Geomechanical characterization of the Miocene ...

Engineering Geology 214 (2016) 79–93

Contents lists available at ScienceDirect

Engineering Geology journal homepage: www.elsevier.com/locate/enggeo

Geomechanical characterization of the Miocene Cuitzeo ignimbrites, Michoacán, Central Mexico A. Pola a,⁎, J. Martínez-Martínez b, J.L. Macías c, N. Fusi d, G. Crosta d, V.H. Garduño-Monroy e, J.A. Núñez-Hurtado f a

Escuela Nacional de Estudios Superiores, Universidad Nacional Autónoma de México, Unidad Morelia, Geociencias, Antigua carretera a Pátzcuaro 8701, 58190 Morelia, Michoacán, Mexico Laboratorio de Petrología Aplicada (Unidad CSIC-UA), Department of Earth and Environmental Sciences, Universidad de Alicante, Campus Sant Vicent del Raspeig, Alicante, Spain c Instituto de Geofísica, Universidad Nacional Autónoma de México, Unidad Michoacán, Antigua carretera a Pátzcuaro 8701, 58190 Morelia, Michoacán, Mexico d Dept. of Earth and Environmental Sciences, Università degli Studi di Milano-Bicocca, Piazza della Scienza 4, 20126 Milano, Italy e Instituto de Investigaciones en Ciencias de la Tierra, Universidad Michoacana de San Nicolás de Hidalgo, Edificio U, Ciudad Universitaria, 58000 Morelia, Michoacán, Mexico f Posgrado en Geociencias y Planificación del Territorio, Instituto de Investigaciones en Ciencias de la Tierra, Universidad Michoacana de San Nicolás de Hidalgo, Edificio U, Ciudad Universitaria, 58000 Morelia, Michoacán, Mexico b

a r t i c l e

i n f o

Article history: Received 19 April 2016 Received in revised form 5 October 2016 Accepted 9 October 2016 Available online 11 October 2016 Keywords: Ignimbrite lithofacies Geomechanical characterization Microtextural characteristics Mechanical behaviour

a b s t r a c t Volcanic rocks, especially ignimbrites, exhibit complex mechanical behaviours due to their large variation in welding, porosity, textural and granulometrical characteristics. Moreover, there is a lack of knowledge of how alteration processes affect their intrinsic properties. Main objective of this research is to characterize the physical, mechanical, and hydraulic behaviour of different lithofacies of the Cuitzeo ignimbrites (Mexico). These volcaniclastic mixtures of lithics and pumice supported by a fine-rich matrix are characterized by different textural and granulometric features. A general lithostratigraphic section and geological mapping of the study area were accomplished to attain the temporal and spatial distribution of the ignimbrites. Strength, deformation and the failure mode of selected lithofacies were directly related to micro-textural and granulometric characteristics by a series of uniaxial and pre- and post-failure non-destructive analyses (e.g. ultrasonic pulse velocity and X-ray image tomographies). Results for four different lithofacies were compared, allowing us to explore the variation in the mechanical properties associated with characteristics derived from diagenesis. Petrophysical properties show that there is a large dependence on the textural characteristics and particularly on the pumice fragments content. Results derived from laboratory observations and X-ray image reconstruction analysis, show that the main differences in the average values of porosity are associated to the geometry and morphology of the grains and pore network. These are closely related to the stress – strength relationship and the mode of failure of each specimen. In general, the strength for all lithofacies tends to increase linearly with several of the physical properties (e.g. ultrasonic pulse velocity, bulk density). Slake durability tests demonstrate that a large part of the mechanical degradation could be attributed to swelling during wetting-drying cycles. This decay is significant in lithofacies with groundmass rich in expansible clays. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The physical and mechanical characterization of rocks is required to build both conceptual and numerical model of the natural phenomena (e.g. pyroclastic density currents; landslides, volcanic instabilities, groundwater flow and heat transport). Several insights into the behaviour of rocks have emerged from laboratory tests (e.g. uniaxial and triaxial compressive strength, total and effective porosity, P and S wave velocity measurements, e.g. Hudyma et al., 2004; Sousa et al., 2005; del Potro and Hürlimann, 2009; Marques et al., 2010; Pola et al., 2012, 2014). Empirical correlations among rock properties have been used to estimate other parameters (e.g. Vernik et al., 1993; Ulusay et al., ⁎ Corresponding author. E-mail address: [email protected] (A. Pola).

http://dx.doi.org/10.1016/j.enggeo.2016.10.003 0013-7952/© 2016 Elsevier B.V. All rights reserved.

1994; Avar et al., 2003; Schöpfer et al., 2009; Baud et al., 2014). These correlations are based on the observation that some specific set of rock characteristics controls both mechanical (e.g. strength, elastic moduli) and physical properties (e.g. elastic wave velocity, Chang et al., 2006; Wong et al., 1996; Stanchits et al., 2003). On the contrary, there is an enormous lack of knowledge about how alteration affects the intrinsic properties of a rock. For example, alteration could drastically change the geometry and morphology of the grain framework and pore network influencing the mechanical and hydraulic behaviour of the altered material (e.g. Frolova et al., 2014; Thörn et al., 2014). The main objective of this paper is to describe the physical and mechanical behaviour of the Cuitzeo ignimbrites (Mexico) and some of their lithofacies. These volcaniclastic mixtures of lithics and pumice supported by a fine-rich matrix are characterized by different textural and granulometric features. To attain the temporal and spatial distribution

80

A. Pola et al. / Engineering Geology 214 (2016) 79–93

of these ignimbrites at a regional scale, both a lithostratigraphic section and geological mapping were carried out. The results cast new light on the reconstruction of conceptual models and scenarios of natural phenomena (e.g. landslides) occurring in the area. 1.1. Study area The study area is located in the northern part of the State of Michoacán (west-central Mexico), and it covers approximately 250 km2 close to the boundary with the State of Guanajuato (Fig. 1) and it is part of the Michoacán-Guanajuato Volcanic Field (MGVF, Gadow,

1930; Luhr and Carmichael, 1985; Hasenaka and Carmichael, 1987; Hasenaka et al., 1994; Newton et al., 2005; Pérez-López et al., 2011; Guilbaud et al., 2011; Pola et al., 2015). The area is bounded by the Cuitzeo lake to the north and the Quinceo-Tetillas volcanic complex (QTVC) to the south. The monogenetic MGVF is composed of more than one thousand volcanic edifices (Hasenaka and Carmichael, 1987) displaying a wide range of eruption styles, from magmatic events that produced cones and lavas to phreatomagmatic events that produced maars, tuff cones and rings (e.g. Ferrari et al., 2000; Ego and Ansan, 2002; Blatter and Hammersley, 2010; Pola et al., 2015). The hilly landscape is made of the Cuitzeo ignimbrites and lava flows and other

Fig. 1. a) Illustrative map of the central part of Mexico. The trace of Michoacán state, the TMVB, the MGVF, and the most representative cities, as well as studied outcrops are included; b) map of Cuitzeo volcanic zone. Images were obtained by merging of two SPOT images (multispectral and panchromatic with 10 and 2.5 m of resolution, respectively). Abbreviations are: Ct = Cuitzeo del Porvenir; CtL = Cuitzeo lake; SA = San Agustín del Maíz; Mo = Morelia; Ta = Tarímbaro; Cp = Copandaro de Galeana; Cu = Cuto; Tj = Téjaro; QTVC = Quinceo-Tetillas Volcanic Complex; Ch = Chucándiro.

A. Pola et al. / Engineering Geology 214 (2016) 79–93

pyroclastic and volcaniclastic deposits. Some of these have been dated at ~19–20 Ma by the 40Ar/39Ar method (Cisneros-Máximo, 2016). The most representative outcrops are located near the Guadalajara-Mexico City highway on the southern shore of the Cuitzeo lake (Figs. 1 and 2). These outcrops extending for several kilometres, allowed the reconstruction of a general stratigraphy for the area. Within the Ignimbrite sequence, several lithofacies display different textural characteristics and also different degrees of alteration and consequently different mechanical behaviour. This provides an excellent scenario to study the evolution of the physical-mechanical properties of porous and weak volcanic materials.

81

Laboratory of the University of Alaska at Fairbanks, USA). Stratigraphy of the study area was constructed on the basis of sections correlation as suggested by 40Ar/39Ar geochronology and geomorphological analyses and of available literature (e.g. Israde-Alcántara and Garduño-Monroy, 1999; Garduño-Monroy et al., 2009; Gómez-Vasconcelos et al., 2015; Pola et al., 2015; Cisneros-Máximo, 2016). Forty blocks of rock (~50 × 50 × 15 cm in size) have been collected during fieldwork from three different lithofacies. 2.2. Mineralogical and petrological characterization

The geologic mapping and the stratigraphic reconstruction of the Cuitzeo ignimbrite were carried out by geological fieldwork (Fig. 2). Once the general stratigraphy was established, the most significant lithofacies (textural features) were identified and petro-physically characterized. All laboratory tests were performed following the procedures established in the international standards (e.g. ISRM, ASTM) and these will be accurately discussed in the following paragraphs. Several thematic maps were produced by processing DEM data with 20 m ∗ 20 m cell resolution) (1:50.000 scale, INEGI, 2010). In addition, two SPOT images (multispectral and panchromatic with 10 and 2.5 m of resolution, respectively) were used for more specific analyses.

Samples were examined by means of optical microscopy (POM, Zeiss Axioskop polarizing microscop) and X-ray powder diffraction (XRD, Bruker D8-Advance diffractometer). Samples were powdered using an agate mortar down to an average particle size of 15 μm, approximately. The diffractometer is equipped with an X-ray Kristalloflex K 760-80F (3000 W, voltage: 20–60 KV and current 5-80 mA) generator with a tube with copper anode. Clay mineral analysis involved the separation of a clay-sized fraction (b2 μm) from the sample by centrifuging - decanting methods. The clay fraction was collected on a filter and subsequently transferred to a glass slide substrate (filter peel method). This method enhances the preferred orientation of the platy clay particles, which helps to obtain a good diffraction signal from the diagnostic basal planes of the clay minerals (Poppe et al., 2001a, 2001b).

