Tiziana Apuani

Claudia Corazzato

Andrea Cancelli

Alessandro Tibaldi

Physical and mechanical properties of rockmasses at Stromboli: a dataset for volcanoinstability evaluation

Received: 29 January 2005Accepted: 25 June 2005Published online: 26 October 2005� Springer-Verlag 2005

Abstract Stromboli island has acomplex geological history with re-peated changes in the volcanicactivity alternating with destructiveevents, caldera collapses and flanklandslides. The last activity resultedin the creation of the Sciara del Fuo-co depression which was modified bythe recent 2002–2003 landslide. Thevariation in lithology, degree oftectonization and disturbance hasresulted in the presence of a widespectrum of geotechnical materials.This paper summarises the physicaland mechanical properties ofStromboli’s intact rocks, rock mas-ses and loose deposits, based on fieldsurveys and laboratory tests. A newclassification of the rock successionis introduced and four lithotechnicalunits defined: Lava, Lava-Breccia,Breccia and Pyroclastic deposit. Therange of variability in bulk volume,porosity, intact rock compressivestrength and geological strength in-dex is presented. The Hoek andBrown’s failure criterion was appliedfor each lithotechnical unit and therock mass friction angle, apparentcohesion, tensile and compressivestrength, global strength and modu-lus of deformation calculated in aspecified stress range.

Keywords Stromboli volcano ÆLithotechnical unit Æ Failurecriterion Æ Rock mass strength

Resume L’histoire geologique del’ıle du Stromboli est complexe, avecune activite volcanique ayant donnelieu a des evenements destructeurs,des effondrements de caldeiras etdes glissements des flancs du volcan.La derniere periode d’activite a eupour consequence la formation de ladepression de la Sciara del Fuoco,qui fut modifiee par le recent gliss-ement de 2002–2003. La nature li-thologique complexe du site, ledegre de tectonisation et les reman-iements des materiaux expliquent ladiversite des proprietes geotech-niques des materiaux presents. Cetarticle synthetise les proprietes phy-siques et mecaniques des roches in-tactes du Stromboli, des massesrocheuses et des depots meubles, apartir de prospections de terrain etd’essais de laboratoire. Une nou-velle classification des roches pre-sentes sur ce site est introduite etquatre unites lithologiques sontdefinies: les laves, les laves-breches,les breches, les pyroclastites. Lesdomaines de variation des parame-tres: densite, porosite, resistance a lacompression simple et GSI - indicede resistance geologique sont pre-sentes. Le critere de rupture deHoek et Brown a ete applique pourchaque unite lithologique et les pa-rametres relatifs a la masse roche-use: angle de frottement, cohesionapparente, resistance a la traction

Bull Eng Geol Env (2005) 64: 419–431DOI 10.1007/s10064-005-0007-0 ORIGINAL PAPER

T. Apuani (&)Dipartimento di Scienze della Terra‘‘A. Desio’’, Universita degli Studi diMilano, Milano, ItalyE-mail: tiziana.apuani@unimi.itTel.: +39-2-50315565Fax: +39-2-50315494

C. Corazzato Æ A. Cancelli Æ A. TibaldiDipartimento di Scienze Geologiche eGeotecnologie, Universita degli Studi diMilano-Bicocca, Milano, Italy

Introduction

Stromboli island, one of the most active volcanoes in theworld, is located in the Tyrrhenian sea off Italy’ssouthern coast (Fig. 1). In the past 100,000 years,Stromboli has evolved as a complex stratovolcano. Itsstructural evolution has been complicated by destructivephases, gradual slope erosion and, in the last13,000 years, four large sector collapses affecting theNW flank (Tibaldi 2001). These collapses, which alter-nate with growth phases, resulted in the creation of theSciara del Fuoco horseshoe-shaped depression, modifiedin December 2002 and January 2003 by the latestlandslide events (Bonaccorso et al. 2003). Several othervolcanoes have recorded a similar history of repeatedlateral collapses (Lenat et al. 1989; Beget and Kienle1992; Komorowski et al. 1994; Siebert et al. 1995).These phenomena are well documented and many au-thors from different geological disciplines have contrib-uted to their understanding. The necessity for aquantitative analysis of volcano stability has beenrecognised and both limit equilibrium methods andnumerical modelling have been applied (Voight et al.1983; Iverson 1995; Sousa and Voight 1995; Elsworthand Voight 1996; Voight and Elsworth 1997; Elsworthand Day 1999; Hurlimann et al. 2000a; 2000b; Donna-dieu et al. 2001; Concha-Dimas and Watters 2003a, b;Zimbelman et al. 2004), even though not always sup-ported by direct measurements of the geotechnical

parameters. Only recently have studies begun to quan-tify volcanic material properties (e.g. Watters et al. 2000;Concha-Dimas and Watters 2003a, b; Zimbelman et al.2003; Thomas et al. 2004). The International Society forRock Mechanics (ISRM) has dedicated a specificworkshop on this theme (Dinis da Gama and Ribeiroand Sousa 2002).

