Mechanical properties of pyroclastic soils in Campania Region

Transcription

1 Mechanical properties of pyroclastic soils in Campania Region L. Picarelli*, A. Evangelista**, G. Rolandi***, A. Paone***, M.V. Nicotera**, L. Olivares*, A. Scotto di Santolo**, S. Lampitiello*, M. Rolandi*** *Department of Civil Engineering, Seconda Università di Napoli, Aversa, Italy **Department of Civil Engineering, Università di Napoli, Aversa, Italy ***Department of Soil Sciences, Università di Napoli Federico II, Napoli, Italy ABSTRACT: Pyroclastic soils cover significant parts of the world s surface, including areas occupied by urban settlements, vital structures and infrastructures. Despite these materials present significant physical and mechanical differences from site to site, posing sometimes severe geotechnical problems, a comprehensive and worldwide accepted geotechnical classification still lacks. The Campania Region, at the centre of which rises the town of Naples, is covered by pyroclastic soils accumulated in the last tens thousands of years as a result of volcanic activity. The examples reported in the paper show that the index, state, hydraulic and mechanical properties of these materials depend on the distance from the eruptive centre and mechanism of deposition. The overall geotechnical framework arising from this review highlights the peculiarities of pyroclastic soils, adding further information to the general knowledge about the properties and behaviour of natural soils. 1 FOREWORD A not negligible part of the World s surface is occupied by pyroclastic deposits generated by the explosive activity of volcanoes. Such materials covered either flat or sloping areas, reaching thicknesses up to many tens of metres. According to the features and history of eruptions, these deposits may be either layered or massive, coarse-grained or fine-grained, bonded or nonbonded, fissured or non-fissured. Their fabric and structure are extremely variable from deposit to deposit and, in the same deposit, along vertical and horizontal directions. As a consequence, the range of hydraulic and mechanical properties is quite wide. Despite the relevance of the problem, a comprehensive geotechnical classification of these deposits still lacks, because of the variability of structure, and relevance at local scale only, of engineering problems. The Campania Region at the centre of which rises its capital, Naples, is covered by pyroclastic deposits generated by different volcanic centres, the most famous of which, are the Phlegrean Fields and the Somma-Vesuvius, which are still active inside the so-called Campanian Volcanic Zone. In this area, pyroclastic soils (mostly ash and pumices) and soft rocks (tuff) have been extensively used since from antiquity for construction purposes. As a matter of fact, the subsoil of Naples and of several other towns of the Region hide very old deep mines, caverns and tunnels (Evangelista et al., 198; 2). Volcanic ash is used for earthworks and is a fundamental component of pozzolanic cements; pumices are used for small light constructions and as a component of light concrete; tuff is an excellent material for masonry constructions, as testified by several monumental constructions rising in the town of Naples. Locally these materials pose severe engineering problems. These include erosion, slope instability, settlement of buildings, piping and failure of old caverns. The most important problem concerns slope instability. In fact, high cliffs, especially alongside the coast, are subject to falls, while slopes covered by ash experience slides and flow-like landslides. As a result of the cease-

2 less growth and spreading of urbanised areas and infrastructures, the risk of landslides increased enormously as testified by hundreds of victims provoked by flowslides in the last fifty years (Cascini & Ferlisi, 22). This paper is mainly concerned with unlithified pyroclastic materials, with particular reference to volcanic ash. It consists of two parts: the first one (sections 2, 3 and 4) is devoted to the origin and volcanological features of deposits outcropping in the Campania Region; the second one summarises the results of extensive investigations carried out on selected deposits outcropping either in the urban area of Naples (section 5) or in a distal zone North to Naples (section 6). 2 PAST AND RECENT GEOLOGICAL HISTORY OF THE CAMPANIA REGION 2.1 Neotectonic evolution of Apennine chain in the Plio-Pleistocene time The structural features of the Campanian Plain, North to Naples (Fig. 1), are the result of extensional tectonic processes caused by opening of Tyrrhenian basin and Europe-Africa compression. During the Pliocene-Quaternary, the Apennine region which bounds the north-eastern part of the Campanian Plain, underwent uplift and widespread sagging of sedimentary basement in graben-like structures occurred along the Tyrrhenian border of the chain. The graben formed along NW-SE and NE-SW faults, whilst outcrops are located in the Apennines chain, at Mt. Massico (NW) and on the Sorrento peninsula (SE) (Fig. 1). Figure 1. The Southern sector of the Campanian Plain graben structure In the graben, drillings down to depths of more than 3 m carried out in the Volturno Plain (Fig. 2) did not found the carbonate basement, whose top, located at about 5 m, has been obtained through analysis of geophysical data (Carrara et al., 1973; Cassano & La Torre, 1987).

3 Figure 2. Graben-like structure of the Campanian Plain inferred from deep borehole data (Ippolito et al., 1973; AGIP, 1977) (modified from Bruno et al., 2) In the graben, the deepening of sedimentary basement was probably due an intense explosive volcanic activity. Large volume of pyroclasts produced by the volcanic activity filled the graben. About 39 ky ago (De Vivo et al., 21), a violent explosive eruption generated the Campanian Ignimbrite (CI) (Fig. 3). Due to large volumes of erupted pyroclasts (about 15 km 3 ), the Plain definitively aggraded. Figure 3. Campanian Ignimbrite deposits near the Monte Massico. Because of the importance of those events, the Campanian Plain, which covers about 2 km 2, has been named Campanian Volcanic Zone (CVZ, in Fig. 2) (Rolandi et al., 23). 2.2 The Campanian Volcanic Zone (CVZ) The Campanian Volcanic Zone (CVZ) has been affected by intense volcanism for at least 6 ky (Ballini et al., 1989; Scandone et al., 1991). Indeed, volcanic successions along the CVZ s Tyrrhenian margin are more than 3 km thick, including the products of a complex sequence of eruptions. The largest one probably was the Campanian Ignimbrite event (Di Girolamo, 1968). According to geophysical surveys, the CVZ is a faulted depression created by regional subsidence of the carbonate basement along a NE SW oriented fault system (Florio et al., 1999; Milia, 1999; Bruno et al., 2). This observation suggests that the Campanian Ignimbrite was fed by

4 tectonically-controlled fissure eruptions across the CVZ. As shown by new stratigraphic data reported below, major ignimbrites appear to have been fed from across the CVZ for at least the last 3 ky. Subsequently, the volcanism migrated towards South. Today, the active volcanic district in the CVZ is restricted to Campi Flegrei and Somma-Vesuvius (Fig. 1). 3 THE ACTIVE VOLCANIC AREA IN THE CVZ 3.1 The Campi Flegrei caldera The Campi Flegrei volcanic district consists in a well defined caldera approximately 6 km across (Fig. 4). Some researchers (Rosi & Sbrana 1987; Orsi et al., 1996) considers this caldera as the CI source. Others studies instead (Di Girolamo et al., 1984; Scandone et al., 1991; Rolandi et al., 23), assume the CI source localized along fissures parallel to the main trends of postorogenetic Apennines extensional tectonics, as previously discussed, and that the caldera structure was formed during the big eruption of Neapolitan Yellow Tuff (NYT), 15 ky ago (Fig. 4). Subsequently, the volcanic edifice from where the NYT originated was destroyed in conjunction with demolition of the top of the magma chamber, taking the morphology of the subsiding caldera. Within such depression, numerous faults weakened the area, where a volcanic field constituted by more than 25 volcanic edifices developed, giving rise to the post 15 ky activity which was characterised by alternating periods of quiescence and of intense volcanism (Di Girolamo et al., 1984). This activity was responsible for repeated sea level variations recorded in the famous marine terrace of la Starza (Cinque et al., 1985). The last eruption took place in September, 1538 AD. Figure 4. The Campi Flegrei volcanic field into the NYT calderic depression 3.2 The Somma Vesuvius volcano Data obtained from the Trecase drill hole (Fig. 2, TC drill hole) reveal an ancient volcanic activity, dated at 4 ky, in the area where Mt. Somma-Vesuvius rises (Brocchini et al., 21). Stratigraphic data also indicate that the Somma-Vesuvius strato-volcano started to form after the CI eruption (39 ky BP). A volcanic edifice 2 m high, the older Somma volcano, was firstly formed mainly by a conspicuous effusive activity. About 25 ky ago, the activity of this volcano changed dramatically into to a succession of explosive Plinian events (Tab. 1). Starting from the Avellino prehistoric eruption (3,55 ky BP), the Somma volcano worked through alternating Plinian and interplinian periods concluded by a repose period (Rolandi et al., 1998). The Vesu-

