 50 weight-%). In addition, the smectite within these clays usually occurs as a component of mixed-layer mineral together with another component (illite, mica, muscovite etc.). The mineralogy and microstructure of clays with mixed-layer minerals is usually quite complex, which makes the determination and characterisation of these clays very challenging. The geological history of these occurrences usually includes processes like weathering, transportation, sedimentation, hydrothermal alteration and in some cases diagenensis and even metamorphosis. Friedland clay is natural swelling clay that has recently been studied as an alternative backfilling material for mixture of bentonite and crushed rock by SKB (Pusch 1998, 1999, 2001a, 2001b). Therefore, this material will be used as a reference material for other natural swelling clays discussed in this chapter. Two alternative clays are presented in this chapter (see chapter 4.2). 4.1 Friedland clay 4.1.1 Friedland clay in general Pusch (1998, 1999, 2001a, 2001b) has investigated the geological and geotechnical properties of mixed-layer clay from North-Eastern Germany. The clay is called Friedland clay according to the location of the quarry near Neubranderburg city. According to the producer (FrieTON: Friedländer Ton-industriegesellschaft GmbH, http://www.frieton.de) the clay was formed during the Eocene epoch approximately 35-57 million years ago. The occurrence is homogeneous and has an area of 330 hectares and thickness of up to 140 m, with a resource of 100 million tons of raw material (FrieTON). According to Pusch (1998) the clay consists of mixed layer minerals (45%) with swelling ability (mica/montmorillonite), quartz (24%), mica (13%), chlorite (11%), feldspar (5%) and carbonates (2%). The mica present in the mineral assemblage is mostly muscovite (Henning 1972). In more recent studies by Carlson (2004) the mineralogical composition of Friedland clay was determined as “mixture of several clay minerals and detrital quartz, feldspars, siderite and small amount of pyrite.” The other clay minerals present are illite, chlorite and kaolinite and the swelling component is mixed-layer illite/smectite or equivalent (Carlson 2004). The chemical composition of the material is presented in table 4-1.

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Table 4-1. Chemical composition of Friedland clay presented as oxide percents (Pusch 1998 & Carlson 2004).

Major element Pusch (1998) Carlson (2004) SiO2 57% 59.9% Al2O3 18% 18.2% Fe2O3 5.5% 6.8% MgO 2% 1.9% CaO 0% 0.4% Na2O 0.9% 1.0% K2O 3.1% 3.0% TiO2 1.0%

The dominant adsorbed cation is Na+ (Carlson 2004), but also other exchangeable cations (Ca2+, Mg2+ & K+) are present in the clay (Henning 1972, Carlson 2004). The variety of exchangeable cations could be one of the facts explaining the good chemical stability of the material. Another possible explanation can be the soluble salt precipitations already existing in the material (so that the external salt does not have very dramatic impact on the properties of the material). The cation exchange capacity (CEC) of Friedland clay is remarkably lower than for bentonites: 60 meq/100 g (Pusch 1999), 37 meq/100 g (Carlson 2004 determined with BaCl2-method and 42 meq/100g (Carlson 2004) determined with 0.05 M Cu (II) ethylenediamine at pH 7. The pH of the bulk material is 8.3 (FrieTON). The material has been dried and ground by FrieTON to desired moisture content and granule size. However, Friedland clay used in SKB backfilling tests in Äspö had some deviations in the water ratio and granule size leading to difficulties in the compaction process (Pusch & Gunnarsson 2001). In further studies, more attention should be given to the moisture control of this material in each step of the process, including also transportation & storage. 66 4.1.2 Geotechnical properties The average amount of clay size (< 2 µm) particles in Friedland clay is 57% (range 40-80%) (Pusch 1998). The producer of the clay informs that the grain size distribution (DIN 18126) of the Friedland clay is the following: clay fraction (< 2 µm) > 70%, coarse clay fraction (2-63 µm) 20-25% and sand fraction (> 63 µm) 3-4%. The hydraulic conductivity of Friedland clay is not solely controlled by montmorillonite minerals, but as well by other clay minerals present in the material. That may be one of the reasons why the material is not as sensitive to the effects of saline groundwater as pure montmorillonite clays. The other reasons for this may include variety of exchangeable cations, complex mixed-layer structure of the swelling minerals and the initial porewater salinity (& soluble salt precipitations) of the material (all the factors controlling the ion exchange and other reactions between the clay and the percolating solution may have an indirect effect on the physical properties of the material). Further

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studies would be required to identify all the reasons for Friedland clays behaviour in saline environment. According to laboratory investigations (Pusch 2001a) the dry density of the compacted material should be approximately 1 900 kg/m3 at saturation) in order to maintain hydraulic conductivity lower than 1E-10 m/s in the salinity of 3.5%. Sufficient swelling pressure (around 200 kPa) is also maintained in this density (Pusch 2001a). According to FrieTON the natural water content for the Friedland clay is 27-30%, the optimum water content is 23-24% and the liquid limit is 90-130%. In practice, the optimal water content is dependent on the compaction method and force (Terzaghi et al. 1996). In addition, the water content at compaction should preferably be a little higher than optimum. In Proctor compaction test made by SKB (Pusch 2001a) the density of the Friedland clay was not very sensitive to variations in the water content. However, this was found not to be the case in practice during the Äspö field test (Pusch & Gunnarsson 2001). The material was compacted to inclined layers (average thickness after compaction 20 cm, angle of the initial slope 35°) with a vibrating plate (compaction time 30 seconds per surface area). The first Äspö field test was made with slightly too dry material (5-7%) which lead to unacceptable dusting and to lower densities than expected (the dry densities were 1 300-1 500 kg/m3 depending on the measuring depth of the compacted layer and on the water content) (Pusch & Gunnarsson 2001). The clay had also separated which also had a negative influence on the density. In the second test series (Pusch & Gunnarsson 2001), the average water content of the material was 12.9% and the clay had coarser granule size than in the previous test, but the problems with separation remained. Unfortunately, the Friedland clay used in the test was not homogeneous in water content and the compaction result was even poorer than in the first test series. It was concluded that higher compaction energy would be needed to gain sufficient density for the bulk material (Pusch & Gunnarsson 2001). Studying of alternative compaction techniques was also recommended. According to Terzaghi & Peck (1967), the compaction effect of vibration decreases greatly with increasing cohesion of the compacted material, because “even slight bond between the particles interferes with their tendency to move into more stable positions”. Therefore, roller compaction (sheepsfoot roller or similar) is recommended for compaction of clays in normal surface conditions (Terzaghi & Peck 1967). However, roller compaction may not apply very well in tunnel conditions due to restricted tunnel dimensions. The in situ compaction of Friedland clay needs to be studied further in order to reach acceptable density for the material. The actual optimal water content in the field compaction needs to be determined with the help of field compaction tests for each compaction method. The upward swelling of the buffer bentonite into the disposal tunnel backfilled with Friedland clay has been estimated by Johannesson & Börgesson (2002). The estimation was based on laboratory tests (hydraulic conductivity, swelling pressure, compressibility) and on the densities achieved for the Friedland clay in Äspö field compaction tests. Due to the low density achieved in field tests (the average dry densities varied between 1 230-1 460 kg/m3) and the high swelling pressure of the buffer bentonite (approximately 4-5 MPa), there is an obvious risk of upward swelling