The collection of geotechnical data is particularlyambitious in volcanic environments, where the geologi-cal complexity and logistical difficulties often limit thefeasibility and reliability of sampling and in-situmechanical and geophysical prospecting commonly ap-plied in non-volcanic areas. In a stratovolcano thematerial constituents vary both laterally and vertically,and their distribution is very difficult to predict. Theassociated petrographical and textural heterogeneity andanisotropy results in a strong and local variability ofmechanical properties: lava rock masses are character-ised by brittle fracturing, whereas weakly welded orloose volcanic deposits behave similarly to soil. In theanalysis of large lateral collapses, the interest in thegeotechnical materials is concentrated on rock masses,which are dominant at that scale.

Considering the present state of Stromboli volcano,the strong correlation between its evolution and collapseoccurrence, the analogies with other volcanoes and thestate of the art on this subject, the research here pre-sented aims at providing a geological and geotechnicalmodel of the volcano. This model, especially addressed

et a la compression, resistance glo-bale et module de deformation ontete calcules pour un domaine decontrainte particulier.

Mots cles Volcan du Stromboli ÆUnite lithotechnique Æ Critere derupture Æ Resistance de masserocheuse

Fig. 1 Geographical setting ofStromboli island in the Aeolianarchipelago. The bathymetry isafter Gabbianelli et al. (1993)

420 Stromboli–physical and mechanical properties

to the northwestern flank (Sciara del Fuoco), includesstratigraphical, lithological and structural characteristics(Tibaldi and Pasquare 2005; Tibaldi et al. 2003), to-gether with geotechnical and geomechanical parametersof the recognised lithotechnical units, and provides themain input data for stability analysis (Apuani et al.2005).

The applied methodology includes:

(a) a geotechnical characterisation of soil-like materialsthat fill the Sciara del Fuoco depression, usingstandard laboratory tests;

(b) a rock mass characterisation and engineering classi-fication using (1) structural and geomechanical sur-veys, (2) measurement of the physical andmechanical properties of rock (intact rocks andjoints) by standard laboratory tests on field samples,and (3) the evaluation of rock mass strength andelastic parameters according to Hoek and Brown’snon-linear strength law.

Geological setting

Stromboli is a basaltic to basaltic-andesitic island vol-cano located at the northern end of the Aeolian Archi-pelago, a late Quaternary volcanic arc related to theNW-dipping Benioff zone. The volcano rises approxi-mately 2.6 km above the sea floor with a summit ele-vation of 924 m above sea level.

The geology, stratigraphy and volcanological evolu-tion of Stromboli have been widely investigated (e.g.Rosi 1980; Francalanci 1987; Pasquare et al. 1993;Hornig-Kjarsgaard et al. 1993; Tibaldi et al. 1994; Ti-baldi 2001; Tibaldi and Pasquare 2005). This complexedifice results from several constructive and destructivephases, represented by a succession of different unitsseparated by major unconformities. The main volcaniccycles are, from the oldest, Palaeostromboli (Palaeo-stromboli I, Palaeostromboli II and PalaeostromboliIII), Vancori (Lower, Middle and Upper), Neostrom-boli, and Recent Stromboli (Hornig-Kjarsgaard et al.1993), whose products have been further subdivided intoRecent Stromboli I (Pizzo), Recent Stromboli II (Pre-Sciara) and Recent Stromboli III (Sciara) by Tibaldi(2001) and Tibaldi and Pasquare (2005). The earlierstages of Stromboli evolution (100,000–24,000 BP) werepredominantly affected by summit vertical collapses ofthe caldera type (Pasquare et al. 1993; Tibaldi et al.1994), while in the last 13,000 years four lateral collapsestook place, the last one resulting in the formation of thepresent Sciara del Fuoco depression (Fig. 2). Each of themain volcanic cycles generally has constant and distinctpetrochemical characteristics, which vary from calc-alkaline to leucite-bearing shoshonitic or potassic,

through high K–calc-alkaline and shoshonitic, showinga general trend of K-enrichment with time. In detail,however, this trend is not regular and displays severalfluctuations (Hornig-Kjarsgaard et al. 1993).

