Quarterly Journal of Engineering Geology and Hydrogeology, 45, 423 –433 DOI: 10.1144/qjegh2012-021.

1470-9236/12 $15.00 © 2012 Geological Society of London

Ophiolitic rocks represent remnants of the Earth’s oceanic crust and upper mantle, derived from mid-ocean ridges or marginal basins, and are made up of an association of typical rocks in a clearly defined sequence. In general, a complete ophiolite suite consists from bottom to top of ultrabasic, basic, hypabyssal and extrusive rocks. The study of the engineering properties of ophiolitic rocks is of special interest, as they include a significant number of lithotypes. They are used extensively as construction materials, including aggregates for highway works, concrete and railway ballast. Ocean-floor metamorphism typically obscures pri-mary petrographic characteristics of ophiolitic rocks, leading to a modification of their strength properties (e.g. Ramana et al. 1986; Marinos et al. 2006; Diamantis et al. 2009; Ozsoy et al. 2010; Rigopoulos et al. 2010; Ündül & Tuğrul 2012).

Petrography is of fundamental importance in understanding the engineering properties of rocks and its use in the evaluation of aggregates dates back to the early part of the 20th century (Knight & Knight 1935; Rhoades & Mielenz 1946). The physico-mechani-cal parameters of rocks mainly depend on their mineralogy, tex-ture, degree of chemical alteration, weathering and deformation (e.g. Hartley 1974; Åkesson et al. 2001; Zorlu et al. 2004; Tamrakar et al. 2007). Ophiolitic rocks exhibit a wide range of chemical com-positions, as well as of mineralogical and textural features, thus the existence of lithotypes with variable mechanical properties is very common even in the same suite.

Fresh ophiolitic rocks have sufficient strength to meet any engi-neering requirement; however, the effect of ocean-floor metamor-phism variously changes their physico-mechanical properties (Rigopoulos 2009). Based on the results of physico-mechanical tests and quantitative analysis of thin sections, this study aims at investigating the effect of microscopic characteristics on rock

strength and especially the relationships between the engineering properties and specific petrographic transformations that take place during alteration processes and are very common in ultrabasic rocks worldwide. The evaluation of the properties of the studied samples as aggregate material is also attempted, with emphasis on the assess-ment of their performance in-service. Fourteen ultrabasic samples from the Pindos, Vourinos and Koziakas ophiolite suites were col-lected and subjected to various laboratory tests. The studies involved the following stages: (1) detailed petrographic examination and quantification of the mineralogical composition using a point-counting system; (2) determination of the physical and mechanical properties; (3) observation of the surface texture of the rock parti-cles with a scanning electron microscope; (4) investigation of the relationships between selected variables using regression analysis.

Geological settingThe Pindos and Vourinos ophiolites belong to the west Greek ophiolitic belt (Fig. 1) and are thought to be continuous beneath the Cenozoic molasse of the Mesohellenic trough (Jones et al. 1991; Rassios & Smith 2000). The Koziakas ophiolite also belongs to the west Greek ophiolitic belt (Fig. 1) and it was probably developed, together with the Pindos, Vourinos and Othrys ophiolite suites, in the same marginal basin volcanic arc regime in the Pindos ocean (Pomonis et al. 2005, 2007). For the present study, ultrabasic samples were collected from these ophiolite suites to investigate the interdependence between pet-rographic features and engineering properties.

The Pindos ophiolite is subdivided into the overlying ultra-mafic massif of the Dramala unit and the underlying, and more

The impact of petrographic characteristics on the engineering properties of ultrabasic rocks from northern

and central Greece

I. Rigopoulos1*, B. Tsikouras1, P. Pomonis2 & K. Hatzipanagiotou1

1University of Patras, Department of Geology, Section of Earth Materials, GR-265 00 Patras, Greece2University of Athens, Department of Geology and Geoenvironment, GR-157 84 Athens, Greece

*Corresponding author (e-mail: rigopoul@upatras.gr)

Abstract: This paper investigates the influence of petrographic characteristics on the engineering properties of ultrabasic rocks from the Pindos, Vourinos and Koziakas ophiolites. The studied litholo-gies include dunites, olivine-rich harzburgites, typical harzburgites and plagioclase-bearing lherzolites. Quantitative petrographic analysis indicates that the samples display various percentages of secondary minerals. The peridotites are tested to determine the following engineering parameters: water absorp-tion, apparent density, total porosity, point load index, Schmidt hammer value, aggregate abrasion value and polished stone value. The relationships between the petrographic features and the engineering parameters are determined using regression analysis. The micropetrographic index Irep and the ratios of secondary to primary (Se/P) and soft to hard (So/H) minerals provide significant information about the impact of ocean-floor metamorphism on the strength and durability of peridotites. The correlations indicate that the formation of secondary minerals results in mechanically weaker rocks. The polished stone value is the only engineering parameter that is positively affected by the presence of low con-tents of alteration products. The results also imply that Cr-spinel grains are a major determinant for the preservation of the microroughness of aggregate particles owing to their very hard nature and different mechanical behaviour compared with the surrounding silicate minerals.