2.1. Fieldwork

2.3. Rock texture and structure

The thematic maps and mosaic of SPOT images were used to trace the geologic contacts and structural features in a preliminary geologic map to support field reconnaissance. Thirty-five stratigraphic sections (Figs. 2 and 3) have been described following the terminology proposed by Branney and Kokelaar (2002). Rocks, sediments and paleosols were collected for laboratory analyses, including 40Ar/39Ar radiometric dating (performed at the Geochronology

Rock texture of each lithofacies, was studied by means of 2D scanned image analysis of thin sections using ImageJ and JMicroVision softwares (http://imagej.nih.gov; http://www.jmicrovision.com). This included the count of pumice and lithic elements, the estimate of matrix content and the analysis of their relative arrangement. Rock structure was studied by means of 3D reconstructions through images obtained from Xray Computed Tomography (micro-CT). In the adopted micro-CT

2. Methodology

Fig. 2. Geological map over a shaded relief model (DEM data with 20 m cell resolution). Red circles indicate the location of each described geological section. Temporal evolution and spatial distribution of each deposit was assigned by stratigraphic correlations and from literature (e.g. Israde-Alcantara and Garduño-Monroy, 1999; Garduño-Monroy et al., 2009; GómezVasconcelos et al., 2015; Cisneros-Máximo, 2016). Abbreviations are: CtL = Cuitzeo lake; SA = San Agustín del Maíz; Ta = Tarímbaro; Cp = Copandaro de Galeana; Cu = Cuto; Tj = Téjaro; QTVC = Quinceo-Tetillas volcanic complex; Ch = Chucándiro.

82

A. Pola et al. / Engineering Geology 214 (2016) 79–93

Fig. 3. General panoramic view of the Cuitzeo ignimbrite sequence at section Cz-7; a) left-central part of the general section, where mpl and spl lithofacies are included; b) left-central part of the general section; a detailed view of mpl and spl lithofacies is included; c) Panoramic view of the top part of the general section, phaneritic dark-greyish, vesiculated, and very fractured lavas; d) hydrothermally altered bands of lavas sequence.

system (BIR Actis 130/150) generator and detector are fixed, while the sample rotates and the scanning plane is horizontal. The 3D reconstruction of each specimen was carried out by means of Avizo software (by Mercury). Effective porosity (ηe) was initially obtained by means of vacuum water saturation test (ISRM, 2007). Samples were dried at a temperature of 70 °C for 48 h until a constant mass was achieved. The dried samples were placed in a vacuum at 20 ± 7-mbar pressure in three 5-hour cycles. The trapped gases in the porous system were eliminated during the first cycle. During the second cycle, samples were slowly introduced into distilled water over a 15-minute period until they were submerged in 5 cm of water. The samples were then submerged during 5 h. Atmospheric pressure was re-established and maintained throughout the last cycle. Some disadvantages of this method are: 1) that complete saturation is seldom reached and consequently the effective porosity is commonly underestimated; 2) that overestimation occurs in samples with high content of clay minerals because of hydration. The pore throat size distribution was quantified on at least 3 specimens for each lithotype using a Micromeritics Autopore IV 9500 mercury porosimeter. The measured pore throat size interval ranges from 0.003 to 200 μm. Bulk density (ρbulk) was calculated from sample mass and total volume. 2.4. Ultrasonic test Compressional and shear wave velocities (Vp and Vs) were measured by means of a signal emitting-receiving equipment (Panametrics-NDT 5058PR) and an oscilloscope (TDS 3012B-Tektronix), which acquired and digitalised the waveforms. Ultrasonic waveform was recorded by direct transmission of a 1 MHz signal in 25 cylindrical samples, using Panametric transducers (5660B, gain 40/60 dB, bandwidth 0.02–

2 MHz). Velocities, under dry and water saturated conditions, and spatial attenuation (αs) were calculated according to Martínez-Martínez et al. (2011). Dynamic Elastic constants (Young's modulus, E and Poisson's coefficient, ν) were obtained according to the following equations (Eqs. (1) and (2)): E ¼ ρbulk Vp2

ð1−2vÞð1 þ vÞ ð1−vÞ

ðVpVsÞ2 −2 Vp2 −2Vs2  i¼  v¼ h 2 ðVpVsÞ2 −1 2 Vp2 Vs2

ð1Þ

ð2Þ

where ρbulk is the bulk density of the material (Machek et al., 2007; Ersoy and Atici, 2007). 2.5. Mechanical characterization Indirect tensile and uniaxial compressive tests were performed following standard procedures (ASTM D2938-95, 2002; ASTM D3148, 2002) on a 25 kN GDS VIS servo-controlled hydraulic testing frame. The height of the cylindrical specimens was sometimes smaller than the recommended (ASTM D 2938, 2002), mainly due to the difficulties in coring larger height (30.70 ± 0.09 mm in diameter and 60.32 ± 10.18 mm in height). Six samples of each lithotype were tested under both uniaxial compression and tensile conditions. Uniaxial compressive strength (UCS) testing on cylindrical samples was performed at a constant displacement rate of 4 mm/h and results are presented in Table 3.

A. Pola et al. / Engineering Geology 214 (2016) 79–93

Splitting tensile tests (ASTM D3967-95a, 2001) were carried out at a constant displacement rate of 6 mm/h on circular disk samples (30.57 ± 0.15 mm in diameter and 9.30 ± 0.77 mm in height). 2.6. Hydraulic characterization Capillarity tests were carried out following standard procedures (UNE-EN 1925, 1999) and those described by Benavente et al. (2015), using a continuous data-recording device due to the high absorption rates of samples. A series of tests at various time steps are required to prepare a complete curve of the capillary-moisture relationship. The capillarity coefficient (Cc), represented by the absorbed water per area of the sample throughout imbibition versus the square root of time (kg/m2·√h), was obtained by measuring the slope of the initial straight portion of the curves. The Coefficients of absorption (Cabs) and desorption (Cds), represented by the absorbed water during imbibition and desorption versus the square root of time (kg·√h), were obtained by measuring the slope of the initial and final straight portions of the curves. 2.7. Durability Slake durability test (ASTM D4644, 1990) was performed on 35 irregular fragments (pieces of about 30–40 g each) by 8 cycles, each one including 10 min in a partially submerged rotating drum and 24 h of oven drying at 65 °C. The slake durability index (Id) was calculated after two, four, and eight cycles as the percentage ratio of final to initial dry weights of rock. Rock durability tests under wetting and drying conditions were carried out following standard procedures (ASTM D5313, 2013). Wet-dry test was carried out on five samples of each lithology. Samples were initially dried at 60 °C for 48 h and then saturated with distilled water for 10 h at atmospheric pressure. During the successive 20 cycles, the samples were oven dried at 65 °C for 14 h after each wetting phase. Sample weight loss was computed after each cycle and a qualitative examination by visual inspection was performed to record any change (e.g. splitting, disintegration, slaking). The wet-dry Index (Iwd) corresponding to the fifth, tenth, and twentieth cycle, was calculated as the percentage ratio of final to initial dry weights of rock. 3. General stratigraphy of Cuitzeo ignimbrites The Quinceo-Tetillas volcanic complex (QTVC), to the south of the study area (Fig. 1b), has been identified as the source of different volcanic materials (e.g. lavas, fall deposits, 1.42 ± 0.12 to 0.33 ± 0.04 Ma), which partially cover the southern portion of the study area (GómezVasconcelos et al., 2015). According to the succession defined in this study, the sequence of the QTVC overlies several ignimbrites alternated with pumice fall deposits and fissural lavas and breccias (Fig. 2). These ignimbrites have been generally called the Cuitzeo ignimbrites (e.g. Pasquaré et al., 1991; Cisneros-Máximo, 2016). The eruptive centre that produced these ignimbrites has not been identified, probably because buried in the Cuitzeo lake or by other volcanic deposits (e.g. QTVC). Most of the outcropping lithofacies (southern shore of Cuitzeo lake, 8 km from San Agustín del Maíz village, Fig. 1b) are deformed, hydrothermally altered and tilted 60° to the north (Fig. 3). Regionally, the tectonic activity is represented by a series of NE-SW oriented faults cutting across the Cuitzeo lake and the east-margin of the Quinceo-Tetillas volcanic sequences (Fig. 1b). These faults promoted hydrothermal activity, modifying several rock characteristics (e.g. colour, texture, mineralogy, porosity, permeability, strength). Hydrothermally altered zones are easily recognizable for their typical spring-green colour (Figs. 2 and 3). Locally, at section Cz-16 (UTM X: 280061; Y: 2194334), and according to statistical structural outcrop data, the tectonic activity is represented by at least two sets of NNW and NE trending fractures cutting the ignimbrite sequences.