These cycles consist of a large number of lithostrati-graphic units; this paper refers to those defined byHornig-Kjarsgaard et al. (1993) and Tibaldi and Pa-square (2005). As commonly occurs in volcanic envi-ronments, the products can be represented by a widevariety of lithotypes, from lava flows (with alternatingmassive and breccia layers), to different types of pyro-clastic deposits, to subvolcanic intrusions and epiclasticdeposits due to erosion. Moreover, these products canvary from massive rocks to completely loose materials.

Geotechnical properties of loose materials

The interest in loose materials was focused on thedeposits related to the Recent Stromboli III (Sciara)activity. The Sciara del Fuoco depression is covered byvolcanic deposits related to the strombolian activity ofthe summit craters, as well as to paroxysmal events.These loose deposits consist of ejecta ranging frommetre-sized bombs to scoria and ash. They appearunconsolidated to depths of some tens of metres with aslope at the limit equilibrium angle. Samples were col-lected at depths of between 0 and 1 m at the base of theSciara del Fuoco depression and in the summit area;they correspond to the grain size fraction smaller thangravel. These deposits behave as ‘‘soil-like material’’ andtheir physical and mechanical properties were estimatedby standard soil laboratory testing. For comparison,some analyses were also performed on deposits relatedto erosion and transport in the Fossetta and RinaGrande-Schicciole depressions (Fig. 2).

According to the Unified Soil Classification System(USCS), adopted by the American Society for Testingand Materials (ASTM 1987), the studied loose depositsconsist mainly of gravel and sands (SP or SW), with acoefficient of uniformity CU<7.3; there are no silt orclay fractions. The natural water content is W<6.3%;the specific gravity of the solid soil particles isGs=28.52–30.23 kN/m3; the maximum and minimumdry unit weight, determined from the grain size fractionless than 9.5 mm are, respectively, cdmax=16.65–17.42 kN/m3 and cdmin=12.96–14.82 kN/m3, fromwhich porosity has been computed n=40–55%.

Consolidated-undrained triaxial compression testsfor the peak and residual values of cohesion and shearstrength angle gave peak values of cp=0, /p

¢ =43–51�and residual values cr=0, /r

¢= 39–49�. The high valuesof the shear strength angle are due to the nature of thegrains (mainly glass and pyroxene), their very lowroundness (r) (r=0.1) and degree of elongation.

T. Apuani et al. 421

The reported values are representative of the loosedeposits where they cover the bedrock, presumably to adepth of a few tens of metres, and do not take intoaccount the grain size fraction larger than gravel. Thecharacterisation of loose deposits is particularlyimportant when analysing superficial landslides possi-bly associated with general slope instability phenom-ena. For this reason they have been included in thispaper, but the main attention is devoted to rock massproperties, which support the analysis of large lateralcollapses.

Rock mass properties: methodology and results

Apart from the distinction between loose materials androck masses, the geological and structural surveys re-vealed that different lithostratigraphic units can be cat-egorised according to their mechanical characteristics.The rock masses were grouped into four lithotechnicalunits based on the relative percentage of the brecciafraction versus lava deposits:

(1) Lava unit (L): lava layers (more than 65%) alter-nating with autoclastic breccia layers;

(2) Lava-Breccia unit (LB): alternation of lava layers(ranging from 35 to 65%) and breccia layers;

(3) Breccia unit (B): alternation of lava layers (less than35%) and autoclastic breccia layers;

(4) Pyroclastic deposit (poorly welded) unit (P): pre-vailing pyroclastic breccias alternating with tuff andlapillistones.

Of the 52 sites where structural surveys were under-taken (Fig. 2), 20 outcrops, mainly in lithotechnical unitswhere lava deposits are dominant, were chosen for rockmechanics characterisation, following ISRM (1981).This procedure identified the number of joint sets andtheir representative orientation, as well as the geometryof each joint (strike, dip and inclination), set spacing,type of movement, amount of dilation, degree of alter-ation, roughness coefficient, presence and nature of infill.Although the stereographic projection of all the fractureorientations reveals a dispersed joint pattern, the mostfrequent joint set strikes NE and dips > 75� NW. Thewestern shoulder of Sciara del Fuoco shows the domi-

Fig. 2 Simplified geological map of Stromboli island and locationof the investigation and sampling sites. The stratigraphic unitsdescribe the main stages of the volcano’s evolution (Hornig-

Kjarsgaard et al. 1993; Tibaldi and Pasquare 2005). Location ofstructural and geomechanical surveys, rock and soil sampling sites,and in situ density measurements are given

422 Stromboli–physical and mechanical properties

nance of another subvertical joint set with a dip directionranging from ENE to ESE (Tibaldi et al. 2003).