5 vius cone was built up in the Somma caldera during the medieval interplinian phase. The volcano is famous for the Plinian eruption of 79 AD, which buried the cities of Pompei, Ercolano and Stabia. The modern historical interplinian activity took place since the 1631 plinian eruption. The last eruption which accomplished the recent hystorical interplinian phase, took place on March, Table I. Eruption, age, and explosive tipology of Plinian and interplinian events in the last 25 ky Eruptions Age Type of activity Dispersion direction Volume (km 3 ) Codola 25 ka BP Plinian S-E 1.4 Ancient Interplinian ka BP Strombolian and effusive Sarno 17 ka BP Plinian E Ancient Interplinian ka BP Strombolian and effusive Novelle ka BP Plinian N-NE 1.5 Ottaviano 8 ka BP Plinian E-NE 2.4 Avellino 355 BP Plinian E-NE 2.5 Protohistorical Interplinian BP Vulcanian and strombolian Pompei AD AD Plinian E-SE 4 Ancient historical Interplinnan AD Vulcanian, strombolian and effusive Pollena AD AD Plinian N-NE 1.2 Medieval Interplinian AD Vulcanian, strombolian and effusive AD Plinian N-NE 1.1 Recent Hystorical Inteplinian AD Vulcanian, strombolian and effusive 4 EXPLOSIVE ERUPTIONS AND THEIR PYROCLASTIC DEPOSITS Pyroclastic deposits are the result of explosive volcanic activity. According to the mode of transport and deposition, they can be classified as falls, flows and surges. Pyroclastic fall-forming eruptions and deposits features. Traditional names for these eruptions are hawaian, strombolian, volcanian and plinian; these are characterized by increasing VEI (Volcanic Explosive Index) from 1 to 6 (Newhall & Self, 1982). Plinian eruptions (VEI=6) are characterized by a steady magma discharge and by the presence of high enough released magma volatile content to ensure a high eruption speed (2-6 m/s). The eruption column formed by magma discharge can be divided into three parts: a) the gas thrust part due to rapid decompression of the gas phase; b) the upper convection plume driven by the release of thermal energy from juvenile particle; c) the umbrella region, where the column spreads radially or downwind (Fig. 5a). In the part b), buoyancy is dominant because air entrained into the basal volcanic jet (the gas thrust part) lowers the bulk density enough for convection to dominate the upper part of the cloud. The top of this region is defined by the level of neutral buoyancy, H b, where the bulk density equals that of the surrounding atmosphere. The umbrella cloud extends from height H b to height H t to which the column rises due to its momentum. Gravity induced fall out of ejects creates pyroclastic fall deposits. These must be distinguished into three kinds of deposits formed by: a) largest fragments of bomb and black scoria (>1 cm), called ballistic clasts, as they are hurled on ballistic trajectories from the vent, unaffected by wind or convection in the plume; b) smaller fragments including coarse lapilli and small bombs and blocks (1 1 cm), mostly lofted by turbu-

6 lent suspension in a convecting volcanic plume, that fall if their terminal velocity exceeds convective updraft in plume margin (Fig.5b). Smallest fragments which include fine lapilli and ash (<1 cm) are also suspended by turbulence. They can be dropped from the cloud when settling velocity is greater than wind velocity. However, fine ash can clump into larger particles in the turbulent plume, if it is wetted by water condensed from cooling steam, forming accretionary lapilli. Pyroclastic flow-forming eruption and deposits features Pyroclastic flow currents are dense, ground hugging, high velocity clouds which travel as high particle concentration gas-solid dispersions. They are gravity controlled hot, and in some partly fluidized. Eruption mechanism generating pyroclastic flows can be split essentially into two main types: a) lava dome collapse; b) eruption column collapse (Fig. 5c). Most pyroclastic flow deposits are composed by more than one flow unit. Within flow units, ungraded structure is usual (Fig. 5d), but larger pumice fragments can be reversely graded, while lithic clasts can show normal grading. Pyroclastic surges-forming eruption and deposits features Pyroclastic surges are horizontally directed, high velocity, turbulent clouds formed by dilute mixture of gas (partly entrained air) and solid particles. They commonly develop as a result of different eruptive mechanisms: a) phreatomagmatic and phreatic eruptions (Fig. 5e); b) pyroclastic flow and fall eruptions (Fig. 5a, c). Surge currents derived from eruptions type a) are named base surges; those derived from type b) are called ground surges and ash cloud surges. Surges travel in turbulent manner to less than a few kilometres from a vent because they have less momentum as they are mostly gas. Surges can develop bed forms similar to those observed in water and wind transport sediments. As in these modes of sediment transport, surge move particles by surface drag in a bed load and by turbulent suspension. Unlike water and wind, density and viscosity of surges can vary during travel creating variations in bed forms. Surge deposits are hence characterized by bed forms low angle cross bedded to wavy beds to planar (Fig. 5f). In volcanological investigations the grain size of deposits is usually represented by histograms reporting the weight of the components separated by mechanical analyses, and by cumulative plots on probability ordinate to determine the Inmam parameters (Inman, 1952): Median grain size: (Md Φ ) = Φ5; Graphic standard deviation: (σ Φ ) = (Φ84-Φ16)/2) Typical parameters and classification of grain size of fall, flow and surge deposits are reported in Tables II and III. Table II. Inman parameters for a fall, a flow and a surge, derived from Fig. 6d Inman parameters Fall Flow Surge Md Φ σ Φ Table III. Sorting of pyroclastic deposits according to volcanologists Range of σ Φ for pyroclastic deposits -1 Very well sorted 1-2 Well sorted 2-4 Poorly sorted >4 Very poorly sorted The relationships between sorting and depositional mechanisms for the three types of pyroclastic deposits described above, put into evidence the following.

7 Fall deposits: both particle size and density control the terminal fall velocity; as a consequence, these deposits consist of particle of similar size: this is the reason for their well sorting. Flow deposits: the deposits left by pyroclastic flow are poorly sorted because of high particle concentration of the currents. Surge deposits: the grain size of surge deposits reflects both the high degree of fragmentation and the competency of the surge to carry particles of given size. Given the different origin of surge deposits, it follows that: a) ash cloud surge deposits are generally better sorted; b) because of low particle concentration, the sorting of base surges and of ground surges is significantly better than the one of pyroclastic flows, but less so than in fall deposits. a) b) d) b) e) d) c) f) Figure. 5. Explosive tipology and associated products: a) eruptive column of a Plinian eruption; b) massive, well-sorted fall pumice deposit; c) flow-forming due to collapsing eruptive column; d) massive ignimbrite deposit; e) surseyan eruptive column and associated base surge; f) cross bedded pyroclastic surge deposit. f)

8 In conclusion, pyroclastic flow and surge deposits are less sorted than fall out deposits and consequently, a better discrimination between them is obtained on a plot (Md Φ - σ Φ ) as in Figure Pyroclastic deposits formed by the explosive Campi Flegrei activity The Neapolitan Yellow Tuff The Neapolitan Yellow Tuff (NYT) is probably the product of a single eruption which provoked the collapse of the Campi Flegrei caldera. The concerned volume was not less than 5 km 3 (Scarpati et al., 1993): about 35 km 3 of this lye within the caldera. The most representative sections of NYT are exposed on the Caldera rim and in proximal locations around the caldera in the Neapolitan urban area. In order to describe the volcanological nature of NYT, the alteration process occurred in the early emplaced deposit must be accounted for. The Neapolitan area is composed of a large variety of pyroclastic deposits related to both magmatic and hydromagmatic activity. In spite of similarities of composition, grain size, age and pre-eruptive environment, these deposits present significant differences in terms of degree and type of alteration of the juvenile components. In particular, the following two facies can be distinguished: a) altered lithified deposits locally named Yellow Tuff; b) unaltered unlithified pyroclastic products named Pozzolana. The transition between Yellow Tuff and Pozzolana corresponds to vanishing of alteration far from the eruptive vent inside the caldera (Fig. 7). Figure 7. Distribution facies of the Neapolitan Yellow Tuff: proximal lithified facies and unlithified distal facies (pozzolana) (modified from Scarpati et al., 1993) The eruptive and emplacement conditions are the major controlling factors of alteration due to secondary mineralization (zeolitization) processes (Fig. 8). The relationships between volcanological features of the deposits and zeolitization are consistent with the glass-fluid phase reaction in a near-closed post-depositional system. Eruptions characterised by separation of water vapour from the pyroclastic particles produce unaltered or poorly altered deposits. In contrast, eruptions characterised by mechanisms permitting the capture and retention of pore water and water vapour produce zeolitized deposits. According to these considerations, zeolitization of the pyroclastic products of Campi Flegrei took place immediately after emplacement, during the cooling of wet deposits (de Gennaro et al., 1999), progressively decreasing towards distal areas from eruptive vent, where the original unaltered pyroclastic materials (pozzolana) is present