2012-021research-articleResearch Article45X10.1144/qjegh2012-021I. RigopoulosEngineering Properties of Ultrabasic Rocks2012

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differentiated, Aspropotamos complex (Jones & Robertson 1991). The Dramala unit is composed of large, variably depleted harzburgite–dunite masses (Economou-Eliopoulos & Vacondios 1995). Peridotites were collected from the central part of the Pindos ophiolite, in a well-preserved mantle section, which belongs to the Dramala unit. The dunite and olivine-rich harzbur-gite exposures of the study area gradually pass into serpentinized harzburgites and minor lherzolites.

The Vourinos ophiolite is a harzburgite–dunite-dominated peri-dotite, with a partly preserved overlying crustal succession, includ-ing gabbros and minor plagiogranites, a sheeted dyke complex (Asprokampos dykes), and locally hypabyssal and extrusive mafic rocks (Krapa series) (Moores 1969; Rassios et al. 1983). Detailed fieldwork in the Vourinos mantle section has revealed that the dun-ite exposures are surrounded by transitional zones of serpentinized harzburgites, which are intruded by veins and lenses of serpenti-nized dunite. Locally, the serpentinized dunites are also intruded by pyroxenitic veins. Contacts between the dunite and the adjacent harzburgite may be either sharp or gradational. Vourinos dunites contain economic chromite deposits (Kapsiotis et al. 2006), whereas chromite is not so abundant in the Pindos ophiolite.

The ultramafic mantle tectonites are, by far, the most volumi-nous rocks of the ophiolitic unit of Koziakas (Pomonis et al. 2005). Sampling was performed in the northern part of the Koziakas ophi-olite, which comprises mantle peridotites with various degrees of serpentinization. These are predominantly harzburgites and sec-ondarily plagioclase-bearing lherzolites.

Methods and resultsSample preparation

Representative samples of peridotites with various degrees of serpentinization and variable proportions of secondary minerals were collected from the Pindos, Vourinos and Koziakas ophiolites. A total of 56 polished thin sections were prepared to thoroughly examine their specific mineralogical and textural characteristics. Among these thin sections, 28 representatives were selected for the quantification of their petrographic features.

The percentages of the mineral phases were determined with a polarizing microscope, by the point-counting method. Approxi-mately 300 equally distributed points were counted on each thin section. Because of the fine-grained character of the Pindos ser-pentinized harzburgite, 500 points were counted on this sample. Scanning electron microscopy (SEM) using energy-dispersive and wavelength-dispersive spectrometric analysis was carried out to determine the chemical composition of certain mineral phases. Also, secondary electron images were acquired from fresh frac-tures of aggregate particles to observe their surface texture.

The material was first crushed with a laboratory jaw crusher and then sieved to achieve the desired fraction for each test. Powdered samples were also prepared with a laboratory rock grinder for the determination of total porosity. Cubic rock specimens of 70 mm edge length were prepared for the Schmidt hammer test, and rec-tangular samples of various sizes were used for the point load test.

Petrography

The studied ultrabasic samples include dunites, olivine-rich harz-burgites, typical harzburgites and plagioclase-bearing lherzolites with various degrees of serpentinization and deformation.

The dunite and olivine-rich harzburgite samples, collected from Pindos and Vourinos, display variable textures. The Pindos samples show mainly porphyroclastic and locally cataclastic texture (Fig. 2a–c). The Vourinos samples display mylonitic (Fig. 2d), por-phyroclastic and/or cataclastic textures, whereas the serpentinized dunites have principally mesh texture (Fig. 2e) with rare relics of primary granular texture. The primary mineralogy of these rocks comprises olivine, orthopyroxene (enstatite) with exsolution lamellae of clinopyroxene (diopside) and disseminated Cr-spinel (Table 1). The only difference between dunites and olivine-rich harz-burgites is the slightly higher content of orthopyroxene in the latter. The secondary phases of these lithotypes include mostly serpentine and minor amounts of chlorite and talc. Veinlets filled with calcite rarely traverse the dunites. Olivine and orthopyroxene occur as both porphyroclasts and neoblasts. Evidence of plastic deformation is shown in strained olivine and orthopyroxene porphyroclasts, which display well-developed undulatory extinction, strain lamellae (Fig. 2a and f), bending and kink-bands. Intense deformation results locally in a preferred orientation of the olivine grains (Fig. 2f). In places, tec-tonized zones of fine-grained olivine and oriented transgranular microcracks were observed as a result of brittle deformation (Fig. 2c).