Following Bieniawski (1989), the rock masses are of‘‘good quality’’ (RMR=66–80). A lower ‘‘quality’’ isexpected for rock masses in which breccia layers domi-nate the lava. However, where disintegrated, poorlyinterlocked rock masses, laminated sheared weak rocksor composite rock masses with a mixture of angular androunded rock pieces are present, it is very difficult andmeaningless to measure some of the parameters neededfor the application of the RMR classification. In thesecases, theGeological Strength Index (GSI), introduced byHoek and Brown (1980) and extended by Hoek et al.(1998) and Hoek et al. (2002), appears to be a better toolfor characterisation. It is based upon the visual impres-sion of rock structures, in terms of blockiness and geo-logical complexity, and of the surface conditions of thediscontinuities expressed by roughness and weathering.The GSI of each lithotechnical unit is reported in Fig. 3.

In-situ density measurements were performed on thebreccia portions alternating with massive ones of lavaflows related to the Vigna Vecchia Lava lithostrati-graphic unit (Neostromboli edifice), both near the coaston the northern side of Sciara del Fuoco and on thenorthern coast of the island between Piscita and Fron-tone (Fig. 2). The third site was chosen on the coast inthe area of the 1985 lava flow (Recent Stromboli III),later covered by the recent December 2002–January2003 lava flow. The method consists of excavating acavity in the breccia layer, keeping the material forweighting and measuring the volume of the irregularhole by water infilling after the positioning of an insu-lating plastic bag. The resulting average unit weightranged from co=8.83–14.82 kN/m3.

Physical and mechanical properties of intact rockand discontinuities

Several samples of intact rock (Fig. 2) were collected formeasurements of: bulk volume, void index and porosity,point load strength index, uniaxial compressive strength,elastic moduli and diametral compressive strength, alldetermined following ISRM recommendations. The ef-fect of confining pressure on strength and elastic moduliwas determined by triaxial compressive tests.

The absence of springs at altitudes over 60 m a.s.l, thegeneral dry state of the investigated outcrops and thelow average annual precipitation (580 mm/year in theperiod 1947–1977) suggest that dry conditions representthe in-situ state, so all the mechanical properties refer todry conditions.

The collected samples represent mainly the massivelava layers which alternate with breccia in the Lavaand Lava–Breccia outcrops, so that the mechanicalproperties refer only to the most competent layers. In

contrast, the point load test itself was carried out bothon single pieces of breccia and on lava samples. All theproperties have been defined taking account of petro-graphical and textural anisotropy, with load appliedparallel (//) or orthogonal (^) to the flow surface of lavalayers or to the main textural element, often defined by apreferential elongation of vesicles.

Physical properties

The tested lava samples belong to the following litho-stratigraphic units: Lower La Petrazza Lava Unit (Pa-leostromboli I), Upper Vancori Lava Unit (UpperVancori) and Vigna Vecchia Lava Unit (Neostromboli)(Fig. 2).

At the microscopic scale, the samples showed a por-phyritic texture, with a microlitic groundmass. Thephenocrysts are typically plagioclase and pyroxene (au-gite), whereas the groundmass is mainly composed ofplagioclase, pyroxene and glass. Accessory minerals areamphibole and olivine. The samples showed differencesin the composition of accessory minerals; in particular,younger products contain less amphibole and moreolivine compared to older material.

The rock specific bulk volume was Gs=25.71–28.62 kN/m3; the dry and saturated bulk volumecd=22.26–25.30 kN/m3 and csat=22.95–25.60 kN/m3

respectively; the void index and porosity range wase=0.031–0.246 and n=3–20% (Table 1).

Point load tests

A rock strength index was obtained using the point loadtest (ISRM 1985): an irregular piece of rock is loadedbetween hardened steel cones, causing failure by thedevelopment of tensile cracks parallel to the axis ofloading. The point load index, corrected assuming thepoints were at the standard 50 mm spacing (Is(50)), iscorrelated with the uniaxial unconfined compressivestrength of cylinders by the frequently used equationrc=23,7 *Is(50) (Brock and Franklin 1972). The proce-dure is easy to perform in the laboratory as well as in-situ. A wide number of samples can be tested and theresults compared to those obtained by the uniaxialcompressive test.

To have a wide distribution of data, 567 point loadtests were carried out with loads parallel (//) andorthogonal (^) to the main textural directions. Sampleswere taken from lava layers or single pieces of breccia.The mean compressive strength, obtained from the pointload index (Is(50)), was rc=134±65 MPa (Fig. 4).Samples from different units (Lower La Petrazza LavaUnit, Upper Vancori Lava Unit and Vigna VecchiaLava Unit) did not show great differences in behaviour,although the Vancori lava appeared to have a higher

T. Apuani et al. 423

strength. The lava elements in the breccia layers gener-ally showed less resistance. No clear evidence ofmechanical anisotropy was recognisable.

