Physical and geotechnical properties of clay phyllites

Applied Clay Science 48 (2010) 307–318

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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y

Physical and geotechnical properties of clay phyllites E. Garzón a,⁎, P.J. Sánchez-Soto b, E. Romero c a

Department of Rural Engineering, University of Almeria, La Cañada de San Urbano – 04120 Almería, Spain Instituto de Ciencia de Materiales, Centro Mixto C.S.I.C. – US. Avda. Américo Vespuccio 49, Isla de la Cartuja – 41092- Sevilla, Spain c Laboratorio de Geotecnia de la Universidad Politecnica de Cataluña, Departamento de Ingeniería del Terreno, Cartográfica y Geofísica. ETSICCP Universidad Politécnica de Cataluña. C/ Gran Capitán s/n; Edificio D22 – 08034 – Barcelona, Spain b

a r t i c l e

i n f o

Article history: Received 2 March 2009 Received in revised form 18 December 2009 Accepted 28 December 2009 Available online 13 January 2010 Keywords: Phyllites Impermeabilising Collapse Stabilisation

a b s t r a c t An experimental programme is presented with the aim of characterising – from physical, microstructural and geotechnical perspectives – the main properties of compacted clay phyllites. These clay phyllites are widely used as waterproofing material for roofs in the Alpujarra (Andalusia, Spain), as sealing liners in irrigation ponds, and as core material of small earthen zoned dams. An exhaustive physical-characterisation programme on the powder fraction has been followed using X-ray fluorescence (XRF), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), chemical analysis by energy dispersive X-ray spectroscopy (EDX), thermal analysis, particle-size distribution analysis, consistency limits, and density of solid particles. From a microstructural standpoint, mercury intrusion porosimetry (MIP) tests, as well as nitrogen-adsorption tests, were carried out to characterise the pore network and surface area of the material in both natural and compacted states. The geotechnical characterisation programme on the compacted material was focused on the water-permeability and water-retention properties, the volume change on soaking (swelling or collapse), the compressibility on loading, the shearstrength properties, and the mechanical-penetration properties. In this way, an important physical and hydro-mechanical data base is provided, which could help in evaluating the suitability for using this material in a wide range of earthen constructions (liners, road subgrades, embankments, core material in zoned dams). It has been found that the material contains illite, chlorite and quartz as the main components, and feldspar, iron oxide and interstratified illite–smectite as minor ones. Despite the presence of active clay minerals, the compacted material did not display an important swelling on soaking at low stresses, as a consequence of its low specific surface and low water-retention ability. The material exhibited good compaction properties and, consequently, low water permeability plus a stiff response on loading. Nevertheless, despite the low porosity attained on the dry-side compaction, the material underwent some collapse on soaking at stresses greater than 100 kPa. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Phyllites are generally considered to be foliated rocks, their essential components being very fine-grained phyllosilicates and quartz. Occasionally, they may contain calcite (calcareous phyllites). Their colours vary between beige, violet, reddish and black. The abundance of fine grain phyllosilicates gives them a soft feel and the existence of preferential cleavage gives them the property of easily breaking up into thin slabs (Valera et al., 2002). In Sierra Nevada (SE Spain) they form a band of Permo-Triassic materials (slate, marble, and clay phyllites). Likewise, in Sierra Alhamilla (Almeria, Spain) there is a phyllite area in blue, violet, and reddish colours, in which limestone and dolomite separated by a transition area of calco-schists ⁎ Corresponding author. Fax: +34 950015491. E-mail addresses: [email protected] (E. Garzón), [email protected] (P.J. Sánchez-Soto). 0169-1317/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.12.022

can be found. All of these materials belong to the Alpujarride complex. However, the phyllites in Cuevas of Almanzora (Almeria, Spain) are associated with chalk, lime, and dolomites. In other parts of eastern Andalusia, such as the Peluca mountain (Malaga, Spain) or the Baza mountain range (Granada, Spain), the presence of phyllites has also been reported (Gómez-Pugnaire et al., 1978). In vast areas of the Andean mountain range, in South America (Venezuela), this material has also been found (Vázquez et al., 2005). In a variable state of trituration in relation to fault areas, this material is successfully utilised for waterproofing roofs in the Alpujarra and in the Baza regions (SE Spain). It has also been used to seal the Beninar dam reservoir (Almeria, Spain). On this subject, Laird (1999) observed that the water retained in phyllosilicates increased the hydration surface, but this effect occurred only in the outer layers. It should be noted that the Alpujarra phyllites have undergone several en-masse movements as a consequence of sliding and/or flow processes. These movements are conditioned, among