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of the buffer resulting in decrease in the density of the buffer. The compression of the Friedland clay (with dry density of 1 400 kg/m3) was calculated to be between 0.2-0.6 m depending on the angle of friction between the buffer and the wall of the deposition hole (Johannesson & Börgesson 2002). Assuming that the angle of friction is 10º, the displacement of the buffer/backfill interface would be 0.3 m and the depth of the zone involved in the swelling would be approximately 2.7 m (Johannesson & Börgesson 2002). In practice this means that the thickness of the buffer should be increased at least to 2.7 m (the current thickness is 1.5 m in the Swedish concept) in order to prohibit the decrease in buffer density around the canister. This would have a significant effect the material costs for the buffer. Another question is, if the decreased density is allowed in the buffer section above the canister either. Due to the smectite-minerals within the clay, the material is supposed to have sufficient self-healing capacity, assuming that sufficient density can be reached for the application. The piping resistance of the material needs to be verified in laboratory. Friedland clay is sensitive to contact erosion if it will be used in heterogeneous concepts and therefore filter layers may be needed depending on the material properties of the material placed adjacent to the Friedland clay blocks. In such case, the risk of contact erosion needs to be evaluated according to requirements set for filter materials by Terzaghi et al. (1996). 4.1.3 Applicability for different concepts Friedland clay may be used in concepts B and D, where the material is either compacted at site or pre-compacted and placed in the tunnel as blocks. According to the experience gained from Äspö field tests, it remains to be demonstrated whether the material can be compacted to sufficient density at site, especially at the roof section. Low density leads to problems with the mechanical properties of the material such as the hydraulic conductivity, swelling pressure, compressibility and sensitivity to surface and internal erosion (piping). According to geologic history of the material and various laboratory tests (Pusch 1998, 1999, 2001a) the mineralogical & chemical stability of the material seems to be fairly good. As a material, Friedland clay is most likely feasible for pre-compaction. Table 4-2 includes a summary on the applicability of Friedland clay as material for the two concepts. The recommended further work is described in following section 4.1.4. 4.1.4 Need for further investigations More basic information should be gathered from the mineralogy, chemistry and geology of the material itself and on the Friedland occurrence to be able to evaluate the mechanical & chemical interaction between the buffer and the backfill. The compaction properties of Friedland clay with different water contents should be further studied with field tests in order to find the (most practical) optimal moisture content for each compaction method. Different compaction methods should be tested in order to enhance the density of the material.

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Quality assurance system needs to be developed in order to enhance the moisture control of the material. The resistance against piping needs to be verified with laboratory tests. If the material would be used in heterogeneous concepts, the risk of contact erosion need to evaluated according to requirements set for filter material (Terzaghi & Peck 1967, Terzaghi et al. 1996). The basic properties (e.g. hydraulic conductivity, swelling ability, homogeneity after saturation, compressibility) of pre-compacted blocks need to be tested in laboratory. Acceptable water inflow to the deposition tunnel in the two different concepts needs to be determined in laboratory or half-scale tests.

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Table 4-2. The applicability of Friedland clay for backfilling concepts B & D.






Depends on the obtained density. According to calculations based on laboratory investigations the compression of the backfill is 0.2-0.6 m for the densities achieved in field tests (Johannesson & Börgesson 2002). This is not considered to be acceptable there is a risk of buffer dislocation due to high compressibility of the material. The resistance against piping is supposed to be relatively good depending on the achieved density. However, the material has self-healing capacity that is supposed to be sufficient enough to seal off the piping induced channels. Contact erosion is not considered as risk in this concept.

Further tests/calculations are required to determine what is the minimum target density for Friedland clay. Development work for in situ compaction techniques is needed to ensure high density for the bulk material. The water ratio of the mixture should be optimal in order to reach maximum density. The resistance against piping need to verified in laboratory.

Design base 2. Low hydraulic conductivity (1E-10 m/s) and swelling pressure (100 kPa) in order to prevent the advection of groundwater along the tunnel axis. Dimensioning water salt content 3.5% (CaCl2/NaCl, 50:50)

According to laboratory investigations (Pusch 2001a), the hydraulic conductivity and swelling pressure of the material is acceptable (in the dimensioning salinity) if the material can be compacted to target dry density of 1 450 kg/m3 (1 900 kg/m3 at saturation). The present status is, that the target density cannot be achieved in the roof section of the tunnel.

Further field studies are required in order to determine the optimal water content and optimal compaction method for the material. This may include development a special compaction tool suitable for compacting cohesive soils in tunnel conditions.

Concept B. Friedland clay / in situ compaction

Design base 3. Long-time stability/long-term performance

The long-term performance of the material depends on the obtained density for the clay phase. Some minor mineralogical changes are possible due to cation exchange, but it seems that mixed-layer clays are relatively stabile in the expected repository conditions. The geological history of the material speaks in favor for good long time stability.

If the requirements and design basis 1 & 2 can be fulfilled, the long-term performance of the material is supposed to be acceptable (see the boxes above).

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Friedland clay can contain some exchangeable potassium (from micas) increasing the risk of gradual illitisation of the buffer, although the process is supposed to be very slow even in geological time scale. The clay can also contain some pyrite that increases the corrosion of the copper canister.

Development work needs to be addressed to enhancing the density of the material within the application. The chemical interaction between the Friedland-clay backfill, groundwater and buffer need to studied further in laboratory. The content of sulphur (pyrite) and organics need to characterized and surveyed in the quality assurance system.


According to field tests (Pusch & Gunnarsson 2001), it is still unclear whether the material properties are suitable for the compaction of the roof section with in situ methods. The sensitivity of Friedland clay for water leakages in the tunnel may induce practical problems during installation.

Development work needs to be addressed to enhancing the density of the material within the application.





Concept D. Friedland clay/pre-compacted blocks.

Design base 1. Low compressibility in order to restrain the upward expansion of the buffer. Resistance against erosion

Friedland clay blocks can most likely be pre-compacted to high densities compressibility is supposed to be very low (assuming that the conditions are confining, i.e. there is no additional free space available in the tunnel, where the material of the blocks could intrude leading to lower density for the block). The resistance against piping is supposed to be relatively good. However, the material has self-healing capacity that is supposed to be sufficient enough to seal off the piping induced channels. Contact erosion is not considered as risk in this concept.

Further tests/calculations are required to determine the minimum target density for the blocks. What density can be achieved for the tunnel cross-section in practice needs to be studied with field tests. The blocks need to be designed and tested both in laboratory and in the field. For design base 1, especially the compressibility of the blocks needs to be tested in laboratory. The erosion resistance needs to be verified in laboratory.