83

The Cuitzeo ignimbrites and lavas are composed of at least ten different lithofacies, well-exposed in the Cz-7 outcrop (UTM X: 274872; Y: 2200513) (Fig. 3a–d) and differentiated on the basis of texture, components proportion, welding, and degree of hydrothermal alteration. Some lithofacies such as mpa (Fig. 4a), are composed of different proportions of mm-to-cm size pumice and lithic lava fragments supported by a fine-rich altered matrix composed of glass, lithics, phenocrysts of plagioclase, quartz, amphiboles, oxides and large amounts of montmorillonite. Some lithofacies (e.g. mpl in Figs. 3 and 4b) are massive, whereas others, such as spl (Figs. 3 and 4c, d), are stratified and composed of an alternation of massive and stratified poorly consolidated layers. Lavas are represented by phaneritic dark-greyish, vesiculated, and very fractured rocks with hydrothermally altered bands (Fig. 3c and d). Some light-greyish dykes, composed of porphyritic rocks with plagioclase and pyroxene embedded in an ashy matrix, transect the Cuitzeo ignimbrites. 4. Studied materials As said above, the sequence of incipient welded ignimbrite lithofacies is strongly affected by the extensional tectonic activity, internal deformation, hydrothermal alteration, tilting, and by two sets of NW and NE trending fractures (Fig. 3). Local slope instabilities were recognized at most of the visited outcrops. Irregular and complex blocks (b5 m in size) are typical, limited by a sub-horizontal stratification and the two pervasive and persistent vertical sets of fractures sometimes characterized by normal displacements. Some zones are characterized by widely spaced fractures and intense hydrothermal activity. Failure surfaces of slope instabilities seem related to specific lithofacies characterized by a large content of pumice fragments into a clayey groundmass. Three lithofacies were selected based on their significant textural characteristics (e.g. grain components and grain-size distributions) to study the influence of mineralogical composition and fabric on physical-mechanical properties. The main physical and textural characteristics of such lithofacies are summarized in Tables 1 and 2, and described in subsequent paragraphs. 4.1. Petrographical description and mineralogical contents Samples were described in terms of colour, mineral composition, texture and structure. They are composed prevalently by vesiculated pumice and dense lava fragments, supported in a fine-ash matrix with large amount of clay minerals. 4.1.1. Pumice lithofacies (mpa) This lithofacies, classified as massive lapilli-tuff (Table 2), is composed prevalently of subrounded and deformed pumice fragments (b 8 mm in size) supported in a fine-rich green-ash matrix with small fragments of dense lithics (mm in size) (Figs. 4a and 5). The mpa is poorly sorted and with no stratification or grading, sometimes showing orientated or lens-shaped pumice fragments. Major constituents are plagioclase, amorphous silica, and phyllosilicates (montmorillonite) in the groundmass (Table 2, Fig. 6). Thin-sections show a recrystallized groundmass, composed prevalently of glass and some euhedral to sub-euhedral phenocrysts of fractured and corroded plagioclases, quartz, pyroxenes and Fe-Ti oxides (Fig. 6a). 4.1.2. Lithic and pumice lithofacies (mpl) This massive lithic-lapilli lithofacies (Table 2) is composed of subrounded and vesiculated lithic fragments (light-gray, black, red) and pumice (from mm to cm in size), supported in a fine-rich lightbrown-ash matrix (Figs. 4b and 5). Major constituents are plagioclase, amorphous silica, and phyllosilicates (montmorillonite) (Table 2 and Fig. 6). The groundmass is composed prevalently of deformed glass with some microlites of plagioclase and quartz. Some euhedral and

84

A. Pola et al. / Engineering Geology 214 (2016) 79–93

Fig. 4. Scanned images of samples of the four studied ignimbrite lithofacies. a) mpa sample composed prevalently by pumice fragments, lens-shape pumice; b) mpl sample composed by lithics and pumice fragments; c) spl-a sample composed by lithic and pumice fragments arranged in vertical layers; d) spl-b sample composed by lithic and pumice fragments arranged in horizontal layers. White circles with abbreviations indicate the most representative texture characteristic of each sample. Abbreviations are: Pu = Pumice fragment; lsP = lens-shape pumice; Li = lithic fragment; vl = vertical stratification arrangement; hl = horizontal stratification arrangement.

sub-euhedral phenocrysts of plagioclase are observed, together with quartz, Fe-Ti oxides and argillitic stains in the groundmass and within some altered minerals (Fig. 6b). 4.1.3. Lithic and pumice stratified lithofacies (spl) This stratified lithic-lapilli lithofacies is classified as (Table 2) is composed of subrounded lithic fragments (from mm to cm in size), supported in a fine-rich brown-ash matrix (Figs. 4c, d, and 5). It presents several layers with specific grain-size, grain shape, sorting, and compositional characteristics (Figs. 4c, d, and 5). Major constituents are plagioclase, amorphous silica, calcite, and phyllosilicates (montmorillonite) (Table 2 and Fig. 6). The groundmass consists of very argillized glass with

some microlites of pyroxenes and a large content of very altered lithics. Samples contain phenocrysts of angular plagioclases, pyroxenes, quartz, and Fe-Ti (Fig. 6c and d). Due to the stratified structure of this lithofacies, the petrophysical characterization was carried out on samples oriented parallel and perpendicularly to layering and named spl-a and spl-b, respectively (Fig. 4c and d). 4.2. Rock structure It is well known that mechanical characteristics decrease with increasing porosity (e.g. Sousa et al., 2005; Heap et al., 2009; Siratovich et al., 2014; Heap et al., 2015); in turn, porosity correlates directly to

Table 1 Summary of physical properties of the Cuitzeo Ignimbrite lithofacies. All values are given as an average, with their corresponding standard deviation. Abbreviations are: γ = dry unit weight; ρ = density; Vp = compressional wave velocity Vs = shear wave velocity; αs = spatial attenuation; ηe = effective porosity; Hg = mercury porosimetry; ρM = bulk-specific weight; Cc = Coefficient of capillarity; Cabs = Coefficient of absorption; Cds = Coefficient of desorption. Sample γ

mpa mpl spl-a spl-b

ρ

Waves

αs

ηe (%)

Cabs

Cds

(kN/m3)

(kg/m3)

Vp (km/s)

Vs (km/s)

Vp (km/s)

Vs (km/s)

(dB/cm)

Hg ρ M kg/m2·h0.5

Kg/m2 · √h

Kg/m2 · √h

14.21 ± 0.71 15.74 ± 0.30 13.78 ± 0.14 14.17 ± 0.54

1449.42 ± 73.12 1594.22 ± 46.16 1405.04 ± 14.97 1444.9 ± 55.06

1.49 ± 0.07 1.65 ± 0.06 2.15 ± 0.13 2.02 ± 0.09

0.79 ± 0.06

1.05 ± 0.05 1.46 ± 0.05 2.17 ± 0.05 1.90 ± 0.09

0.54 ± 0.04

27.37 1.11 22.19 1.99 18.57 3.37 16.90 2.60

±

38

32

±

34

24

±

31

30

0.023 ± 0.009 0.011 ± 0.006 0.012 ± 0.001

0.0026 ± 0.0002 0.0016 ± 0.0003 0.0017 ± 0.0002

Waves (wet)

0.933 ± 0.02 1.25 ± 0.15 1.25 ± 0.05

0.76 ± 0.01 1.039 ± 0.07 0.96 ± 0.05

±

Cc

0.51 ± 0.25 7.19 ± 0.74 7.01 ± 1.25

A. Pola et al. / Engineering Geology 214 (2016) 79–93

85

Table 2 Summary of the textural characteristics of the Cuitzeo Ignimbrite lithofacies obtained by means of 2D scanned image analyses using ImageJ and JMicroVision software (http:// imagej.nih.gov; http://www.jmicrovision.com). All values are given as an average, with their corresponding standard deviation. Abbreviations are: Gr = granules (2–4 mm), CoS = coarse sand (0.5–2 mm), MeS = medium sand (0.25–0.5 mm), FiS = fine sand (0.06–0.25 mm), Si = silt (0.06 N mm). Sample

mpa mpl spl-a spl-b

Pumice proportion (%)

Lithics proportion (%)

Gr

CoS

MeS

4.8 ± 0.11 1.02 ± 0.26 0.52 ± 0.08 0.13 ± 0.05

2.96 ± 0.07 1.02 ± 0.22 0.7 ± 0.19 0.36 ± 0.08

1.67 1.35 0.68 0.41

FiS ± ± ± ±

0.23 0.41 0.18 0.10

1.09 1.11 0.44 0.41

± ± ± ±

0.17 0.26 0.08 0.11

Si

Gr

CoS

– – – –

0.15 ± 0.04 0.1 ± 0.05 – –

0.20 0.21 0.03 0.02

permeability, and inversely to the P-wave propagation velocity. In order to establish a robust relationship between these properties (Pereira and Arson, 2013; Pola et al., 2014; Heap et al., 2015) we analyzed the porosity of each sample, pore geometry and spatial distribution and interconnection. As described in several studies (Pola et al., 2012, 2014; Frolova et al., 2014; Wyering et al., 2014) the physical and the mechanical properties of the rocks are largely modified by pervasive hydrothermal alteration processes (e.g. mineral degradation, secondary mineralization, pores filling precipitation). These processes are strongly related to the variation in fluids content and temperature of the rocks, promoting diverse degrees of mechanical alteration and consequently different rock behaviours (e.g. del Potro and Hürlimann, 2009; Coggan et al., 2013; Frolova et al., 2014; Heap et al., 2015). In this study, the main differences among lithofacies were determined on the basis of image analysis and laboratory measurements of effective porosity, pore throat size patterns distribution, bulk density and ultrasonic wave velocity. As shown in Fig. 4, the mpa and mpl lithofacies are massive with a diffuse preferential orientation of the clasts. On the contrary, the spl-a, and b lithofacies are stratified with oriented clasts (e.g. pumice, lithics) forming bands, basically composed of the same constituents in the matrix with minor lithics (Fig. 4 and Table 2). The mpa lithofacies consists, in order of abundance, of fine sand to gravel of altered pumice and minor coarse-ash dense red and gray lava. Pumice diameter ranges between 4 and 0.10 mm, mostly concentrated in 2–4 mm class, which represents the 4.8 ± 0.11% of the total (Table 2). The mpl lithofacies consists of well-distributed fine sand to gravel altered pumice and dense lava with large amounts of silt to fine sand fragments of vesiculated scoria. In the case of this vesiculated scoria, particles with mean 0.10 mm size represent the 1.02 ± 0.04% of the total (Table 2). In turn, the spl lithofacies consists of a little proportion of