The harzburgite samples, collected from the Pindos, Vourinos and Koziakas ophiolites, are variably serpentinized and usually display cataclastic and/or porphyroclastic texture. Their primary mineralogy comprises olivine, orthopyroxene, clinopyroxene and spinel (Table 1). Olivine forms porphyroclastic grains, as well as smaller polygonal neoblasts with 120° triple junctions. As a result of hydrothermal alteration, the primary minerals are transformed into secondary products (olivine into hydrated associations of serpentine minerals, orthopyroxene mainly into bastite and talc, clinopyroxene into actinolite, tremolite and chlorite, and spinel into ferrian chromite and magnetite). The areas occupied by serpentine usually display pseudomorphic mesh and ribbon textures (Fig. 2g) (O’Hanley 1996). The deformation of these samples is primarily brittle, as expressed by the numerous intragranular and transgranu-lar microcracks. The olivine and orthopyroxene crystals also dis-play structures caused by plastic deformation such as undulose extinction and bending.

The plagioclase-bearing lherzolites collected from the Koziakas ophiolite show granular to porphyroclastic and locally cataclastic texture. Their primary mineralogy includes olivine, orthopyroxene, clinopyroxene (diopside), interstitial plagioclase (bytownite– anorthite) and Cr-spinel (Table 1). Olivine, orthopyroxene and clinopyroxene neoblasts occur as small, polygonal strain-free

Fig. 1. Simplified map showing the study areas and the distribution of the ophiolitic outcrops of Greece and neighbouring countries.

EnGInEERInG PRoPERTIES of UlTRAbASIC RoCKS 425

Fig. 2. Photomicrographs (cross-polarized light) of the studied peridotites: (a) dunite with cataclastic texture showing strain lamellae in olivine porphy-roclast (sample PKZ1); (b) orthopyroxene porphyroclast in olivine-rich harzburgite (sample PKZ2); (c) oriented transgranular microcracks in olivine-rich harzburgite (sample PKZ2); (d) typical mylonitic texture in dunite (sample VVD1); (e) mesh texture in serpentinized dunite (sample VAET2); (f) elon-gated olivine grains showing strain lamellae (sample VKOR1); (g) ribbon texture in serpentinized harzburgite (sample PKZSR1); (h) interstitial plagio-clase crystal in plagioclase-bearing lherzolite (sample KOZ1). Ol, olivine; Opx, orthopyroxene; Serp, serpentine; Pl, plagioclase.

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grains around the larger porphyroclasts or as fine-grained assem-blages, forming the rock matrix. Plagioclase occurs as single crys-tals with relatively wide straight twin lamellae and contains much less evidence of intracrystalline deformation than the olivine and orthopyroxene crystals of the host peridotite (Fig. 2h). Ocean-floor metamorphism resulted in the development of serpentine, talc, chlorite, actinolite, tremolite and Fe-oxides. Evidence of plastic deformation is shown in strained olivine and orthopyroxene por-phyroclasts, which display undulatory extinction and kink-bands.

The investigated ultrabasic samples from the Pindos, Vourinos and Koziakas ophiolites contain mainly intragranular microcracks, which are confined to the interior of a single grain, and sparsely transgranular microcracks, which cross more than one grain. Most of the latter type of microcracks have been sealed with serpentine and rarely with chlorite or calcite.

Surface texture

Surface texture is an expression of the microroughness of aggre-gate particles, and mainly depends on the petrographic features of rocks. SEM images were acquired from aggregate particles of the studied mantle peridotites to observe their microtopography. According to BS 812 (BSI 1975), there are the following quali-tative categories for the characterization of the surface texture: glassy, smooth, granular, rough, crystalline, honeycombed and porous. The studied fresh to moderately altered ultrabasic sam-ples exhibit rough surfaces with topographic highs of olivine, orthopyroxene and Cr-spinel, and lows occupied by serpentine (Fig. 3a–d). The Vourinos serpentinized dunites have slightly lower relief as can be seen in Figure 3e, whereas the highly ser-pentinized harzburgites exhibit mainly smooth surfaces, formed by serpentine (Fig. 3f).

Physical properties

The apparent density, water absorption and total porosity of the ultrabasic samples were determined. The results for each prop-erty are given in Table 2.

