Mineralogical controls on the engineering behavior of hydrothermally altered granites under uniaxial compression

Engineering Geology 160 (2013) 89–102

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Mineralogical controls on the engineering behavior of hydrothermally altered granites under uniaxial compression J.S. Coggan a,⁎, D. Stead b, J.H. Howe c, C.I. Faulks a a b c

Camborne School of Mines, University of Exeter, Cornwall Campus, United Kingdom Department of Earth Sciences, Simon Fraser University, Burnaby, B.C., Canada IMERYS Minerals Ltd., Cornwall, United Kingdom

a r t i c l e

i n f o

Article history: Received 28 September 2012 Received in revised form 2 April 2013 Accepted 7 April 2013 Available online 13 April 2013 Keywords: Mineralogy Altered granites Kaolinization Strength Acoustic emission Crack propagation

a b s t r a c t Effective engineering characterization of granites altered through processes of kaolinization is critical for safe extraction and design of slopes for the china clay industry in south-west England. By considering this important issue, representative samples taken from a range of decomposition grades of altered granite were used to assess the controlling influence of changes in mineralogy on uniaxial compressive strength, elastic modulus, dry density and ultrasonic velocity. Dramatic reduction in uniaxial compressive strength occurs between alteration Grades II and III. Analysis of acoustic emission data, constrained by evaluation of Scanning Electron Microscope (SEM) images and strain gauge deformation readings, has been used to identify crack propagation stages during uniaxial compression. Quantitative evaluation of mineralogy was determined from X-ray diffraction (XRD) analysis of representative samples in order to establish relationships between mineralogy, such as %kaolinite, and uniaxial compressive strength for the different alteration grade material. The results of the testing confirm that the degree of alteration or kaolinization, and associated changes in mineralogy of the granite, is directly related to reduction in uniaxial compressive strength and dry density. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Quantification of the engineering characteristics of hydrothermally altered or kaolinized granite is essential for effective extraction and design of excavations in the china clay pits of south-west England. Stead et al. (2000) provided a review of the approaches to engineering characterization of altered granites, taking into account the previous work undertaken in south-west England and also extensive work performed in Hong Kong. Characterization of altered granites usually involves categorization into classes, zones or grades according to readily recognized or simply measured variations in their characteristics. The assessment of material properties is a fundamental component of geotechnical characterization of slopes in altered granite. This task may be extremely challenging due to the inherent variability of alteration within a particular slope. The kaolinization intensity can vary both vertically and horizontally along a slope profile. Stead et al. (2000), for example, noted that kaolinization is structurally controlled and generally occurs in association with sheeted greisen, tourmaline and quartz veining. Recent investigations on the engineering characteristics of hydrothermally altered granites from south-west England have included

⁎ Corresponding author at: Camborne School of Mines, University of Exeter, Cornwall Campus, Penryn, Cornwall, TR10 9EZ, United Kingdom. Tel/fax.: +44 1326 371824. E-mail address: [email protected] (J.S. Coggan). 0013-7952/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enggeo.2013.04.001

multi-stage triaxial and uniaxial studies undertaken by Imperial College London Consultants (2004), analysis of index testing and petrographic analysis (Suringar, 2004), uniaxial studies and Scanning Electron Microscope (SEM) image analysis (Chilton, 2006) and fieldbased mapping to establish a correlation between the Geological Strength Index (GSI) and decomposition or alteration grade. A summary of some of the results from the 2004 series of tests is provided in Coggan et al. (2007). The focus of the current study was to review existing test data, create an up-to-date south-west England database and subsequently undertake further complementary testwork to provide greater insight into the influence of changes in mineralogy on the observed marked strength reduction from slightly altered (Grade II) to moderately altered (Grade III) material and establish relationships between mineralogy, dry density and uniaxial compressive strength for varying grade material. This was undertaken using a range of diverse tools, including a series of instrumented uniaxial compression tests that were performed at the Camborne School of Mines, University of Exeter, on representative samples taken from varying grades of altered material from the Wheal Martyn Pit, Cornwall, England. Analysis of acoustic emission (AE) and strain gauge deformation measurements, combined with Scanning Electron Microscope (SEM) images taken at various stages of failure, were also used to characterize the micro-fracture development and damage processes occurring during uniaxial compression. The AE response for Grade III samples showed more continuous AE activity over a wider amplitude range throughout uniaxial loading than Grade II samples. All samples,

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whether Grade II or III, exhibit increased AE activity approaching failure, with a tendency for increased event amplitude which suggests that larger amplitude emissions approaching failure are probably a result of fracture propagation, coalescence and ultimately macro-crack development. Variability of crack initiation (CI) and crack damage (CD) threshold values reflect the potential for sudden brittle failure, and the mechanical instability associated with Grade II material. Quantitative mineralogy was undertaken through X-ray diffraction (XRD) analysis of representative samples in order to establish relationships between changes in mineralogy, such as %kaolinite, and uniaxial compressive strength for the different alteration grade material.

2. Engineering and petrographic characterization Wheal Martyn china clay pit is located on the Hensbarrow Downs which form part of the St Austell granite cupola, Fig. 1. The pit, shown in Fig. 2, is approximately 100 m deep, and 700 m by 300 m in plan area. The deepest part of the pit intersects a northeast–southwest trending zone containing sub-vertical and sub-parallel quartz–tourmaline sheeted vein-systems and stockworks. According to Bristow and Exley (1994) and Selwood et al. (1998) the St Austell granites were emplaced at the end of the Varsican Orogeny (300–275 Ma) into metasediments of early and middle Devonian age (410–380 Ma). Differences in texture, composition and age of the granites between, as well as within, the outcropping bodies indicate that different magmas were emplaced in several phases. The lithology of the St Austell granite has been described by Manning et al. (1996) who recognized four different varieties based on whether biotite, tourmaline, lithium mica or topaz was the main mineral in addition to quartz and feldspar. Wheal Martyn Pit is situated within the tourmaline granite.