Uniaxial and triaxial compressive strength

Uniaxial compressive strength (rc) and elastic modulus(Et50) were determined from uniaxial compressive tests(ISRM 1981) on 45 lava samples, mostly from the VignaVecchia Lava Unit.

The results, given in Fig. 5 and Table 1, indicatethat:

– for all the data, the mean compressive strength wasrc=95±48 MPa and the elastic modulus wasEt50=23±8 GPa, where the range represents onestandard deviation;

– the Vigna Vecchia and La Petrazza lava samplesshowed similar behaviours and, grouped together,give rc=78±20 MPa and Et50=22±8 GPa;

Fig. 3 Geological strength in-dex (GSI) of the rock masstypes recognised at Stromboli,grouped into lithotechnicalunits. Classification table fromHoek et al. (1998)

424 Stromboli–physical and mechanical properties

– the Vancori lava samples reached high compressivestrengths (up to 242 MPa), but their elastic modulusfalls within the general distribution;

– the Vigna Vecchia lava samples showed a wider va-riability of rc and Et50, probably reflecting the greaternumber of tests on these lavas;

– there was no clear evidence of mechanical anisotropyat the macroscopic scale; thus, for the Vigna Vec-chia lavas rc^=73±17 MPa, rc//=79±19 MPa;Et50^=18±4 GPa, and Et50//=21±9 GPa (the sub-scripts // and ^ identify load parallel and perpendic-ular to the direction of flow texture).

Seventeen cylindrical samples, randomly orientedwith respect to the in-situ orientation, were tested undertriaxial compression using a confining pressure ranging

from 4 to 20 MPa with the deviator stress being pro-gressively increased to failure. The axial and diametricdeformation was calculated from linear variable dis-placement transducer (LVDT) measurements and theresults used to determine the elastic modulus (E) andPoisson’s ratio (m) at a constant confining pressure(Drh=0), assuming a linear elastic and isotropicbehaviour. The tested material exhibited a certain degreeof anisotropy, as demonstrated by significant differencesbetween axial and diametric deformation during theapplication of isotropic stress. E and m must therefore betaken as conventional measures of stiffness and de-formability, better called ‘‘axial stiffness modulus’’ and‘‘transverse contraction coefficient’’, the average valuesof which are: E=17.409 ± 4.237 GPa and m=0.245 ±0.094 (mean ± SD) (Fig. 6).

Brazilian tensile strength

The indirect tensile strength was calculated by theBrazilian test (ISRM 1981), also named the diametral

Table 1 Physical and mechanical properties of intact rock

Sample Site Lithostratigraphicunit

Gs

(kN/m3)cd(kN/m3)

csat(kN/m3)

n(%)

rt mean±SD*(MPa)

rc mean±SD**(MPa)

Et50±SD**(GPa)

C6 Filli di Baraona Vignavecchia Lava 28.62 23.63 24.22 17.2 36±16 70±11 15.0±1.5C7 Filo del Fuoco Vignavecchia Lava 27.45 23.92 24.53 12.9 56±15 70±15 8.5 1.06C8, C14 La Fossetta Vignavecchia Lava 25.90 22.50 23.62 13.1 63±16 76±26 26.9±10.7C25, C26 Ginostra Vignavecchia Lava 27.72 22.26 22.95 19.7 40±17 67±9 14.9±3.3C9, C10 Liscione Vignavecchia Lava 25.71 22.47 23.09 12.6 82±14 79±10 26.9±5.6C20, C22 Frontone –

FaraglioneVignavecchia Lava 25.92 22.40 23.03 13.6 73±27 95±22 18.4±1.8

Mean of all above Vignavecchia Lava 26.89 22.86 23.57 14.9 59±24 76±18 20.0±7.5C12, C13 I Vancori Upper Vancori Lava 26.09 25.30 25.60 3.0 95±24 201±39 29.5±3.9C19 Malpasso Lower La Petrazza Lava 26.51 23.81 24.24 10.2 70±12 90±31 29.1±3.4Mean of all samples 26.74 23.29 23.91 12.8 65±25 95±48 22.6±7.9

*indirect tensile strength by Brazilian test**by uniaxial compressive tests

Fig. 4 Point load index frequency histogram. Lava elements inbreccia layers generally show a lower resistance. No strongevidence of mechanical anisotropy is recognisable

Fig. 5 Strength and elastic properties of lava samples mainlybelonging to the Vigna Vecchia Lava Unit: uniaxial compressivestrength (UCS) versus Elastic Moduli (Et50). The dashed ellipsesdelineate samples belonging to the same lithostratigraphic unit

T. Apuani et al. 425

compressive strength test. The test consists of theapplication of a diametral compressive load to a disc-shaped rock sample until failure occurs in tension.