The apparent density (ρa) was determined according to the pro-cedure described in EN 1097-6 (CEN 2000). This property is related to the mineralogical and textural characteristics of the aggregates. The ρa values of the peridotites range between 2450 and 3350 kg m−3 (Table 2) and can be classified as moderate to very high according to the International Association of Engineering Geology (IAEG 1979).

The water absorption (wa) of the peridotites was determined in accordance with EN 1097-6 (CEN 2000) and it ranges from 0.14 to 3.17% (Table 2). It is a major property in evaluating the durability of rocks as aggregates, as it has been shown that rocks with wa values greater than 3% are susceptible to freeze–thaw damage (Shakoor et al. 1982).

Total porosity (nt) is an important factor in rock strength, as a small change in pore volume can produce an appreciable mechani-cal effect (ISRM 1981). In this study, the nt values were calculated by grinding the rock samples to fine powder and measuring the absolute volume of powder by fluid displacement in a pycnometer according to the ISRM (1981) specification. The nt values obtained for the mantle peridotites range between 2.75 and 9.59% (Table 2) and can be classified as low to medium according to the International Association of Engineering Geology (IAEG 1979).

Mechanical properties

The mechanical properties of the investigated ultrabasic rocks were determined by a variety of laboratory tests, including the Schmidt hammer value, the point load index, the aggregate abra-sion value and the polished stone value (Table 3).

The Schmidt hammer test is considered as a non-destructive method widely used in rock mechanics practice for a quick rock hardness and strength characterization. It is a significant mechani-cal parameter owing to its rapidity, portability and simplicity, and is usually related to the uniaxial compressive strength (e.g. Dinçer et al. 2004; Aydin & Basu 2005; Sabatakakis et al. 2008). The Schmidt hammer value (SHV) was determined using an L type Schmidt hammer on cubic rock specimens of 70 mm edge length, according to the procedure of ISRM (1981). The SHV results obtained vary between 38 and 64, with the most serpentinized and

Table 1. Modal compositions of the investigated samples, based on point-counting analysis with a polarizing microscope

Sample code/locality Lithotype Ol Opx Cpx Pl Sp Serp Tc Chl Tr/ Act

Cc Ox AP (%)

PM (%)

Irep

PKZ1/Pindos Dunite 85.5 2.5 – – 1.5 9.5 1.0 – – – – 10.50 88.00 0.12PKZ2/Pindos Ol-rich

harzburgite74.0 13.5 – – 1.5 10.0 1.0 – – – – 11.00 87.50 0.13

PKZSR1/Pindos Harzburgite 35.5 4.5 0.5 – 0.5 47.5 0.5 1.5 0.5 0.5 8.5 50.00 40.50 1.23VVD1/Vourinos Dunite 90.3 7.0 – – 1.6 1.1 – – – – – 1.10 97.30 0.01VVD2/Vourinos Ol-rich

harzburgite85.3 11.7 – – 1.7 1.3 – – – – – 1.30 97.00 0.01

VKOR1/Vourinos Dunite 85.5 3.5 – – 3.0 8.0 – – – – – 8.00 89.00 0.09VKOR2/Vourinos Ol-rich

harzburgite78.0 10.0 – – 2.5 9.5 – – – – – 9.50 88.00 0.11

VRZ1/Vourinos Dunite 88.0 2.7 – – 2.3 7.0 – – – – – 7.00 90.70 0.08VRZ2/Vourinos Dunite 84.0 8.0 – – 1.5 6.0 – – – 0.5 6.00 92.00 0.07VAET1/Vourinos Dunite 51.5 – – – 1.5 46.5 – 0.5 – – – 47.00 51.50 0.91VAET2/Vourinos Dunite 51.0 – – – 2.0 44.5 – 2.5 – – – 47.00 51.00 0.92VVDSR1/Vourinos Harzburgite 39.0 12.5 – – 1.0 38.0 – – – 0.5 9.0 38.00 51.50 0.74KOZ1/Koziakas Pl-bearing

lherzolite41.5 10.5 4.0 1.0 1.0 28.5 1.5 1.5 7.5 – 3.0 39.00 56.00 0.70

KOZ2/Koziakas Harzburgite 47.0 16.0 – – 1.0 30.5 2.0 1.0 – – 2.5 33.50 63.00 0.53

A total of 300 equally distributed points were counted per polished thin section. Because of the fine-grained character of the Pindos serpentinized harzbur-gite, 500 points were counted on this sample. Ol, olivine; Opx, orthopyroxene; Cpx, clinopyroxene; Pl, plagioclase; Sp, spinel; Serp, serpentine; Tc, talc; Chl, chlorite; Tr/Act, tremolite and/or actinolite; Cc, calcite; Ox, oxides; Irep, replacement index; AP, Serp + Tc + Chl + Tr/Act; PM, Ol + Opx + Cpx.