Friedland clay blocks can most likely be pre-compacted to high enough density (ρsat > 1 900 kg/m3) (Pusch 2001a) requirements set for the hydraulic conductivity and swelling pressure can be achieved in the dimensioning salinity (assuming that the installation is successful).

The blocks need to be designed and tested both in laboratory and at field.

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The long-term performance of the material itself in a pre-compacted form is assumed to be fairly good. The long-term performance of the concept is more dependent on the successful installation than the material. Some minor mineralogical changes are possible due to cation exchange, but it seems that mixed-layer clays are relatively stable in the expected repository conditions. The geological history of the material speaks in favor for relatively good long-time stability.

Field tests are required to ensure that the blocks can be installed to yield backfill fulfilling the design basis 1 & 2.

Design base 4. No harmful effect on the other barriers.

Friedland clay may include exchangeable potassium (from micas) increasing the risk of gradual illitisation of the smectites, although the process is supposed to be very slow even in geologic time scale. The clay can also contain some pyrite that increases the corrosion of the copper canister.

The chemical interaction between the Friedland-clay backfill, groundwater and buffer need to be studied further in laboratory. In addition, the content of pyrite (sulphur) and organics need to characterized and surveyed in the quality assurance system.


It can be assumed that the clay has suitable properties to allow successful pre-compaction. The resulting density in the roof section depends on the size & shape of the blocks, installation technique and dimensions of the backfilled tunnel. The free space in the tunnel should be limited to minimum, so that density of the backfill would remain acceptable after saturation.

The suitability of the material for block production will be tested. The blocks need to have a good mechanical stability not to brake during installation. Field tests are required to ensure that the blocks can be installed to yield backfill fulfilling the design basis 1 & 2. The effect of the tunnel excavation technique to the backfill needs to be evaluated. In order to reach tight contact between the backfill and the tunnel face, the tunnel may have to be quarried with a tunnel boring machine for smooth and even surface.

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4.2 Other natural swelling clays The division of smectite bearing clays into bentonites and natural swelling clays is not always very clear. As an example, this chapter discusses the smectite-rich clays occurring in Czech Republic and in Estonia. The Czech Radioactive Waste Repository Authority (RAWRA) has been studying the smectite-rich clays occurring in Czech Republic in order to replace foreign expensive bentonites to cheaper domestic clays (Pŕikyl & Woller 2002). Posiva and SKB have been taken part in these studies aiming at finding the most potential occurrences located in the Western and Southern Czech. The smectite-bearing clays in Czech have been divided into bentonites and smectite-rich clays due to the differences in their origin (Pŕikyl & Woller 2002, Ryndová 2001). According to Pŕikyl & Woller (2002), the major bentonite deposits in Czech Republic formed during the Tertiary, in the time when volcanism linked to intercontinental drifting affected the Western part of the country. Tuffites and pyroclastic rocks altered to Ca/Mg-bentonites by argillization in lacustrine environment and by weathering (Pŕikyl & Woller 2002, Ryndová 2001). The Tertiary smecite-rich clays in the Southern part of the Czech Republic originate from various sedimentary processes (Pŕikyl & Woller 2002, Ryndová 2001). According to Pŕikyl & Woller (2002) & Ryndová (2001) these clays usually contain minerals such as montmorillonite, illite, kaolinite, calcite, quartz and feldspar. Mixed-layer minerals (e.g. montmorillonite/illite) and ferric oxides/hydroxides have also been found within these clays. The Rokle deposit near town of Kadan is one of the most promising occurrences chosen for further studies. This deposit has already been exploited by a company named Keramost. The bentonite clay at Rokle occurs in thin layers requiring selective quarrying in order to obtain high quality material. The heterogeneity of the Czech clay deposits may be problematic when quarrying large volumes, and therefore, the potential deposits need to be characterized in detail. The Rokle clay consists of montmorillonite (main component), quartz, calcite, illite, kaolinite and goethite (Carlson 2004). The chemical composition of the Rokle clay is the following: SiO2 (48%), Al2O3 (14%), Fe2O3 (13%), MgO (3%), CaO (5%), Na2O (0.1%), K2O (1%) and TiO2 (4.5%) (Carlson 2004). The CEC of the Rokle clay is approximately 60 meq/100g determined with BaCl2 method (Carlson 2004). As a summary on Czech bentonites it can be stated that further studies are needed in order to evaluate their suitability to different backfilling concepts. However, the pros and cons of Czech clays may be quite similar to Friedland-clay discussed in chapter 3. Another example area discussed in this chapter is the Baltic Paleobasin in North Estonia. This area has been chosen for an example due to the fact that there are no significant deposits of smectite bearing clays in Finland, and Estonia is simply the nearest source of such clays. In addition, North Estonia could be a desirable source of raw material due to the suitable transportation location (outcrops are located near sea and ports) and due to low production and material costs.

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According to Kirsimäe et al. (1999) and Kirsimäe & Jørgensen (2000) the mixed-layer clays occurring in North Estonia represent originally Lower Cambrian phyllosilicates and tectosilicates that were altered during shallow burial diagenesis during Middle to Late Devonian period. The Lower Cambrian unlithified clays and silty clays have thickness of 150 m. These clays occur in inclined layers and outcrop at the northern margin of the Baltic paleobasin. The burial increases slowly (2.5-3.5 m/km) southwards to > 2 000 m depth in the southeastern Lithuania. The amount of the clay fraction (< 2 µm) is > 40-45% of the bulk material and the natural water content of the clay is 8-28% (Kirsimäe & Jørgensen 2000). Illite and mixed-layer illite/smecite compose the dominant phase of the clay fraction (> 70%). Other clay minerals present in the clay fraction are kaolinite and chlorite. Quartz and orthoclase are the most abundant non-clay minerals present in the clay/silty clay (Kirsimäe & Jørgensen 2000). According to Kirsimäe et al. (1999) there are outcrops and quarries of this material in Tallinn, Kopli, Kolgakũla and Kunda located in the northern seaside of Estonia. The outcrop is approximately 60 m thick and occurs along the northern coast of Estonia. The clay is called the Blue clay and the biggest operating quarry is at Kunda. The clay is used mainly for cement production but also for sealing purposes. The Blue clay has recently been used as a sealing material in the new Tallinn landfill, and according to tests, the material fulfills the criteria set for mineral landfill sealing materials. The clay has relatively low permeability and some swelling ability, although the amount of swelling minerals (illite/smectite) with the clay is low (only 6-16% in the clay fraction) (Kirsimäe et al. 1999). Further studies would be required in order to evaluate the suitability of Estonian Blue clay for different backfilling concepts. The need for further studies concerns especially the swelling properties and permeability of the material in saline conditions. 4.3 SUMMARY Natural smectite-rich clays may be used in backfilling concepts B & D including in situ and pre-compaction techniques. The previous backfill studies with mixed-layer smecite-rich material have been done with the German Friedland clay. Development of compaction procedure is needed to gain sufficient density and tolerable mechanical properties (compressibility) for the material also at the roof section of the tunnel (concept B). Other smectite-rich clays may possibly be used as an alternative material for Friedland clay. However, this would require further studies on potential alternative clays, because every natural clay material and deposit has their own characteristics and properties. Smectite-rich natural clays with adequate swelling ability are possibly suitable for pre-compaction (concept D). The mechanical & chemical interaction between such backfill blocks and bentonite buffer need to be evaluated according to laboratory data and calculations.