2.1. Engineering characterization of altered granites Several researchers including Baynes and Dearman (1978), Dearman et al. (1978), Hencher et al. (1990), Irfan and Dearman (1978) and Stead et al. (2000) provided important guidelines on the description and characterization of kaolinized granite in the southwest of England. Similarly, Hencher and Lee (2010), Hencher and McNicholl (1995) and Irfan (1996, 1999) provided details of the characterization of the weathered granites of Hong Kong. Table 1 summarizes typical alteration grades for granites, which provide a basis for fieldbased categorization and use of the Schmidt Hammer for index testing. Hencher et al. (1990) indicated that the kaolinized zones of St Austell granite show many similarities to tropically weathered granites and can be classified for engineering purposes according to similar criteria. They noted good correlation between alteration and Schmidt Hammer rebound value, feldspar decomposition, field strength and degree of slaking. Other researchers have also investigated the effects of alteration on the engineering behavior of granites. For example, Gunes Yilmaz et al. (2009, 2011) provide relationships between petrographic characteristics and mechanical strength properties of granitic building stones selected from various granite producing countries (Spain, Italy, Brazil, Turkey, Ukraine and Finland). Basu et al. (2009) evaluated the engineering characteristics of varying altered granites in relation to their petrology from Brazil. They noted that deterioration of uniaxial compressive strength and elastic modulus and increase in Poisson's ratio with increasing weathering intensity could be attributed to alteration of minerals, disruption of rock skeleton and microcrack augmentation. Tugrul and Zarif (1999) assessed both textural and mineralogical properties on the engineering properties of granites from Turkey. They suggested that the influence of the textural characteristics on the engineering properties appears to be more important than the mineralogy, and also noted that the types of contacts,

Fig. 1. Map of Cornwall showing the distribution of granite types. After Selwood et al. (1998).

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Fig. 2. Image of the western slope at Wheal Martyn china clay pit.

grain (mineral) shape and size significantly influence the engineering properties of the granitic rocks tested. Sousa et al. (2005) investigated the influence of microfractures and porosity on the physicomechanical properties and alteration of ornamental granites from Portugal. They developed a microfracture index (called linear crack density (LCD)), based on inter-granular, intra-granular and transgranular cracks, and revealed that uniaxial compressive strength and P-wave velocity (VP) appear to decrease as linear crack density increases, albeit with low correlation coefficients. Chen et al. (2011) studied the crack growth in Westerly Granite during cyclic loading tests. Crack growth patterns were analyzed by microscopic observation and image analysis techniques. They noted the different crack growth behaviors of quartz and feldspar minerals during the different loading stages. Aydin and Basu (2006) showed that the Brazilian test may be used as a quantitative measure of alteration of Hong Kong granites. 2.2. Petrology and kaolinization of hydrothermally altered granites from Wheal Martyn Pit Mineralogical changes that occur during kaolinization of the granites in the St Austell area of south-west England have been previously described by Psyrillos (1996) and Psyrillos et al. (1997, 1998). Quartz, Table 1 Engineering classification of weathered uniform materials. From Anon (1995). Grade

Description

Characteristics

I II

Fresh rock Slightly altered

III

Moderately altered

IV

Highly altered

V

Completely altered

No visible alteration. Slight discoloration and weakening. Schmidt Hammer ‘N’ > 45. Considerable weakening. Penetrative. Discoloration. Schmidt Hammer ‘N’ 25–45. Large pieces broken by hand. Schmidt Hammer ‘N’ 0–25. Considerably weakened. Geological pick penetrates. Original texture preserved. Slakes readily in water. Hand penetrometer, 50–250 kPa.

tourmaline and muscovite remain essentially unaltered during kaolinization. Feldspars, however, are dissolved by water passing through vein and fracture systems permeating the adjoining granite through micro-fractures. Kaolin and smectite are formed in the dissolution cavities inside the feldspars. Alteration fronts expand away from fluid pathways into fresh granite resulting in varying degrees of alteration. These can be described as fresh granite, slightly kaolinized granite with partially dissolved plagioclase and dissolution cavities filled with kaolin and smectite, moderately altered granite with increased alteration of plagioclase and partial alteration of K-feldspar, highly kaolinized granite with fully altered plagioclase and increasingly altered K-feldspar, kaolinite being more abundant than smectitie, and completely kaolinized granite with all feldspar dissolved and kaolinite being the primary reaction product present. The degrees of alteration described above relate to the alteration grades shown in Table 1. Suringar (2004) described the granite within Wheal Martyn as a porphyritic medium-grained muscovite tourmaline leuco-granite with K-feldspar megacrysts up to 6 cm and granular quartz crystals up to 1 cm. Representative samples from the varying alteration grades were taken from the Wheal Martyn Pit by Suringar for subsequent petrographic and X-ray diffraction analysis. In addition, the dry bulk density and porosity were also determined. Due to the friable nature of some samples, all sample materials selected for thin section analysis by Suringar (2004) using a Nikon petrographic microscope were impregnated with resin under a vacuum. Thin sections of 50 by 75 mm and of 0.03 mm thickness were prepared for alteration Grades II to IV, whereas 20 by 30 mm sections were prepared for Grade V material. Petrographic analysis by Suringar (2004) suggested that changes in mineralogy were gradational within and between the alteration grades. Observations indicate that in Grade II material plagioclases are increasingly affected by alteration and change to secondary mica, while K-feldspars remain essentially unchanged. For typical Grade III material all plagioclase crystals contain 10 to 50% secondary mica, and some K-feldspar crystals are 20% altered to secondary mica. K-feldspars are then progressively converted from the exsolution lamellae outward. For Grade IV material, only about 10% of the original plagioclase remains; some crystals are 100% altered to secondary mica, others are altered to a mixture of 50% secondary mica and 50% kaolinite

