Serre Massif, southern Calabria, Italy

Catena 145 (2016) 301–315

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Geotechnical and landslide aspects in weathered granitoid rock masses (Serre Massif, southern Calabria, Italy) F. Ietto ⁎, F. Perri, F. Cella Università della Calabria, Dip. di Biologia, Ecologia e Scienze della Terra, Arcavacata di Rende, CS, Italy

a r t i c l e

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Article history: Received 10 March 2016 Received in revised form 15 June 2016 Accepted 17 June 2016 Available online xxxx Keywords: Granitoid rocks Weathering profiles Geotechnical parameters Landslides Southern Calabria Italy

a b s t r a c t The weathering processes produce important chemical and physical transformations in rock masses producing a decay of their strength and then stability. In this study different methodologies were employed to define the physical-mechanical properties and the minero-petrographical changes at each grade of weathering, useful to determine a geological model of the weathered granitoids involved in mass movements. The paper focuses on a multidisciplinary research on the weathering profiles of the Paleozoic granitoid rocks surrounding the village of Fabrizia located in the Serre Massif (southern Calabria, Italy). Fabrizia lies on an old erosion surface composed of basement rocks made up of weathered granitoids, belonging to the Serre Massif granitoid complex, and overlain by thin eluvial-colluvial deposits. The tectonic uplift of the area combined with Mediterranean climatic conditions, both active at least since the Quaternary, are the main causes of the deep weathering of the plutonic rocks. The weathered profiles of the granitoid complex have been studied through field observations, borehole explorations with geotechnical tests and seismic surveys coupled to minero-petrographical analysis and laboratory tests. Weathered profiles are mainly composed by rock masses varying from completely weathered and residual soil (classes V–VI) to moderately weathered rocks (class III), characterized by a progressive increase of geotechnical properties (e.g., seismic wave velocity, rock quality designation, internal friction angle, uniaxial compressive strength) with a decrease of weathering. The weathered rocks display a wide variety of mass movement. Landslide types are rockfalls for the fresher rocks (class III); translational slides occur between different weathering degrees (classes IV and V) and rotational slides dominate at the top of the slopes and generally involve the weathering grades from class IV to classes V–VI. The shallow horizons are composed of thin eluvial-colluvial deposits and more weathered granitoids (classes V–VI) that may generate dangerous debris flows or debris avalanches during intense and prolonged rainfall. The landslide surface is usually at a depth lower than 2 m in the debris flow type, while deeper landslides (depth N 2 m) are rotational and translational types with sliding surfaces occurring mainly in the transition between the classes IV and V. The field survey coupled to geotechnical, geophysical and minero-petrographical data allowed the recognition of the different weathered horizons and relative geotechnical properties useful to the reconstruction of the reliable geological model. The model represents an important support for the study of the landslide processes and for a sustainable management of the territory. The used approach provided results to set a general reference framework for establishing more precise correlations between mass movement phenomena and weathering processes. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Weathering processes occur in the Mediterranean area, where greater attention has been paid to weathering-related slope instabilities. Several studies on weathering processes and their relation to landslides have been carried out both in the Calabria region (e.g., Borrelli et al., ⁎ Corresponding author at: Università della Calabria, Dip. di Biologia, Ecologia e Scienze della Terra, 87036 Arcavacata di Rende, CS, Italy. E-mail address: [email protected] (F. Ietto).

http://dx.doi.org/10.1016/j.catena.2016.06.027 0341-8162/© 2016 Elsevier B.V. All rights reserved.