EnGInEERInG PRoPERTIES of UlTRAbASIC RoCKS 427

tectonized peridotites from both Pindos and Vourinos (samples PKZSR1 and VVDSR1, respectively) having the lowest strength (Table 3).

The point load test is an ‘indirect’ test method used to determine rock strength in geotechnical studies. The results of this test are usually related to those of other design parameters, such as uniaxial compressive strength. The point load index (I(S50)) was determined

according to the ISRM (1985) specification. The I(S50) values vary between 1.46 and 7.96 MPa, with the less serpentinized peridotites having the highest values (Table 3).

The aggregate abrasion value (AAV) test (BS 812, BSI 1990) provides a relative measure of the ability of an aggregate to tolerate surface wear. It reflects the hardness and brittleness of the constitu-ent minerals, the influence of mineral cleavage, and the strength of

Fig. 3. SEM images showing the surface texture of representative ultrabasic samples related to their mineralogical and textural features: (a) PKZ1; (b) PKZ2; (c) VVD1; (d) VKOR1; (e) VAET2; (f) VVDSR1.

I. RIGoPoUloS et al.428

the intergranular bond (Smith & Collis 2001). According to Hawkes & Hosking (1972), inadequate resistance to abrasion of road surfacing aggregates causes an early loss of the texture depth that is necessary to maintain high-speed skid resistance. The distri-bution of the AAV results obtained is wide, ranging between 6.94 and 26.20 (Table 3). The serpentinized harzburgite samples PKZSR1 and VVDSR1, from Pindos and Vourinos respectively, have the highest susceptibility to abrasion.

The aggregate skid resistance was assessed in the laboratory by the polished stone value (PSV) test, which was performed accord-ing to EN 1097-8 (CEN 1999). This test simulates the action of dust-laden tyres on samples of aggregate; hence it is of fundamen-tal importance in road construction. As shown in Table 3, PSV ranges between 48 and 62, with the most hydrothermally altered ultrabasic samples having the lowest resistance to polishing.

Correlation analysisRegression analysis was applied to the petrographic data and physico-mechanical properties of the ultrabasic rocks, with con-fidence limits of 95%, to investigate the interrelationships between the petrographic characteristics and the engineering properties. The regression procedure was to fit a line through the

points, which is computed so that the squared deviations of the measured points from that line are minimized. The coefficients of determination (R2) and equations of the fitted lines were also calculated.

The apparent density of the studied peridotites shows strong relationships with the percentage of serpentine (Serp), as well as with the ratio of secondary to primary minerals (Se/P). As illus-trated in Figure 4a and b, the values of this physical property tend to decrease when the content of secondary products increases. The correlations between these parameters can be expressed by the fol-lowing equations:

ρaSerpe= =−3229 7583 0 89550 005 2. , .. R (1)

ρa/ )e e= =−3177 3665 0 87140 2317 2. , .. (S P R (2)

The replacement index (Irep), proposed by Rigopoulos et al. (2010) for basic ophiolitic rocks, was also used in correlation anal-ysis. This index is an expression of the degree of preservation of the primary mafic minerals during ocean-floor metamorphism and is described by the following equation:

I ii

n

jj

mrep

AP

PM= =

=

∑∑

1

1

(3)

where AP is alteration products of primary mafic minerals (in vol. %) and PM is primary mafic minerals (in vol. %). A strong negative correlation was observed between the apparent density and Irep (Fig. 4c):

ρa e rep= =−3173 5826 0 8970 2091 2. , .. I R (4)

The water absorption is positively correlated with the percent-age of serpentine, the ratio Se/P and the Irep index (Fig. 4d–f). The statistical linkage between these parameters can be depicted by the following equations:

w Ra Serp= + =0 0312 0 2059 0 41092. . , . (5)

w S P Ra e= + =1 4641 0 307 0 40262. ( / ) . , . (6)

w I Ra rep= + =1 4243 0 2728 0 48192. . , . (7)

The total porosity of the peridotites is also positively correlated with the percentage of serpentine and the Irep index (Fig. 4g and h). Although the correlations between these variables are poor, they indicate that the coherence of the rock decreases with increasing degree of ocean-floor metamorphism. The calculated equations are the following:

n RtSerpe= =3 1481 0 35410 0107 2. , .. (8)

n I Rt rep= + =2 7693 3 0326 0 43752. . , . (9)

The IS(50) is negatively correlated with the percentage of serpen-tine and the ratio Se/P, indicating that the mechanical strength of the ultrabasic samples tends to decrease with increasing degree of alteration (Fig. 5a and b). The calculated linear equations between these properties are:

I RS(50) Serp= − + =0 0917 7 3882 0 61922. . , . (10)

I S P RS(50) e= − + =4 3542 7 1078 0 61992. ( / ) . , . (11)

Table 2. Physical properties of the investigated samples

Sample code/locality Water absorption

wa (%)

Apparent density ρa (kg m−3)

Total porosity nt (%)

PKZ1/Pindos 0.32 3180 2.75PKZ2/Pindos 0.39 3120 3.70PKZSR1/Pindos 3.17 2450 9.59VVD1/Vourinos 0.21 3150 3.37VVD2/Vourinos 0.14 3350 3.18VKOR1/Vourinos 0.72 3050 3.48VKOR2/Vourinos 0.58 3100 3.43VRZ1/Vourinos 0.68 3070 3.76VRZ2/Vourinos 0.85 3010 4.14VAET1/Vourinos 0.95 2690 3.93VAET2/Vourinos 1.02 2640 4.00VVDSR1/Vourinos 2.29 2570 6.20KOZ1/Koziakas 0.23 2830 3.08KOZ2/Koziakas 0.31 2790 3.46

Table 3. Mechanical properties of the investigated samples

Sample code/locality

Schmidt hammer

value SHV

Point load index IS(50)

(MPa)

Aggregate abrasion

value AAV

Polished stone value

PSV

PKZ1/Pindos 59 7.96 10.52 58PKZ2/Pindos 57 7.41 8.86 59PKZSR1/Pindos 38 1.46 26.20 48VVD1/Vourinos 63 6.88 9.48 56VVD2/Vourinos 64 7.10 8.93 57VKOR1/Vourinos 54 7.56 12.98 62VKOR2/Vourinos 55 7.87 10.89 61VRZ1/Vourinos 52 5.79 12.85 59VRZ2/Vourinos 54 4.07 12.32 59VAET1/Vourinos 49 4.38 11.60 57VAET2/Vourinos 49 3.98 12.55 58VVDSR1/Vourinos 40 2.50 19.10 50KOZ1/Koziakas 60 5.36 6.94 56KOZ2/Koziakas 58 4.70 8.58 57

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Similar correlations can be observed from the plots of the Schmidt hammer value versus the percentage of serpentine and the ratio Se/P (Fig. 5c and d). These relationships indicate that with increasing degree of alteration of an ultrabasic sample, the mechan-ical behaviour will deteriorate according to the following equations:

SHV Serp= − + =0 3114 60 1179 0 51412. . , .R (12)

SHV e e= =−58 977 0 46650 2819 2. , .. ( / )S P R (13)

As can be seen in Figure 5e and f, the Irep index is negatively correlated with the IS(50) and SHV, indicating the reduction of the mechanical strength with increasing degree of ocean-floor meta-morphism. The correlations between these variables can be expressed by the following equations:

I I RS(50) rep= − + =4 0012 7 1147 0 66192. . , . (14)

SHV e rep= =−59 1657 0 52460 2659 2. , .. I R (15)

Poor positive correlation was observed between the ratio of soft to hard minerals (So/H) and the aggregate abrasion values (Fig. 5g); however, this trend indicates that soft minerals have a negative effect on the resistance of aggregates to abrasion. This relationship can be expressed by the following equation:

AAV o= + =7 4127 9 6425 0 29702. ( / ) . , .S H R (16)

As can be seen in Figure 5h two different trends have been calculated for the correlation between the So/H ratio and PSV. A positive correlation is observed for So/H < 0.2, indicating that low contents of soft minerals improve the resistance of aggregates to polishing. A weak negative trend can be seen for higher values of this ratio, suggesting that higher contents of soft minerals are accompanied by high susceptibility of the aggregates to polishing. Only the equation of the positive trend is quoted, considering that the negative one is characterized by a significant low coefficient of determination:

PSV o= =63 2185 0 53080 0249 2. ( / ) , ..S H R (17)

The content of Cr-spinel (Sp) shows a good positive correlation with the polished stone values, implying that the presence of this mineral phase can enhance the polishing resistance of ultrabasic rocks. As illustrated in Figure 6, this relationship is expressed by a logarithmic trend, which is described by the following equation:

PSV Sp= + =7 2338 54 1021 0 76252. ln . , .R (18)

DiscussionOcean-floor metamorphism has variably affected the collected ultrabasic samples. The results of this study demonstrate the importance of petrography to the understanding of the mechani-cal behaviour of mantle peridotites.