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other factors, by the dipping angle of the slopes, and the low shear strength and the humidity of the material (Alcántara-Ayala, 1999). The occurrence of expansive phenomena may also cause serious problems in arid-climate regions. In such regions, the clay is so dry that even a small amount of water supply can lead to major swelling (Lamara et al., 2005). During the 90s, residential buildings built on clay phyllites (Vicar — Almería, Spain) sustained severe damages due to water uptake. Due to the high variability in the composition of phyllites on which their specific properties depend, as well as on their potential uses, it is necessary to characterise these materials from each region before their application. Only through a good state-of-the-art knowledge of the composition and the hydro-mechanical behaviour of phyllites in each region, it is possible to understand their traditional uses and to try to improve them. This knowledge is also important to assess the properties of phyllites in other regions and to evaluate their potential uses in processes other than traditional ones. In this sense, the present study examines the physical and microstructural properties as well as the main hydro-mechanical behavioural features of phyllites from the district of Berja of the Alpujarra (Almeria, SE Spain). These clay phyllites are commonly used to waterproof roofs and water reservoirs due to their low permeability, their relative low cost, and their abundance. Specifically, this experimental programme focuses on three complementary characterisation studies (physical, microstructural, and geotechnical) carried out on destructured, natural and compacted material. The physical-characterisation study involves the analysis of the mineralogical and chemical composition, the particlesize distribution, and the density of solid particles of the destructured material. The structure of the pore network (pore-size distribution) and the specific surface of the natural and compacted material were analysed using microstructural techniques (mercury intrusion porosimetry and nitrogen-adsorption tests). These microstructural studies were used to upscale hydraulic properties as described by Romero and Simms (2008). Finally, the geotechnical characterisation performed on the compacted material focused on the hydraulic properties (water-permeability and water-retention properties) and the mechanical features, such as the volume-change behaviour on soaking (swelling or collapse), the compressibility on loading, the shear-strength properties, and mechanical-penetration properties. 2. Experimental programme 2.1. Physical characterisation (mineralogical and chemical composition, particle size and remoulded-state characterisations) For the present study, a representative sample of greyish phyllite, easily exfoliable, from a quarry located in Berja (Almería, SW Spain) was considered (Fig. 1). A sample was taken through successive quartering, crushed, lightly ground and sieved to pass 63 μm. This powdered sample was dried at 110 °C during 24h. Aliquots of dried sample (1–2 g) were gently ground using an agate mortar for further analysis. Chemical composition was obtained by X-ray fluorescence (XRF) analysis using a sequential spectrometer (Siemens SRS-3000) and cylindrical compacted samples. X-ray powder diffraction diagrams (XRD) were obtained using a Bruker diffractometer, model D-501. The patterns in disoriented preparations were obtained with Ni-filtered CuKα radiation, graphite monochromator, at 36 kV and 26 mA and a scanning speed of 1° in °2θ min− 1. Morphology of the phyllites was investigated by scanning electron microscopy (SEM, JEOL JSM-5400 equipment) on a sample previously prepared with gold by sputtering, chemically analysed under the SEM using energy dispersive X-ray spectroscopy (EDX, Oxford Link detector Si-Li). Particle-size distribution analyses of this material were carried out (Mastersizer X Laser, Malver Instruments), after sieving and disper-

sion in aqueous medium using sodium hexametaphosphate as dispersant (Sánchez-Soto et al., 2000). The physical properties were complemented with the determination of the consistency limits of remoulded states (liquid and plastic limits), and another particle-size analysis was made with sieve and hydrometer methods, and the density of soil solids (specific gravity of soil solids) using water picnometry, according to the procedures outlined in ASTM Standards (ASTM D 420 – D 5611, 2006). 2.2. Microstructural characterisation (pore-size distribution and specific surface) The mercury intrusion porosimetry (MIP) test was used to gain information on the multiple-porosity network of the phyllite (poresize distribution and specific surface), both in natural and compacted states. This information will be helpful to improve the understanding of the hydro-mechanical macroscopic behaviour of the material, such as the water-retention and water-permeability properties, as well as the collapse or swelling response on soaking. MIP tests were performed on ‘Micromeritics-AutoPore IV’ equipment, attaining maximum intrusion pressures of 220 MPa. The largest and smallest pore sizes that can be measured are 400 μm and 6 nm (Washburn equation), respectively, following a previous study (Romero and Simms, 2008). MIP tests were performed on a natural sample at a dry density of 2.03 Mg/m3 and water content of wo = 1.8%, as well as on compacted samples under the following conditions: samples compacted at Standard Proctor SP energy (ASTM Standard) at water contents of wo = 5% and 9.4%, and sample compacted at Modified Proctor MP energy (ASTM Standard) with wo = 5%. A freeze-drying process (Delage et al., 1982) was used to dry the samples from the initial moisture and to preserve the pore network. A complementary test to analyse the surface area and pore structure of the natural sample was the nitrogen adsorption in pore spaces. A ‘Micrometrics-ASAP 2010’ equipment was used. Specific surface was estimated from Brunauer, Emmett and Teller (BET) and Langmuir surface areas along the adsorption branch. The pore-size distribution and cumulative pore area were estimated following the Barrett, Joyner, and Halenda (BJH) method using the desorption information. Comparisons of the pore-size distribution of the natural sample were performed using the aforementioned desorption information and the intrusion data of MIP. Details on the different data-reduction methods can be found in Webb and Orr (1997). 2.3. Geotechnical characterisation (macroscopic hydro-mechanical behaviour) This macroscopic characterisation programme started with the study of the dynamic-compaction properties of the destructured material. Different dynamic-compaction energies were applied to the material (passing #4 ASTM sieve) at different water contents: 0.30 MJ/m3 for half of the Standard Proctor SP (ASTM Standard); 0.59 MJ/m3 for SP; 1.18 MJ/ m3 for double energy of SP; and 2.69 MJ/m3 for the Modified Proctor MP (ASTM Standard). The objective of these tests was to analyse the work input per unit volume required to bring the destructured material as close as possible to the dry density of the natural state (ρd = 2.03 Mg/m3), as well as to define the optimum water contents, which lead to the highest dry densities for a given energy input. Different mechanical tests were carried out at different compaction states (material passing #4 ASTM sieve) to explore the volumechange behaviour on soaking (swelling or collapse), the compressibility on loading, and the shear-strength properties. Wherever possible, the testing protocols followed ASTM Standards. Oedometer cells were used to describe the compressibility properties of the compacted material at SP on loading at different water contents (loading tests at constant water contents of w = 5% and 9.4%; and loading/unloading paths under saturated conditions). Oedometer