9 (Fig. 8). The NYT deposit consists of two members (Cole & Scarpati, 1993): the basal one (Member A) is made up of at least six fall units, while the upper one (Member B) consists of several flow depositional units. Figure 8. Neapolitan Yellow Tuff, B Member: a) radial clusters of acicular crystals of phillipsite; b) rhombohedral chabazite crystals; c) phillipsite crystals embedded in chabazite; d) Mofete Red Tuff: authigenic feldspars and analcime crystals. A representative stratigraphic section of the unlithified facies (pozzolana) is present in the urban area of Naples, at the Ponti Rossi site. There the pyroclastic deposit fully represents the volcanic stratigraphy of the NYT (15 ky BP). Because of lacking of zeolitic alteration, the sindepositional texture of the deposit is well highlighted. The deposit is composed by several members (Fig. 9). At the bottom, is present the Member A which is 2 m thick. In the basal part this is constituted by numerous fine grained ash levels alternating with accretionary lapilli-bearing ash layers and by fall pumice levels. In the top section, the member ends with a whitish ash level. The ash layers present a grain size distribution and SEM characters of phreatomagmatic origin. In contrast, either the grain size or the morphometric features of pumice lapilli beds suggest a dry magmatic origin. Member B is constituted by at least 5 m thick layer showing textural characters varying from massive to sand-waves structures. Grain size distribution and SEM analyses of the massive layer reveal the presence of coarse grained small fragments. In both members, A and B, accretionary lapilli layers suggest are related to high water content currents varying from low-particle-concentration turbulent flow (base surge) to high particle concentration laminar flow (pyroclastic flow), suggesting an increase in mass discharge rate during the eruption and repeated eruption column collapse generating several pyroclastic flow alternating with pyroclastic surge currents. A distinct unconformity separates the upper layers of Member B, which show a massive and chaotic texture with high concentration of lapilli sized-clasts embedded in a coarse ash matrix (Fig. 9). In conclusion, the presence of Member A indicates that the first phase of the eruption was characterised by a discrete phreatoplinian explosion with a variable water ratio that produced pyroclastic surge deposits. The high percentage of vesciculated pumice clasts in surge layers indicates that prior to magma-water interaction, an intense loss of magmatic volatile occurred. Fall pumice layers of Member A also indicate a coexistence of a pulsating phreatoplinian phase. Member B indicates a second volcanic phase characterized by a complex depositional sequence of both pyroclastic flow and pyroclastic base surge deposits related to repeated eruption column collapse. In the proximal area (Quarto, Camaldoli) welded exposures of both members A and B are present and include large sized (5-6 cm) lithic and pumice clasts.

10 Figure 9. Composite section of the unlithified Neapolitan Yellow Tuff (pozzolana) at the Ponti Rossi site: 1) paleosoil; 2) ash; 3) accretionary lapilli; 4) pumice and lithic lapilli. Grain size istograms illustrate the variation of texture characters between individual layers of the members. In addition, SEM micrographs of typical surface features of pyroclasts from pozzolana deposit are shown The past 15 ky explosive intracaldera activity The post NYT intracaldera pyroclastic deposits were produced by the overall eruptive centres whose activity was frequently of hydromagmatic type. Pyroclastic fall and flows explosive eruptions occurred less frequently, but were important too. Excellent examples of hydromagmatic activity are represented by the Astroni, Averno and Monte S. Angelo eruptions, whereas pyroclastic flow activity is well represented at Agnano Monte Spina locality. Plinian fall activity also occurred and is represented by the Pomici Principali (1.3 kyr BP) and Agnano Monte Spina (4.3 ky BP) eruptions, whose fall-out products reached the Appenines and interbedded with Somma-Vesuvius fall products. At proximal locations within the caldera depression, there are several distinctive lithofacies of these deposits, but a description of the stratigraphic relationships between them is more complicated in the whole caldera area.

11 Figure 1. Stratigraphic sections in proximal (A) and distal (B) areas outside the Campi Flegrei caldera, showing the complex eruption sequence in the post 15 ky Campi Flegrei fall activity. Note the absence of several fall layers (b to e) from proximal to distal outcrops: 1) paleosoil; 2) pumice bedded lapilli; 3) ash; 4) scoria-lapilli; 5) reworked pyroclastic material In the following are described two sections located outside the caldera depression, which includes several pyroclastic beds of the post-caldera explosive activity. Pyroclastic fall layers of each eruption are covered by fall deposits of the next eruptions (plinian, strombolian and/or phreatomagmatic), forming a complete sequence of events that can be examined outside the Campi Flegrei caldera as distal fall deposits throughout a large area around the caldera, in the urban Naples area. The thickness varies from about 2 m in proximal areas, to about 4 m, in distal outcrops. A very complete sequence is made up of at least six fall units (from a to f, from base to top, in Fig. 1), interbedded with numerous ash layers (b and d) for which there are some evidences of phreatomagmatic origin. Ash layers are interbedded with pumice fall layers of the Agnano (a) and Agnano-Monte Spina (f) plinian eruptions. In the sequence, other two fall units formed by typical dark scoria layer overlaying a dark ash bed can be recognized (c and e). These deposits are associated with two violent Strombolian eruptions, locally named as Minopoli (8 kyr), and Montagna Spaccata (5 ky). The thickness of the post 15 ky sequence vanishes far from

12 the caldera rim, being controlled by the energy of the eruptions. Ash beds (b and d in Fig. 1) where probably deposited from dilute turbulent pyroclastic surge clouds, gradually thinning and vanishing due to erosion at distal area (Ponti Rossi, in Fig 1). Pumice fall beds of Campi Flegrei plinian eruptions (a and f) can be well correlated between proximal and distal exposures because they maintain an almost constant thickness even within Naples urban area. Plinian pumice layers are very distinctive and readily identifiable throughout the urban area of Naples, even at most distal exposures, interbedded between fall layers of the Somma volcano, forming a useful marker horizon. 4.2 Explosive activity of Somma-Vesuvius Pyroclastic sequences along the Apennine chain The products erupted by Somma-Vesuvius in the last 25 ky range in composition from silica saturated K-basalt and K-trachyte to silica-unsaturated K-tephrite and K-phonolite, which, on the base of geochemical data, have been grouped as follow: i) Group I, concerning the volcanic activity from 39.ky to 14 ky; ii) Group II, from 8 ky to 2,7 ky; iii) Group III, from 79 to 1944 AD (Ayuso et al., 1998). The central volcanic activity of Somma-Vesuvius in the proximal area gave rise to deposits of pyroclastic fall, flow and surges for the collapse of the eruptive column. (Fig.11). a) b) Figure 11. A.D. 79 Plinian eruption: a) proximal white and gray pumice fall deposit at Pompei; b) proximal flow deposit at Ercolano. In the distal area of Somma-Vesuvius region only fall deposits are well represented since pyroclastic flow and surge do not travel to great distance from the volcano. Somma Plinian eruptions were characterized by a column as high as 3-35 km, so that pyroclastic fall and pyroclastic flow and surges deposits associated with column collapse, formed very thick deposits in proximal area. At the same time, the eruptive column dispersions were influenced by the stratospheric wind pushing the products mostly along N-NE dispersion direction (only the products of the 79 AD eruption were pushed in the S-SE direction), towards the Apennine relief. Ancient products of fine-grained, highly humified ash cloud fall deposit generated from loss of fines at the top of ancient pyroclastic flows (elutriation process), originated from 3 to 39 ky, are not well preserved on the western Apennine slope, because of erosional processes, being accumulated by time at the base of the relief as conoid sediments. On the other hand, recent pyroclastic fall deposits of ash and highly vesciculated pumice fragments of Somma-Vesuvius activity are well preserved in distal areas, where the Apennines sedimentary relief was mantled by tephra of the plinian eruptions.

13 Figure 12a shows the dispersion directions of the fall deposits of each Plinian event, through the representation of the isopaches of 1 cm. In the last 25 ky, air-fall deposition along the main dispersal axis oriented at E-NE and E-SE created a very thick fall mantle bedding, ranging between 3 and 1 m, drapping the steep topography of the Apennines chain (Fig. 12b). Pyroclastic fall layers of each plinian event is covered by fall deposits of next eruptions, whose repose periods are testified by humified pyroclastic ash layers interbedded between fall layers (Fig. 13). Along the Apennines chain, fall deposits are well sorted (σ Φ <2) because of aeolian fractionation during the transport, i.e. at any location the fall deposit consists of vesciculated particles mostly of similar size (see also Fig. 5b) Pyroclastic fall deposits at very distal area Small deep pyroclastic sequences are exposed on the Partenio massif, that is the Apennines relief located between Avellino and Benevento cities (Fig. 12). Mantle bedding of the Apennines sedimentary basement inside this area is mainly due to 5-1 cm of fine-grained, highly humified, ash cloud fall deposit generated from elutriation (loss of fines at the top of ancient pyroclastic flows) in the Campanian Plain (CVZ). A case study section located near Cervinara town, on northeast-facing slope of Partenio massif, is represented by a section approximately 1.5 m deep (see also Fig. 43a). In the upper part, the profile includes two pumice layers, each one 25-3 cm thick, belonging to 4.5 ky (Campi Flegrei) and to 3.55 ky (Somma) plinian eruptions respectively, separated by thin weakly humified ash. The recent deposits overlie a more ancient, 8 cm thick, fine grained, strongly humified pyroclastic ash fall deposit, with brownish color (andosol), directly in contact with limestone bedrock. In the lower ancient part of the soil the contents of poorly ordered allumosilicates (alloa Figure 12. a) Distribution map of pyroclastic fall deposits of the Somma-Vesuvius deposited in the last 8 ky; b) distribution of the overall fall products from single eruptions corresponding to the layers of Fig. 11a (modified from Lirer et al., 21) b