SEM images taken from aggregate particles show that the micro-roughness of the studied fresh to moderately altered ultrabasic sam-ples is significantly high, probably owing to the combined existence of the ‘soft’ serpentine, which belongs to the phyllosilicate subclass of minerals and forms smooth surfaces, with the ‘hard’ olivine, orthopyroxene and Cr-spinel, which form rough humps. These fea-tures positively affect frictional properties and intergranular slip in

unbound aggregate and the adhesion of binders or cement. They also play an important role in road construction, as they improve the adhesion between tyres and the road surface, principally at low speeds (Sabey 1965). The Vourinos serpentinized dunites exhibit slightly lower relief than the other dunites; however, the differential hardness of their constituent minerals is considered to result in the preservation of their surface roughness when used in the wearing surface of a road. In particular, the uniform distribution of soft and hard minerals throughout the rock specimen, which is attributed to the mesh texture, is considered to promote the polishing resistance of these samples. On the other hand, the intensively serpentinized harzburgites from the Pindos and Vourinos complexes exhibit mainly smooth surfaces. This can be assigned to the high content of serpentine in these samples, which creates a low microroughness.

The minerals of ultramafic rocks are often converted to second-ary mineral phases as a result of ocean-floor metamorphism pro-cesses. The replacement index Irep gives a measure of the degree of replacement of the primary mafic constituents by secondary miner-als. This index was used for the first time by Rigopoulos et al. (2010) in dolerites from the Pindos and Vourinos ophiolites. The relation-ships of the Irep index, the content of serpentine and the ratio Se/P with the apparent density, the water absorption and the total porosity values express the negative impact of ocean-floor metamorphism on the physical parameters of ultrabasic rocks. The main reason for this is the presence of serpentine in peridotites, which absorbs water within the layers of its structure and generally lowers the coherence of the rock. Similar results have also been found for basic ophiolitic rocks owing to the presence of chlorite (Rigopoulos et al. 2010), which is also a phyllosilicate mineral. The tectonic deformation that has affected the collected samples is another reason for the modifi-cation of their physico-mechanical properties. The serpentinized harzburgites from the Pindos and Vourinos ophiolites (samples PKZSR1 and VVDSR1, respectively) contain a significant number of microcracks. The high density of microcracks in these samples is considered to be the main reason for the reduction of their strength.

The mechanical properties of the studied samples were also cor-related with specific petrographic parameters. The negative corre-lations of the IS(50) and SHV parameters with the percentage of serpentine and the ratio Se/P suggest that the mechanical strength of the studied ultrabasic samples tends to decrease with increasing degree of alteration. The modification of the mineralogical and tex-tural features of the studied mantle rocks during the serpentiniza-tion process is considered to be the reason for the reduction of their coherence. It is suggested that continuous replacement of the pri-mary minerals by secondary products involves further obliteration of the primary textures.

The relationships between Irep and selected mechanical parame-ters indicate that hydrothermal alteration generally results in mechan-ically weaker rocks. This is assigned to the fact that the secondary minerals have substantially different mechanical behaviour com-pared with the primary ones, leading to a modification of rock strength (Rigopoulos et al. 2010). Serpentine, talc and chlorite cause deterioration in the mechanical behaviour of ultrabasic rocks because their hardness sufficiently differs from that of the primary mineral phases. On the other hand, actinolite and tremolite have approxi-mately the same hardness as the primary clinopyroxene; however, their brittle behaviour results in mechanically weaker rocks. This is in agreement with Rigopoulos et al. (2010), who mentioned the neg-ative influence of actinolite on the strength and durability of basic ophiolitic rocks. They also noted that the main reason for the brittle behaviour of actinolite is the excellent cleavage on (110) planes in combination with the fact that actinolite usually forms fibrous overgrowths on the short prismatic clinopyroxene crystals.

The aggregate abrasion value test gives a measure of the ability of an aggregate to preserve its macroroughness in a road surface (formed by the projection of the aggregate particles from the

I. RIGoPoUloS et al.430

Fig. 4. The apparent density (ρa) of the studied peridotites plotted against (a) the content of serpentine (Serp), (b) the ratio of secondary to primary minerals (Se/P), (c) the Irep index. The water absorption (wa) plotted against (d) the content of serpentine, (e) the ratio of secondary to primary minerals, (f) the Irep index. The total porosity (nt) plotted against (g) the content of serpentine, (h) the Irep index.

EnGInEERInG PRoPERTIES of UlTRAbASIC RoCKS 431

Fig. 5. The point load index (IS(50)) of the studied peridotites plotted against (a) the content of serpentine (Serp), (b) the ratio of secondary to primary minerals (Se/P). The Schmidt hammer value (SHV) plotted against (c) the content of serpentine, (d) the ratio of secondary to primary minerals. The Irep index plotted against (e) the point load index (f) the Schmidt hammer value. The ratio of soft to hard minerals (So/H) plotted against (g) the aggregate abrasion value (AAV), (h) the polished stone value (PSV).