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5 NATURAL NON-SWELLING CLAYS AND OTHER POTENTIAL SOIL TYPES

5.1 Introduction Suitable non-swelling natural soil types for tunnel backfilling have low hydraulic conductivity, and therefore, they have been used in various civil engineering applications e.g. in dam and landfill sealing structures. Such soil types, typical in Finland & Sweden, include natural non-swelling clays and basal tills with high content of fineries. Hydraulic conductivity of a soil is dependent on several factors: porosity, effective porosity (porosity available for fluid flow), grain size, grains size distribution, packing, shape of the grains and electrostatic charges (Fetter 1994). The porosity of clays is higher than of any other soil type, 33-60% (Fetter 1994), yet clays still obtain low hydraulic conductivity. This is due to small size of the particles/grains, small size of the pores, irregular shape of the particles/grains, large surface area increasing frictional resistance to flow and high negative charges of the clay particles leading to adsorption (Fetter 1994). In addition, not all of the pores in clay are interconnected (Fetter 1994). The low hydraulic conductivity of tills is based on poorly sorted grain size distribution and irregular grain shapes leading to effective packing and very small porosity (10-20%) (Fetter 1994). The more fineries (grains and particles with size smaller than 0.063 mm) the till contains, the lower is the hydraulic conductivity of the material. In addition, the gradation of tills is such that the material has self-filtering capacity, meaning that it has relatively good capacity to resist internal erosion (piping). The possibilities to use natural non-swelling clays and tills in different concept are discussed in this chapter. In theory these materials could be used in the following concepts: - Concept C, where the bulk volume of the tunnel is filled with in-situ compacted non-

swelling clay (or till) complemented with pre-compacted bentonite blocks at the roof section.

- Concept D, where the total volume of the tunnel is filled with pre-compacted blocks made of non-swelling clay.

- Concepts A, E, F, where till can be used as substitute for crushed rock. Non-swelling clays The majority of clay minerals are layer silicates that comprise of sheets of corner-linked tetrahedra and edge-linked octahedra (Velde 1995). The sheets are linked together in two possible ways. The 1:1 layer silicate structure comprises of one tetrahedral and one octahedral sheet and the repeat unit is 7Å (Fig. 5-1). An example of 1:1 minerals is kaolinite. The 2:1 layer silicate structure comprises of one octahedral sheet sandwiched between two tetrahedral sheets and the repeat unit is basically 10 Å. Because of the substitutions in the tetrahedral (e.g. Al3+ for Si4+) and/or octahedral (e.g. Mg2+ for Al3+) sheets, the layers have negative charge (Velde 1995). The charge is balanced by hydrated (Na, Ca, Mg) or non-hydrated (K) interlayer cations. The repeat unit of 2:1 layer silicates depends on the interlayer cation and its degree of hydration. The interlayer cation of illite (mica) is K and the repeat unit is 10 Å. The repeat unit of

38

tetrahedra

tetrahedra

tetrahedra

tetrahedra

tetrahedra

hydroxyoctahedra

octahedra

octahedra

octahedra

7 Å�

10 Å�

14 Å�

Figure 5-1. Block diagrams of clay mineral types according to the combination of octahedral and tetrahedral coordinated sheet (Velde 1995). vermiculite and members of smectite group varies between 12 and 15 Å, depending on the interlayer cation and the number of water molecule layers (1 or 2). Chlorite has one octahedral sheet in the interlayer position and the repeat unit is 14 Å. Smectites, and to some degree vermiculites, are expandable. The swelling capacity of smectite is due to its ability to absorb more than 2 layers of water in the interlayer space as a function of the activity of the water (Velde 1995). Also ammonium and organic molecules can enter the interlayer space causing the repeat unit to grow. Mixed-layer clay minerals are made up of layers of two or more components that are stacked either regularly or randomly. Most common mixed-layer mineral is illite/smectite. The identification of clay minerals using X-ray powder diffraction is based on determining the repeat unit in untreated and treated oriented mounts. Sepiolite and palygorskite are clay minerals with other than sheet like structure (Velde 1995). Finnish clays This chapter discusses non-swelling clays commonly occurring in South-West Finland and having relatively short transportation distance to the Olkiluoto site. The corresponding Swedish clays are described in Pusch (2001b). The majority of Finnish clays formed during the deglaciation process approximately 10 000-7 000 years ago (Eronen & Haila 1981, Miettinen et al. 1999). The deglaciation process is divided in to four stages: Baltic ice lake stage, Yoldia sea stage, Anculys lake stage and Litorina sea stage (Eronen & Haila 1981). The clays formed during the first three stages are considered glacial clays, and the clays formed during and after Litorina sea stage are post-glacial. The glacial clays include clays with varved structures while

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the Litorina clays are homogeneous. Mineralogically, the Finnish clays consist of illite (illite is the main component in Finnish clays), chlorite, vermiculite and mixed-layer minerals (illite-chlorite-vermiculite) (Soveri 1956). Mixed-layer minerals and vermiculite occur mainly in the surface zone (dry crust) that has been affected by weathering. Kaolinite and smectites occur rarely in Finnish clays. The non-clay minerals commonly present in the Finnish clays are quartz, feldspars and amphiboles (Soveri 1956). The organic content of glacial clays is relatively low (less than or around 1%), while the more recent clays have organic content of 2-6% (Gardemeister 1975). The average amount of clay fraction (< 2 µm) in Western Finland is 15-30% for Litorina clays, 40-70% for Anculys clays and 70-90% for Yoldia stage clays (Gardemeister 1975). The clays formed during the Baltic ice lake stage are very rich in silt fraction (Gardemeister 1975). The Finnish clays have traditionally been used as raw-material for bricks and lately also as a sealing material in the new EU-standard landfills. The clays in the Satakunta-area (South-West Finland) represent typical glacial and postglacial clays in Finland (Lindroos et al. 1983). The clay resources of the area are considerable. Glacial clays in S-W Finland are used as raw-material for brick industry due their low organic content and suitable gradation. Due to the relatively high sulphate and organic content of Litorina clays, only glacial clays should be considered to be used in tunnel backfilling. These clays outcrop in the areas locating approximately 40 m above the sea level and on hillsides. One potential source of clays is the Loimaa district located approximately 100 km South-East from Olkiluoto. More than half of the surface-area in this district is covered with Yoldia and Anculys stage clays with thickness up to 30 m (Kukkonen et al. 1993). The average content of clay fraction (< 2 µm) is > 70% and the average content of organics is only 0.6% (Kukkonen et al. 1993). Tills with high content of fine fraction Basal till is “a firm clay-rich till containing many abraded stones dragged along beneath a moving glacier and deposited upon bedrock or other glacial deposits” (Bates & Jackson 1987). Basal till is the most common surficial sediment in Finland covering almost 50% of Finland’s land area and having average thickness of 6-7 m (Lintinen 1995). Basal till in Finland was formed and deposited during the last glaciation (Weichselian glaciation). Some of these basal tills are rich in fines and suitable for backfilling purposes. A fine-rich till contains at least 30% fines (< 0.063 mm) and at least 5% clay fraction (< 2 µm) (Lintinen 1995). Such tills occur in Ostrobothnia, southwestern Finland and in Lake Ladoga-Bothian bay zone (Lintinen 1995) (see figure 5-2). Similar basal tills rich in fines occur also in the crystalline bedrock area in Sweden. The Swedish backfill material of glacial origin has been discussed in Pusch (2001b).