2007; Pellegrino and Prestininzi, 2007; Calcaterra and Parise, 2010) and in some other areas such as Spain (e.g., Calcaterra et al., 1998; Palacios et al., 2003), Portugal (e.g., Fernando and Marques, 2003; Pereira et al., 2014), Turkey (e.g., Gokceoglu and Aksoy, 1996; Karsli et al., 2009) and Japan and Malaysia (e.g., Chigira et al., 2002, 2011). The Calabria region (southern Italy) is mainly composed of igneous and metamorphic rocks, which underwent intense weathering processes, overlaid by sedimentary deposits. Weathering processes combined with morphological, lithological, tectonic and climatic factors, have produced significant decay of the physical-mechanical properties of the

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F. Ietto et al. / Catena 145 (2016) 301–315

parent rocks making this area prone to erosion and landslides (e.g., Calcaterra and Parise, 2005, 2010; Pellegrino and Prestininzi, 2007; Borrelli et al., 2007; Ietto et al., 2015). Instability in weathered granitoid rocks are widespread in the Sila and Serre Massifs (Calabria, southern Italy), partly due to the recent strong regional uplift started in the Lower-Middle Pleistocene (Ghisetti and Vezzani, 1980; Ghisetti, 1980, 1981; Westaway, 1993; Ietto and Bernasconi, 2005; Critelli et al., 2013 and references therein). Therefore, in the Serre Massif the tectonic uplift combined with intense weathering processes produces a wide variety of mass movement. Generally, debris flows are the main mass movement affecting the most weathered rocks during intense rainfall events, while rockfalls and slides occur within the least weathered rock masses (e.g., Calcaterra and Parise, 2005, 2010; Ietto et al., 2007, 2015). This paper is part of a wider project aimed to describe and characterize the mass movement in weathered rocks, their alteration processes in the frame of the geomorphic evolution in the south-central sectors of the Calabria-Peloritani Arc (e.g., Ietto A. et al., 2003; Ietto F. et al., 2007, 2013, 2015; Perri et al., 2012, 2016). As confirmed by several authors the study of weathered rock behavior is complex, depending on several factors such as the minero-petrographical and geotechnical properties coupled to geomorphology and hydrogeology (e.g., Scarciglia et al., 2005, 2016; Apollaro et al., 2009; Borrelli et al., 2012, 2014; Perri et al., 2014; Perri, 2014; Perri and Ohta, 2014). Therefore, the use of different and multidisciplinary survey methodologies is required in order to study the weathering processes of a rock mass. The studied area is located close to the Fabrizia village in the Serre Massif (southern Calabria) and is characterized by certain weathering processes of the granitoid rocks causing a very high susceptibility to landslides. The slopes of the Fabrizia surroundings and the neighboring valley of the Allaro River have a long history of landslides and flooding. Thus, the Fabrizia surroundings are a favorable setting to study and characterize the relationships between the landslide phenomena and the features of the granitoid weathering profiles. Some authors have only studied the granitoid weathering processes in the Serre Massif (e.g., Moresi, 1987; Mongelli and Moresi, 1990; Mongelli, 1993; Mongelli et al., 1998), whereas, others have also associated this study to the landslides (e.g., Genevois and Prestininzi 1979; Calcaterra et al., 1996a, 1996b; Calcaterra and Parise, 2005, 2010; Ietto et al., 2007; Pellegrino and Prestininzi, 2007; Bretschneider et al., 2013). In this paper the study and characterization of the weathered granitoid rocks involved in instability phenomena was determined in two ways. The first step was based on a field survey, aimed to recognize the main features of slope movements and characterize the weathering grades of the granitoid rocks. The second step was based on several geognostic surveys, coupled to geophysical surveys, in order to determine the change of the main geotechnical features in the weathered rock masses investigated. Furthermore, integration of the previous step, laboratory tests and petrographic analyses on undisturbed borehole samples were performed in order to better characterize the geotechnical properties and petrographic differences of the various degrees of weathering. These survey methodologies are consistent with those used to study weathering contexts both in Calabria region and elsewhere (e.g., Calcaterra and Parise, 2005, 2010; Moon and Jayawardane, 2004; Viana da Fonseca et al., 2006; Borrelli et al., 2007, 2014). This work uses different and multidisciplinary methodologies to study and characterize the granitoid rocks involved in weathering processes which were affected by instability. The different methodologies aim to collect geological, geotechnical, hydrogeological, geophysical and minero-petrographical data, used to define a geological model of the weathered granitoids involved in instability phenomena. 2. Geological framework The Western Alpine Mediterranean Chains are characterized by several outcrops of granitoid complexes that are involved in important