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and others only 20 to 50% altered. Some K-feldspar crystals show 50% alteration to secondary mica and kaolinite. The final alteration phase consists of a groundmass of kaolinite with the remaining unaltered quartz, muscovite and tourmaline. Fig. 3a–d shows typical photomicrographs to illustrate the alteration process, and represent Grade II to Grade V material. Fig. 3a shows a large K-feldspar megacryst surrounded by smaller quartz, feldspar, tourmaline and muscovite crystals, characteristic of Grade II material. Fig. 3b shows an image of a 75% altered plagioclase crystal which is typical of a Grade III, moderately altered material. In the lower center of the plagioclase there is an area that is relatively unaltered. The linear features are cracks filled with secondary mica. Fig. 3c shows dissolution holes in a K-feldspar crystal filled with secondary mica and kaolinite, typical of Grade IV material. Also shown are masses of kaolinite and secondary mica. Fig. 3d shows a higher magnification image of a kaolinite groundmass, representative of Grade V material. The images shown in Fig. 3 have been taken with crossed polars, as these provide better detail than images taken under plane polarized light. Table 2 provides example results for quantitative mineralogy, using X-ray diffraction (XRD), performed on samples of varying alteration grade. Density values highlighted with an asterisk (10 samples) in Table 2 are taken from samples tested by Suringar (2004). The remaining 8 sample results presented in Table 2 are taken from XRD tests undertaken by IMERYS minerals Limited on samples sent to Imperial College London Consultants for strength testing. The results are presented for samples of increasing magnitude of dry density and assigned alteration grade. During the sampling process intermediary classes, such as II–III and III–IV, were assigned to the varying grade material by the same experienced geologist using the criterion in Table 1. The respective alteration grades are also provided in Table 2. No unaltered granite (Grade I) was present within the Wheal Martyn Pit. Table 2 shows the relationship between dry density, alteration and porosity for the different grade material determined by Suringar (2004). Marked changes in density and porosity are observed in Grades IV and V material, which can be linked to the observed variations in mineralogy. Fig. 4 emphasizes the relationship between alteration grade and porosity. X-ray diffraction analyses were undertaken on an

Table 2 Mineralogy from XRD analyses, dry density, porosity and alteration grade for representative samples of kaolinized granites (adapted from Suringar (2004) and ICLC (2004)). Samples marked with an asterisk are taken from Suringar (2004). Density Alteration Por. grade % kg/m3 1850* 1880* 1910* 2136 2162 2186 2199 2209 2210 2210* 2320* 2425 2520* 2540* 2540* 2570* 2570* 2604

V V V IV/V IV/V III/IV IV IV IV IV IV III III II/III II/III II II II

Quartz Feldspar Albite Tourmaline Mica Kaolinite % % % % % %

28.0 40 31.0 41 29.0 45 32 33 34 30 36 32 15.0 34 12.0 37 29 4.5 30 3.4 34 4.5 30 2.7 29 3.7 29 27

0 1 0 30 35 33 37 34 30 34 33 31 33 36 36 35 36 41

0 0 0 0 0 0 0 0 0 0 3 6 19 17 14 12 20 9

8 3 2 3 1 2 2 4 2 5 5 5 8 7 7 7 8 8

14 14 10 15 18 17 12 14 18 13 14 21 8 6 12 13 5 14

38 41 43 20 13 14 19 18 18 14 8 8 2 0 1 4 2 1

automated Siemens X-ray diffractometer model D5000. The sample powders are presented to the machine in cavity mount sample holders and were only very lightly compressed in order to minimize the degree of orientation of the platy clay particles and minimize any biased response of the reflections of kaolinite and mica. Proprietary software is used to convert the diffractograms to quantitative determinations. The machine is calibrated before and after a test run using a standard made with quartz from Arkansas sandstone. The results are normalized to 100% before reporting. 2.3. Uniaxial compressive strength and kaolinization of hydrothermally altered granites from Wheal Martyn Pit Fig. 5 summarizes the results of previous strength testing undertaken during 2004 on different grades of altered material from Wheal Martyn

Fig. 3. Crossed-polars optical micrograph images of a) slightly altered, b) moderately altered, c) highly altered and d) completely altered granite. Note: k = K-feldspar, p = plagioclase, q = quartz, m = muscovite, t = tourmaline and ka = kaolinite. Adapted from Suringar (2004).

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Fig. 4. Increased porosity with increased alteration. Adapted from Suringar (2004).

Pit. As part of the investigation field strength testing, using a Schmidt Hammer type ‘N’, was undertaken and representative drill core samples were also sent to Imperial College London Consultants (ICLC) (2004) for uniaxial and multistage triaxial testing. The uniaxial compressive strength (UCS) data presented in Fig. 5 is taken from the Imperial College London Consultants report. The data confirm that there is an obvious strength reduction with increased alteration; with major decrease in uniaxial compressive strength between Grade II and Grade III material. The results also reconfirmed that the Schmidt Hammer can be used as a useful field guide for evaluation of alteration grade. However, the sampling regime emphasized the variability of alteration even within samples taken in close proximity to each other. Given this variability there can be difficulty in assigning a specific alteration grade to in-situ samples (hence the occasional use of terms II/III, III/IV or IV/V). In order to provide further evaluation of the engineering characteristics of altered granites from Wheal Martyn, and in particular the dramatic reduction in uniaxial compressive strength between Grade II and Grade III, representative block samples were taken for further laboratory evaluation to complement and supplement the previous testwork of Chilton (2006), Suringar (2004) and Imperial College London Consultants (ICLC) (2004). To ensure consistency, allocation of alteration grade was undertaken by the same experienced geologist