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Figure 5-2. Regional distribution of clay fraction content (< 2 µm) in till fines in Finland (Lintinen 1995). 5.2 Geotechnical properties The hydraulic conductivity of natural non-swelling clays is < 1*10-9 m/s and for silty till 1*10-7 – 1*10-9 m/s (SYKE 2002). The conductivity depends on the density and clay & silt content of the material. In practice, this means that approximately 3-6% of bentonite needs to be added to the till in order to gain the target conductivity of less than 1*10-10 m/s. According to experiences on dam construction, it is possible to gain the target conductivity for such mixture in fresh water conditions. Further testing is, however, needed to study the hydraulic conductivity of till-based backfilling material in saline water and in order to determine the swelling ability of such material. Only tills with sufficient amount of fineries (approximately 20-50% of grains smaller than 0.063 mm) should be considered for further testing. Due to the optimal grain size distribution, basal tills have good compaction properties and low compressibility compared to non-swelling clays, which tend to consolidate in nature after sedimentation. Consolidation of clays is a slow geological process where

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the clay gradually reduces in volume and increases in density by squeezing the pore fluids form the pore spaces in response to increased load or effective compressive strength (Bates & Jackson 1987). Therefore, the compaction of 100% clays should be very effective in order to avoid loss in volume after installation. In addition, the water content of the clay needs to optimal (not too dry or too wet) to enable the efficient compaction. Ground water leakages or excessive drying will disturb the compaction process. It is probable that sufficient density cannot be reached for the non-swelling clay with current in situ compaction techniques due to the difficult moisture control and uniform grain size distribution of the material. Therefore, it is not likely, that the in situ compacted non-swelling clay would fulfill the requirements set for the compressibility and hydraulic conductivity. However, it is possible to manufacture blocks with high density, tolerable hydraulic conductivity and compressibility from non-swelling clay, but they would naturally have no swelling ability. The advantage of the material compared to smectite-bearing clays is that the hydraulic conductivity of non-swelling clays is not sensitive to the salt content of the percolating water. Further testing would be required in order to define the hydraulic conductivity and mechanical of the pre-compacted blocks in saline conditions. Non-swelling clay may be used in the other parts of the repository, e.g. in shafts, where more robust techniques could be used for compaction and where the material would compact by itself in time due to high overburden. It is not certain whether non-swelling clays have enough self-healing capacity to resist piping. The material is also relatively sensitive to contact erosion, if used in contact with coarser materials. Fine-rich tills with unsorted grain size distribution and low porosity have a relative good resistance against piping and other erosive processes. The erosion resistance of these materials needs to be verified in laboratory. 5.3 Applicability for different concepts The applicability of non-swelling clay is questionable in concept C, where the bulk of the material would be installed with in situ compaction technique (see table 5-1). However, non-swelling clay may possibly be used combined with blocks made of bentonite clay in concept D (e.g. in the core of a plug structure, where the outer surface is made of bentonite blocks) (see table 5-1). Non-swelling clay can probably also be used in other parts of the repository (e.g. filling of shafts). The advantage of non-swelling clay is that the hydraulic conductivity of the material is not as sensitive to the salt content of the percolating water as the hydraulic conductivity of smectitic clays. Basal tills can probably be used for tunnel backfilling (concept C) (see table 5-2) if approximately 3-6% of bentonite is added to the material and the basal till has unsorted grain size distribution including at least 20-50% grains of silt and clay size. The advantages of till-based backfill are good compaction properties, low compressibility, low porosity and relatively good resistance against surface and internal erosion. This kind of material may have some swelling pressure and self-healing capacity. However, bentonite blocks (or pellets) may be needed in the roof section (depends on the swelling ability of the mixture). Basal till is probably not suitable material for block manufacturing due to risk of block breakage during the installation (low plasticity

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fragile blocks). The applicability of basal tills for concepts B & C is summarized in table 5-2. The long-term stability of non-swelling clays and fine-rich tills can be assessed according to the behavior of these materials in geological processes. There might be some mineralogical alterations of fine particles into clay minerals or into another clay minerals in both of these materials in the expected repository conditions, but these changes are assumed to be slow and they probably would not harm the mechanical or chemical properties of these materials significantly. Only clays and basal tills with low content of organics (< 1%) and sulphur are recommended to be used in repository backfilling. 5.4 Need for further investigations - Can blocks made of non-swelling clay be combined with blocks made of bentonite to

a functional plug structure (e.g. in concepts E & F), where the non-swelling clay would be used only in the core of the structure? This would require more laboratory tests on blocks made of non-swelling clay, e.g. to the test the hydraulic conductivity, compressibility and chemical interaction between the non-swelling clay and bentonite.

- Can non-swelling clay be used as backfilling material in other parts of the repository

than the disposal tunnel, e.g. in shafts? - Does the in situ compacted mixture of fine-rich till and bentonite (3-6%) fulfill the

requirements set for concept C in the dimensioning salinity? To obtain this knowledge, laboratory tests and a small-scale compaction test need to be done with the potential material. The laboratory tests would include determination of the basic geotechnical (hydraulic properties, swelling pressure etc.), chemical & mineralogical properties of the material.

- The potential resources for fine-rich till needs to be studied as well as the possibilities to manufacture synthetic fine-rich till (crushed rock combined with silt and sieved and mixed to a till like composition)?

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Table 5-1. Applicability of non-swelling clay for concepts C & D.






The compressibility depends on the obtained density. It is not probable that sufficient density can be reached for the non-swelling clay with in-situ compaction technique at full tunnel scale. This is due to the material properties (e.g. difficult moisture control) of the non-swelling clay The material may not be able to resist piping nor contact erosion depending on the achieved density.