weathering processes (e.g., Sequeira Braga et al., 2002; Olona et al., 2010). These processes are closely related to the geodynamic and tectonic evolution of the Western Mediterranean area starting from the Permian–Triassic boundary (e.g., Guerrera et al., 2012; Alcalá et al., 2013; Perri and Ohta, 2014). The Calabria-Peloritani Arc (CPA) represents the maximum deformation of the Apennine–Maghrebian chain, correlated with a chain– foredeep–foreland system due to the convergence of the Euro-Asiatic and the African plates (e.g., Vai, 1992; Pellegrino and Prestininzi, 2007). The CPA is a stack of crystalline basement nappes, some of them covered by a Meso-Cenozoic sedimentary cover. The geodynamic and tectonic evolution of the CPA is mainly related to a trend with prevalently compressional tectonic movements (Bonardi et al., 1982; Vai, 1992; Critelli et al., 2013; Tripodi et al., 2013). The CPA is mainly composed of three structural sectors: Coastal Chain-Sila, Serre and Aspromonte Massifs (Fig. 1A). The Serre Massif (southern Calabria, Italy) is located among the Catanzaro Graben to the north, the Mesima valley to the west, the Gioia Tauro plain to the south and the Ionian coast to the east (Fig. 1A). The Serre Massif constitutes the linkage between the southern (Aspromonte Massif and Peloritani Mountains) and the northern (Sila Massif and Coastal Chain) sectors of the Calabria-Peloritani Arc and is mainly characterized by Late Hercynian granitoids and granodiorites (e.g., Ferrara and Longinelli, 1961; Borsi et al., 1976; Angì et al., 2010). The latter, discontinuously are surrounded by high-grade metamorphic rocks and locally covered by unmetamorphosed sedimentary deposits from Miocene to Quaternary in age (Graeßner et al., 2000) (Fig. 1B). The Serre is one of the few places in the world showing the complete tilted continental crustal section, represented by upper, intermediate and lower crust (Graeßner et al., 2000 in Fig. 1B). The Serre granitoid complex represents the intermediate crustal section of a Late Hercynian batholith (Serre Batholith) and is composed of foliated tonalite with minor Qtzdiorite and gabbro, with increasing felsic and peraluminous granitoid in the upper crustal levels (D'Amico et al., 1982; Rottura et al., 1990; De Vivo et al., 1992; Del Moro et al., 1994; Fornelli et al., 1994). The hydrographic network is mainly characterized by subparallel basins, extending W-E and NW-SE toward the Ionian coastline. Extended peneplanation areas are widespread within the higher parts of the massif (at elevations between 850 and 1200 m a.s.l.), showing evidence of continental erosion of the granitoid basement related to the Quaternary uplift (e.g., Ghisetti, 1980). The neotectonic pattern is represented by the Apennine and anti-Apennine high-angle normal fault systems (e.g., Brogan et al., 1975; Ghisetti, 1979; Ghisetti and Vezzani, 1980; Westaway, 1993). These fault systems have given rise to the main morphostructural lineaments of the region since the Late Miocene, and nowadays the same fault systems control the uplifting process (e.g., Tortorici et al., 1995; Monaco and Tortorici, 2000; Critelli et al., 2013). In particular, the Serre Massif is marked by the NE–SW Neogenic graben of the Mesima Valley on the west side and by two WNW–ESE tectonic lineaments corresponding to the Catanzaro and the Nicotera– Gioiosa Jonica faults (Ghisetti, 1979; Ghisetti and Vezzani, 1980; Meulenkamp et al., 1986) (Fig. 1A). Fabrizia lies on an old erosional surface composed of weathered granitoids (the Serre Massif granitoid complex) and is overlain by thin eluvial-colluvial deposits (Fig. 1C). The tectonic pattern consists of normal fault systems that are NE-SW and NW-SE directed, as shown in the rose diagram in Fig. 2. The granitoid rocks of the Calabria region were affected by several weathering processes that mainly occurred during the Pleistocene in an environment characterized by a Mediterranean climate (e.g., Moresi, 1987; Le Pera and Sorriso Valvo, 2000; Calcaterra and Parise 2005). Guzzetta (1974) suggested tropical conditions and a consequently earlier age for the weathering processes. Previous studies suggest that the beginning of the weathering processes in the Calabria granitoids dates back to at least a pre-Tortonian age and that the outcrops of granitoid alterites are the erosive residual of more ancient mantles (Ietto and Ietto, 2004; Ietto et al., 2007). The denudation of these