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Fig. 1. Geological sketch of Berja and localisation of the phyllite pits studied in the present work (scale 1/50,000).

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cells were also used to monitor the volume changes undergone by the compacted materials at SP and MP — and starting from the same initial water content wo = 5% on soaking at different constant vertical stresses. The expansive behaviour was studied with the denser specimens (MP) and lower stresses (2.5 kPa). The collapse response was analysed by testing samples compacted at SP and soaked at vertical stresses of σv = 100 kPa and 200 kPa. With respect to the shear-strength properties, two different tests were carried out, namely direct shear tests and uniaxial compression tests under unconfined conditions. Direct shear tests were performed on SPcompacted material at different constant water contents (w = 5%, 7.4% and 9.4%) and under saturated conditions. These tests enabled the determination of the shear-strength envelopes and parameters at different water contents. Uniaxial compression tests were performed on samples compacted at different energies (1/2 SP, SP and 2 SP) and under different constant water contents (ranging from 7.2% to 12.2%). Regarding the hydraulic behaviour, attention was focused on the characterisation of the water-permeability and water-retention properties of the SP-compacted material. The water-retention curve is a useful way to provide information on the hydraulic state of the soil, since it is related to the relative humidity surrounding the material through the psychrometric law (Fredlund and Rahardjo, 1993). In addition, coupling between the mechanical and hydraulic behaviour is provided in most constitutive frameworks through the water-retention curve (Romero and Jommi, 2008). Saturated-water permeability was determined directly on compacted samples at wo = 5% using a rigid-wall permeameter under variable-head conditions and a flexible wall permeameter under constant confining stress and controlled-gradient conditions. Variable-head tests were also performed on destructured samples. Indirect calculations of saturated-water permeability were performed using consolidation test results (loading paths under saturated conditions using oedometer cells). Time evolution of soil deformation was interpreted using a non-linear curve-fitting algorithm to determine the different parameters used in consolidation analysis following Terzaghi's theory. The dial displacement d under an effective stress increment δ σv′ can be theoretically expressed as: d = do +

2h δðσv −uw Þ Uðt; Cv Þ + 2h Cα logðt = t90 Þ 0 Ek

3. Test results: Interpretation and discussion 3.1. Mineralogical and chemical characterisation Fig. 2 shows selected SEM and TEM micrographs of a phyllite sample separated after dispersion in distilled water and after 24 h sedimentation. Layered morphologies, associated to the presence of clay minerals, and quartz grains, with their characteristic morphology, were clearly visible. The XRD analysis of a representative phyllite sample showed a phase composition of mica (muscovite or illite), and chlorite (possibly identified as clinochlore), both clay minerals, quartz and minor feldspars (microcline) and iron oxide (Fig. 3). A semiquantitative assessment of the sample, gave average figures (in mass %) of 50–60% illite, 20–15% quartz, 10–15% chlorite, 5–10% feldspars (microcline) and ca. 5% iron oxide. The presence of a mixed-layer or interstratified phase (possibly illite–smectite or illite–chlorite) as a minor

ð1Þ

where do is the initial compression due mainly to equipment deformability, 2h the thickness of the specimen, Ek′ the drained constrained modulus of elasticity, U(t,Cv) the average degree of consolidation that is a function of time t and the coefficient of consolidation Cv (Lambe and Whitman, 1998), Cα the coefficient of secondary consolidation, and t90 the time required for 90% average consolidation. The fitted parameters Cv and Ek′ were related to water permeability through (γw is the unit weight of water): kw =

1 0 Cv γw Ek

ð2Þ

SMI transistor psychrometers (Woodurn et al., 1993) with an extended measuring range were used to plot the water-retention curve on drying in the total suction range from 0.5 to 80 MPa. The output signal of the transistors was related to the relative humidity (or total suction via the psychrometric law) through a suitable and extended calibration (Mata et al., 2002). Measurements were performed on a SP-compacted sample (passing #10 ASTM) at an initial wo = 9.4%. The material was dried in steps, stored for one day equalisation, weighed and the relative humidity of the air surrounding the soil measured. At the end of the multi-stage drying path, the specimen was weighed, oven-dried and the water contents backcalculated.