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that carried out the task for the previous testwork. Instrumented uniaxial compression tests on the various alteration grade samples were carried out on a MTS servo-controlled hydraulic testing machine in accordance with ISRM guidelines (ISRM, 2007). Axial displacement was recorded by an LVDT (Linear Variable Displacement Transformer). In addition, a lateral micro-measurement N2A series electric resistance strain gauge (2.54 mm length) was secured to each sample, with MBond 610 epoxy resin, to record lateral strain development on loading. Care was taken to ensure adequate bonding of the strain gauges at representative locations (avoiding large feldspar grains, if present). Representative samples were cored from the Grades II and III altered granite blocks for testing. The cored samples were measured and weighed so that analysis of the results could also be evaluated against dry density, rather than solely relying on alteration grade determined from Schmidt Hammer tests. The following parameters were investigated: uniaxial compressive strength, dry density, ultrasonic P-wave velocity, elastic modulus and, importantly, %kaolinite to establish any relationships between the parameters. Where possible, data from previous tests undertaken by Suringar (2004), Imperial College London Consultants (ICLC) (2004) and Chilton (2006) was included in the database. Perceived Grade IV and, in particular IV/V, samples were not subjected to strength testing as part of the current study, but were included in testwork by Imperial College London Consultants (ICLC) (2004) where more altered samples were obtained by triple tube core drilling undertaken by IMERYS Minerals Limited. Further petrographic analysis was also undertaken on the Grades II and III samples that were taken for strength determination, and is summarized in Tables 3 and 4, which agreed with previous petrographic work undertaken by Suringar (2004). The key differences between Grades III and II are more intense alteration of plagioclase to secondary mica, initial growth of secondary mica on K-feldspars and the increased occurrence of intra-granular fractures. These observations are similar to the previous findings of Suringar (2004). Fig. 6 summarizes the results for the combined updated database (including results from previous tests and tests undertaken as part of the current study) for uniaxial compressive strength and dry density; highlighting the marked reduction in strength as a result of increased alteration (up to 70% reduction in strength from Grade II to Grade III). Fig. 6 includes the strength data from Imperial College London Consultants (ICLC) (2004) and data from 5 Grade II to 5 Grade III samples tested under uniaxial loading conditions as part of the current study. Importantly, there is consistency in strength magnitude for the Grades II and III samples when the data from both testing regimes are compared. Table 5 provides a summary of the test results for Grades II and III samples. Table 3 Petrographic description of Grade II mineralogy with typical modal percentages. Texture:

Slightly altered, porphyrytic medium grained anhedral granular with feldspar megacrysts from 5 to 15 mm.

Mineralogy

Description

Quartz:

Fig. 5. Uniaxial compressive strength and Schmidt rebound reduction with increased alteration grade.

Equant subhedral to anhedral grains (2–5 mm). Also occurs as fine grains in groundmass. Inclusions of mica. K-feldspar: Microperthic; euhedral megacrysts; 5 to 15 mm; 5% of megacysts consist of inclusions of 0.4 mm anhedral quartz, 0.3 mm muscovite and 1 mm plagioclase, that is 10–40% altered to secondary mica; also occurs as medium grained anhedral crystals. Plagioclase: Columnar subhedral grains up to 5 mm; 7o extinction angle suggests albite or oligoclase; inclusions of 250 μm muscovite; commonly shows advanced seritisation and growth of secondary mica (covers 10–20% of grain area, rarely 50–60%). Tourmaline: Anhedral, 2 mm; equant as well as elongated grains; show intergrowth with quartz. Muscovite: Anhedral columnar grains, 0.5–2 mm; can occur as intergrowths with feldspars and within groundmass. Biotite: Slightly ragged grains, up to 0.5 mm; may show minor chloritization.

Modal % 20–25 30–35

30–35

1 1–2 2–5

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Table 4 Petrographic description of Grade III mineralogy with typical modal percentages. Texture:

Mineralogy

Moderately altered, porphyritic medium-grained anhedral granular with feldspar megacrysts. Plagioclase and K-feldspars more obvious by slight yellowing and buff-cream coloration respectively. Description

Modal %

Quartz

Equant sub-hedral to anhedral grains, up to 5 mm; contain 30–35 inclusions of mica and feldspars; some crystals have fractures (intragranular cracks) filled with secondary micas. K-feldspar Megacrystic; euhedral; 5 to 15 mm; Albite/plagioclase 20–25 inclusions in microperthitic texture generally altered to secondary mica; minor growth of secondary mica on K-feldspar crystals; intragranular cracks filled with secondary mica. Plagioclase Euhedral to sub-hedral grains, up to 4 mm; intense seritisation 30–35 common; all crystals contain 10–70% secondary mica. Tourmaline Euhedral to anhedral, 3–5 mm. 1–3 Muscovite Anhedral columnar grains; can occur as intergrowth and 1–2 groundmass. Biotite Ragged grains, up to 0.5 mm, with chloritization and alteration 2–4 halos surrounding apatite inclusions.

Fig. 7 highlights the corresponding increase in %kaolinite with reduction in dry density and associated increase in alteration using the combined data from the testwork of Suringar (2004) and Imperial College London Consultants (ICLC) (2004). Figs. 6 and 7 can be also used to compare alteration grades (in terms of Grades II, III, IV and V) against %kaolinite and uniaxial compressive strength using the dry density range for a specific alteration grade identified by previous testwork from Suringar (2004) and Imperial College London Consultants (ICLC) (2004), shown in Table 2. Figs. 8 and 9 show relationships between ultrasonic P-wave velocity and dry density and uniaxial compressive strength respectively for the recent tests undertaken on Grades II and III samples. The results confirm that P-wave velocity can be used as a useful indicator of both uniaxial compressive strength and dry density for the south-west England hydrothermally altered granites (Grades II and III). Fig. 10 highlights the changing engineering behavior, in terms of reducing elastic modulus and uniaxial compressive strength of the samples, as a result of alteration from Grade II to Grade III. The majority of the data plot in the modulus ratio range between 200:1 and 500:1, which is typical of most rock types. The change in engineering behavior is also emphasized in Fig. 11 which provides typical axial stress versus axial and lateral strain responses for the recently tested samples of varying dry density. There is a marked difference between the relatively stiff response of higher density samples (>2600 kg/m3), typical of Grade II

Fig. 6. Reduction in uniaxial compressive strength with reduction in dry density.