MeS ± ± ± ±

0.047 0.06 0.01 0.01

0.49 0.33 0.16 0.04

FiS ± ± ± ±

0.37 0.08 0.09 0.01

0.59 0.45 0.22 0.14

Si ± ± ± ±

0.14 0.09 0.08 0.03

1.02 ± 0.04 1.97 ± 0.66 0.40 ± 0.05 1.0 ± 0.08

pumice and fragments of dense lava and vesiculated scoria (Fig. 4 and Table 2). Unit-weight values (γ) range from 13.78 (spl-a) to 15.74 kN/m3 (mpl) and are in accordance with those reported for similar volcanic rocks (Topal and Doyuran, 1997; Özbek, 2014; Korkanç, 2013; Frolova et al., 2014; Aydan and Ulusay, 2013). They are conditioned by the proportion of matrix, pumice, and lithics contained in each specimen. In particular, values in mpl lithofacies are largely controlled by the proportion of the matrix within the sample. The effective porosity values (ηe) of all lithofacies are very high, varying from 25.35 (mpl) to 32.01% (mpa) after the vacuum water saturation test (Table 1). However, as described in Section 2.3 this method presents several disadvantages, particularly in samples with high content of clay minerals. In fact, in the mpa lithofacies, the ηe value could be wrongly estimated due to the hydration effects of clay minerals and to the large proportion of pores in pumice particles, that are both difficult to be taken into account (Fig. 4). On the contrary, the ηe mercury porosimetry value better reflects the general distribution of pores, principally those contained in pumice samples, with values ranging from 31.20 (spl) to 38.68% (mpa). It is worth to remember that due to the small size of fragments required for the mercury porosimetry analysis, the pore frequency of the entire specimen could not be totally representative. Nevertheless, these analyses carefully identified the micromeso scale pore populations. In general terms, all the lithofacies show similar pore distribution patterns in which, two populations can be easily identified (Fig. 7): the first characterized by radius between 1 and 10 μm and corresponding to the porosity of pumice fragments, and the second one characterized by small sizes, lower than 0.1 μm, corresponding to the porosity of the rock matrix. The pore distribution in the mpa lithofacies is concentrated in the first group with the large

Fig. 5. example of 2D image analysis of the textural characteristics of each lithofacies. Components proportions (e.g. pumice and lithics) are included in Table 2; a) Scanned image of mpa sample, where pumices and lithics are easily identified; b) black lithics distribution as obtained from thresholding of filtered scanned image. Automatic acquisition of values of the spatial and geometrical characteristics of every single grain was performed with Image-J software.

86

A. Pola et al. / Engineering Geology 214 (2016) 79–93

Fig. 6. Photomicrographs of selected thin-sections of the four studied lithofacies showing a) recrystallized and argillized groundmass with altered rock fragments and very fractured plagioclases; b) groundmass prevalently composed by deformed glass with some microlites of plagioclase and quartz; c) Inequigranular groundmass with plagioclase and quartz microlites embedded in interstitial glass; d) plagioclase embedded in an argillized groundmass, where some oxides are visible. Abbreviations are: plg = plagioclase, arg = Al-lh = altered lithics, Ar = argillization, Mi = microlites of quartz and plagioclase.

radius being basically controlled by the large amount of pumice fragments (Table 2). 5. Results and discussion As described above, samples were selected according to their textural characteristics and not only to the specific lithofacies. Rocks were systematically sampled on the field following their orientation with respect to the stratigraphic contact plane. 25 cylindrical specimens of each

lithology (30.70 ± 0.09 mm in diameter and 60.32 ± 10.18 mm in height) were prepared from the large blocks. The obtained results are described, compared and discussed in next paragraphs also in the light of the existing literature on similar lithologies. 5.1. Ultrasonic characteristics Compressional (Vp) and shear wave velocities (Vs) in a rock are controlled by factors like mineralogy, texture, density, grain size and shape,

Fig. 7. Pore size distribution curves of sampled rocks. The light gray areas correspond to the identified pore families within 0.01–10 μm size range.

A. Pola et al. / Engineering Geology 214 (2016) 79–93

porosity, anisotropy, water content, stress, and temperature (Kahraman, 2007). The variation of Vp and Vs in volcanic rocks as a function of textural characteristics, vesicle density, shape, and size distribution, microcrack density and anisotropy, and water saturation, has been investigated in the literature (Topal and Doyuran, 1997; Aydan and Ulusay, 2013; Korkanç, 2013; Heap et al., 2014; Özbek, 2014). Authors suggest that the low measured Vp and Vs of andesites, in comparison to similar rocks (from 2.34 to 3.11 km/s and 1.09 to 1.45 km/s, respectively), correspond more to the large sensitivity to micro-crack porosity than vesicle porosity. This is corroborated by measuring a significant increment of Vp on water-saturated samples. The ultrasonic wave values obtained for the studied rocks (Table 1) are in good agreement with those published for similar rocks (Topal and Doyuran, 1997; Korkanç, 2013; Frolova et al., 2014). The highest and the lowest wave propagation velocities (both Vp and Vs) are computed for the spl and the mpa lithology, respectively. An inverse relationship between ultrasonic wave propagation and both porosity and rock density is observed, probably controlled by the pumice content of each lithofacies, which largely contributes to the vesicle porosity. The same trend is obtained for the spatial attenuation of ultrasonic waves (αs). This parameter measures the wave energy lost during the propagation, and higher values correspond to the most scattered front waves. Scattering is due to the interaction between ultrasonic wave and dispersive elements (e.g. pores, cracks, and dense particles; Martínez-Martínez et al., 2011). The most and less dispersive lithofacies are mpa and spl, respectively. In the case of mpa, the high values of αs are associated to its high porosity (Table 1) and the highest pumice content and larger pumice and lithic fragment sizes (Table 2) among the various lithologies. On the other hand, spl is the least dispersive variety corresponding to both the less porous rock (after Hg-porosimetry, Table 1) and the lowest pumice and lithic contents and sizes (Table 2). Clay content also affects the ultrasonic wave propagation (Schön, 2011), with lower wave propagation velocities for high clay contents. This influence is even more evident when velocities measured under dry and wet conditions are compared (Table 1). Spl shows a small decrease (b100 m/s) or even a slight increase in Vp values. On the other hand, mpl and especially mpa register significant velocity reductions around 200 m/s and N400 m/s, respectively). Similar drops in Vp values of saturated samples have been reported for similar rocks (Topal and Doyuran, 1997; Aydan and Ulusay, 2013; Frolova et al., 2014) and attributed to the swelling of montmorillonite (‘softening of the matrix’ after Schön, 2011). Swelling depends on the clay micro-fabric, the chemical compositions and the type of exchangeable cations (Katti and Katti, 2003; Deriszadeh and Wong, 2014) and these are associated to the behaviour of wave propagation. 5.2. Mechanical properties As extensively described in literature (e.g. Fuenkajorn and Daemen, 1992; Moon, 1993b; Saotome et al., 2002; Yılmaz et al., 2011; Wyering et al., 2014) the mechanical properties for heterogeneous rocks are directly related to their intrinsic characteristics, strongly modified by hydrothermal alteration processes (e.g. Coggan et al., 2013). Again, as for physical properties the mechanical behaviour of rocks is firmly governed by distribution of pores, grain size, and grain bonding characteristics, mineralogy and clay minerals content. Zhu et al. (2011) affirm that tuffs constituted by similar fragments with comparable porosities but different arrangements may show significant variability in strength. Microstructural observations suggest that stress is distributed and concentrated according to the sample textural characteristics inducing for example compaction associated with pore collapse and grain crushing (Zhu et al., 2010; Pola et al., 2014; Özbek, 2014; Heap et al., 2016). The effects of water on mechanical properties, including uniaxial compressive strength (UCS), tensile strength (TS), and elastic modulus (E) are well known (e.g. Morales Demarco et al., 2008; Wong et al.,

87

2016). A significant wakening of the porous structure for saturated tuff samples has been observed (Zhu et al., 2010). Baud et al. (2000) have shown that clay rich rocks are weaker when they are water-saturated with a decrease in UCS for argillitic rocks up to 50–75%. In general terms, ignimbrites are highly porous (42–63%) weak rocks (UCS = 2– 10 MPa), which frequently present high contents of clay minerals replacing volcanic glass (Frolova et al., 2014). Table 3 shows the mechanical results obtained in this study. mpa and spl are the weakest and softest and the strongest and stiffest ignimbrites, respectively. An inverse relationship between porosity and both UCS and Est is found as suggested by several studies (Kahraman et al., 2005; Yildiz et al., 2010; Coggan et al., 2013; Pola et al., 2014; Wyering et al., 2014; Sousa et al., 2005). Moreover, all mechanical properties in our lithologies are controlled by the size, content and distribution of the pumice fragments (Table 2 and Fig. 8), which provides very weak structures where stress is easily concentrated. In particular, the contact between pumices, lithics and matrix in mpa and mpl samples is weaker than the matrix. This is observed in the 3D micro-CT reconstructions of samples (Figs. 8a and b), which show the relationship between the fracture pattern and distribution of pumice and lithic fragments, as well as failed pumice derived from stress concentration (Fig. 8a). The mpa samples, which contain a larger proportion of pumice fragments, have lower compressive strengths (4.33 ± 0.58 MPa) than those with more homogeneous textures such as mpl (8.29 ± 1.23 MPa) and spl (a and b, 9.57 ± 1.32 and 8.93 ± 0.80 MPa, respectively) (Table 2 and Fig. 4). The differences in the peak strength are closely related to the arrangement of the pore throat size patterns and structure, which in turn depends on the matrix and pumice individual characteristics. The strength in spl samples seems to be largely controlled by the thin stratification layers (Table 3). Samples tested with loading direction parallel to layers are stronger and stiffer than those tested perpendicular to them. In general, for anisotropic rocks (e.g. banded, layered or foliated rocks) the highest strength is registered when the sample is loaded perpendicular to banding. This assertion is not completely valid when bands of heterogeneous porous layers (weaker) are placed among more massive ones (stronger). When such bands are oriented parallel to the load, the mechanical behaviour of the whole rock is conditioned by the strength of the low-porosity bands. On the contrary, when the bands are perpendicular, the rock structure fails in favour of its weakest sections (porous bands). Similar behaviours were found for banded anisotropic travertines (García-del-Cura et al., 2011). However, the previous assumption is observed in Fig. 8c and d where a clear control of the fracture pattern and propagation is induced by the geometrical (e.g. strike and dip) and the spatial distribution of the stratification layers. Particularly, the stratification layers in spl-a constitute preferential planes of fracture propagation (Fig. 8c). On the other hand, the strain-strength trend in other samples (e.g. mpl, mpa) depends basically on the distribution of the pore structure and grain arrangement of each layer, which seems to be also affecting the peak strength and the static Young's modulus. In particular, the decrease in strength of the mpa depends principally on the interaction between the border of the pumice and clay rich matrix (Fig. 8a). The stress-strain curves for all tested lithofacies are presented in Fig. 9. All Curves present the typical brittle behaviour under uniaxial compressive loading (e.g. Martin and Chandler, 1994; Eberhardt et al., 1999; Martínez-Martínez et al., 2016). As expected, the slope of the curves (quantified by the static Young's modulus, Table 3) decreases according to the proportion of pumice and matrix clay minerals (Table 2, Figs. 4 and 9). As found for similar rocks (Moon, 1993a; Sonmez et al., 2006), it is expected that crystals, lithics and pumice provide zones of contrasting elastic modulus with the groundmass and so zones of stress and crack concentration. Cracks will tend to initiate at, and propagate away from, such areas. In the case of crystals and lithics, the cracks concentrate around their margins, while in the case of pumice fragments the cracks propagate both around and through the soft clasts (Fig. 8a).