Fig. 2. (a) SEM micrograph of a phyllite sample separated after dispersion in distilled water and sedimentation (24 h). (b) TEM micrograph of the same sample.

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Fig. 3. (a) XRD powder diagram of the phyllite sample. The main crystalline minerals identified are M = mica (illite, muscovite), C = chlorite (possibly clinochlore), Q = quartz, F = potassium feldspar (microcline) and O = iron oxide. (b) XRD powder diagram in zone 2 (theta) = 5–30° to show the presence of a mixed-layer or interstratified phase (possibly illite–smectite or illite–chlorite) denoted by asterisks (*), with the relative intensities of each peak.

component could be also mentioned, but it was difficult to identify and quantify at the present. Although the proportion must be low, the influence of this kind of minerals in the physical and geotechnical properties of the phyllites cannot be neglected. The mixed-layer minerals, their proportion, and the pattern of interstratification can be obtained directly from mathematical analysis of the positions and intensities of XRD basal reflections, but it was a complex analysis for additional work. The chemical composition of this sample, determined by XRF (Table 1), was in well agreement with the mineralogical composition as seen by XRD. The results, expressed in oxide percentage, confirms the presence of silica (49%) and alumina (26%) from the silicates and quartz present, besides alkaline elements, mainly potassium oxide associated with illite and microcline content. The iron oxide content stood out at around 10%, as well as calcium and magnesium oxides (both around 3%), the constituents comprising the remaining 4.49% Table 1 Summary of chemical composition of the phyllite sample using XRF. SiO2

Al2O3

Fe2O3

K2 O

CaO

MgO

Na2O

TiO2

49%

26%

10%

4.2%

3.29%

3.02%

2.5%

1.4%

being hardly relevant (Table 1). This chemical composition was checked by EDX analysis, as illustrated in Fig. 4. The retained water content which is lost at 110 °C (with a permanence time of 24 h) was very low, its value ranging between 1– 2 mass %. The mass loss through heating at 1000 °C was 7.02%, which was attributed to the presence of hydroxylated phyllosilicates. Additional thermal analyses traced the evolution of this raw material when subjected to thermal treatment and confirmed the presence of the mineral components. On the other hand the average particle size, determined by laser sedimentation analysis, was 23 μm (particle sizes less than this value in a proportion of 70 mass %). The particle-size distribution by sieve and hydrometer analysis displayed a wide range of sizes from gravel to clay sizes, as shown in Table 2. Consistency limits of the remoulded sample shown in Table 2 correspond to a medium activity soil with a relatively low liquid limit and plasticity index, which could be classified as SC according to the Unified Soil Classification System (ASTM Standard). The density of the solid particles was between 2.75 and 2.82 Mg/m3. This value was possibly related to the main components of the phyllite, such as illite with a density of solid particles between 2.6 and 2.86 Mg/m3 and chlorite (2.6–2.9 Mg/m3) (Lambe and Whitman, 1998).

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Fig. 4. EDX analysis of the phyllite sample.

3.2. Microstructural characterisation MIP was used to estimate the cumulated specific surface of the natural material as a function of the pore size by assuming pores with cylindrical shape (Fig. 5). This figure also includes the cumulative pore area determined from BJH nitrogen-desorption data reduction on natural material. Maximum specific surface values were 6.9 m2/g (MIP) and 7.7 m2/g (nitrogen-desorption data). These values matched well with BET and Langmuir surface areas listed in Table 2. These values were relatively low, although they were in agreement with the low activity, the low liquid limit, and the low plasticity index of the material. Fig. 6 presents the cumulative pore volume vs. pore size established from MIP intrusion results and BJH nitrogen-desorption data on natural phyllites. A wide range of pore sizes were covered when using this complementary information. Maximum intruded void ratio (volume of voids to volume of solids) was about 0.17, which was slightly lower than the void ratio of the natural material (around 0.39 for a dry density of 2.03 Mg/m3). Differences arised due to the non-intruded porosity with entrance pore sizes lower than 2 nm and the non-detectable porosity for pore sizes larger than 400 μm. MIP was also used to find the pore-size density distribution, relating the log differential intrusion curve vs. the pore size, which aids in the visual detection of the dominant pore modes. Fig. 7a shows the pore-size density curve of the natural material (complemented with the BJH nitrogen-desorption data). As observed, the dense natural material displayed a unique dominant pore mode at around 15 nm. On the other hand, the SP-compacted material at wo = 5% appeared to display two dominant modes, one at 100 nm and another at 6 μm. This compaction process led to a microstructure formed by large inter-aggregate pores between clay aggregations or grains, and smaller intra-aggregate pores inside aggregates. A similar double

porosity network has been previously described (Juang and Holtz, 1986; Delage et al., 1996) on compacted materials. On further energy application, the MP-compacted material at the same wo = 5% showed a marked decrease in the volume of macropores, while the micropores

Fig. 5. Evolution of the cumulative specific surface with pore size by mercury intrusion porosimetry and BJH desorption of phyllite samples at their natural state.