14 phane and imogolite) formed with weathering of volcanic ashes, is very high, i.e. about of 25% (Terribile et al., 2). Its worth to note that since such allumosilicates have a hallow tabular structure that has a high surface-to volume ratio, they play significant effects on ion exchange and water adsorption in the soil, playing an important role in the landslide processes. Figure 13. a) The Somma-Vesuvius volcanic region extend towards the western Apennines chain; b) the fall volcanic succession of Somma-Vesuvius cropping out at Sarno site. Fall products separated by dark paleosoils mantle the Apennine relief. 5 PHYSICAL AND MECHANICAL PROPERTIES OF PYROCLASTIC DEPOSITS IN THE URBAN AREA OF NAPLES 5.1 Investigated sites and materials In spite of similarities in age, composition, grain size and pre-eruptive environment, the pyroclastic deposits outcropping in the Neapolitan area present significant local differences. In particular, they display both lithified and unlithified facies related to eruptive and emplacement conditions, which significantly affect their engineering behaviour. A review of the mechanical properties of pyroclastic materials outcropping in the town of Naples and in its neighbours is reported below, referring to the following representative deposits: 1) unlithified Neapolitan Yellow Tuff, or Pozzolana (YTP); 2) lithified Neapolitan Yellow Tuff (NYT); 3) pyroclastic products of the volcanic activity of Campi Flegrei younger than 15 ky (Intracaldera Phlegrean pyroclastic Deposits, IPD), which crop out above the Neapolitan Yellow Tuff formation. In the urban area, the Neapolitan Yellow Tuff is widespread and represents the subsoil that most concerns geotechnical engineers. Figure 7 above shows the outcrops of this formation in distal and proximal areas with a clear interface between lithified and unlithified zones throughout the city. In particular, the NYT is exposed along natural cliffs facing the sea or facing inland, and along man-made cuts of significant height (Fig. 14). The experimental data presented in the following have been obtained by laboratory tests performed on undisturbed samples recovered in rather a wide area (few square kilometres extended) located to the North of the city centre: this area is delimited to the South side by a deep valley (Cavone di Miano in Figure 15). Further to the South, at 8 m distance (Ponti Rossi area, PR in Figure 15), man-made cuts show the complete stratigraphic section of the unlithified facies (YTP) lying under other younger pyroclastic deposits. The area is highly urbanised and a number of tunnels have been bored in the subsoil to accommodate aqueducts and sewerage. The data regarding pozzolana have been obtained either from geotechnical investigations related to design and construction of these works, either from a number of research projects developed at the De-

15 partment of Geotechnical Engineering (DIG) of the Università di Napoli Federico II (Evangelista et al., 1998; Nicotera, 1998). Figure 14. Lithified Neapolitan Yellow Tuff: Posillipo hill, Naples Figure 15. Geological map of the Campi Flegrei and location of the investigated area The data about lithified NYT concern samples recovered at different sites in the town and from blocks sampled at two quarries located respectively within the urban area and in the outskirts of the city. The data about IPD have been obtained in the realm of a research project on shallow landslides and hence they are restricted only to low stress levels (Evangelista & Scotto di Santolo, 2). These deposits are delimited by either thin paleosols or erosional discontinuities that provide clear demarcations between the various eruptions of the recent volcanic history. In general, IPD consists in a sequence of fine to coarse unsaturated ash with intercalated pumice layers. The paper reports only data regarding the finer materials (ash layers) since they yielded the only undisturbed samples that have been recovered, and since they form the matrix of the coarser levels.

16 Laboratory tests have been performed on samples taken from boreholes drilled in different sites (Fig. 15) and actually they can be considered representative of the shallower deposits present in the hilly areas of the city. 5.2 Yellow Tuff Pozzolana (YTP) As discussed above, in the area considered in this paper (Fig 15) the pyroclastic soil deposits are completely unlithified (Pozzolana, YTP). The transition between yellow tuff and pozzolana corresponds with the vanishing of the alteration process far from the eruptive vent inside the caldera (Fig. 7); however in the distal areas, even if the alteration due to the zeolitization did not occur, the hydration processes could give rise to some weak bonding between particles. In the investigated area YTP is composed by two members (Figure 1): Member A, which is constituted, in the basal part, by numerous fine grained ash levels alternating with accretionary lapilli-bearing ash layers and by fall pumice levels, and in the upper part, by a whitish ash level; Member B, which is constituted by at least 5 thick layers showing textural characters varying from massive to sand-waves structures. This last member is probably the product of different density flow currents which vary from low-particle-concentration turbulent flow (base surge) to high particle concentration laminar flow (pyroclastic flow). In this case, pozzolana is located above the groundwater table and is partially saturated Physical properties Figure 16 shows the grain size distribution of YTP determined on samples recovered at depth ranging from 1.3 m to 24.8 m. The envelope exhibits a limited scatter, which demonstrates the uniformity of the deposit. The material is well graded, spanning from sand to silt with a little amount of clay fraction. This wide sorting is related to the origin of the YTP as massive pyroclastic flow. However, the two boxes in Figure 16 report the grain size distribution of more uniform and fine-grained materials recovered respectively at 24 m and 15 m. Probably these soil levels are originated by base surges embedded in the YTP deposit. The histograms in Figure 17 report the frequency distribution of dry unit weight, void ratio and degree of saturation. The same data have been drawn as a function of depth below the ground level. A careful analysis of data indicates that the physical properties of YTP present large variances and it is not possible to identify any trend in their variation with depth. This could be justified by the emplacement of YTP by a series of high concentration massive pyroclastic flows silt sand gravel N = finer by weight (%) z = 24 m finer by weight (%) d (mm) finer by weight (%) z = 15 m d (mm) d (mm) Figure 16. Grain size distribution of Pozzolana

17 γ d (kn/m 3 ) e S r N = (%) z (m) Figure 17. Main physical properties of pozzolana Hydraulic properties A volume extractor, a pressure plate (Fredlund & Rahardjo, 1993) and an oedometer suction controlled apparatus (Aversa & Nicotera, 22) have been used to determine the water retention curve and the permeability function of YTP (Aversa et al. 1998: Nicotera et al. 1999a). Specimens 56 mm in diameter and 2 mm in height have been tested at zero net vertical stress in the volume extractor and in the pressure plate, and at vertical stresses ranging from zero to 4 MPa in the suction controlled oedometer. Specimens tested in the pressure plate have been dried rising by step the matric suction (values: 1, 2, 4, 7, 1 kpa) starting from the natural water content which corresponds to a mean value of the degree of saturation around.5. A dryingwetting cycle has been performed in the volume extractor on either saturated or natural water content specimens. In this last case, the matric suction has been varied by step in the range from to 15 kpa. Oedometer tests consisted in the following phases (Fig. 18): determination of the initial suction by means of the axis translation technique (point 1 in Fig. 18); application of a prescribed value of suction (path from point 1 to point 2); increase of the vertical net stress with constant

18 suction and constant stress rate (path from point 2 to point 3); reduction of suction by steps up to the lowest possible value without loosing the control of the apparatus (path from point 3 to point 8); increase of the vertical stress with constant suction and constant stress rate (path from point 8 to point 9); unloading at constant stress rate and constant suction (path from point 9 to point 1) Figure 18. Adopted procedure for suction controlled oedometer tests The experimental results obtained with the three different apparatuses are reported in Figure 19 where the volumetric water content is drawn as a function of the matric suction. It is worth noting that in oedometer tests the volumetric water content has been evaluated taking into account the measured vertical strains, while the value determined with pressure plate or volume extractor tests disregard any strain. Bearing on results of investigations carried out by Aversa et al. (1998) and Nicotera et al. (1999a), the empirical expression suggested by Brooks & Corey (1964) has been employed to interpolate all the experimental data neglecting both hysteresis and net stress. The best fit of data is reported in Figure 19a (θ ws =.56; θ wr =.72; α=.1787 kpa-1; λ=.5731). The transient phases of suction equalisation in the tests performed in the volume extractor and in the oedometer have been interpreted to determine permeability. Each phase has been analysed with the methods proposed by Gardner (1956) and by Rijtema (1959). The first one has been used to analyse equalisation steps performed at very high suction when the soil permeability is very low. The second one has been adopted to analyse the equalisation process at low suction level, in order to take into account of the impedence of the porous stone. The obtained results are reported in Figure 19b where the data are interpolated by means of the empirical relation proposed by Gardner (1958) (α=.3553; n=2.634). The permeability varies in a very wide range, assuming a value of cm/s for saturated soil, and a value of ~ cm/s for a suction of 35 kpa. A large part of the reduction in permeability is concentrated in the range -5 kpa. It is worth to mention that the results are in a good agreement with each other regardless of the type of test. In conclusion, the main features of the water retention curve and of the permeability function are the following: the air entry value is very low (approximately 5 kpa) and drying is almost completed in a few tens of kpa; consequently, the permeability decreases very suddenly at low values of suction; the data about soil permeability are quite comparable regardless of the type of test; both functions are not significantly influenced by the net stress in the range of values used in these tests;