Table 5 Test results showing dry density, uniaxial compressive strength, P-wave velocity, elastic modulus for Grades II and III samples. Density kg/m3

Alteration grade

P-wave velocity m/s

UCS MPa

Modulus, E GPa

Schmidt Hammer Type N

Schmidt Hammer Type L

2405 2424 2459 2488 2523 2604 2612 2619 2625 2639

III III III III III II II II II II

1846 2298 2941 2570 2816 4820 4853 4807 4667 5034

30 38 35 63 51 115 186 138 122 156

3 7 9 16 24 44 52 49 51 49

30–39

24–30

50–65

42–58

samples, when compared to the less dense samples (density between 2400 and 2520 kg/m3) which are more typically associated with Grade III. This difference in deformational behavior between Grade II and Grade III samples is evaluated in further detail in the following section. 3. Fracture development and deformation behavior under uniaxial loading Eberhardt (1998), Eberhardt et al. (1998), Diederichs et al. (2004) and, more recently, Ghazvinian et al. (2012) have shown how instrumented uniaxial compression tests can be used to observe and record the deformational behavior and brittle fracture propagation of rock under compressive loading conditions. Analysis of the axial, lateral and volumetric strain versus axial stress and acoustic response under compressive loading allows identification of specific deformation phases and damage thresholds including crack closure (σcc), linear elastic deformation, crack initiation (σci or CI), stable crack propagation and subsequent coalescence (σcs) and crack damage (σcd or CD) leading to ultimate failure. Guidance for identification of the appropriate thresholds from laboratory data is provided in Eberhardt et al. (1998), Diederichs et al. (2004), Ghazvinian et al. (2012) and Martin and Chandler (1994). Fig. 12 shows the stages of crack propagation suggested by Martin and Chandler (1994). Diederichs (2003), Diederichs et al. (2004) and Ghazvinian et al. (2012) also discuss the concepts of long-term strength and short-term (yielding) in-situ strength of rock associated with crack initiation (CI) and crack damage (CD), and

Fig. 7. Increase in %kaolinite with reduction in dry density (adapted from data provided in Suringar (2004) and XRD test results for samples sent to Imperial College London Consultants for strength testing. Corresponding data is shown in Table 2).

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Fig. 8. Increase in ultrasonic P-wave velocity with increase in dry density (Grade II and Grade III).

development of the spalling limit envelope for brittle rock behavior. Damjanac and Fairhurst (2010) also discuss the concept of long-term strength of crystalline rock. Eberhardt et al. (1999) showed the importance of mean grain size on the uniaxial compressive strength of granites and granodiorites at the underground research laboratory in Pinawa, Manitoba (confirming the inverse relationship between uniaxial compressive strength and grain size). Tugrul and Zarif (1999) also show the influence of grain size and the quartz to feldspar ratio on the strength of granites from Turkey. Suorineni et al. (2009) discuss the importance of rock, mineralogical composition, texture, foliation, damage (microcracking) and porosity microstructure on the tenacity/toughness of rocks including granites and granodiorites. Suorineni et al. (2009) also discuss the concept of the “brittleness index”, which is usually defined as the ratio of uniaxial compressive to tensile strength and used to assess how brittle a rock is, the higher the ratio the more brittle the rock. They emphasize that this parameter has no reference to the rock's resistance to stressinduced damage, which is important when assessing the likely performance of a mine slope. These authors prefer the use of the terminology rock tenacity or toughness to describe the rocks ability to withstand changes in stress. In assessing the tenacity or toughness of rock types Suorineni et al. (2009) use the dimensionless texture coefficient of a

Fig. 9. Increase in ultrasonic P-wave velocity with increase in uniaxial compressive strength (Grade II and Grade III).

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Fig. 10. Relationship between modulus and uniaxial compressive strength for samples of different alteration grade (Grade II and Grade III). Data plotted of log v log plot with respect to modulus ratio from 200:1 to 500:1.

rock which accounts for its grain shape, orientation, degree of grain interlocking, packing density and grain size distribution. In order to provide further insight into factors influencing the deformational behavior of altered material the series of instrumented uniaxial compression tests also included recording of acoustic emission (AE) activity, together with Scanning Electron Microscope (SEM) images taken at various stages of failure in order to characterize the fracture development and damage processes. A minimum of 2 Pico AE sensors was used for each test to detect emission data. The AE sensors were attached to the either side of the sample using tape (upper left and bottom right). Prior to starting each uniaxial test a ‘pencil break’ test was undertaken to ensure good contact between the sensor and the rock surface and the sensor was operating effectively. The acoustic data was processed using the MISTRAS system (PAC, 1995) and NOESIS software (Envirocoustics, 2010), allowing either graphic display via the PC monitor or conversion to a text file for use in Excel. The system used the AEDSP (Acoustic Emission Digital Signal Processing)

Fig. 11. Varying axial stress versus axial and lateral strain response for samples of different dry density/alteration grade.

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Fig. 12. Stress–strain diagram showing the stages of crack development. After Martin and Chandler (1994).

board to process all necessary data including time, amplitude, duration, counts and rise time. Several researchers have used SEM images to examine the development, interaction and propagation of fractures within different rock types (Wong, 1982; Bobet and Einstein, 1998; Sousa et al., 2005). Chilton (2006) used SEM images of polished mounts prepared from samples tested to specific stages of a uniaxial test to observe fracture development and associated material behavior. Prior to mounting and polishing the samples were first impregnated with resin and allowed to harden. Preparation of the polished mount was then carried out in a number of stages beginning with coarse grade silicon powder and decreasing to a very fine diamond paste (25 μm). Prior to analysis the polished surface was coated with carbon to prevent surface charging under the SEM electron beam.