The usage of non-swelling clay as a component in other backfilling concepts would require further studies with the material.


It is questionable whether in-situ compacted non-swelling clay will have hydraulic conductivity of 1E-10 m/s in the dimensioning salt content. In addition, non-swelling clay will not have any swelling pressure in any conditions.

No further testing is required, if not used in other applications.

Design base 3. Long time stability/long-term performance

Some mineralogical changes are probable due to cation exchange, but the geological history of the material speaks in favor for relatively good mineralogical stability.



The compressibility of the material within this particular concept is assumed to be intolerable

harmful effect on the buffer. In addition, some of the non-swelling clays contain too high content of sulphur and organics. Some clays may also include exchangeable potassium possibly leading to gradual illitisation of the buffer, although the process is supposed to be very slow even in the geologic time-scale.

If non-swelling clay will be used in some backfilling applications, the content of sulphur and organics need to be tested.

Concept C. In situ compacted non-swelling clay + bentonite blocks at the roof


The material has not very good compaction properties for in situ applications in tunnel conditions (the optimum water content range is limited moisture control is difficult). It is questionable whether sufficient density will be gained close the roof.


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Concept D. Pre-compacted blocks made of non-swelling clay


It may be possible to obtain sufficient density for the blocks by pre-compaction, so that the compressibility would be acceptable. The material has limited self-healing capacity and resistance against piping. Contact erosion is possible depending on the grain size distribution of the adjacent material.

Tests would be needed to determine the compression & erosion properties of the blocks.


It may be possible to reach the target conductivity with pre-compacted blocks. The material is supposed to be less sensitive to groundwater salt content compared to bentonites. However, the material has no swelling ability and there might occur pathways along the contact zones between the blocks and the roof. Therefore, it is recommended that non-swelling blocks would be used only when combined with swelling blocks.

Tests would be needed to verify the hydraulic conductivity of the blocks in the dimensioning salt content.


Some mineralogical changes are probable due to cation exchange, but the geological history of the material speaks in favor for relatively good mineralogical stability.

No further testing is required at this stage.


Some of the non-swelling clays may contain too high content of sulphur and organics. Some clays may also include exchangeable potassium increasing the risk of gradual illitisation of the buffer, although the process is supposed to be very slow even in geologic time scale.

The content of sulphur and organics need to characterized and surveyed in the quality assurance system.


The blocks will probably have sufficient density, but without swelling pressure there will be a gap between the roof and the blocks. Therefore, is recommended that non-swelling blocks would be used only when combined with swelling blocks.

Testing of block compaction may be required, if used together with swelling blocks.

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Table 5-2. Applicability of basal-till mixed with bentonite (3-6%) to concepts C & D.



Fulfillment of requirements for the material Needed research & development work in order to fulfill the requirements


Depends on the obtained density. Due to the low porosity of the material, the compressibility is supposed to be low. The resistance against piping and other erosive processes is supposed to be relatively good. The bulk material has only very limited self-healing capacity.

The compression properties need to be tested in the laboratory and possibly also at field. The resistance against erosion needs to be tested in laboratory.


It may be possible that the material will have the target hydraulic conductivity in the dimensioning groundwater salinity. The material is also supposed to have some swelling ability, but not as high as 100 kPa.

The hydraulic and swelling properties of the material need to be tested in the dimensioning salt content in laboratory.

Design base 3. Long time stability/long-term performance

The long-term performance of the material depends on the obtained density for the material. The geological history of the tills speaks in favor for good long time stability.

No further investigation is needed at this stage.


The material is not supposed to have any significant harmful effects on the other barriers.


Concept C. Fine-rich till mixed with bentonite (3-6%) and pre-compacted bentonite blocks at the roof.


The material is supposed to have good compaction properties when in situ compaction is considered.

The compaction properties need to be verified in larger than laboratory scale tests.

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Fulfillment of requirements for the material Needed research & development work in order to fulfill the requirements

Concept D. Pre-compacted mixture of fine-rich till and bentonite (3-6%)

Design base 1. Low compressibility in order to restrain the upward expansion of the buffer. Sensitivity to erosion.

The blocks are supposed to have low compressibility, but it is uncertain whether durable blocks made of this kind of material (low plasticity compared to 100% clays) can be manufactured without high risk of degradation during the installation. If sufficient density is obtained and the gradation of the material is suitable, the material is supposed to be relative insensitive to piping or other erosive processes after saturation. However, the bulk material has only limited self-healing capacity.

The mechanical & compression properties of the blocks need to be determined in laboratory.

Design base 2. Low hydraulic conductivity (1E-10 m/s) and swelling pressure (100 kPa) in order to prevent the advection of groundwater between adjacent blocks, in the roof section of the tunnel and along the tunnel axis. Dimensioning water salt content 3.5% (CaCl2/NaCl, 50:50).

The blocks may reach the target hydraulic conductivity in the dimensioning salt content, but there is a risk of flow between the blocks due to restricted swelling ability.

The hydraulic and swelling properties of the blocks need to be tested in the dimensioning salt content in laboratory.


The geological history of tills speaks in favor for good long time stability.

No further testing is required at this stage.


The material is not supposed to have any remarkable harmful effects on the other barriers.



It may be possible to compact blocks of the mixture, but the blocks would be relatively fragile and sensitive to breaking during the installation process.

The mechanical properties of the blocks need to be tested in laboratory.

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6 COSTS The market situation for bentonites and other industrial minerals in the year 2002 has been described by Harben (2002). Generally, the price-range of commercial bentonites is wide as well as the price range of other clay materials. The world’s biggest producer of bentonites is USA, but remarkable amounts of bentonite is produced also in Europe (e.g. in Greece, Italy, Germany), in the former USSR countries, South-America and in the Far East (e.g. India, Japan) (Harben 2002). Germany and UK are the most remarkable exporting countries for the European market (Harben 2002). In the year 2002 the average price for “foundry grade” Wyoming Na-bentonite with high montmorillonite content was 230-250 e/t delivered to an European country through UK in 1 000 kg big bags (Harben 2002). The international price-development for Wyoming Na-bentonite “depends on the oil/gas drilling activity which is influenced by the price of oil & gas, the world economy, international politics, drilling conditions and drilling technology (Harben 2002).” Generally, more economical alternatives can be found in the bentonite markets, but in each case, the quality of the bentonite need to be compared to the reference Wyoming Na-bentonite (MX-80). The quality of the bentonite depends on the smectite content, mineralogy, chemistry, cation exchange capacity, swelling ability and homogeneity of the material. When comparing the raw-material prices per tonne, one should take into account also the transportation costs and the water content of the material. Especially long road transportation distances (> 500 km) increase the costs remarkably. However, most commercial bentonites are distributed by shipping and the prices usually include the transportation to destination, but exclude the unloading costs or transportation from the harbour to the storage. In addition, need for further processing needs to be considered when evaluating the true costs for each material. Storage of bulk clay materials in silos should be considered as an alternative for big bags (a 1 000 kg), especially if the material is mixed with another component (the storage silos can be combined with the mixing plant to form a functional and highly automatized processing plant). The costs for smectite-bearing mixed-layer clays depend on various factors, but it can be stated that generally the mixed-layer clays are considerably more economical than bentonites with high smectite content. The raw-material cost for natural domestic soil-types is considered to be comparable with the material costs of crushed rock. In both cases transportation costs and the need for further processing usually affect the final costs significantly.