88

A. Pola et al. / Engineering Geology 214 (2016) 79–93

Table 3 Summary of mechanical properties of the Cuitzeo Ignimbrite lithofacies. All values are given as an average, with their corresponding standard deviation. Abbreviations are: UCS = uniaxial compressive strength; Et50 = Young's modulus from UCS; ED = dynamic Young's modulus; υD = dynamic Poisson's ratio; TS = tensile strength; Id2, Id4, Id8 are Slake durability index values after 2, 4 and 8 cycles); Iwd = wet-dry Index after 5, 10 and 20 cycles; Di = disgregation; Fr = fractures; Ldi = light disgregation. Sample

mpa mpl spl-a spl-b

Mechanical properties

Durability tests

UCS

Et50

ED

υD

Ts

Id2

(MPa)

(GPa)

(GPa)

(−)

(Mpa)

(%)

4.33 8.29 9.57 8.93

2.30 4.20 8.20 6.50

2.46 3.54 5.46 5.36

0.30 0.27 0.24 0.19

± 0.58 ± 1.23 ± 1.32 ± 0.80

± ± ± ±

0.50 0.80 0.80 0.70

± 0.29 ± 0.24 ± 1.02 ± 0.58

± ± ± ±

0.04 0.02 0.07 0.02

0.42 1.08 1.07 1.83

± ± ± ±

0.21 0.27 0.49 0.38

Rao and Noferesti (2008) affirm that the behaviour of a very heterogeneous system is controlled by the geometrical, spatial and individual characteristics of the elements arranged in each specimen. In these cases, the interlocking pattern of elements drastically reduces the brittleness of the specimen while the presence of considerable amount of stronger elements strengthens the specimens, and a little amount of weaker elements causes immediate reduction of elastic modulus. We observed a significant difference between static and dynamic Young's modulus (Est and ED, respectively) with Est values in general higher than ED (Table 3). This is well known and it is in accordance with observations that a comparison of Est and ED is significant only if the values of the Est are measured at low strain-stress levels (e.g. Al-Shayea, 2004; Ciccotti and Mulargia, 2004). This difference is explained again by the presence and abundance of defects (e.g. fractures, cracks, cavities, planes of weakness and foliation, Al-Shayea, 2004). The more defects in the rock, the lower the Young's modulus is and the higher the discrepancy between the static and dynamic values. The Est values in our samples seem to be controlled by the size and amount of pumice fragments organized in a complex interlocked porous system. This assumption could be supported by Figs. 8, 9 and Table 2. The slope of the stress-strain curves (Fig. 9) decreases according to the proportion of pumice content (Table 2 and Fig. 8). The mpa sample contains the major proportion of pumice (Table 2) corresponding to the minor slope of stress-strain curve (Fig. 9). As shown in Fig. 10, the comparison between the porosity (ηe, %) and uniaxial compressive strength (UCS, MPa) dataset presented in this paper with literature data (Ottaviani, 1988; Topal and Doyuran, 1997; Tommasi and Ribacchi, 1998; Evangelista et al., 1998; Rotonda et al., 2002; Vásárhelyi, 2002; Pola et al., 2012, 2014; Aydan and Ulusay, 2013; Korkanç, 2013; Özbek, 2014) supports the trend proposed in the literature. According to the welding scale proposed by Quane and Russell (2005), the Cuitzeo ignimbrite lithofacies could be classified as partially to incipiently welded (Fig. 10). This characteristic is evident in mpa specimens where components (principally pumice fragments) are immersed in a soft, poorly made, and chemically transformed matrix. In addition, the Cuitzeo data set shows a compatible trend with a progressive increase in alteration. 5.3. Hydraulic characteristics Hydraulic properties of rock specimens depend on many variables such as the percentage of interconnected pores and the arrangement of microstructures. The permeability is directly related to the confining pressure and largely depends on the stress history. Therefore, it is reasonable to assume that an increment in the stress level causes a reduction of permeability (because of the closure of micro-pores and microfractures) and that this reduction could not be recovered fully if the stress is released. In turn, capillarity is the main mechanism of water retention in the porous system and it is directly related to the pore throat sizes, pore network geometry, and consequently permeability, and to the adhesion forces controlled by the nature of both the solid and liquid. Benavente et al. (2007) suggest that the Capillary coefficient (Cc) is related to the square root of the permeability for homogeneous and

Id4

27.15 84.83 88.42

16.08 69.34 78.98

Id8

6.28 43.98 66.45

Wet-dry test Iwd5

Iwd10

Iwd20 (%)

Decay patterns

22.77 ± 25.54 95.59 ± 0.44 96.39 ± 0.26

0 94.62 ± 0.56 95.89 ± 0.33

0 92.44 ± 1.95 94.56 ± 2.66

Di Fr, Ldi Fr, Ldi

porous rock materials. It is also clear that a porous system characterized by large pores and by high effective porosity will present, generally, the highest hydraulic transport coefficient values (Benavente et al., 2002; Beck et al., 2003; Sengun et al., 2014). As described in Fig. 11a and Table 1, the behaviour of capillarity curves, quantified by Cc, for mpl and spl lithofacies is identical, while for mpa lithofacies is drastically lower. In general, Cc is well correlated to the pore network characteristics of each lithofacies (e.g. pore size and relative proportion). The Cc values of mpa lithofacies are related to the pore size, which according to the mercury porosimeter analyses are mostly concentrated in a range of 4 to 7 μm (Fig. 7). Differences in the absorption-desorption curves between samples are directly related to the clay content and fabric, which in turn control the retained amount of water. This is supported by the curve for the mpa sample with a clay rich matrix, which forces an increment of water absorption (Fig. 11).

5.4. Rock durability Results from the SDT tests (Id2, Id4 and Id8) and from the wet-dry tests (Iwd5, Iwd10 and Iwd20) are summarized in Table 3. According to the Slake Durability Classification proposed by Franklin and Chandra (1972), Cuitzeo ignimbrites are classified as very low-to-low durability for mpa, and medium-high durability for mpl and spl. Interpreting slake durability data is difficult as they are the result of both simple slaking, through wetting and drying, and mechanical abrasion which occurs during rotation (Moon, 1993b). In order to discriminate both effects, the wet-dry test was performed. Results obtained from this test are directly related to wetting and drying exclusively. Data in Table 3 show that mpa samples decay intensely after only 5 wet-dry cycles, and are completely destroyed after 10 cycles, whereas mpl and spl decay by means of fracturing and light disgregation but the total weight loss at the end of 20 cycles was lower than 10%. The data in Table 3 also demonstrate that the slake durability causes a progressive decrease in mass which does not occur in wetting and drying tests, witnessing again for the poor resistance of such materials to abrasion. The durability of rocks related to wet-dry processes depends on: i) permeability and porosity of the material, since these control the entry and retention of pore fluids and their mobility once inside the rock; ii) the action of fluids once they have penetrated the rock, due to surface energy changes, swelling, dissolution of cement, appearing of disruptive forces by pore-pressure generation; and iii) the resistance of the rock to disruptive forces (Franklin and Chandra, 1972). The extremely low durability of mpa samples to wet-dry test reflects the prevalence of swelling decay processes related to high content of expansive clays (montmorillonite). This fact, added to the high porosity of this lithofacies (~ 32%) and its very low uniaxial compressive strength (~ 4.3 MPa) (Table 1), explain the low resistance of this rock to both mechanical abrasion and wetting and drying processes. On the other hand, mpl and spl show moderately better characteristics in terms of durability, being related to a less effective clay swelling, a slightly lower porosity and stronger mechanical behaviour.

A. Pola et al. / Engineering Geology 214 (2016) 79–93

89

Fig. 8. 3D tomography image reconstructions of. a) mpa sample; b) mpl sample; c) spl-a sample. All images include the specimen top view and the pre and post-fracture 3D reconstruction of the most representative sample of each lithofacies. Abbreviations are: Pu = pumice fragment, APu = altered pumice fragment, FPu = failed pumice fragment, Dl = dense lithic, RDl = red dense lithic, Sl = stratified layers, Pf = principal fracture, Sf = secondary fracture; black arrows indicate the fracture pattern developed in samples under unconfined compression.