Fig. 14 provides example axial, lateral and volumetric strain and stiffness versus axial stress plots, together with identified strainderived damage thresholds, according to Eberhardt et al. (1998), for one of the Grade II samples tested (density 2619 kg/m 3, strength 138 MPa). The sample exhibits a distinct bedding-in phase (crack closure), an extensive linear axial stress versus axial strain response and a relatively short non-linear phase prior to failure. The lateral strain plot shows a slope gradient change around 124 MPa which may be related to sudden deformation or localized damage in close proximity to the strain gauge. Fig. 14 also shows the clear slope reversal on the volumetric strain and stiffness plot, where the slope of the curve

3.1. Stress–strain behavior of Grade II and Grade III material under uniaxial loading Fig. 11 shows typical axial stress versus axial and lateral strain responses for selected samples across the Grades II–III range of dry densities. Comparison between the deformational response of samples with a dry density of 2639 kg/m3 and a dry density 2405 kg/m3 emphasize the variation within the Grades II–III density range. A summary of the deformational behavior of Grade II and Grade III material is provided below. 3.1.1. Grade II, slightly altered granite Fig. 13 shows typical axial stress versus axial and lateral strain responses for recently tested samples of Grade II material, with dry densities typically above 2600 kg/m 3 (range 2604 to 2639 kg/m 3). All samples tested had similar elastic moduli (49 ± 3 GPa), but the uniaxial compressive strength varied between 122 and 184 MPa (140 ± 28 MPa). This range in strength can be attributed to the porphyritic medium-grained texture of the samples and potential influence of large feldspars, resulting in sudden brittle failure of several samples.

Fig. 13. Axial stress versus axial, lateral and volumetric strain for slightly altered granite (Grade II) under uniaxial compression.

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as depicted in Fig. 15. The results are presented for the same Grade II sample shown in Fig. 14. The ITLS approach emphasizes the change in slope of the lateral strain versus axial stress plot and provides an improved methodology for determination of the crack initiation (CI) threshold. Acoustic Event or AE-derived threshold evaluation is considered in Section 3.2.

3.1.2. Grade III, moderately altered granite Fig. 6 emphasized the dramatic reduction in uniaxial compressive strength for Grade III samples when compared to Grade II samples (typically 70% reduction in strength). Fig. 16 provides typical axial stress versus strain responses (axial, lateral and volumetric) for several Grade III samples within the range of dry densities tested (2405 to 2523 kg/m3). There is clear variability in both uniaxial compressive strength (43 ± 13 MPa) and elastic modulus (12 ± 8 GPa) for Grade III material, but there is overall reduction in both uniaxial compressive strength and elastic modulus with reduction in dry density, as indicated in Figs. 6 and 11. Grade III samples, with dry density values towards the lower end of their dry density range, show an extensive bedding-in phase, a relatively short linear elastic phase and a marked non-linear response prior to reaching their ultimate strength. A more ‘brittle’ response is associated with Grade III samples that have dry density values towards the upper end of their dry density range. Fig. 17 shows axial, lateral and volumetric strain versus stiffness plots, together with identified damage thresholds for one of the Grade III samples tested (density 2405 kg/m 3, strength 30 MPa). The crack damage threshold (slope reversal on the volumetric strain and stiffness plot) is clearly observed at approximately 18 MPa (60% of peak strength) which is markedly different to that observed for the Grade II sample in Fig. 14 (122 MPa or 88% of peak strength). Fig. 18 shows use of the ITLS approach for determination of the crack initiation (CI) threshold for the Grade III sample, where there is an obvious gradient change around 6 MPa. Using this approach the crack damage (CD) threshold is less obvious when compared to the clearly observed volumetric strain and stiffness reversal seen on Fig. 17 (equivalent CDstrain derived value highlighted on Fig. 18 for comparison purposes) due to the curved nature of the ITLS response and difficulty in assigning potential gradient changes.

Fig. 14. Axial strain and axial stiffness, lateral strain and lateral stiffness and volumetric strain and volumetric stiffness versus axial stress plots for an example Grade II sample. A, B, C, D and E represent crack closure, crack initiation, crack coalescence, crack damage and peak strength values.

changes from positive to negative, which is indicative of the crack damage (CD) threshold. Other Grade II samples tested did not have such a clear slope reversal on the volumetric stiffness plot in view of sudden brittle failure. Recent work by Diederichs et al. (2004) and Ghazvinian et al. (2012) suggest that evaluation of crack thresholds using the strain-derived approach should focus on the use of lateral strain through utilization of the Inverse Tangent Lateral Stiffness (ITLS) plot,

Fig. 15. Use of Inverse Tangent Lateral Stiffness (ITLS) approach to determine crack initiation threshold for example Grade II sample.

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Fig. 16. Axial stress versus axial, lateral and volumetric strain for moderately altered granite (Grade III, dry density range 2405–2523 kg/m3) under uniaxial compression.