19 the analytical expressions of Brooks and Corey (for the water retention curve) and Gardner (for the permeability function) interpolate with an acceptable degree of accuracy the experimental data. a) b) Figure 19. Water retention curve (a) and permeability function (b) of pozzolana Compressibility Compressibility of YTP in the urban area of Naples was investigated by a number of researchers in the past in order to analyse problems of soil-foundation interaction (Croce, 1954; Penta et al. 1961; Pellegrino, 1967). Typical results of standard oedometer tests on unsaturated specimens are reported in Figure 2. The compression curves are highly non-linear, being strongly related to the initial density; however, they do not merge a unique and clearly defined virgin-compression line. In contrast, unloading curves (not represented in the figure) are practically linear and almost horizontal. This is in good agreement with the theoretical framework proposed by a number of authors (e.g. Liu

20 et al., 26) for granular materials. However further development is needed in order to take into account matric suction effects. Pellegrino in the Sixties (1967) stressed the structural collapse of unsaturated pozzolana upon wetting. The experimental procedure developed in those years has been then routinely adopted in the laboratory and a large amount of data have been collected. Some experimental results are reported in Figure 21. In the diagram, the results of three standard oedometer compression tests on unsaturated natural specimens (air lines with symbols) are compared to those of three tests (tight lines) in which the soil has been submerged under water at a constant vertical net load of 2 kpa. After collapse, compression proceeds along the same curve of unsaturated material having a smaller initial void ratio. However, the reduction in void ratio caused by wetting decreases as the initial void ratio decreases; wetting effects become practically nil for the densest specimen. 1.8 S r = e c αε 1.5% 1.%.5%.% σ v (MPa) σ v (MPa) Figure 2: Results of standard oedometer tests on unsaturated pozzolana, and secondary compression index as a function of the vertical stress pitch 1 e S r =.225 S r =.28 S r =.32 S r =.33 S r = wetting σ v (MPa) Figure 21. Results of oedometer compression tests on unsaturated pozzolana with a wetting phase A further insight into YTP collapse has been recently achieved by means of suction controlled oedometer tests (Nicotera, 1998). Figure 18 shows the stress path of a test performed on an un-

21 saturated specimen. Due to the low initial degree of saturation (Sr =.588), no significant volumetric strains are induced by suction increase up to 4 kpa under constant vertical net stress (path 1-2), and no significant variation in the degree of saturation is caused by vertical net stress increase up to 259 kpa at a constant suction (path 2-3). The volume decrease induced by saturation is represented by the steps 3-4 to 7-8, corresponding to a decrease of suction from 4 kpa to 1 kpa. Collapse mostly occurs during the last suction decrease, with the larger saturation increase. Therefore, if structural collapse is of concern, the suction effect is neither progressive nor linear. The test has been continued along a loading-unloading path at constant suction (steps 8-9 and 9-1 respectively). Creeping behaviour is another interesting feature of YTP. Ever since the 6 s (Croce, 1954), it has been remarked that both unsaturated and saturated pozzolana display significant secondary deformations. Data collected by Pellegrino (1967) by oedometer tests allow to evaluate the secondary compression index c αε as a function of the vertical net stress A relationship between c αε and the vertical net stress (Nicotera, 1998) is reported in the diagram inserted in Figure Shear Strength The analysis of standard triaxial compression tests adopted in the past do not permit to examine the influence of all variables affecting the mechanical behaviour of the unsaturated YTP. In general, for engineering applications two pragmatic approaches have been adopted. According to the first one, undisturbed samples are tested at their natural water content; following the more conservative second one, undisturbed samples are tested after saturation in order to reduce to zero the matric suction. In the former case the results are described in terms of total stress, while in the latter case the analysis is conducted in terms of effective stress. However, both approaches are quite unsatisfactory. In the first case, the volumetric strain cannot be measured, thus even the stress state is affected by a number of uncertainties (the correct axial stress can not be determined taking into account the radial strain; pore fluids pressures are unknown). As a consequence, the analysis of experimental data must be based on two implicit assumptions: 1) the shear strength can be investigated disregarding water content variation as the scatter of water content is small; 2) due to low saturation degree (usually less than 7%), the variations in pore fluid pressure during compression and shearing are negligible. In the second case, even though pore pressures and volumetric strains can be measured during the test, the strains induced by saturation cannot be determined. Actually, as shown above, the YTP presents a remarkable collapsible behaviour upon wetting. A number of data coming out from standard triaxial compression tests performed according to both approaches are available. Here, the results of 125 tests on natural samples have been considered. Since YTP behaves as a granular material, it may be acceptable to seek correlations of strength with initial density and confining stress. Tests results have been subdivided in four classes on the basis of the initial dry unit weight γ d. Strength envelopes (Fig. 22) clearly show the influence of the initial dry unit weight. In particular, a significant cohesive intercept comes out from the linear regression of the data for γ d >8.517 kn/m 3. Assuming the absence of bonds, the cohesive intercept should be due to the coupled effect of dilatancy and matric suction. Nevertheless, the regression of data regarding loose samples (γ d = kn/m 3 ) gives a nil value of the cohesive intercept while the soil shows a strain hardening behaviour, thus the corresponding friction angle (ϕ=36.49 ) could be considered as a lower bound for the critical state friction angle. However, since the correct axial stress can not be determined accounting for the radial strain, the deviator stress, and hence the friction angle, is underestimated. It is worth mentioning that the reported values of the strength parameters have been obtained by means of a statistical procedure (least-squares method). This approach is justified by the intrinsic variability of the YTP. Confidence regions of the regression parameters c and ϕ with probability equals to 7% are reported in Figure 23. Each of the ellipses drawn in the figure has at his centre the point representative of parameters least-squares estimate and contains the point corresponding to the value of c and ϕ with probability 7%. The differences between the shear strength of the different classes clearly come out (the loosest samples have not been considered due to the nil cohesive intercept). Moreover the different size of the ellipses shows that the re-

22 ported estimates have different reliability. As a matter of fact, smaller is the confidence region higher is the reliability of the least square estimate. Since the peak strength not only depends only on the initial conditions, but also on the state of stress, further analyses have been performed in order to assess the influence of the confining stress. The analyses have been carried out only on the data classes containing a significant number of experimental points, i.e. in the range of γ d between kn/m 3 and 1.31 kn/m 3 and in the range between 1.31 kn/m 3 and kn/m 3. The results are summarised in Table IV where they are compared to the strength parameters of the remaining classes. The influence of the state of stress is evident. 17 (σ 1 -σ 3 )/2 (kpa) (σ 1 -σ 3 )/2 (kpa) (σ 1 -σ 3 )/2 (kpa) (σ 1 -σ 3 )/2 (kpa)15 ϕ=36.49; c= kpa N = 14 γ d = kn/m (σ 1 +σ 3 )/2 (kpa) ϕ=32.13; c=42.73 kpa N = 38 γ d = kn/m (σ 1 +σ 3 )/2 (kpa) ϕ=35.69; c=49.91 kpa N = 6 γ d = kn/m (σ 1 +σ 3 )/2 (kpa) ϕ=37.81; c=97.4 kpa N = 13 γ d = kn/m (σ 1 +σ 3 )/2 (kpa) Figure 22. Strength envelopes of unsaturated undisturbed pozzolana specimens (empty circles indicate outliers data identified by the regression analysis)

23 ϕ ( ) all data (1-α)=7% γ d = kn/m 3 γ d γ = kn/m 3 d = kn/m c (kpa) Figure 23. Confidence regions of strength parameters of unsaturated undisturbed specimens of pozzolana with different dry density Table IV. Shear strength parameters of pozzolana in standard triaxial tests γ d σ 3 S r φ ( ) c (kpa) N R 2 (kn/m 3 ) (kpa) (1) min max min max mean Confidence regions of the regression parameters c and ϕ with probability equals to 7% are reported in Figure 24. If the lower bound of the range of the confining stress is fixed, the higher is the upper bound the higher the cohesive intercept and the lower the friction angle. The observed trend can be justified in the usual framework for granular materials (e.g. Wood, 24). In this framework, for a given range of initial dry unit weight, the peak strength envelope of natural specimens is described by a non linear relationship. As a consequence, the strength parameters are not only influenced by the initial dry unit weight and matric suction, but also by the state of stress. Further data about the shear strength of unsaturated YTP have been achieved by means of a suction controlled triaxial apparatus which allows to account for the influence of suction (Fig. 25), The cell was designed for samples measuring 68 mm in diameter and 14 mm in height. The suction control is based on the axis translation technique. Radial strains are determined by accurate measurement of the difference in pressure between the water filling an inner cell coaxial to the sample and the water filling a reference double walled burette, submitted to the same pressure. The variations in water content are obtained by measuring the difference in pressure in two double walled burettes, one of which being connected to the drainage circuit, the other one operating as a reference.