Moon (1993a) explains the direct relationship observed between slake durability, compressive strength and petrological characteristics in ignimbrites. This author asserts that ignimbrites with closely aligned lithic fragments, low porosity and welded structures are highly durable. Under these circumstances, only small quantities of water can reach the

inner rock. Consequently, the relatively weak forces involved in wetting and drying or mechanical abrasion will be unable to disrupt the tightly packed structure. In contrast, non-welded ignimbrites will be susceptible to destruction by a weak abrasive force. They will also admit considerable quantities of water into their structure, making the forces

90

A. Pola et al. / Engineering Geology 214 (2016) 79–93

Fig. 9. Stress - strain curves for the Cuitzeo ignimbrite lithofacies constructed by unconfined compressive test values. Only some representative curve of each lithofacies is included.

involved in wetting and drying much larger (Moon, 1993a). This reasoning is supported by previous works (del Potro and Hürlimann, 2008; Ergüler, 2009; Koralay et al., 2011) and it applies also to the Cuitzeo ignimbrites with mpa classified as ‘partially-to-incipiently welded’ and the remaining lithofacies as slightly stronger welded (partially welded) (Fig. 10). Moradian et al. (2010) suggest different predictive equations for estimating the slake durability index (Id) of soft rocks using several petrophysic parameters. They conclude that the most useful, accurate and simple equation uses only two parameters: porosity (ηe, %) and Pwave velocity (Vp, km/s): Id ¼ 88:54–0:51  ηe þ 12:58  ln ðVpÞ

ð3Þ

The estimated Id for the mpa, mpl and spl lithofacies are 77.3%, 82.8% and 83.2%, respectively. The Id values calculated for mpl and spl are quite

Fig. 11. hydraulic properties of the studied ignimbrite lithofacies. a) capillarity behaviour curves; b) absorption and desorption curves. Coefficients derived from this curves are included in Table 3.

similar to those obtained directly from the slake durability test with differences vary between 2.03 and 5.22%, respectively (Table 3). The proposed equation can be used to attain the slake durability of rocks from non-destructive measurements (ηe and Vp). On the contrary, the Id value assessed for mpa was strongly overestimated. Most of the mpa

Fig. 10. Porosity (η) vs UCS plot. The arrow represents the general expected trend for increasing alteration degree, while the horizontal lines represent the ranking welding intensity developed on the basis of petrographic textural observations (Quane and Rusell, 2005). Data from the literature (Ottaviani, 1988; Topal and Doyuran, 1997; Tommasi and Ribacchi, 1998; Evangelista et al., 1998; Rotonda et al., 2002; Vásárhelyi, 2002; Pola et al., 2012, 2014; Aydan and Ulusay, 2013; Korkanç, 2013; Özbek, 2014; Heap et al., 2015) are for similar rocks from different volcanic zones.

A. Pola et al. / Engineering Geology 214 (2016) 79–93

decay is caused by swelling, and consequently, previous equation cannot be directly applied. Unfortunately, empirical best-fit equations are strongly influenced by the used dataset and availability of larger datasets mixing different lithologies can cause even larger differences. 5.5. Geotechnical considerations Several geotechnical classifications have been established for volcanic materials (see Moradian et al., 2010 and references therein) but are prevalently limited to strong massive materials. The parameters used for the geotechnical characterization derive from Schmidt hammer, point load test, ultrasonic wave propagation velocity and/or slake durability test. Scarce efforts have been spent on the geomechanical characterization of tuffs and ignimbrites basically due to the problems related to sampling, coring and testing. The geotechnical classification proposed by Moon (1993b): i) it was specifically developed for ignimbrites; ii) it was based on a simply an easily measurable petrophysic parameter (slake durability test); and iii) it offers different scenarios for engineering and slope stability purposes. According to Moon's classification Cuitzeo rocks are classified as ‘non-durable’ (for mpa lithofacies) and ‘intermediate durability’ (for mpl and spl). Non-durable ignimbrites are characterized by breaking down under a weak slaking process (Id2 ≤ 30%), low bulk density and highly porous materials (ηe ≥ 40%), and are very weak in compression (UCS ≤ 5 MPa), showing considerable plastic deformation prior to failure. The mpa lithofacies falls perfectly in this description and consequently it is expected that despite the fact that mpa deposits can maintain steep slopes for long periods and failures involve only small amounts of material, these non-durable ignimbrites may be susceptible to loss of strength caused by structural changes or elevated pore water pressures. ‘Ignimbrites with intermediate characteristics’ is a mixed group defined between the ‘non-durable’ and ‘durable’ rocks. Ignimbrites included in this class show properties intermediate to the ranges given for ‘durable’ and ‘non-durable’ groups, or because they share properties for each range. mpl and spl lithofacies are included in this group because of: i) their Id2 values (between the 30% and the 90% limits which define the ‘durable’ and ‘non-durable’ classes); and ii) their moderately high porosity (between 24 and 30%) and their low strength (around 9 MPa). Therefore, these deposits can show high durability (quantified by relatively high values of slake durability test), but the low strength and high porosity indicate potential problems for relevant loading conditions. It is also important to point out that ignimbrites with high content of expansible clays (mpa, for example) can suffer a continuous deterioration of their mechanical properties. Results from Vergara and Triantafyllidis (2015) highlight the fact that rocks with expansible clays suffer an increase of their swelling behaviour during wetting and drying cycles due to the progressive breakdown and disaggregation of the rock during swelling, allowing a greater volume of clay particles to adsorb water. Consequently, the rock records a continuous irreversible loss of strength and stiffness due to changes in the rock structure. All these considerations should be taken into account, as this deposit constitutes the support and slopes of part of the Morelia-Guadalajara highway, particularly along the southern shore of the Cuitzeo lake. 6. Conclusions The results presented in this work provide the physical and mechanical characterization of four different ignimbrite lithofacies of the Cuitzeo zone. The investigated ignimbrites show that there is a large dependence of all the physical mechanical properties on the textural characteristics of each sample, particularly on the pumice content. The average values of γ (13.78 for spl-a to 15.74 kN/m3 for mpl) coincide with values of similar rocks described in several studies. The average values of ηe are very similar for all lithofacies, varying from 24 to 32%.

91

X-ray micro CT image reconstruction shows that the main differences in these average values are directly associated with the geometry and morphology of the grains and pore network, which in turn are closely related to the mode of failure of each specimen. The geometry and arrangement of clasts in each specimen largely contribute to determining the total porosity value. In particular, the abundance of mm-to-cm size pumice fragments largely influences the porosity in the mpa lithofacies. In addition, the higher degree of anisotropy observed in the mpa lithofacies confirms the large variation in its properties (UCS from 3.74 to 5.04 MPa). In general, the UCS for all lithofacies tends to increase linearly with several of the physical properties (e.g. Vp, ρbulk), with large dispersion in the mpa samples. The decrease in the mechanical properties (e.g. UCS, TS) indicates that the strength and deformability of each lithofacies are largely influenced by the intrinsic characteristics of each specimen inherited by transportation and emplacement mechanisms and the successive degradation processes. Slake durability tests demonstrate that great part of the mechanical degradation could be attributed to swelling during wetting-drying processes. This decay is especially significant in mpa due to the combined effects of the content of expansible clays (montmorillonite), the high porosity and extremely low strength. Geotechnical considerations exposed in this paper highlight that extreme caution needs to be taken into account, when these materials are used to design structures. However, the Cuitzeo ignimbrite is composed of at least sixteen lithofacies and therefore important differences are found among them. Thus is clear that further studies related to geomechanical characterization and detailed stratigraphy needs to be performed on these and similar rocks. Additionally, all of the existing values need to be evaluated to explore the variation in the rock behaviour. This evaluation will emphasize the importance of the physical-mechanical characterization of the area to construct conceptual and numerical models of different phenomena occurring in the region, as the slope instabilities along the Morelia-Guadalajara highway. Acknowledgements We gratefully acknowledge (PNPC) SEP-CONACYT, DGAPA-UNAM for the postdoctoral financial support, PAPIIT project (IA105016), and CEMIE-Geo projects (P.15, P17 and P-LABS-UMSNH). We also gratefully acknowledge The Applied Petrological Laboratory of the Alicante University (CSIC-UA), especially, Feli Martínez and Manuel Palomo. We also gratefully acknowledge Denis Avellán for their advices and assistance in the field and Guillermo Cisneros who provided the DEM data and SPOT images. Finally, we also thank Hsein Juang, Mike Heap and anonymous reviewer for their invaluable comments that largely improved the manuscript. References Al-Shayea, N.A., 2004. Effects of testing methods and conditions on the elastic properties of limestone rock. Eng. Geol. 74, 139–156. American Society for Testing and Materials, ASTM D4644-90, 1990,. Standard Test Method for Slake Durability of Shales and Similar Weak Rocks. American Society for Testing Materials, Pennsylvania, USA. American Society for Testing Materials, ASTM D2938-95, 2002. Standard Test Method for Unconfined Compressive Strength of Intact Rock Core Specimens. American Society for Testing Materials, Pennsylvania, USA (Reapproved). American Society for Testing Materials, ASTM D3967-95a, 2001. Standard Test Method for Splitting Tensile Strength of Intact Rock Core Specimens. American Society for Testing Materials, Pennsylvania, USA (Reapproved). American Society for Testing Materials, ASTM D3148-02, 2002. Standard Test Method for Elastic Moduli of Intact Rock Core Specimens in Uniaxial Compression. American Society for Testing Materials, Pennsylvania, USA. American Society for Testing Materials, ASTM D5313-13, 2013. Standard Test Method for Evaluation of Durability of Rock for Erosion Control Under Wetting and Drying Conditions. American Society for Testing Materials, Pennsylvania, USA. Avar, B.B., Hudyma, N., Karakouzian, M., 2003. Porosity dependence of the elastic modulus of lithophysae-rich tuff: numerical and experimental investigations. Int. J. Rock Mech. Min. Sci. 40 (6), 919–928. Aydan, Ö., Ulusay, R., 2013. Geomechanical evaluation of Derinkuyu antique underground city and its implications in geoengineering. Rock Mech. Rock. Eng. 46 (4), 731–754.