One hundred and fifty one lava samples were tested:85 from the Vigna Vecchia Lava unit, 42 from theUpper Vancori Lava unit and 24 from the Lower LaPetrazza Lava unit. Core axis and load directions werechanged in order to induce fractures normal, parallel orat 45� with respect to the lava flow layers, assumingthese to be probable planes of transverse anisotropy. Awide range of values were obtained, but altogether thetested lava samples showed a quite high indirect tensilestrength rt(BT)=59±24 MPa (mean ± SD; Table 1). Itmust be remembered that if the elastic properties ofrocks in tension are different from those in compres-sion, then the calculated tensile strength could be

overestimated with respect to the values obtained fromdirect tensile tests (Hudson et al. 1972; Sundaram andCorrales 1980).

The spectrum of values cannot be related to a precisetextural anisotropy, but instead to variations in poros-ity, both in total amount and in geometry (pore spatialdistribution, aspect ratio and preferential elongation).The tested Vancori lavas, with their lower porosity,appeared to be more resistant, consistent with their highcompressive strengths.

Direct shear strength of rock discontinuities

The direct shear test of rock discontinuities measurespeak (p) and residual (r) shear strength as a function ofstress normal to the sheared plane (ISRM 1981). Planes

Fig. 6 Results of triaxial com-pressive tests on Vigna VecchiaLava samples. Triaxial com-pressive strength (rc) and axialstiffness modulus (E) versusconfining pressure

Table 2 Physical and mechanical properties of the lithotechnical units: input data necessary to define the Mohr–Coulomb equivalentparameters and calculated rock mass values of friction angle, cohesion, strength and elastic parameters analysis

Lithotechnical units: physical-mechanical properties

Unit 1Lava

Unit 2Lava+Breccia

Unit 3Breccia

Unit 4Pyroclastic Deposits

Bulk Volume cd (kN/m3) 19.71–26.68 (22.56)Lb (18.63) 8.83–17.46 (14.71)s (14.71)Porosity n (%) 3–20Lb – – –Intact rock rci (MPa) 50–162 (50)Lb 40–100 (40)un 30–80 (30)un 10–50 (20)un

mi 25±5 (25)Tr 22±5 (19)Tr 19±5 (18)Tr 13±5 (15)Tr

Geological strength index GSI 40)55 (40)s 30–45 (30)s 15–35 (20)s 8–20 (15)s

Disturbance factor D 0–0.6 (0)s 0–0.6 (0)s 0–0.6 (0)s 0–0.6 (0)s

Base friction angle /b (�) 22–32Lb

Hoek–Brown failure criterion Mohr–Coulomb equivalent parameters function of r3max (5–20) MPaFriction angle /b (�) 43–31 35–25 29–20 23–15Apparent cohesion c (MPa) 1.5–3.9 1.1–2.8 0.8–2.0 0.6–1.4Tensile strength r¢tm(MPa) )0.022 )0.011 )0.004 )0.002Uniaxial compressive strength UCSm (MPa) 1.654 0.688 0.239 0.100Global strength r¢cm (MPa) 11.05 6.109 3.348 1.682Modulus of deformation Em (MPa) 3976 2000 974 596

Lb Laboratory tests, s in situ direct tests and evaluations, Tr theorethical data, un uncertain data, in brackets input data for stabilityanalysis

426 Stromboli–physical and mechanical properties

of weakness represented by joints in the lavas have beencharacterised in terms of apparent cohesion (c¢), frictionangle (/¢) and shear modulus (Ks) by means of directshear strength tests undertaken at a normal stress rn<4 MPa. The tested discontinuities, belonging to the mainsets 320�/80� and 98�/73� (average dip-direction/dip),gave the following shear strength parameters:c’p = 0.03–1.05 MPa, /’p=39�–54�, Ks=28–38 GPa/m,c’r=0 MPa, /’r=25�–44�, However, the set 148�/82�gave c’p=1.05 MPa, /’p=56�, Ks=11 GPa/m andc’r=0 MPa, /’r=46�. The basic friction angle (/b) fromthe tilt test was /b=22�–32�.