3.2. AE behavior of Grade II and Grade III material under uniaxial loading Fig. 19 highlights the varying AE response recorded during uniaxial loading for typical Grade II and Grade III samples. The axes shown on the 3D charts are counts, parametric 1 (Load in kN) and event amplitude (dB). Only AE recorded above 50 dB is provided on Fig. 19. The respective dry densities for samples a) to d) shown on Fig. 19 are: a) 2405 kg/m3 (Grade III), b) 2488 kg/m3 (Grade III), c) 2619 kg/m3 (Grade II) and d) 2625 kg/m 3 (Grade II). The associated axial stress versus axial and lateral strain behavior for samples labeled a) and b) on Fig. 19 is provided in Fig. 16 and for samples c) and d) in Fig. 13. Axial, lateral and volumetric strain and stiffness versus axial stress plots, together with identified strain-derived damage thresholds for samples a) and c) are given in Figs. 17 and 14 respectively. Diederichs et al. (2004) and Ghazvinian et al. (2012) summarized the methodology to determine AE-derived damage thresholds, based on changes in slope using a cumulative counts versus axial stress plot (logarithmic axes). Fig. 20 provides cumulative counts versus axial stress plots for the Grade III and Grade II samples depicted in Fig. 19. Fig. 21 shows AEderived crack initiation and crack damage thresholds for the same Grade II sample used to determine strain-derived thresholds given on Figs. 14 and 15. Grade II samples tend to have AE activity associated with beddingin or crack closure followed by a relatively ‘quiet’ response period during linear elastic behavior. Figs. 19d) and 20d) indicate that localized damage resulting in increased AE activity can be associated with Grade II material. Bursts of activity, related to crack initiation and subsequent fracture development, occur prior to increased AE activity approaching failure. Grade III samples, Fig. 19a) and b), tend to show more continuous AE activity over a wider amplitude range throughout uniaxial loading than Grade II samples (Figure 19c) and d)). Identification of any AE-derived crack damage (CD) threshold for Grade III samples is more difficult than for Grade II samples. This is highlighted in Fig. 20a) where there is monotonic increase in AE activity throughout the Grade III test until immediately prior to peak stress. The clear volumetric strain and stiffness reversal shown on Fig. 17 (strain-derived CD) is not associated with increased AE activity on Fig. 20d). All samples, whether Grade II or III, exhibit increased AE activity approaching failure, with a tendency for increased event amplitude which suggests that larger amplitude emissions approaching failure

Fig. 17. Axial strain and axial stiffness, lateral strain and lateral stiffness and volumetric strain and volumetric stiffness versus axial stress plots for an example Grade III sample. A, B, C, D and E represent crack closure, crack initiation, crack coalescence, crack damage and peak strength values.

are probably a result of fracture propagation, coalescence and ultimately macro-crack development. Fig. 22 provides a comparative summary of identified crack thresholds for the Grade II and Grade III samples tested. The observed crack threshold levels have been normalized with respect to the maximum stress or uniaxial compressive strength of the samples tested.

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Crack initiation (CI) thresholds vary across the Grade II and Grade III samples. For Grade II samples the average strain-derived CI threshold is 42 ± 14%, whereas for Grade III is 30 ± 8%. This variability is not unexpected given the porphryritic nature of the material, as large grains will result in heterogeneity and mechanical instability, the presence of high cleavage feldspars and quartz that will result in mechanical incompatibility and alteration of feldspars that may create damage nucleation sites. The average strain-derived crack damage threshold determined for Grade II samples is 89 ± 9% of the peak strength, whereas for Grade III is 57 ± 15%. The higher CD values observed for Grade II samples are associated with sudden, brittle failures of the stiffer material. AE-derived CD thresholds for Grade III samples are not included on Fig. 22 as they could not be easily identified. Further work is required to assess to suitability of both strain and AE-derived threshold techniques for varying grade material. The techniques proposed in the literature have principally been developed for ‘brittle rock’ and are therefore more appropriate for Grade II material.

3.3. SEM analysis of altered granites

Fig. 18. Use of Inverse Tangent Lateral Stiffness (ITLS) approach to determine crack initiation threshold for example Grade III sample.

Chilton (2006) used SEM images of polished mounts prepared from samples tested to specific stages of a uniaxial test to observe fracture development and associated material behavior. Analysis of Grade II

Fig. 19. Example AE behavior for different Grade II and Grade III samples. The axes on the 3D bar charts are counts, parametric 1 (load) and event amplitude (dB). The respective densities for samples a) to d) are: 2405, 2488, 2619 and 2625 kg/m3.

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Fig. 20. Cumulative counts versus axial stress plots for Grade III and Grade II samples shown in Fig. 19. The respective densities for samples a) to d) are: 2405, 2488, 2619 and 2625 kg/m3.

samples from Wheal Martyn Pit indicated that prior to testing grain boundary cracks are present between all minerals and in all orientations. Examination of the internal structure of samples tested within their linear elastic response revealed no new damage below crack initiation. Subsequent loading beyond crack initiation resulted in propagation of grain boundary cracks in directions sub-parallel to the loading direction. Following the crack damage threshold there is a significant increase in AE emission, with associated increases in amplitude and rise-time. SEM analysis suggests that this is associated with further coalescence of grain boundary cracks leading to subsequent failure of the sample. Feldspars aligned perpendicular to major fractures

Fig. 21. AE-derived crack initiation and crack damage thresholds for Grade II sample.

displayed a stepped fracture pattern, while feldspars aligned parallel to major fractures showed evidence of axial splitting along cleavage planes, as shown in Fig. 23. Previous SEM analysis of Grade III material from Wheal Martyn Pit by Chilton (2006) indicated that prior to testing samples showed the presence of grain boundary fractures between unaltered minerals; the kaolinized feldspars showed evidence of both cleavage splitting and voids. Some cracks contained secondary mica. Following crack initiation SEM analysis suggests that in some areas the material showed evidence of minor initiation of grain boundary cracks, with

Fig. 22. Observed crack initiation and crack damage threshold values for Grade II and Grade III samples.

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Fig. 23. SEM evidence of both stepped failure and splitting within cleavage adjacent to major failure surface in Grade II sample.

unaltered minerals remaining undamaged, while crack initiation and subsequent growth is concentrated in kaolinized regions. During the unstable crack propagation phase of the axial stress versus axial strain response zones of coalescing microfractures propagate between kaolinized regions. This results in clear linkages between kaolinized feldspars, as shown in Fig. 24. 4. Discussion Instrumented uniaxial compressive strength tests on Grades II and III materials have been used to characterize the axial stress versus axial strain, lateral strain and volumetric strain and acoustic response under uniaxial loading conditions. The results highlight the more