Table 2 Particle-size distribution, consistency limits and specific surface. Particle-size distribution (wt.%) Gravel (N2 mm)

Sand (0.074–2 mm)

Silt (0.002–0.074 mm)

Clay (b0.002 mm)

6.3

33.8

25.9

34

Consistency limits (wt.%) LL

PL

PI

25.5

17.2

8.4

Specific surface (m2/g) BET Langmuir LL = liquid limit; PL = plastic limit; PI = plasticity index.

7.6 10.6

Fig. 6. Evolution of the ratio pore volume/volume of solids with pore size by mercury intrusion porosimetry and BJH desorption of phyllite samples at their natural state.

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Fig. 8. Dynamic-compaction curves for different compaction energies. Soil conditions for the natural state.

indicated in the graph (dry density ρd = 2.03 Mg/m3, water content w = 1.8% and degree of saturation Sr = 13%). As reflected in the figure, the destructured material could be easily compacted, and the natural density was recovered by applying an energy corresponding to the SP (maximum ρd = 2.11 Mg/m3 at an optimum w = 9.4% and Sr = 79%). The MP test induced a very dense state, reaching a ρd = 2.25 Mg/m3 at an optimum w = 7.3% and Sr = 81%. Although the compaction allowed easily reached dry densities above the natural condition, it was important to emphasise that this density was achieved with dominant

Fig. 7. Pore-size density (PSD) function of phyllite samples at natural and compacted (SP) states. (a) Mercury intrusion porosimetry results compared to BJH desorption data in samples at their natural state. Effects on PSD at different compaction energies (SP and MP) and w = 5%. (b) PSD of SP compactions at w = 9.4% compared to natural state.

were not affected, as also reported by Romero and Simms (2008). As illustrated in the figure, the compaction process was unable to bring the material at its natural state, which displayed a much lower dominant pore size. When compacting at SP at larger water contents (wo = 9.4% in Fig. 7b), the dominant pore sizes were slightly shifted towards larger values (around 185 nm) compared to the dominant sizes of the drier states (around 100 nm). As observed in Fig. 7b, the SP compaction at a wetter state brought the pore-size density distribution to a unique dominant pore size. It appeared that the softer clay aggregations fused together on compaction, somewhat reducing the inter-aggregate macroporosity. 3.3. Mechanical characterisation (volume-change behaviour and shear strength) Fig. 8 presents the dynamic-compaction diagram of the material for the different input energies per unit volume considered. Lines of constant degree of saturation (volume of water to volume of pores) were also included in the plot. As a reference, the natural state was

Fig. 9. Void ratio evolution along loading, unloading and soaking paths at constant vertical stress (10, 100 and 200 kPa) performed under oedometer conditions on SPcompacted samples.

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pore sizes larger than those found for the natural condition (see previous section). The results of oedometer tests on SP-compacted samples at wo = 5% (dry of optimum) in terms of void ratio (void volume per volume of solids) and vertical stress, are included in Fig. 9. The stress paths followed were: loading under constant water content (the initial one); soaking under a constant vertical stress (σv = 10, 100 or 200 kPa); and finally, loading and unloading under saturated conditions. The dry materials displayed a lower compressibility on loading. On soaking at constant vertical stress, the dry material underwent compressive irreversible strains, which increased with greater vertical stress. The soaked material tended to reach the compressibility curve of the compacted material initially saturated at low stress. As discussed by Barden et al. (1973), there were different conditions for this wetting-induced volume decrease – known as collapse – to occur: a) an open partly unstable fabric; b) high enough total stresses; c) the loss of water menisci that stabilise the structure when dry; and d) the addition of water. Each of these must be present to produce a collapse phenomenon, the degree to which each was present influences the resulting collapse observed. Accordingly, the irreversible collapse observed in Fig. 9 was associated with this loss of the initial structure (contact between aggregates or grains), which was maintained by the initial suction and was associated with the cementing effect of water menisci (Alonso et al., 1987). Usually, larger stresses were related to higher collapses due to the destabilising effect of total stress that causes the structure to be metastable, as previously mentioned. As observed in Fig. 9, the magnitude of the volumetric strain change on collapse was around 2.63% at 100 kPa and larger (8.65%) at 200 kPa. Nevertheless, this trend could revert when at high stresses (larger than 200 kPa) an elevated degree of saturation was attained on loading previous to the flooding stage, which was not the case of the relatively low stresses used in the experimental programme. Fig. 10 shows an equivalent plot comparing the response in oedometer tests of SP-compacted materials at two different water contents: wo = 5% (dry of optimum) and wo = 9.4% (optimum conditions). Soaking was induced at a constant vertical stress of 200 kPa, after which the samples were loaded and unloaded under saturation. As shown in the figure, the wetter sample at optimum water content did not display any collapse on soaking. This was

Fig. 10. Void ratio evolution along loading, unloading and soaking paths at constant vertical stress (200 kPa) performed on SP-compacted samples at two water contents (w = 5% — dry of optimum, and w = 9.4% at optimum).