Rock mass strength and elastic properties

The rock mass strength and its elastic parameters wereevaluated applying the Hoek and Brown failure cri-terion (Hoek 1983; Hoek and Brown 1988, 1997;Hoek et al. 2002). The input data for the analysis are:the uniaxial compressive strength of the intact rockconstituting the rock mass (rci); the material constantmi depending on the textural and petrographicalcharacters of the intact rock; the GSI of the rock massand the factor of disturbance D due to excavations

or stress release. The physical and mechanical prop-erties of each lithotechnical unit are reported inTable 2.

The field observations and the laboratory test re-sults provide well-defined input data for applying theHoek-Brown failure criterion to the lithotechnical‘‘Lava unit’’ (L) as specified in Table 2 by the super-scripts Lb-laboratory or S-in-situ tests and evalua-tions. They also allow estimates of the values whichbest represent the weaker lithotechnical units, ‘‘Lava-Breccia unit’’ (LB), ‘‘Breccia unit’’ (B) and ‘‘Pyro-clastic deposit unit’’ (P). For these, the uniaxial com-pressive strength (rc) is estimated based on the manualindex test (MIT) recommended by ISRM (1981), awide literature review (Lama and Vutukuri 1978 andcited references; Dinis da Gama et al. 2002), in addi-tion to considering the results obtained for the lavasamples.

According to the Hoek and Brown failure criterion,the maximum and minimum effective stresses at failure(r1

¢ and r3¢ ) for jointed masses are related by the equa-

tion:

r01 ¼ r03 þ rci � mb �r01rciþ s

� �a

;

where mb is a reduced value of the material constant mi,and the constants s and a depend upon the degree offracturing and joint conditions for the rock mass.

First, the parameters mb, s and a are calculated fromthe input data as a function of GSI and D by theempirical relations:

mb ¼ mi expGSI� 100

28� 14D

� �

s ¼ expGSI� 100

9� 3D

� �

a ¼ 1

2þ 1

6� e�GSI=15 � e�20=3� �

The stress state at failure is then plotted on a r1¢ ) r3

¢

stress diagram and the equivalent Mohr-Coulombstrength parameters, friction angle /¢ and cohesivestrength c¢, calculated for each rock mass and stressrange. Preferred values were obtained by fitting anaverage linear relationship to the curve generated bysolving the generalized Hoek-Brown equation for arange

r0tm\r03\r03max:

Other rock mass parameters, rock mass tensilestrength rtm

¢ , uniaxial rock mass compressive strengthUCSm,global rock mass compressive strength rcm

¢ esti-mated from the Mohr-Coulomb parameters, rock massmodulus of deformation Em are calculated as follows:

Fig. 7 Effect of the disturbance factor (D) and stress state,expressed by the extreme value of the range of the minimumeffective stress (r3max), on Mohr-Coulomb strength parameters:friction angle and cohesion. The example refers to the ‘‘Lava’’lithotechnical unit

T. Apuani et al. 427

rtm ¼ �s � rci

mb; UCSm ¼ rci � sa; rcm ¼

2c0 � cos/0

1 � sen/0

EmðGPaÞ ¼ 1� D2

� ��ffiffiffiffiffiffiffiffirci

100

r� 10 GSI�10ð Þ=40ð Þ

Figure 7 shows how rock mass strength and elasticproperties, in particular friction angle and cohesivestrength, are strongly sensitive to variation of the factorD. Similar results have been presented by Thomaset al. 2004. The D factor ranges from 0 (no distur-bance) to 1 (maximum disturbance). D=0.6 is appro-priate to describe the outcrops or slope investigated,but for the same lithotechnical unit at increasing depthD=0 is a more reasonable assumption. In view of theabove, it is considered that, in drawing a lithotechnicalsection, each unit, which is assumed to be homoge-neous, is better characterised by the average value ofstrength and deformability calculated in the stress stateD=0.

For the Lava Unit, the calculated effective stress stateat failure was compared with the ultimate triaxial com-pressive stress field and the equivalent Mohr-Coulombstrength diagram (Fig. 8).

The effect of rock mass discontinuities and geologi-cal complexities at outcrop scale results in a reductionof the rock mass compressive strength. At low confin-ing pressures, assuming D=0, this reduction is up to57%; with increasing minimum principal stress itdecreases to 27%.

Repeating the analysis for each lithotechnical unit, acomplete model of the rock mass strength and elasticproperties was inferred (Table 2). It is well known thatthe strength of a rock mass and its elastic properties arestrongly dependent upon the stress range of the analysis;hence the stress state (which is a function of depth) mustbe specified. The Mohr-Coulomb equivalent parametersin Table 2 refer to a normal stress 5<r3max<20 MPa.Once the range of variability of all input data had beendefined, a mean representative value to be introducedinto the stability analysis was chosen (values in bracketsin Table 2).