‘brittle’/stiffer response of Grade II material when compared to Grade III material. This variability is not unexpected given the porphryritic nature of the material, as large grains result in heterogeneity and deformation modulus contrasts between minerals resulting on potential stress concentrations. The presence of high cleavage feldspars and quartz result in mechanical incompatibility. Scanning Electron Microscope (SEM) image analysis of fracture development has provided useful additional data for interpretation of axial stress versus axial, lateral and volumetric strain and acoustic emission (AE) data. Fig. 24 highlights the brittle fracture propagation occurring in the quartz crystals linking the more ductile regions of kaolinization. Analysis of the acoustic emission data, constrained by evaluation of strain gauge deformation readings, has been used to identify crack propagation stages during uniaxial compression. The data shows variability within both the strain-derived and AE-derived crack initiation (CI) threshold for both Grade II and Grade III samples. The typical CI values fall within the 20 to 50% of peak strength suggested by Diederichs et al. (2004). Variability of crack initiation (CI) and crack damage (CD) threshold values reflects the potential for sudden brittle failure, and the mechanical instability associated with Grade II material (porphyritic texture, presence of quartz and feldspar megacrysts). Diederichs (2007) also discussed how sample heterogeneity can cause complex stress distribution inside a sample, that can trigger different failure mechanisms inside the sample resulting in axial splitting and shear failure. The use of the Inverse Tangent Lateral Stiffness (ITLS) plot proved successful in the identification of crack initiation (CI), but less robust for identification of the crack damage (CD) threshold, particularly for Grade III material when compared to the clearly observed volumetric strain and stiffness reversal due to the curved nature of the ITLS response and difficulty in assigning potential gradient changes. The use of the AE-technique for determination of the CD threshold was less successful than the strain-based technique (slope reversal on the volumetric strain and stiffness plot) for Grade III samples, which may relate to the observed ‘less-brittle’ deformational behavior on the axial stress versus axial strain plot. The AE response for Grade III samples tends to show more continuous AE activity over a wider amplitude range throughout uniaxial loading than Grade II samples. All samples, whether Grade II or III, exhibit increased AE activity approaching failure, with a tendency for increased event amplitude which suggests that larger amplitude emissions approaching failure are probably a result of fracture propagation, coalescence and ultimately macro-crack development. 5. Conclusions

Fig. 24. SEM image of fracture coalescence/linkage between kaolinized regions within moderately altered, Grade III sample.

Index testing has been used to establish relationships between dry density, %kaolinite, P-wave velocity, uniaxial compressive strength

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and elastic modulus for the hydrothermally altered granites at Wheal Martyn china clay pit. Correct field identification of alteration grade is particularly important for planned extraction, mining and subsequent processing of the altered granites. It is also critical for pit optimization and reconciliation purposes. The results of the testwork undertaken highlight the benefits of combining a variety of different tools such as instrumented uniaxial compressive tests to capture deformational and acoustic emission data with petrographic, X-ray diffraction and SEM analysis to evaluate the mineralogical controls on the engineering properties of altered granites. Increased levels of alteration result in a major reduction in the uniaxial compressive strength of the material. This is directly associated with changes in mineralogy, increase in %kaolinite, reduction in dry density and associated increase in porosity. A 70% reduction in uniaxial compressive strength typically occurs between alteration Grades II and III. This marked reduction in strength is also reflected in reduction of ultrasonic P-wave velocity and reduction in elastic modulus. Variability of crack initiation (CI) and crack damage (CD) threshold values reflects the potential for sudden brittle failure, and the mechanical instability associated with Grade II material (porphyritic texture, presence of quartz and feldspar megacrysts). Application and use of both strain-derived and AE-derived techniques for evaluation of crack thresholds suggest that further work is required to validate/optimize the methodologies for different rock types. Acknowledgments The authors would like to acknowledge the petrographic work undertaken by Ben Suringar and Ben Mason and previous testwork undertaken by Jody Chilton. The views expressed are not necessarily those of IMERYS Minerals Ltd. References Anon, 1995. The description and classification of weathered rocks for engineering purposes. Geological Society working Party Report. The Quarterly Journal of Engineering Geology 28, 207–242. Aydin, A., Basu, A., 2006. The use of Brazilian test as a quantitative measure of rock weathering. Rock Mechanics and Rock Engineering 39 (1), 77–85. Basu, A., Celestino, T.B., Bortolucci, A.A., 2009. Evaluation of rock mechanical behaviours under uniaxial compression with reference to assessed weathering grades. Rock Mechanics and Rock Engineering 42, 73–93. Baynes, F.J., Dearman, W.R., 1978. The relationship between the microfabric and the engineering properties of engineering granite. Bulletin of the International Association of Engineering Geology 18, 191–197. Bobet, A., Einstein, H.H., 1998. Fracture coalescence in rock-type materials under uniaxial and biaxial compression. International Journal of Rock Mechanics and Mining Science 37 (7), 863–888. Bristow, C.M., Exley, C.S., 1994. Historical and geological aspects of china clay industry of southwest England. Transactions of the Royal Geological Society of Cornwall 21, 247–314. Chen, Y., Watanabe, K., Kusada, H., Kusaka, E., Mabuchi, M., 2011. Crack growth in westerly granite during cyclic loading. Engineering Geology 117, 189–197. Chilton, J.L., 2006. The Evolution of the Brittle Deformation Process in Uniaxial Compression. Unpublished PhD thesis University of Exeter (324 pp.). Coggan, J.S., Stead, D., Chilton, J., Howe, J.H., Collins, R.C., 2007. Effects of alteration on the engineering behaviour and intact rock fracture characteristics of granite under uniaxial compression. In: Eberhardt, E., Stead, D., Morrison, T. (Eds.), Proceedings of 1st Canadian-US Rock Mechanics Symposium, Vancouver, 27–31 May, vol. 1. Taylor & Francis, pp. 547–555. Damjanac, B., Fairhurst, C., 2010. Evidence for a long-term strength threshold in crystalline rock. Rock Mechanics and Rock Engineering 43, 513–531. Dearman, W.R., Baynes, F.J., Irfan, T.Y., 1978. Engineering geology of weathered granite. Engineering Geology 12, 345–374. Diederichs, M.S., 2003. Rock fracture and collapse under low confinement conditions. Rock Mechanics and Rock Engineering 36 (5), 339–381.

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