consistent with results of collapse tests performed at different initial water contents, and discussed by Lawton et al. (1992), in which collapse was detected only in dry-side compacted samples subjected to heavy stresses. It was expected that the dry-side compaction induces a relatively open and partly unstable aggregated structure with dominant inter-aggregate pores, as shown in Fig. 7a (double porosity network with macropores at around 6 μm). On wetting at high enough stress, this partially saturated and open structure tends to present a larger collapse. On the other hand, the sample prepared at optimum conditions displays a stable matrix-type fabric with a dominant pore size at a lower size of 185 nm (Fig. 7b), which was not sensitive to the addition of water at high stresses. Fig. 11 summarises the evolution of the compressibility on loading and unloading for different hydraulic states (constant w = 5% and w = 9.4%, and saturated states). The compressibility on loading of the material increased with stress and levelled off at high values, indicating that ‘virgin’ (post-yield) conditions were achieved. The maximum ‘virgin’ compressibility values were found under saturated conditions, which were higher than at the driest state. ‘Virgin’ compressibility average values on loading under saturated and dry conditions were indicated in the figure. The higher compressibility under saturation was consistent with the increasing collapse observed with stress in Fig. 9, where on soaking the compressibility curve of the dry material tends to the saturated (and more compressible) one. Compressibility values on unloading, which were associated with the pre-yield reversible response, were somewhat lower as indicated in the figure. This information was useful for modelling purposes of the deformational response on loading and wetting (collapse) using an elastoplastic framework (see for example, Romero and Jommi, 2008). Fig. 12 shows the volume-change response undergone by the dryside compacted material on soaking at constant vertical stress and under oedometer conditions. The figure showed the development of swelling strains of a dry-side compacted material at MP and at low vertical stresses. Despite the presence of some clay minerals (illite, and chlorite) and the interstratified phase, the densely MP-compacted phyllite did not display notable swelling on soaking at low stresses, as a consequence of its low specific surface. Nevertheless, this expansivity limits its use in earth constructions were low stresses were

Fig. 11. Evolution of compressibility in loading/unloading stages as a function of vertical stress for different hydraulic states (partially saturated and saturated states).

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Fig. 12. Volumetric strains developed on soaking at different stresses and initial conditions.

envisaged, such as road subgrades. On the other hand, the SPcompacted material on the dry side experiences collapse strains when soaked at stresses exceeding σv = 100 kPa (this value could be seen us an upper bound of the swelling pressure of the SP-compacted material). Despite the good compaction properties of the material and the low porosity attained on dry-side SP compaction, the material still presents a well-developed macroporosity with relatively large pores (Fig. 7a). The presence of this macroporosity contributed to the development of collapsible strains, as previously discussed. Direct shear tests performed on the dry-side SP-compacted material at different hydraulic states (constant w = 5% and saturated conditions) are reported in Fig. 13, in which the values of the shear-stress evolution are plotted against the horizontal displacements for different vertical stresses (σv = 50, 100 and 200 kPa). The saturated material displayed a monotonic increase of the shear stress that levelled off at horizontal displacements greater than 3 mm (a typical behaviour of ductile materials). On the contrary, the driest state showed a monotonic increase up to an intermediate peak (horizontal displacements between 1 and 2 mm), followed by a shear-stress reduction that appears to level off at displacements larger than 5 mm (a typical behavioural feature of a fragile material). Maximum shear stresses were systematically greatest at the driest state. In this regard, Fig. 14 summarises the maximum shear stresses for different vertical stresses and hydraulic conditions. The envelopes were plotted in terms of the linear Mohr–Coulomb criterion. An increase in the shear strength was clearly observed at lower water contents. This increase was a consequence not only of the total cohesion variation, but also of an increase in the internal friction angle, which varied between ϕ′ = 24° under saturated conditions and ϕ′ = 32° at the driest state. The variation of total cohesion for the different hydraulic states is reported in Fig. 15. Instead of plotting its variation against water content, it has been represented against the estimated suction using the fitted waterretention drying curve indicated by Eq. (3) (being a = 200 MPa, n = 0.34, m = 0.48, α = 0.92 MPa− 1 and wsat = 13.8% for the SPcompacted material), which is shown in Fig. 17. No levelling off of the total cohesion was found at high suctions (low water contents), attaining a maximum value of c = 76 kPa at w = 5% (suction of around 32 MPa). Suction has been preferred as a measure of the hydraulic state for representing this variation since it could be easier related to

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Fig. 13. Evolution of shear stress with horizontal displacement in direct shear tests performed on SP-compacted material (samples under saturated and constant w = 5% conditions).

the relative humidity of the environment through the psychrometric law (Fredlund and Rahardjo, 1993). In addition, models that incorporate this shear-strength variation with the hydraulic states are usually formulated in terms of suction (the suction-dependent elasto-plastic model proposed by Alonso et al., 1990). Fig. 16 complements the information of the uniaxial compression strength with water content for different compaction energies. The main dependence of the compressive strength was related again to the hydraulic state. In this case, water content has been preferred rather than suction to represent its variation. Eq. (3), related to the water-retention curve described in the next section, was useful to translate this dependency in terms of suction. Lower compressive

Fig. 14. Shear-strength envelopes at different water contents. SP-compacted material.