24 ϕ ( ) γ d = kn/m 3 σ 3 < 2 kpa 36 σ 3 < 5 kpa all data σ 3 < 69 kpa 33 all data σ 3 < 78 kpa) 32 (1-α)=7% 31 3 (1-α)=7% c (kpa) c (kpa) Figure 24. Confidence region of strength parameters of unsaturated undisturbed pozzolana specimens as function of stress level: a) (γ d = kn/m 3 ); b) (γ d = kn/m 3 ) ϕ ( ) a) b) γ d = kn/m 3 σ 3 < 24 kpa σ 3 < 5 kpa Fig. 25. The adopted suction controlled triaxial apparatus (Nicotera & Aversa, 1999) The tests have been carried out as standard ones, but controlling all the variables which cannot be managed by the standard apparatus (e.g. matric suction, volumetric strains). The testing procedure consists in the following phases: i) determination of the initial suction by the axis translation technique; ii) isotropic compression, carried out under either constant suction or constant water content, generally at a constant rate of loading; iii) strain controlled shearing under either constant suction or constant water content. Generally, in tests at constant water content the variations in matric suction throughout isotropic compression and shearing phases have been negligible (Nicotera et al., 1999b). This is justified by the low initial saturation degree (mean value.475). On the other hand, the matric suction was quite low, ranging from 5 kpa to 2 kpa. This suggests that standard triaxial tests performed on unsaturated samples having a low saturation

25 degree could be interpreted as constant suction tests, even though the actual matric suction remains unknown. Some results of suction controlled triaxial tests are reported in Figure 26, where the net stress ratio q/(p-ua) and the volumetric strain ε v are plotted as a function of the axial strain ε a. In the same figure some results of standard tests on saturated specimens (dashed lines) are plotted in terms of the effective stress ratio q/p. Almost all unsaturated specimens show a brittle behaviour, while saturated ones display a hardening behaviour. In the investigated stress ranges (in suction controlled tests, p-ua= kpa corresponding at peak to σ 3 -u a =41 24 kpa; in standard tests σ 3 = kpa) the behaviour of unsaturated specimens having a dry unit weight, γ d, ranging from kn/m 3 to kn/m 3 (mean value kn/m 3 ) is dilative, while the one of saturated specimens with γ d ranging from 9.17 kn/m 3 to 11.3 kn/m 3 (mean value kn/m 3 ) is contractive. As a matter of fact, the net and the effective stress ratios seem to converge towards clearly defined steady-state values that are quite similar, corresponding respectively to (ϕ=37.49 ) and (ϕ =37.44 ). These values are fairly greater than the lower bound of the critical friction angle obtained through standard triaxial tests on loose samples (ϕ = ). Hence, for YTP it can be concluded that the matric suction does not affect the critical friction angle but only the peak strength, and that a unique critical friction angle can be assumed, irrespective of saturation conditions. In Table V the strength parameters obtained by suction controlled triaxial tests are compared to those determined by standard triaxial tests (Table IV). The data are quite consistent: standard triaxial tests show slightly higher values of the friction angle and of the cohesive intercept probably due to underestimation of the radial strains. 2. S r < 1; u a - u w = 5 2 kpa; p-u a = kpa q/(p-u a ), q/p' S r = 1 ; u a - u w = kpa; σ' 3 = kpa M = (ϕ = ) ε a (%) ε v (%) S r = 1 S r < ε a (%) Figure 26. Results of suction controlled triaxial tests (continuous lines) versus standard triaxial tests on saturated pozzolana specimens (dashed lines) dil. comp.

26 Some additional details about the influence of density and confining stress can be discussed by further analysis of the experimental data. In Figure 27 the net stress ratio at peak is reported as a function of the mean net stress. The data are subdivided as a function of γ d in the same four classes considered above. Each point represents the stress ratio at peak determined as the mean value obtained in triaxial tests carried out at the same confining stress. The range of the critical stress ratio as previously estimated, is reported as comparison. The figure shows that, as the mean net stress increases, the net stress ratio of unsaturated specimens decreases towards the critical value. The gradient of the net stress ratio reduction and of the mean net value behind which the stress ratio is close to the critical value depends on the initial dry unit weight. However, the maximum stress ratio of looser samples is lower or equal to the critical value measured by suction controlled triaxial tests. In the same diagram the maximum values of the effective stress ratio determined by triaxial tests on saturated specimens are reported as a function of the mean effective stress. The range of the initial dry unit weight of these specimens is also reported (however, these values refer to the initial conditions ignoring volume reduction caused by the saturation process). Saturated specimens show a maximum effective stress ratio higher than the critical one only in the lower stress range. It is worth noting that these present a peak strength significantly lower than unsaturated specimens of similar initial density. It follows that saturation reduces the stress range in which the peak strength is affected by dilation. Therefore, the proposed analysis indicates that the matric suction affects the peak strength by modifying the stress-dilation relationship; this point will be further developed in the following section. Table V. Shear strength parameters of pozzolana in suction controlled triaxial tests Test Loading Stress level ua-uw γd Sr path (kpa) (kpa) (kn/m3) (1) φ ( ) c (kpa) TXT DS Suction controlled Conventional Natural water content Wetted at peak p-ua = const σ 3-ua < peak final σ3 = const σ 3 < 25 n.a peak σ '3 = const σ '3 < peak final σ v = const σ v < 35 n.a peak final Figure 27. Net stress and effective stress ratios as a function of the mean net stress and of the mean effective stress

27 5.2.5 Shear response upon wetting Some observations concerning the mechanical behaviour of pozzolana upon wetting have been obtained by direct shear tests (Nicotera 1998; 2). Two different kinds of tests have been performed: a) tests on natural specimens wetted during shearing at peak strength; b) tests on specimens saturated in the consolidation stage. The first ones have been used to investigate the phenomena that occur when saturation is accomplished in proximity of failure. Direct shear tests on samples saturated during consolidation have been performed for comparison. The tests have been carried out at three different values of the vertical stress, i.e. 19, 177 and 345 kpa. Typical results are reported in Figure 28. YTPs at natural water content show a brittle behaviour when sheared at the lower and medium normal stress, which corresponds to the in situ vertical stress; in contrast, they show a hardening behaviour either at the higher normal stress or saturated (no matter of the normal stress). Wetting at peak causes a sudden and relevant strength decrease and volumetric compression. With increasing shearing, both shear strength and compression increase reaching a steady-state value. In the first part of the test, the samples subjected at lower vertical stresses tend to dilate, while those subjected to higher values contract. However, after saturation, all specimens display a contracting behaviour a) τ (kpa) dx (mm) -.5 dil. b). dy (mm) σ v (kpa): comp dx (mm) Figure 28. Results of direct shear test on pozzolana (saturated samples represented by a continuous line; samples wetted at peak represented by a dashed line) The observed behaviour can be interpreted in the framework of the Critical Soil Mechanics extended to unsaturated soils (Alonso et al., 1987). Wetting and the associated suction decrease produce a contraction of the actual yield locus and hence, at strain controlled conditions, a reduction of the mobilised shear strength. Then, volumetric strain hardening causes a recovery of

28 the shear strength as shearing progresses. More generally, plastic strains occurring upon collapse affect the soil behaviour and must be taken into account in modelling the soil behaviour upon wetting. It follows that the geotechnical design based on saturated shear strength parameters is not necessarily conservative. The data obtained by direct shear tests have been interpreted in terms of net stress (i.e. total stress) to obtain the strength envelope. They have been subdivided into different classes as a function of the initial dry unit weight. The cohesion intercept at natural water content has been obtained by means of a linear interpolation of the points representative of the maximum (peak) strength. For the material saturated during consolidation, the peak and final strength envelopes differ very little one from the other. The cohesive intercept of the peak envelope is obviously influenced by the curvature of envelope itself. In contrast, the final strength envelope approximates the critical state strength at zero matric suction. In Table V the friction angle and the cohesive intercept relative to unsaturated and saturated specimens having a mean value of γ d equals respectively to kn/m 3 and kn/m 3 are compared to the strength parameters obtained by standard and suction controlled triaxial tests. The comparison concerns the tests performed at comparable states of stress (σ 3 <29 kpa in triaxial tests and σ 3 <35 kpa in direct shear test). The strength parameters are in quite a good agreement. In particular, as expected, the peak strength determined by direct shear tests is to some extent higher than the one obtained from triaxial. In contrast, the final shear strength in direct shear tests is slightly lower. This result is probably due to the incomplete development of hardening following wetting at peak. 5.3 Lithified Neapolitan Yellow Tuff The lithified NYT is constituted by three components: an ashy matrix, highly porous inclusions (pumices and/or scoriae) and lithic inclusions. The quality and amount of porous and lithic inclusions depend on the distance from the source. The minerals present in the ashy matrix have a strong influence on properties. The most common crystalline phases are: phillipsite, cabasite, sanidino (K-feldspar); an amorphous silicatic phase is also present. The phillipsite and cabasite minerals constitute the zeolitic fraction (Fig. 29). Figure 29. SEM view of zeolities: a) cabasite; b) phillipsite Only relevant properties of NYT are presented in this section. More details can be found in previous papers (Evangelista, 198; Evangelista & Pellegrino, 199; Evangelista & Aversa, 1993, 1998; Aversa et al., 1993). Figure 3a reports the uniaxial compressive strength as a function of dry density. Single dots concern groups of samples taken from different sites where the rock may be considered uniform: in some cases the reported value represents the average strength of numerous samples, while in others cases, it refers to a few samples or to single values. The figure shows that the uniaxial compressive strength strongly depends on dry density. In Figure 3b the coefficient of variability of the uniaxial strength is plotted against the variability of the dry density of groups of sam-