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7 CONCLUSIONS & DISCUSSION Bentonites, smectite-bearing clays with swelling ability, non-swelling clays and other potential soil types were studied in order to evaluate their applicability for six preliminary backfilling concepts designed for KBS-3V type repository. These preliminary backfilling concepts were chosen for further evaluation by Posiva and SKB in the end of year 2002. The concepts considered were: A. In situ compacted mixture of crushed rock and bentonite B. In situ compacted swelling clay (other than bentonite) C. In situ compacted non-swelling clay/other soil type with bentonite blocks at the roof

section D. Pre-compacted blocks of all materials E. “Sandwich” concept (heterogeneous concept: crushed rock & pre-compacted

bentonite blocks) F. Compartment concept (heterogeneous concept: crushed rock & bentonite plugs

restraining the flow along the tunnel and excavated disturbed zone). The set of requirements for these concepts aim at restraining advection along the tunnel axis, control of erosion, inhibiting the upward expansion of the buffer and at minimizing all harmful processes between the buffer and the backfill. Therefore the backfilling material should have low compressibility, low effective porosity, low permeability, preferably some swelling pressure, and chemical, physical & mineralogical stability in the long-term. In addition, practical issues like compactibility of the material play an important role when assessing the technical feasibility of the concept. General conclusions on different backfilling materials The compaction result depends both on the properties of the compacted materials and the compaction technique (see figure 7-1). Figure 7-1 is a simplified sketch, where the end member 100% clays represent all clays discussed in this memo. The in situ compaction of pure clays in tunnel conditions is questionable. This is due to the properties of the clay (e.g. cohesion, uniform grain size distribution, difficult moisture control) that may result in insufficient density of the compacted layers, especially in the roof section of the tunnel where the compaction is difficult with normal compaction devices. Low density leads to other problems, like unacceptable compressibility and sensitivity to erosion. So far, this has been the situation also in practice with the Friedland-clay (Pusch & Gunnarsson 2001). Further investigations are planned in order to determine whether Friedland clay could be in situ compacted to sufficient density by varying & enhancing the compaction technique & force and by optimizing the water content of the material.

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Figure 7-1. Compactability of backfilling materials containing different proportions of clay. End member 100% represents all clay materials discussed in this memo and the end member 100% aggregate represent any non-cohesive coarse material (e.g. crushed rock, till or sand) that could be used in tunnel backfilling. Bentonites and most of the smectite-bearing clays with swelling ability are suitable materials for block compaction. It is also possible to manufacture blocks made of non-swelling clay, e.g. from illitic glacial clay. Pre-compacted blocks made of non-swelling clay naturally do not have any swelling ability, but they would still have low hydraulic conductivity and low compressibility after pre-compaction. It should be considered whether such blocks could be used together with blocks made of swelling clay (e.g. in the core of a plug-like structure). Both swelling and non-swelling blocks require special storage facilities (regulated humidity inhibiting drying) and sensitive handling.

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Pre-compacted blocks made of smectitic clay have relatively good properties against piping due to the self-healing capacity of the material, while this is not necessary the situation with blocks made of non-swelling clay. Contact erosion is possible between clay based blocks and coarser backfill materials in heterogeneous concepts (depending on the grain size distribution of the coarser material). The mineralogical and chemical stability of clay blocks is supposed to be relatively good also in the long-term. However, the raw-material used for the blocks should be regularly checked for organics and sulphur. The mechanical & chemical interaction between the pre-compacted backfilling blocks and the bentonite buffer should be taken into account in the further studies with these materials. The term aggregate used in figure 7-1 can be applied to any coarse material from tills to crushed rock. In clay/aggregate mixtures, the aggregate is supposed to form a skeleton and the clay is supposed to fill the voids between the aggregate grains. Generally speaking, in situ compaction may be effective technique only for materials including an aggregate skeleton. The skeleton grains transmit the force of the compaction device trough the material leading to effective compaction. The salt resistance of the clay within an aggregate/clay mixture depends on the achieved density for the clay fraction and on the chemical stability of the clay. The achieved density for the clay fraction depends on the compaction force and on the gradation curve of the aggregate material (see figure 7-2). If the aggregate consists of sorted grains, the clay between the aggregate grains does not compact effectively (large voids are left between the aggregate grains). In addition, the risk of internal erosion (piping) is higher for mixtures composing of clay and sorted aggregate with uniform grain size distribution (figure 7-2). In such mixtures heavy water flow may wash away the clay component and leave permanent water conducting channels to the material. In a material with unsorted grain size distribution, the transportation of the clay particles between the aggregate grains is restricted due to clogging of the pathways. Therefore, the aggregate should preferably have very unsorted grain size distribution. This applies both to crushed rock and natural soils. Whether a truly unsorted grain size distribution can be obtained for crushed rock depends on the crushing technique together with the geology, mineralogy and texture of the crushed rock. Adding of fines (e.g. milled rock with a grain size range corresponding to silt) should also be considered to enhance the composition of the material. Fine-rich tills have naturally suitable gradation curve for backfilling purposes.

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Figure 7-2. The effect of the aggregate grain size distribution on the density of the clay fraction. This figure is a simplified sketch. Most of the aggregate materials have other than spherical grain shapes. E.g. in the case of tills the grains are usually angular giving the material better compaction properties than spherical grains. Material A is more sensitive to piping compared to material B. Generally speaking, the feasibility of pre-compaction is better for clays than for coarser materials. A block consisting of e.g. crushed rock (70%) and clay (30%), especially non-swelling clay, would be fragile & relatively sensitive to drying and degradation and would require special storage facilities and sensitive handling. This kind of blocks would also have limited swelling ability. The advantage of this kind of blocks would be that the crushed rock would carry the mechanical load induced by the swelling pressure of the buffer, and the compression of such blocks would be very limited. Applicability of backfilling materials for different concepts The applicability of different backfilling materials for each concept is summarized in table 7-1. MX-80 is the reference material for bentonites and Friedland-clay for smectite-bearing clays other than bentonites (the difference between bentonites and smectite-bearing clays is the content of smectites and smectitic mixed-layer minerals within the clay, but in some cases this division can be somewhat artificial). In addition to swelling clays, non-swelling clays and fine-rich tills are also discussed in table 7-1.

A) B)

A) Clay & sorted aggregate with uniform grain� size distribution��B) Clay & aggregate with unsorted grain size� distribution

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Table 7-1. Applicability of different materials for different backfilling concepts.