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Fig. 15. Variation of total cohesion with estimated suction using data from Fig. 17.

strengths were systematically detected at higher water contents. The evolution of the compressive strength of compacted soils with water content (or suction) and its relationship with tensile strength has been discussed by Peters and Leavell (1988). 3.4. Hydraulic characterisation (water-retention and water-permeability properties) Fig. 17 shows the water-retention properties on drying of a SPcompacted phyllite at wo = 9.4%. MIP results were also used to determine the relationships between suction and water content. The injection of non-wetting mercury was assumed to be equivalent to the ejection of water by the non-wetting front advance of air (drying branch) for the same diameter of pores being intruded. Therefore, the volume of pores not intruded by mercury – assuming a nondeformable soil – was used to evaluate the water content corresponding to the equivalent in applied suction, as proposed by Romero and Simms (2008). SMI measurements were consistent with those using MIP data for the SP-compacted material. The waterretention properties of the MP-compacted material estimated from MIP data were slightly shifted towards lower water contents in the low suction range, due to the lower water-storage capacity of the

Fig. 16. Variation of uniaxial compression strength with water content and different compaction conditions.

Fig. 17. Water-retention curves for natural and compacted states.

denser material. At elevated suctions it was assumed that both curves tend to be equivalent to water-retention properties (Romero et al., 1999). Test results were fitted using a modified form of the van Genuchten's equation (1980), in which water content w was defined as a function of suction s according to Romero and Vaunat (2000):    m ln 1 + as w 1 = CðsÞ ; CðsÞ = 1 − lnð2Þ wsat 1 + ðα sÞn

ð3Þ

Fig. 18. Evolution of water permeability under saturated conditions as a function of void ratio. Direct methods and indirect estimations (back-analysis from consolidation data and estimations from mercury intrusion porosimetry). Fitted curve.

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Parameters n, m and α are the same as used in van Genuchten's expression, and wsat represents the water content stored under saturated conditions. The expression forces the curve to be linear in a semi-log scale in the high suction range. Parameter a represents the intersection with the y-axis at null water content of this linear part. A non-linear curve-fitting algorithm using least-squares method was used to determine parameters n, m, and α, assuming a = 200 MPa and wsat = 13.8% for the SP-compacted material and wsat = 10% for the MP-compacted sample. Fitted parameters were n = 0.34, m = 0.48, and α = 0.92 MPa− 1 for the SP-compacted material and α = 0.383 MPa− 1 for the MP-compacted sample. As previously indicated, Eq. (3) was very useful in describing the hydraulic state of the compacted material both in terms of water content or suction (an intensive stress state variable frequently used in elasto-plastic constitutive frameworks). The variation in water permeability kw with void ratio e is presented in Fig. 18 for both compacted and destructured states using different direct and indirect methods. Maximum permeability values corresponded to the destructured state (powdered sample at e = 1.13): kw = 10− 7 m/s. When compacted, the water permeability decreases below kw = 10− 8 m/s (at around e = 0.5), reaching a very low value of kw = 10− 10 m/s at around e = 0.2. An expression to account for this variation with void ratio in the range 0.2 b e b 0.5 is given in the figure. MIP data was also used to estimate indirectly the water permeability. The model was based in the Hagen–Poiseuille equation for laminar flow through a cylindrical capillary of known diameter (García-Bengochea et al., 1979; Lapierre et al., 1990). To simplify the calculation, only cross-sections with identical pore-size density functions were considered to be connected in a correlated way (Romero and Simms, 2008), according to the following expression: kw ðxm Þ =

ρw gn xm 2 ∫ x f ðxÞdx 32μ 0

ð4Þ

where ρw is the water density, g the gravitational acceleration, μ the coefficient of absolute viscosity of water, n the porosity of the soil, f(x) the pore-size density function and xm the maximum pore size. In this way, water permeability was mainly related to the distribution of the larger pore sizes. A good agreement between predicted water-permeability values using indirect methods (back-analysis from consolidation tests), estimated values using MIP results, and direct determinations with controlled-gradient tests is reflected in Fig. 18. The material presented relatively low permeability (below kw = 10− 8 m/s) when compacted, evidencing the suitability of the compacted phyllites as sealing liners in irrigation ponds and as core material for earth-zoned dams. 4. Conclusions An experimental programme was undertaken and a substantial data base was established aimed at characterising – physically, microstructurally, and geotechnically – the main properties of compacted clay phyllites from SE Spain in view of their potential use as materials in a wide range of earth constructions (liners, road subgrades, embankments and core material in zoned dams). These phyllites contain muscovite, illite, chlorite, quartz and iron oxides, as major components, besides feldspars and an interstratified phase as minor components. The natural material displays a relative dense state (dry density 2.03 Mg/m3) with a dominant pore size of around 15 nm (MIP), which leads to low water permeability under natural conditions. When compacted, the material easily reaches quite high dry densities, even higher than under natural conditions (around 2.11 Mg/m3 for Standard Proctor and around 2.25 Mg/m3 for Modified Proctor). Nevertheless, on dry-side compaction, these elevated dry densities are achieved with a completely different pore-size distribution with