I. RIGoPoUloS et al.432

binder), which is a crucial parameter at high speeds (Sabey 1965). Although the correlation between AAV and the ratio So/H is poor, it expresses that soft minerals tend to reduce the resistance of ultra-basic aggregates to abrasion. The negative influence of soft miner-als on the abrasion resistance of aggregates has also been mentioned by Hartley (1974).

The mineralogical composition is the main factor controlling the susceptibility of aggregates to polishing, whereas grain size in gen-eral appears to have only a slight effect (Hartley 1974). The polishing resistance of the studied ultrabasic samples, determined by the PSV test, increases for low contents of soft minerals. This is in line with the findings of Hartley (1974), Smith & Collis (2001) and Rigopoulos et al. (2010), who noted that soft minerals increase the differential hardness of aggregates in-service. Moreover, this relationship indi-cates that alteration is not always detrimental to the mechanical prop-erties of rocks. On the other hand, higher contents of soft minerals seem to be accompanied by high susceptibility of the aggregates to polishing, as indicated by the PSV of the highly serpentinized peri-dotites (Table 3). Also, the content of Cr-spinel shows a good posi-tive correlation with the polished stone values, implying that this mineral phase can enhance the polishing resistance of ultrabasic rocks. This can probably be attributed to the different mechanical behaviour of the very hard Cr-spinel grains compared with the main silicate minerals (Rigopoulos et al. 2011). The uniform distribution of Cr-spinel grains in the studied samples also contributes to the low susceptibility of aggregates to polishing (Knill 1960).

The mechanical properties of the studied peridotites vary throughout the alteration spectrum. The dunites and olivine-rich harzburgites from the Pindos and Vourinos ophiolites are slightly to moderately altered lithologies, except for the samples VAET1 and VAET2, which are highly serpentinized. Among the serpenti-nized harzburgites, the Pindos one is evidently the most altered and tectonized sample compared with the Vourinos sample, whereas the harzburgite and plagioclase-bearing lherzolite from Koziakas exhibit a lower degree of serpentinization. The mechanically weaker samples are the harzburgites PKZSR1 and VVDSR1, as indicated by the results of physical and mechanical tests. In gen-eral, the results of the laboratory tests indicate that among the col-lected samples, the dunites and olivine-rich harzburgites are the most suitable samples for use as aggregate material.

ConclusionsThis paper sheds light on the effect of ocean-floor metamor-phism on the engineering properties of ultrabasic ophiolitic rocks. The investigated samples from the Pindos, Vourinos and

Koziakas ophiolites include dunites, olivine-rich harzburgites, typical harzburgites and plagioclase-bearing lherzolites with var-iable contents of secondary minerals.

On the basis of petrographic evidence and statistical analyses of the mineralogical and engineering parameters, it is proved that ocean-floor metamorphism is the major determinant for the physico- mechanical properties of mantle peridotites and hence for their quality as aggregate material. The replacement index Irep has also been used as a petrographic parameter in regression analysis. This micropetrographic index reflects and quantifies the degree of replacement of the primary mafic minerals by secondary products during ocean-floor metamorphism. The correlations of Irep with the physico-mechanical parameters indicate that the strength and dura-bility of ultrabasic rocks tend to decrease with increasing degree of hydrothermal alteration.

The variation of the rock strength is related to the degree of preservation of the primary mineral phases, the development of soft minerals during alteration and the degree of tectonic deformation. Ocean-floor metamorphism generally results in mechanically weaker rocks, as indicated by the relationships between specific petrographic parameters and the engineering properties of the stud-ied samples. Exceptionally, the polishing resistance is positively affected by the participation of low contents of soft minerals. This is attributed to the differential hardness provided by these minerals to the aggregate particles. However, higher contents of soft miner-als seem to be accompanied by high susceptibility of the aggregates to polishing. Cr-spinel also improves the polishing resistance of ultrabasic rocks owing to (1) its different mechanical behaviour compared with the main silicate minerals and (2) the uniform dis-tribution of its grains in the studied peridotites. All the above petro-graphic features are critical factors for the mechanical behaviour of ultrabasic aggregates in-service and hence for the quality of a wide range of engineering works.

Acknowledgements. We thank the European Social Fund (ESF), Operational Program for Educational and Vocational Training II (EPEAEK II), and particularly the Program PYTHAGORAS, for funding this work. We also thank an anonymous journal referee for useful comments on the paper.

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Received 15 March 2012; accepted 4 June 2012.

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