92

A. Pola et al. / Engineering Geology 214 (2016) 79–93

Baud, P., Zhu, W., Wong, T.F., 2000. Failure mode and weakening effect of water on sandstone. J. Geophys Res. B Solid Earth 105 (B7), 16,371–16,389. Baud, P., Wong, T., Zhu, W., 2014. Effects of porosity and crack density on the compressive strength of rocks. Int. J. Rock Mech. Min. Sci. 67, 202–211. Beck, K., Al-Mukhtar, M., Rozenbaum, O., Rautureau, M., 2003. Characterization, water transfer properties and deterioration in tuffeau: building material in the Loire valley-France. Build. Environ. 38, 1151–1162. Benavente, D., Cueto, N., Martínez-Martínez, J., Garcia-del-Cura, M.A., Cañaveras, J.C., 2007. The influence of petrophysical properties on the salt weathering of porous building rocks. Environ. Geol. 52, 215–224. Benavente, D., Lock, P., García-del-Cura, M.A., Ordóñez, S., 2002. Predicting the capillary imbibition of porous rocks from microstructure. Transp. Porous Media 49 (1), 59–76. Benavente, D., Pla, C., Cueto, N., Galvañ, S., Martínez-Martínez, J., García-del-Cura, M.A., Ordóñez, S., 2015. Predicting water permeability in sedimentary rocks from capillary imbibition and pore structure. Eng. Geol. 195, 301–311. Blatter, D.L., Hammersley, L., 2010. Impact of the Orozco fracture zone on the central Mexican volcanic belt. J. Volcanol. Geotherm. Res. 197 (1–4), 67–84. Branney, M.J., Kokelaar, P., 2002. Pyroclastic density currents and the sedimentation of ignimbrites. Geol. Soc. Lond. Spec. Mem. 27, 143. Chang, C., Zoback, M.D., Khaksar, A., 2006. Empirical relations between rock strength and physical properties in sedimentary rocks. J. Pet. Sci. Eng. 51 (3–4), 223–237. Ciccotti, M., Mulargia, F., 2004. Differences between static and dynamic elastic moduli of a typical seismogenic rock. Geophys. J. Int. 157, 474–477. Cisneros-Máximo, G., 2016. Estudio geológico-geomorfológico de los volcanes TetillasQuinceo, Morelia, Michoacán. Propuesta de cartografía Jerárquica. Maestría en geografía. Facultad de Filosofía y Letras; Universidad Nacional autónoma de México (290 p). Coggan, J.S., Stead, D., Howe, J.H., Faulks, C.I., 2013. Mineralogical controls on the engineering behavior of hydrothermally altered granites under uniaxial compression. Eng. Geol. 160, 89–102. del Potro, R., Hürlimann, M., 2008. Geotechnical classification and characterisation of materials for stability analyses of large volcanic slopes. Eng. Geol. 98, 1–17. del Potro, R., Hürlimann, M., 2009. A comparison of different indirect techniques to evaluate volcanic intact rock strength. Rock Mech. Rock. Eng. 42, 931–938. Deriszadeh, M., Wong, R.C.K., 2014. One-dimensional swelling behaviour of clay and shale under electrical potential gradient. Transp. Porous Media 101, 35. Eberhardt, E., Stead, D., Stimpson, B., 1999. Quantifying progressive pre-peak brittle fracture damage in rock during uniaxial compression. Int. J. Rock Mech. Min. Sci. 36 (3), 361–380. Ego, F., Ansan, V., 2002. Why is the Central Trans-Mexican Volcanic Belt (102°–99° W) in transtensive deformation? Tectonophysics 359 (1), 189–208. Ergüler, Z.A., 2009. Field-based experimental determination of the weathering rates of the Cappadocian tuffs. Eng. Geol. 105 (3–4), 186–199. Ersoy, A., Atici, U., 2007. Correlation of P and S-waves with cutting specific energy and dominant properties of volcanic and carbonate rocks. Rock Mech. Rock. Eng. 40 (5), 491–504. Evangelista, A., Aversa, S., Pescatore, T.S., Pinto, F., 1998. Soft rocks in Southern Italy and the role of volcanic tuffs in the urbanization of Naples. The Geotechnics of Hard Soils–Soft Rocks. Proc. 2nd Int. Symp. vol. 3, pp. 1243–1267 Ferrari, L., Conticelli, S., Vaggelli, G., Petrone, C.M., Manetti, P., 2000. Late Miocene volcanism and intra-arc tectonics during the early development of the Trans-Mexican Volcanic Belt. Tectonophysics 318 (1–4), 161–185. Franklin, J.A., Chandra, R., 1972. The slake-durability test. Int. J. Rock Mech. Min. Sci. 9 (3), 325–341. Frolova, J., Ladygin, V., Rychagov, S., Zukhubaya, D., 2014. Effects of hydrothermal alterations on physical and mechanical properties of rocks in the Kuril–Kamchatka island arc. Eng. Geol. 183, 80–95. Fuenkajorn, K., Daemen, J.J.K., 1992. An empirical strength criterion for heterogeneous tuff. Eng. Geol. 32 (4), 209–223. Gadow, H., 1930. Jorullo: The History of the Volcano Jorullo and the Reclamation of the Devastated District by Animals and Plants. Cambridge University Press, p. 101. García-del-Cura, M.A., Benvante, D., Martínez-Martínez, J., Cueto, N., 2011. Sedimentary structures and physical properties of travertines and carbonate tufa building stone. Constr. Build. Mater. 28 (1), 456–467. Garduño-Monroy, V.H., Pérez-López, R., Israde-Alcantara, I., Rodríguez-Pascua, M.A., Szynka-ruk, E., Hernández-Madrigal, V.M., García-Zepeda, M.L., Corona-Chávez, P., Ostroumov, M., Medina-Vega, V.H., García-Estrada, G., Carranza, O., López-Granados, E., Mora-Chaparro, J.C., 2009. Paleoseismology of the southwestern Morelia-Acambay fault system, central México. Geofis. Int. 48, 319–335. Gómez-Vasconcelos, M.G., Garduño-Monroy, V.H., Macías, J.L., Layer, P.W., Benowitz, J.A., 2015. The Sierra de Mil Cumbres, Michoacán, México: transitional volcanism between the Sierra Madre Occidental and the trans-Mexican Volcanic Belt. J. Volcanol. Geotherm. Res. 301, 128–147. Guilbaud, M.N., Siebe, C., Layer, P., Salinas, S., Castro-Govea, R., Garduño-Monroy, V.H., Le Corvec, N., 2011. Geology, geochronology, and tectonic setting of the Jorullo Volcano region, Michoacán México. J. Volcanol. Geotherm. Res. 201, 97–112. Hasenaka, T., Carmichael, I.S.E., 1987. The cinder cones of Michoacán-Guanajuato, central Mexico: petrology and chemistry. J. Petrol. 28, 241–269. Hasenaka, T., Ban, M., Delgado-Granados, H., 1994. Contrasting volcanism in the Michoacán-Guanajuato volcanic field, central Mexico: shield volcanoes vs. cinder cones. Geofis. Int. 33 (1), 125–138. Heap, M.J., Vicinguerra, S., Meredith, P.G., 2009. The evolution of elastic moduli with increasing crack damage during cyclic stressing of basalt from Mt. Etna volcano. Tectonophysics 471, 153–160. Heap, M.J., Lavallée, Y., Petrakova, L., Baud, P., Reuschlé, T., Varley, N., Dingwell, D.B., 2014. Microstructural controls on the physical and mechanical properties of edificeforming andesites at Volcán de Colima, Mexico. J. Geophys. Res. 119, 2925–2963.