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larger inter-aggregation pore sizes (around 6 μm). Despite the larger void sizes, the compacted material still displayed low water permeability with values lower than 10− 8 m/s, adequate when using this compacted material as sealing liner in irrigation ponds and as core material of small earth-zoned dams. Mechanically, the compacted phyllites are quite rigid when subjected to loading/unloading paths under partially saturated and saturated conditions. Despite this adequate rigidity on loading and shear-strength properties under partially saturated conditions, they are slightly sensitive to the addition of water. At vertical stresses greater than 100 kPa, the SP-compacted material displayed some collapse as a consequence of the availability of large inter-aggregate pores in its structure. On the contrary, at MP-compacted conditions the material also displayed some expansion at low stresses, which was not so important due to its reduced specific surface and low water-retention capacity. Acknowledgements The authors thank the contribution of Dr. Antonio Ruiz-Conde from the ICMS (CSIC-US) in the X-ray diffraction work and interpretation of results. Partial financial support of research project CTQ2008-00188/BQU, Junta de Andalucía through project TEP-02550 and Research Group TEP-204 are acknowledged. References Alcántara-Ayala, I., 1999. The Torvizcon, Spain landslide of February 1996: the role of lithology in a semi-arid climate. Geophysica Internacional 38, 1–3. Alonso, E.E., Gens, A., Hight, D.W., 1987. Special problem soils. General report. Proceedings of the 9th European Conference on soil Mechanics and Foundation Engineering, Dublín, Vol. 3, pp. 1087–1146. Alonso, E.E., Gens, A., Josa, A., 1990. A constitutive model for partially saturated soils. Geotechnique 40 (3), 405–430. ASTM D 420 – D 5611, 2006. Construction: Soil and Rock, Vol. 04.08. ASTM, Philadelphia, USA. Barden, L., McGown, A., Collins, K., 1973. The collapse mechanism in partly saturated soil. Eng. Geol. 7, 49–60. Delage, P., Tessier, D., Audiguier, M.M., 1982. Use of the cryoscan apparatus for observation of freeze-fractured planes of a sensitive Quebec clay in scanning electron microscopy. Can. Geotech. J. 19, 111–114. Delage, P., Audiguier, M.M., Cui, Y.J., Howatt, M.D., 1996. Microstructure of a compacted silt. Can. Geotech. J. 33, 150–158. Fredlund, D.G., Rahardjo, H., 1993. Soil Mechanics for Unsaturated Soils. John Wiley & Sons, Inc, New York. García-Bengochea, I., Lovell, C.W., Altschaeffl, A.G., 1979. Pore distribution and permeability of silty clays. J. Geotech. Eng. ASCE 105 (7), 839–856. Gómez-Pugnaire, M.T., Sassi, F.P., Visona, D., 1978. Sobre la presencia de paragonita y pirofilita en las filitas del complejo Nevado-Filabride en la Sierra de Baza (Cordilleras Béticas, España). Bol. Geol. Min. 89, 468–474. Juang, C.H., Holtz, R.D., 1986. Fabrics pore distribution and permeability of sandy soils. J. Geotech. Eng. Div. ASCE 112 (GT9), 855–868. Laird, D.A., 1999. Layer charge influences on the hydration of expandable 2:1 phyllosilicates. Clays Clay Minerals 51, 630–636. Lamara, M., Derriche, Z., Romero, E., 2005. Case study of swelling induced damage and characterisation of an arid climate soil. Proceedings of International Conference on Problematic soils, Eastern Mediterranean University, Famagusta, N. Cyprus, pp. 25–27. Lambe, W.T., Whitman, R.W., 1998. Mecánica de suelos. Editorial Limusa S.A, México. Lapierre, C., Leroueil, S., Locat, J., 1990. Mercury intrusion and permeability of Louiseville clay. Can. Geotech. J. 27, 761–773. Lawton, E.C., Fragaszy, R.J., Herington, M.D., 1992. Review of wetting-induced collapse in compacted soil. Journal of Geotechnical Engineering, ASCE 118 (9), 1376–1394. Mata, C., Romero, E., Ledesma, A., 2002. Hydro-chemical effects on water retention in bentonite–sand mixtures, in: Proc. 3 rd Int. Conf. on Unsaturated Soils, Recife, Brazil. Unsaturated Soils, vol. 1. Jucá, de Campos & Marinho (eds.). Swet & Zeitlinger, Lisse, pp. 283–288. Peters, J.F., Leavell, D.A., 1988. In: Donaghe, R.T., Chaney, R.C., Silver, M.L. (Eds.), Relationship between tensile and compressive strengths of compacted soils, ASTM STP 977 ‘Advanced triaxial testing of soil and rock’. American Society for Testing and Materials, Philadelphia, pp. 169–188. Romero, E., Jommi, C., 2008. An insight into the role of hydraulic history on the volume changes of anisotropic clayey soils. Water Resour. Res. 44, W12412. doi:10.1029/ 2007WR006558. Romero, E., Simms, P.H., 2008. Microstructure investigation in unsaturated soils: a review with special attention to contribution of mercury intrusion porosimetry and environmental scanning electron microscopy. Geotech. Geol. Eng. 26 (6), 705–727.

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