Discussion

The identified lithotechnical units have been character-ised in terms of physical and mechanical behaviour by:

Fig. 8 Rock mass strength andelastic properties determined byHoek and Brown’s failurecriterion. a Diagram represent-ing the effective stress state atfailure. b EquivalentMohr-Coulomb strengthdiagram

428 Stromboli–physical and mechanical properties

(1) standard laboratory tests on intact rocks and joints,(2) rock mass characterisation and engineering classifi-cation by structural and geomechanical surveys, (3)evaluation of rock mass strength and elastic parametersaccording to Hoek and Brown’s empirical failurecriteria.

As regards laboratory results, the rheologicalbehaviour of the tested volcanic materials, mainlybasaltic and basaltic-andesitic lavas, is generally inaccordance with the few reported in the rock mechanicsliterature. This is particularly true for bulk volume,porosity and intact rock compressive strength.Kwasniewski (2002), for instance, comparing the prop-erties of a selection of andesites, basalts, diabases andliparites reports bulk volume, porosity and uniaxialcompressive strength ranging from 18.14 to 30.01 kN/m3, 0.3–22.8% and 13.1–487 MPa respectively. As re-gards tensile strength, the same author underlines thedifficulty in finding clear data on uniaxial tensile strengthin the literature. Frequently the published mechanicalproperties are not accompanied by details of the pro-cedure by which they were obtained and it is well knownthat tensile strengths are strongly dependent upon thetesting method. Direct tensile strength tests generallygive lower values than indirect tests. The values obtainedin the present work (rt=59 MPa) are very high com-pared with the few presented in the literature; in Lamaand Vutukuri (1978) tensile strength values are generallyless than 20 MPa, although the authors cite values up to50 MPa for some diorites with rc=180 MPa and55 MPa for diabase-basalt alternations (rc=321 MPa).

Although the Hoek and Brown’s empirical failurecriterion has been widely documented for non-volcanicmasses, only a few results have been published on thevolcanic rock mass strength and elastic parameters(Watters et al. 2000; Serrano et al. 2002, Thomas et al.2004) although the procedure is gaining consent. Wat-ters et al. (2000), testing andesite lava flows with differ-ent alteration grades, estimated equivalent cohesion andfriction angle of 0.4 – 0.2 MPa and 45�–24� respectively.These results fit well with those obtained for Stromboli’sLava and Lava-Breccia units.

It is particularly important to compare the input datafor the stress-field. In particular, the GSI values have awide range with Serrano et al. (2002) suggestingGSI=25–75 for pyroclastic materials with differentdegrees of imbrication and welding. Generally sensitivityanalyses are preferred to deterministic ones and the

effect of variation in GSI, density and porosity should beevaluated. In the present work, the effect of the distur-bance factor D has been described.

Conclusions

As a result of its geological history, with repeatedchanges in volcanic behaviour from effusive to explo-sive activity interplaying with caldera collapses andflank landslides, the Stromboli stratovolcano has avery complex stratigraphy and mechanical structure.The variation in lithology, degree of tectonization andlocal disturbance has produced a wide spectrum ofgeotechnical materials, ranging from hard lavas tounconsolidated or poorly welded pyroclastic deposits,which overlie each other with an unknown lateraldistribution.

In order to prepare a geological and geotechnicalmodel, the writers have provided a new classification ofthe rock succession in terms of lithotechnical units,based on in situ and laboratory experimental investiga-tions as well as lithostratigraphic and geomorphologicalsurveys.

The main interest has been devoted to rock massproperties. Geological surveys, together with structuraland geomechanical rock mass characterisation, haveallowed the GSI to be evaluated. Four lithotechnicalunits have been defined on the basis of the relativepercentage of the breccia fraction versus lava deposits:Lava unit (L), Lava-Breccia unit (LB), Breccia unit(B), Pyroclastic deposits (P). The range of variabilityof their main physical and mechanical properties (bulkvolume, porosity, intact rock compressive strength,geological strength index) has been measured by lab-oratory or in-situ direct tests or estimated, and meanrepresentative values assigned in order to apply theHoek and Brown’s failure criterion. The rock massfriction angle, apparent cohesion, tensile and com-pressive strength, global strength and modulus ofdeformation have been calculated in a specified stressrange.

Acknowledgements The research was supported by Gruppo Naz-ionale per la Vulcanologia—INGV (Italy) and FIRB-MIUR 2001.It is a contribution to the UNESCO-IUGS-IGCP project 455‘‘Effects of basement structural and stratigraphic heritage on vol-cano behaviour and implications for human activities’’.

T. Apuani et al. 429

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