Heap, M.J., Kennedy, B.M., Pernin, N., Jacquemard, L., Baud, P., Farquharson, J.I., Scheu, B., Lavallée, Y., Gilg, H.A., Letham-Brake, M., Mayer, K., Jolly, A.D., Reuschlé, T., Dingwell, D.B., 2015. Mechanical behaviour and failure modes in the Whakaari (White Island volcano) hydrothermal system, New Zealand. J. Volcanol. Geotherm. Res. 295, 26–42. Heap, M.J., Wadsworth, F.B., Xu, T., Chen, C.F., 2016. The strength of heterogeneous volcanic rocks: a 2D approximation. J. Volcanol. Geotherm. Res. 319, 1–11. Hudyma, N., Avar, B.B., Karakouzian, M., 2004. Compressive strength and failure modes of lithophysae-rich Topopah Spring Tuff specimens and analogue models containing cavities. Eng. Geol. 73 (1–2), 179–190. International Society on Rock Mechanics, 2007. In: Ulusay, R., Hudson, J.A. (Eds.), The Complete ISRM Suggested Methods for Rock Characterization, Testing and Monitoring. ISRM, pp. 1974–2006 (628 pp). Instituto Nacional de Estadística y Geograía (INEGI), 2010. http://www.inegi.org.mx/geo/ contenidos/geodesia/default.aspx. Israde-Alcántara, I., Garduño-Monroy, V.H., 1999. Lacustrine record in a volcanic intra-arc setting: the evolution of the Late Neogene Cuitzeo basin system (central-western Mexico, Michoacán). Palaeogeogr. Palaeoclimatol. Palaeoecol. 151 (1), 209–227. Kahraman, S., Gunaydin, O., Fener, M., 2005. The effect of porosity on the relation between uniaxial compressive strength and point load index. Rock Mech. Min. Sci. 42, 584–589. Kahraman, S., 2007. The correlations between the saturated and dry P-wave velocity of rocks. Ultrasonics 46 (4), 341–348. Katti, K., Katti, D., 2003. Effect of Clay-water Interactions on Swelling in Montmorillonite Clay. Proc. ASCE Engineering Mechanics Conference, Seattle, WA, USA. Koralay, T., Özkul, M., Kumsar, H., Çelik, S.B., Pektaş, K., 2011. The effect of welding degree on geotechnical properties of an ignimbrite flow unit: the Bitlis castle case (eastern Turkey). Environ. Earth Sci. 64, 869–881. Korkanç, M., 2013. Deterioration of different stones used in historical buildings within Nigde province, Cappadocia. Constr. Build. Mater. 48, 789–803. Luhr, J.F., Carmichael, I.S.E., 1985. Jorullo volcano, Michoacán Mexico (1759–1774): the earliest stages of fractionation in calcalkaline magmas. Contrib. Mineral. Petrol. 90, 142–161. Machek, M., Spacek, P., Ulrich, S., Heidelbach, F., 2007. Origin and orientation of microporosity in eclogites of different microstructure studied by ultrasound and microfabric analysis. Eng. Geol. 89, 266–277. Marques, E.A.G., Barroso, E.V., Menezes-Filho, A.P., Vargas Jr., E.d.A., 2010. Weathering zones on metamorphic rocks from Rio de Janeiro-physical, mineralogical and geomechanical characterization. Eng. Geol. 111 (1–4), 1–18. Martin, C.D., Chandler, N.A., 1994. The progressive fracture of Lac du Bonnet Granite. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 31 (6), 643–659. Martínez-Martínez, J., Benavente, D., García-del-Cura, M.A., 2011. Spatial attenuation: The most sensitive ultrasonic parameter for detecting petrographic features and decay processes in carbonate rocks. Eng. Geol. 119 (3–4), 84–95. Martínez-Martínez, J., Fusi, N., Galiana-Merino, J.J., Benavente, D., Crosta, G.B., 2016. Ultrasonic and X-ray computed tomography characterization of progressive fracture damage in low-porous carbonate rocks. Eng. Geol. 47–57. Moon, V.G., 1993a. Microstructural controls on the geomechanical behaviour of ignimbrite. Eng. Geol. 35 (1–2), 19–31. Moon, V.G., 1993b. Geotechnical characteristics of ignimbrite: a soft pyroclastic rock type. Eng. Geol. 35 (1–2), 33–48. Moradian, Z.A., Ghazvinian, A.H., Ahmadi, M., Behnia, M., 2010. Predicting slake durability index of soft sandstone using indirect tests. Int. J. Rock Min. Sci. 47, 666–671. Morales Demarco, M., Jahns, E., Rüdrich, J., Oyhantcabal, P., Siegesmund, S., 2008. The impact of partial water saturation on rock strength: an experimental study on sandstone. Z. dt. Ges. Geowiss. 158 (4), 869–882. Newton, A.J., Metcalfe, S.E., Davies, S.J., Cook, G., Barker, P., Telfor, R.J., 2005. Late Quaternary volcanic record from lakes of Michoacán, central Mexico. Quat. Sci. Rev. 24 (1– 2), 91–104. Ottaviani, M., 1988. Proprietà geotecniche di tufi vulcanici italiani. Riv. Ital. Geotecnica. pp. 173–178. Özbek, A., 2014. Investigation of the effect of wetting-drying and freezing-thawing cycles on some physical and mechanical properties of selected ignimbrites. Bull. Eng. Geol. Environ. 73 (2), 595–609. Pasquaré, G., Ferrari, L., Garduño, V., Tibaldi, A., Vezzoli, L., 1991. Geology of the central sector of the Mexican Volcanic Belt, states of Guanajuato and Michoacan: Boulder, CO, Geological Society of America, Map and Chart Series MCH072. Pereira, J.M., Arson, C., 2013. Retention and permeability properties of damaged porous rocks. Comput. Geotech. 48, 272–282. Pérez-López, R., Legrand, D., Garduño-Monroy, V.H., Rodríguez-Pascua, M.A., GinerRobles, J.L., 2011. Scaling laws of the size-distribution of monogenetic volcanoes within the Michoacán-Guanajuato volcanic field (Mexico). J. Volcanol. Geotherm. Res. 201, 65–72. Pola, A., Crosta, G.B., Fusi, N., Barberini, V., Norini, G., 2012. Influence of alteration on physical properties of volcanic rocks. Tectonophysics 566–567, 67–86. Pola, A., Crosta, G.B., Fusi, N., Castellanza, R., 2014. General characterization of the mechanical behaviour of different volcanic rocks with respect to alteration. Eng. Geol. 169, 1–13. Pola, A., Macías, J.L., Osorio-Ocampo, S., Sosa-Ceballos, G., Garduño-Monroy, V.H., Martínez-Martínez, J., 2015. El Estribo Volcanic Complex: evolution from a shield volcano to a cinder cone, Pátzcuaro Lake, Michoacán, Mexico. J. Volcanol. Geotherm. Res. 303, 130–145. Poppe, L.J., Paskevich, V.F., Hathaway, J.C., Blackwood, D.S., 2001a. A laboratory manual for X-ray powder diffraction. Separation of silt and clay by decantation for X-ray powder diffraction. U.S. Geological Survey Open-file Report 01-041a.

A. Pola et al. / Engineering Geology 214 (2016) 79–93 Poppe, L.J., Paskevich, V.F., Hathaway, J.C., Blackwood, D.S., 2001b. A laboratory manual for X-ray powder diffraction. Separation of silt and clay by decantation for X-ray powder diffraction. U.S. Geological Survey Open-file Report 01-041b. Quane, S.L., Russell, J.K., 2005. Ranking welding intensity in pyroclastic deposits. Bull. Volcanol. 67 (2), 129–143. Rao, K.S., Noferesti, H., 2008. Brittle fialure in heterogeneous crystalline rocks. Def. Sci. J. 58 (2), 285–294. Rotonda, T., Tommasi, P., Ribacchi, R., 2002. Physical and mechanical characterization of the soft pyroclastic rocks forming the Orvieto cliff. In: da Gama, C.D., Ribeiro e Sousa, L. (Eds.), Workshop on Volcanic Rocks, ISRM International Symposium on Rock Engineering for Mountainous Regions, Eurock. Saotome, A., Yoshinaka, R., Osada, M., Sugiyama, H., 2002. Constituent material properties and clast-size distribution of volcanic breccia. Eng. Geol. 64, 1–17. Schön, J.H., 2011. Physical Properties of Rocks: A Workbook (8). Elsevier. Schöpfer, M.P.J., Abe, S., Childs, C., Walsh, J.J., 2009. The impact of porosity and crack density on the elasticity, strength and friction of cohesive granular materials: Insights from DEM modelling. Int. J. Rock Mech. Min. Sci. 46 (2), 250–261. Sengun, N., Demirdag, S., Akbay, D., Ugur, I., Altindag, R., Akbulut, A., 2014. Investigation of the Relationships Between Capillary Water Absorption Coefficients and Other Rock Properties of Some Natural Stones. V. Dünya Doğaltaş Kongresi (Globalstone 2014), Antalya-Türkiye, pp. 22–25. Siratovich, P.A., Heap, M.J., Villenueve, M.C., Cole, J.W., Reuschlé, T., 2014. Physical property relationships of the Rotokawa Andesite, a significant geothermal reservoir rock in the Taupo Volcanic Zone, New Zealand. Geother. Energy 2 (10), 2–31. Sonmez, H., Gokceoglu, C., Medley, E.W., Tuncay, E., Nefeslioglu, H.A., 2006. Estimating the uniaxial compressive strength of a volcanic bimrock. Int. J. Rock Mech. Min. Sci. 43, 554–561. Sousa, L.M.O., Suárez del Río, L.M., Calleja, L., Ruiz de Argandoña, V.G., Rodríguez Rey, A., 2005. Influence of microfractures and porosity on the physico-mechanical properties and weathering of ornamental granites. Eng. Geol. 77 (1–2), 153–168. Stanchits, S.A., Lockner, D.A., Ponomarev, A.V., 2003. Anisotropic changes in P-wave velocity and attenuation during deformation and fluid infiltration of granite. Bull. Seismol. Soc. Am. 93 (4), 1803–1822. Thörn, J., Ericsson, L., Fransson, Å., 2014. Hydraulic and hydromechanical laboratory testing of large crystalline rock cores. Rock Mech. Rock. Eng. 48, 61–73. Tommasi, P., Ribacchi, R., 1998. Mechanical behaviour of the Orvieto tuff. In: Evangelista, A., Picarelli, L. (Eds.), The Geotechnics of Hard Soils–soft Rocks. Proc. 2nd Int. Symp. on Hard Soils–soft Rocks, Naples, Italy, Balkema, Rotterdam, pp. 901–909.

93

Topal, T., Doyuran, V., 1997. Engineering geological properties and durability assessment of the Cappadocian tuff. Eng. Geol. 47 (1–2), 175–187. Ulusay, R., Türeli, K., Ider, M.H., 1994. Prediction of engineering properties of selected litharenite sandstone from its petrographic characteristics using correlation and multivariate statistical techniques. Eng. Geol. 37, 135–157. UNE-EN 1925, 1999. Natural Stone Test Methods. Determination of Water Absorption Coefficient by Capillarity. Vásárhelyi, B., 2002. Influence of the water saturation on the strength of volcanic tuffs. In: da Gama, C.D., Ribeiro e Sousa, L. (Eds.), Workshop on Volcanic Rocks, ISRM International Symposium on Rock Engineering for Mountainous Regions, Eurock, pp. 89–96. Vergara, M.R., Triantafyllidis, T., 2015. Swelling behaviour of volcanic rocks under cyclic wetting and drying. Int. J. Rock Mech. Min. Sci. 80, 231–240. Vernik, L., Bruno, M., Bovberg, C., 1993. Empirical relations between compressive strength and porosity of siliciclastic rocks. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 30 (7), 677–680. Wong, R.H.C., Chaud, K.T., Wang, P., 1996. Microcracking and grain size effect in Yuen long marbles. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 33 (5), 479–485. Wong, L.N.Y., Maruvanchery, V., Liu, G., 2016. Water effects on rock strength and stiffness degradation. Acta Geotech. 11, 713–737. Wyering, L.D., Villeneuve, M.C., Wallis, I.C., Siratovich, P.A., Kennedy, B.M., Gravley, D.M., Cant, J.L., 2014. Mechanical and physical properties of hydrothermally altered rocks, Taupo Volcanic Zone, New Zealand. J. Volcanol. Geotherm. Res. 288, 76–93. Yildiz, A., Kuşcu, M., Dumlupunar, I., Krrem Aritan, A., Bağci, M., 2010. The determination of the mineralogical alteration index and the investigation of the efficiency of the hydrothermal alteration on physic-mechanical properties in volcanic rocks from Koprulu, Afyonkarahisar, West Turkey. Bull. Eng. Geol. Environ. 69, 51–61. Yılmaz, N.G., Goktan, R.M., Kibici, Y., 2011. Relation between some quantitative petrographic characteristics and mechanical strength properties of granitic building stones. Int. J. Rock Mech. Min. Sci. 48 (3), 506–513. Zhu, W., Baud, P., Vinciguerra, S., Wong, T., 2011. Micromechanics of brittle faulting and cataclastic flow in Alban Hills tuff. J. Geophys. Res. 116 (B6). Zhu, W., Baud, P., Wong, T., 2010. Micromechanics of cataclastic pore collapse in limestone. J. Geophys. Res. 115 (B4).