29 ples considered homogeneous. The strength variability is considerably higher than that of dry density; thus suggests that the structure of the material plays a fundamental role. Looking at the results of triaxial tests, Pellegrino (1968, 197) recognised two different stress domains characterised by different behaviours (Figure 31): the rock-like behaviour, observed at small states of stress, and the soil-like behaviour observed at high states of stress. The transition from the rock-like behaviour to the soil-like behaviour is associated with rupture of bonds which progressively transforms the rock into an assembly of fragments or of single grains. Leroueil and Vaughan (199) call this phenomenon destructuration. The scattering of the triaxial strength is usually very high at low confining pressures. Figure 32 shows the strength envelope of samples taken from two sites. Generally, the friction angle varies between 22 and 3, while the cohesion depends on porosity. Within the yield surface the rock stiffness is approximately constant. For this reason, the Young s modulus may be considered a rock property. In the Miller diagrams the tuff is classified as weak having a E/σ c ratio comprised between 5 and a) b) 4 35 σc (MPa) 1 1 σc (C.V.%) , γ d (kn/m 3 ) γ d (C.V.%) Figure 3. Uniaxial compressive strength of lithified tuff vs. dry unit weight a) and coefficient of variability of strength vs. coefficient of variability of dry unit weight (b) (σ 1 σ 3 )/2 (MPa) peack strength yield stress (σ 1 +σ 3 )/2 (MPa) Figure 31. Rock-like and soil-like domains for the lithified tuff (after Pellegrino, 197b) The interaction between tuff and water has been investigated by Evangelista (198) who notices that soaking in water causes a volumetric strain. The magnitude of expansion is inversely proportional to the initial degree of saturation (Fig. 33). It is worth to note that the experimental data show a reduction of the peak strength and a contraction of the elastic domain as a consequence of saturation. This difference has been ascribed to a decrease of suction and to interaction of water and zeolitic minerals.

30 15 1 Quarto 1 c (MPa) 1 (σ1-σ3)/2 (MPa) 5,1,4,5,6 n San Rocco Quarto San Rocco (σ 1+σ 3)/2 (MPa) Figure 32. Strength envelope of two types of lithified tuff: San Rocco tuff and Quarto tuff (after Evangelista and Pellegrino, 199) and influence of porosity on cohesion 1 Axial strain (%),1,1,1,1,1,1 1 Initial degree of saturation Figure 33. Uniaxial expansion of lithified tuff due to saturation as a function of the degree of saturation. Marino et al. (1991) stress the influence of the mineralogical composition on swelling or shrinkage. Investigations have been carried out on different types of material: 1) tuff characterised by different zeolitic minerals; 2) unzeolitized tuff. Wetting-expansion and drying-shrinkage curves relative to a zeolitized tuff and to a unzeolitized tuff are shown in Figure 34. The zeolitized tuff shows both the greatest expansion and the greatest shrinkage. Deformation increases with the content of zeolite minerals. Further experiences show that after thermal treatment, the behaviour of a zeolitized tuff becomes similar to that of a unzeolitized tuff. Figure 34 Wetting-expansion and drying-shrinkage curves relative to zeolitized (YL1) and unzeolitized (GR1) tuffs (after Marino et al., 1991)

31 5.4 Pyroclastic deposits of Campi Flegrei younger than 15ky Physical properties As discussed above, the past 15ky intracaldera deposits of Campi Flegrei (IPD) are the result of different hydromagmatic eruptions. Soil samples, which are usually unsaturated, have been collected at depths ranging from 1. m to about 15. m (Fig. 15). The test programme mainly consisted of routine triaxial tests. The grain size distribution is reported in Figure 35 (44 samples). The material ranges from fine sandy silt (A in Fig. 35) to well graded silty sand, and finally to sandy gravel (D). The clay fraction is usually less than 1% (except than for the material A), while the sandy fraction is usually less than 4% (except for the material D). This variability is related to the origin of the material, constituted by hydromagmatic eruption-style fall, flow and surge deposits. Figure 35. Grain size distribution curves of IPD deposits The physical properties of IPD in the are investigated sites is reported in Figure 36 as a function of the depth from the ground surface Hydraulic properties The water retention curve of material B (Fig. 37) has been determined on undisturbed specimens using a Richards pressure plate and volume extractor (Scotto di Santolo, 2). The experimental points (N=48) have been interpolated with the correlation proposed by Brooks e Corey (1964). The air-entry pressure (u a -u w )e is assumed to be less than 1 kpa; the major variation has been recorded for values less than 1 kpa. The hysteresis is negligible. The experimental data and curve fit are shown in Figure 38 in terms of relative volumetric water content Θ, defined as the ratio (θ w - θ wr / θ ws - θ wr ), where θ w is the volumetric water content (ratio of water volume to the total soil volume), θ wr is the residual volumetric water content and θ ws is the saturated volumetric water content. The permeability function in terms of k r (k w /k sat ), plotted in Figure 38 as well, has been obtained by the indirect method proposed by van Genuchten (198): the saturated permeability k sat is equal to m/s Shear strength The shear strength has been investigated by means of 175 triaxial tests (CID) carried out on samples of the types A and B: 132 tests have been performed on specimens at their natural water

32 content and 43 on saturated specimens. Also direct shear tests have been carried out, under natural, dry and saturated conditions and for normal stresses ranging from 7 to 1 kpa (Scotto di Santolo, 2). All tests performed on saturated specimens show the typical mechanical response of sands, i.e. brittle and dilative in case of dense specimens, and ductile and contractive in case of loose specimens. The influence of the initial degree of saturation is shown in Figure 39 which compares the behaviour of a saturated and of a unsaturated specimen characterised by the same dry density (γ d = 1.76 kn/m 3 ), both subjected to a confining pressure of 2kPa. The figure shows that the a peak strength is displayed only by the unsaturated specimen. This behaviour is consistent with data reported in section for unsaturated YTP and with the results of direct shear tests. γ d (kn/m 3 ) e S r N = (%) z (m) Figure 36. Main physical properties of IPD (N=175),6,5,4 θw,3,2,1 N= u a -u w (kpa) Figure 37. Soil retention curve of IPD - soil B (Fig. 35)

33 1,2 1,8 Brooks & Corey [1964] Mualem [1976a] Θ kr,6 k r Θ,4, u a -u w (kpa) Figure 38. Soil retention curve and permeability function of IPD soil B 2,5 2 S r =,31 σ 3 =σ' 3 =2 kpa γ d =1,76kN/m3 1,5 η 1 S r =1, ε a (%) Figure 39. Stress-strain response in triaxial tests of two specimens of IPD characterised by the same dry density and a different degree of saturation 3 γd= kN/m 3 2,5 γd= kN/m 3 η, η' 2 1,5 M=1.4 (ϕ~35 ) S r = σ 3, σ' 3 (kpa) Figure 4. Stress ratio (η=q/p; η =q/p ) vs confining stress for specimens of IPD characterised by a different density and degree of saturation

34 As for the YTP the specimens have been subdivided into four classes as a function of γ d. If a linear strength envelope is assumed, the peak friction angle rises as density increases. However, the friction angle depends on the range of stress, rising as the normal effective stress decreases. In Figure 4 the peak stress ratio (η =q/p ; or η=q/p for unsaturated specimens) is plotted against the confining pressure. Once again the peak strength increases as γ d increases and the degree of saturation decreases. The value of the critical state angle is around 35. Four kinds of direct shear tests have been performed: a) on specimens wetted at peak during the shear stage; b) on specimens saturated in the consolidation stage; c) on specimens at their natural water content; d) on specimens dried in the pressure plate apparatus at different matric suction values (Scotto di Santolo, 2). As shown above, wetting causes either a shear strength decrease either a modification of dilatancy. Figure 41 shows the results of tests of types a, b, and c carried out under the same vertical stress (σ v = 19 kpa). The results are reported in terms of the ratio τ/τ u (where τ is the applied shear stress and τ u is the shear stress at the end of the test) as a function of the horizontal displacement dx. Any increase of the degree of saturation causes a change of the soil behaviour which turns from brittle to ductile. Wetting at peak (tests of type a) causes a relevant compression and a strength decrease. However, as the shear displacement increases, a progressive recovering of the shear strength occurs until the steady state value is attained. 1,8 1,6 1,4 1,2 H 2 O test a test b test c τ/τu (kpa) 1,,8,6,4,2 a) σ v =19 kpa, dx (mm) 1,2, b),4 dy (mm), -,4 -,8-1,2 Figure 41. Results of direct shear tests on IPD carried out for different saturation conditions In the assumption that the matric suction experiences only small changes during the test and that the pore-air pressure is atmospheric, the shear strength envelope can be obtained as a function of the matric suction. Since this is in turn a function of the water content, a direct relationship can be established between the water content and the shear strength. Therefore, this last can be written as follows, as a function of the value measured for complete saturation accounting for the characteristic curve (Vanapalli et al., 1996):