CONCEPT Bentonites Smectite-bearing clays

Non-swelling clays Fine-rich till

A. In situ compacted mixture of crushed rock and bentonite

(+/-) Not certain, demands further studies. Main problem: salt resistance.

B. In situ compacted swelling clay

(+/-) Not certain, demands further studies. Main problems: poor compaction properties leading to intolerable compressibility.

C. In situ compacted non-swelling clay or other soil type + bentonite blocks on the roof

(+) Suitable material for pre-compacted blocks.

(-) Questionable. Main problems: compaction properties, compressibility, permeability and lack of swelling pressure.

(+/?) Possibly suitable material, if ~ 3-6% of bentonite is mixed to the material. Demands further studies.

E. Pre-compacted blocks


(+/?) Suitable material for pre-compacted blocks. Demands further studies.

(+/?) Not certain, demands further studies in order to determine whether non-swelling block could be used together with bentonite blocks.

(-) Questionable due to the assumed fragility of the blocks.

F. Sandwich concept (bentonite blocks & crushed rock)


(+/?) Till can possibly be used as a substitute for crushed rock.

G. Compartment concept (bentonite blocks & crushed rock)


(+/?) Till can possibly be used as a substitute for crushed rock.

(+) Suitable (+/-) Questionable, based on previous tests & current knowledge. Requires further studies. (+/?) Possibly suitable, requires further studies. (-) Not suitable.

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Bentonite is a suitable material for block production (for concepts C, D, E & F) because bentonite compacted to high density is supposed to fulfill all the requirements set for the backfilling concepts (assuming that the density of the block at saturation is > 2 000 kg/m3). In addition, bentonite is a suitable material for pellet production (pellets can be used in various concepts to enhance the density of the system). It remains to be questioned whether bentonite can retain its physical properties in repository conditions in concept A (bentonite mixed with crushed rock, 30:70). Generally, most of the commercial bentonites are suitable materials for backfilling of a disposal tunnel, although in the long-term there might be some chemical interaction between the backfill and the buffer if they do not consist of the same material. This requires knowledge on the mineralogy of the potential bentonite clays. The feasibility of in situ compaction of Friedland (or similar) clay in tunnel conditions (concept B) will be studied further to reach higher density and lower compressibility for the material. Friedland-clay is most likely suitable for block production (concept D). The chemical & physical interaction between the buffer and the Friedland-clay needs to be taken into account in further studies. A variety of similar smectite-rich clays occur in Europe (e.g. in Czech and Estonia), but the usage of these clays would require further studies both on the basic material properties (in the foreseen salinity) and on the geology & resources of the source occurrences. Non-swelling clays are not suitable for in-situ compaction due to poor compaction properties (easily leading to unacceptable compressibility) and lack of swelling pressure (concept C). However, it may be possible to pre-compact blocks of non-swelling clay and they could possibly be used together with blocks with swelling ability within concepts D, E & F. This would require further laboratory tests to determine the basic material properties the non-swelling blocks in the foreseen salinity. Fine-rich tills have very unsorted grain size distribution, and hence, low porosity, low compressibility and good compaction properties. Therefore, in situ compacted fine-rich till mixed with small amount of bentonite (3-6%) can be applied in concept C, where the advection in the roof section of the tunnel is limited with bentonite blocks (or pellets). Further laboratory tests in 3.5% salinity would be needed to determine the physical properties of the material in repository conditions. All of the materials discussed in this memo represent natural materials that may have remarkable variations in quality due to the geological limitations of the occurrence. Very few occurrences consist of evenly homogeneous clay. The geologic history of these sediments can be seen e.g. in decreasing smectite content towards the boundary of the formation. Although the producer of these materials tends to homogenize the excavated material, there might still be relatively large variations between different batches. Therefore, it is important to be aware of the geologic limitations of the source occurrence and to develop an effective quality assurance system. Due to the long time-span of the backfilling project, SKB and Posiva should be prepared to change the backfilling material to another during the backfilling operation as a consequence to quality related problems.

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Fetter, C.W. 1994. Applied Hydrogeology, 3rd edition. Prentice-Hall Inc. 691 pages. Fuentes-Cantillana, J.L. & Huertas, F. 2002. Backfill on high active waste repositories: technological considerations. 6th International Workshop on Design and Construction of Final Repositories. “Backfilling in Radioactive Waste Disposal.” ONDRAF/NIRAS Brussels, 11-13 March 2002. Transactions, Session III, paper 8 A. Gardemeister, R. 1975. On engineering geological properties of fine-grained sediments in Finland. Building Technology and Community Development, Publication 9. Technical Research Center of Finland. 91 pages. GLR 1993. Geotechnics of Landfill Design and Remedial Works Technical Recommendations. 2nd edition. Edited by the German Geotechnical Society for the International Society of Soil Mechanics and Foundation Engineering. Ernst & Sohn. 158 pages. Grim, R.E. & Güven, N. 1978. Bentonites: Geology, Mineralogy and Uses. Elsevier Scientific Publishing Company. Amsterdam-Oxford-New York. 256 pages. Gunnarsson, D., Hansen, J., Börgesson, L., Keto, P. & Tolppanen, P. 2003. Backfilling and Closure of the Deep Repository. Assessment of Backfill Concepts for the Deep Repository. Swedish Nuclear Fuel and Waste Management Co (SKB), Stockholm, and Posiva Working Report 2003-77. Posiva Oy. Güven, N. 1988. Smectites. In: S.W.Bailey (editor), Hydrous Phyllosilicates. Mineralogical Society of America. Reviews in Mineralogy 19, pages 497-559. Güven, N. 1990. Longevity of bentonites as buffer material in a nuclear waste repository. Eng. Geol. 28, pages 233-247. Harben, P. W. 2002. The Industrial Minerals Handybook: a Guide to the Markets, Specifications & Prices – 4th ed. Industrial Minerals Information. 409 pages. Henning, K. H. 1972. Mineralogische Untersuchung des eozänen Tones der Lagerstätte Friedland (Bezirk Neubrandenburg). Ber. Deutsch. Ges. geol. Wiss. B Miner. Lagerstättenf. 16 (1971) 1, pages 5-39. Berlin 1972. Herbert, H.J. & Moog, H.C. 1999. Cation exchange, interlayer spacing, and water content of MX-80 bentonite in high molar saline solutions. Engineering Geology 54, pages 54-65. Johannesson, L.-E. 2002. Äspö Hard Rock Laboratory. Manufacturing of bentonite buffer fore the Prototype Repository. Swedish Nuclear Fuel and Waste Management Co (SKB), Stockholm. International Progress Report, IPR-02-19 Johannesson, L-E. & Börgesson, L. 2002. Friedland clay as backfill material. Results of laboratory tests and swelling/compression calculations. Swedish Nuclear Fuel and Waste Management Co (SKB), Stockholm. International Progress Report, IPR-02-50.

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