35 [ c ' + ( σ u ) tanϕ' ] + ( u u ) ( Θ) ϕ k τ = tan ' a a w In previous expression: τ is the shear strength in unsaturated conditions; c and ϕ are the shear strength parameters for saturated conditions; (σ - u a ) is the net normal stress on the failure plane; (u a -u w ) is the matric suction; Θ is the relative volumetric water content and k is a fitting parameter. The main advantage of this approach is that it requires the use of standard laboratory tests and a pressure plate. In addition, it provides a practical value of the unsaturated shear strength through quite a fast procedure. Substituting in previous expression the equation proposed by Brooks and Corey, for suction greater than the air entry value the following shear strength can be obtained: τ = c ' + ( σ u ) tan ϕ' + a λ ( u u ) a w e ( ) ( u ) tan ϕ' u u u a w a w As an example, this function is plotted in Figure 42 for a net stress equal to 19 kpa. The matric suction has been obtained through the value of the water content and by direct measurements. The figure shows that the predicted shear strength is generally lower than measured one at least for the range of stress tested (Evangelista & Scotto di Santolo, 21) τ (kpa) experimental prediction σv - u a = 19 kpa u a - u w (kpa) Figure 42. Comparison between measured and predicted values of shear strength of IPD as a function of the matric suction. 6. PHYSICAL AND MECHANICAL PROPERTIES OF THE CERVINARA VOLCANIC ASH, NORTH TO NAPLES 6.1 Investigated site In the last decade a number of researches have been addressed to investigation of the properties of air-fall unsaturated unlithified pyroclastic soils involved in catastrophic flowslides, present to the North and East from Naples (Guadagno et al., 1988; Cascini et al., 2; Esposito et al., 23; Olivares & Picarelli, 23; Calcaterra & Santo, 24). Figure 43 reports grain size curves of some deposits involved in flowslides or judged prone to such slope movements. It is shown that these materials are highly uniform; in particular, volcanic ash displays a high sandy component but also a significant amount of non-plastic silt. A high porosity, ranging between about 65% and 75%, is another typical property of these deposits.

36 a) Campi Flegrei Caserta Campanian plain Naples Vesuvius S.Felice a Cancello Palma C. Benevento Forchia S. Martino V. C. Cervinara Baiano Lauro Sarno Quindici Bracigliano Gulf of Naples Capri Castellammare Sorrentina peninsula Siano Nocera Vietri Salerno km Sites involved in catastrophic flowslides in the last fifty years Investigated sites Gulf of Salerno Passer by weight [%] Passer by weight [%] Passer by weight [%] Bracigliano Volcanic ash d [mm] 1 Monteforte Irpino 8 Volcanic ash Pumices d [mm] 1 Lauro Volcanic ash Pumices d [mm] 1 Cervinara Volcanic ash 8 Pumices d [mm] 1 Sarno (Episcopio site) 8 Volcanic ash d [mm] 1 Forchia 8 Volcanic ash d [mm] 1 Sarno (Lavorate site) 8 Volcanic ash Pumices d [mm] 1 Baiano Volcanic ash 8 Pumices d [mm] b) Figure 43. The area of Campania Region involved or judged prone to flow-like slope movements (a), and grain size distribution (b) of samples taken from different sites Cervinara is a representative site among those mentioned above. For this reason, wide investigations including laboratory and flume tests, and site monitoring have been carried out on the materials involved in the flowslide of December, 1999 (Olivares et al., 22; Damiano, 24; Lampitiello, 24). Some data obtained in laboratory investigations are discussed in the following.

37 6.2 Fabric and physical properties The tested material has been recovered just aside the flowslide of Figure 44 shows a schematic picture of the cross section of the slope which is around 4. The slope is quite flat and regular and consists of unsaturated layered air-fall pyroclastic soils resting on fractured limestone. The average thickness of the cover is 2.5 m. The top soil (5-8 cm thick) consists of humified ashes including roots and organic matter (V). Below the top soil, are present two 3 cm thick pumice layers (A and C), with a 5 to 1 cm interbedded layer of volcanic ash (B). The deepest layer, which directly mantles the bedrock, consists of a slightly altered volcanic ash (D) whose thickness ranges between 3 and 5 cm. Figure 44. Cross section and stratigraphy of the Cervinara slope (from Olivares et al., 22) The layers V, A, B and C are cohesionless. As a consequence, sampling is not easy. It can be successfully carried out only in the volcanic ash (layer B), thanks to suction which normally ranges between 5 and 5 kpa, and in the layer D which is slightly plastic and cohesive. Sampling has been carried out through a number of pits and boreholes performed respectively along the slope and at its foot. The samples of highest quality have been taken from pits, using steel cylindrical samples that have been gently pushed in the soil by hand or by the help of a hydraulic jack. Layers A, B and C are quite uniform thanks to the highly selective deposition mechanism of these deposits. Figure 45 reports the grain size distribution of the single layers. Layer B consists of a silty sand with a fine-grained component, generally less than 3%. Depending on its amount (higher or less than 1%), this fine-grained component can significantly affect the coefficient of uniformity, which in fact varies between 11 and 6. However, fines are non-plastic, being essentially constituted by very poor ordered minerals, sanidine and pyroxenes. The altered ash of layer D is finer than ash of layer B. Pumices fall in the domain of sandy gravel (layer A) and of gravely sand (C). The specific weight of ash (B) measured with conventional procedures varies within quite a wide range (Gs= ). Since this variability is due to external and internal voids affecting single particles, different procedures have been used to obtain the best value of Gs. The results reported in Figure 46 suggest that the influence of voids can be minimized working on crushed particles through a helium precision pycnometer. However, all procedures show that the specific weight of particles having a size comprised between.1 and.5 mm is much higher than the average, with a peak of 2.9 for a size around.3 mm. This has been explained with the help of mineralogical analyses, which show that this granulometric class includes a significant crystal fraction characterised by a higher specific weight (Lampitiello, 24). The variability of Gs implies some approximation in the determination of soil porosity and of saturation degree (Olivares et al., 23). However, the porosity of layer B is quite high, ranging between 66 and 7 %, with an average value of 67%, while the porosity of layer D is much

38 smaller, averaging 54%. The saturation degree, thus suction, depends on environmental conditions, but is generally much less than one. According to retention curves obtained by suctioncontrolled triaxial tests (Fig. 48b) and accounting for suction which is regularly measured with fixed tensiometers (see Fig. 44), a maximum value around 97% has been assessed for the most rainy periods for layer B. 1 9 Ash (Layer B) Percent by weight Altered ash (Layer D) a) Upper pumices (Layer A) Percent by weight Lower pumices (Layer C) b) d [mm] Figure 45. Grain size of single layers (from Olivares & Picarelli 23) Specific Weight, Gs 3. 1) Water pycnometer 2) Helium pycnometer ) He pyc. on shattered particles Weighted mean: Gs= 2.63 d [mm] d [mm] Figure 46. Specific weight of volcanic ash (layer B) (from Lampitiello, 24) A number of photographs have been taken with the SEM in order to examine either the features of single particles or the fabric of intact samples. The shape of pumices is from sub-angular to sub-rounded. They are vescicular, presenting small holes separated by glassy diaphragms caused by quick quenching in air after explosion (Fig. 47a). Layer B is characterised by macropores having a size comparable to the one of the coarsest particles. Some macropores are empty, others are filled with silt (Fig. 47b, c). Sandy particles are covered by a sort of adhering dust, the largest ones being bridged each other through chains of silty particles (Figs. 47d). In the following sections are reported the main results of laboratory investigations. As discussed above, due to difficulty in sampling pumices, mechanical tests have been primarily conducted on the intermediate ash layer (B). In addition, a few triaxial tests have been performed on the basal altered ash (D). Even though the layer B is generally less than 1 m thick, it is highly representative of the ash deposits mantling slopes to the North and East from Naples (Fig. 43a). The tests have been conducted on undisturbed and reconstituted specimens; these have been prepared following the moist tamping procedure. The main goal of the experimental program on reconstituted specimens concerns the role of the void ratio on the properties and mechanical behaviour of these materials.

39 a) b) Vetro bridge Macropores Macropori c) d) Figure 47. SEM photographs: a) pumice; b) fabric of layer B showing large macropores, often filled with silt (c); d) silty chains bridging coarser particles (from Lampitiello, 24). 6.3 Hydraulic properties The saturated coefficient of permeability of natural samples taken from the layer B has been measured in the triaxial apparatus by constant head tests for an effective confining pressure comprised between 1 and 5 kpa, and a differential pore pressure of 1 kpa between the top and the base of the specimen. The size of this was 38 mm in diameter and 76 mm in height. Systematic field measurements carried out by fixed tensiometers from the beginning of 22 to the end of 23 show that suction in layer B generally fluctuates between 4 and 8 kpa in the wet season and between 2 and 5 kpa in the dry one (Olivares et al., 24). Since suction plays a fundamental role on permeability, compressibility and shear strength, a significant part of the testing programme has been carried out using the suction controlled triaxial apparatus already described in section (Fig. 25).