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Fig. 1. A: location of study area; B: geological sketch-map of Serre Massif (modified after Graeßner et al., 2000); C: Fabrizia geological sketch-map (modified from CASMEZ, 1967).

weathered rocks occurred because of an uplift stage, starting from the Lower-Middle Pleistocene (Ghisetti, 1980, 1981; Dumas et al., 1982, 1987; Ibbeken and Schleyer, 1991; Westaway, 1993; Ietto and Bernasconi, 2005; Critelli et al., 2013), producing very significant reliefs accentuated by the erosive action of the drainage network. The combination of these natural phenomena leads to intense erosion and landslides. 3. Geomorphological framework The geomorphological pattern of the Serre Massif seems to control the distribution of instability phenomena. Because of different topography in the head and middle–lower sectors of the catchments, mass movements appear to act as the main process only on the slopes where steeper gradients lead to fast transport rates of weathered materials (Calcaterra et al., 2004; Garfì et al., 2007; Bruno et al., 2008). The smoothed topography of the top plateaus, instead, is dominated by limited transport and erosion causing thickening of the soils. A sharp increase of slope gradients marks the downstream area, where deep and narrow valleys originate and where the thickness of weathered materials strongly decreases due to the mass movement. Generally, the unconsolidated materials are involved in shallow landslides, whereas, several meters of weathered rocks are involved in deep-seated landslides. Calcaterra and Parise (2010) show that the granitoid weathered horizons of the Serre Massif were involved in slope movements, and the most of debris flow phenomena affect the weathering classes from III to V (according to the classification scheme of G.C.O., 1988), whereas the weathering classes from II to III (G.C.O., 1988) are involved mainly in slides, rockfalls and complex landslides. During the 20th century, several landslide phenomena have been recorded in the Serre Massif. The most severe and dangerous instability events probably occurred between the 15th December 1972 and the 2nd January 1973 (Giangrossi, 1973; Petrucci et al., 1996). In this period about 300 mm of rainfall were recorded at the Mammone rain gauge

(elevation 981 m a.s.l. in Serre Massif) in just a few days (December 15th and 22nd, 1972 and January 2nd, 1973) that was about 16–17% of the mean annual precipitation. During these intense rainfalls the activation or reactivation of several landslides occurred in the Fabrizia surroundings (Ietto et al., 2012), mainly along the western and southern slopes causing severe damage to inhabited areas. Fabrizia is located on the left side of the Molini River, a left-hand tributary of the Allaro River (Fig. 1A and C). The Molini River is characterized by a strongly vertical erosion of its riverbed, due to the regional uplift responsible of continuous rejuvenation of the hydrographic network in the Serre Massif (Ietto and Ietto, 2004; Calcaterra and Parise, 2005). This process favors steep riverbed gradients and steep slopes dominated by widespread instability. In the Fabrizia surroundings, several mass movements have been identified and mapped (Fig. 2) based on aerial photo interpretation and field surveying. The landslide phenomena involve large amounts of debris related to weathered profiles of granitoid rocks. The mass movements were classified using the classification scheme proposed by Varnes (1978). In the studied area slope movements include: rockfall, translational slides and rotational slides, and most of them are in an active state (Fig. 2). The activity of the landslides are mainly highlighted by the presence of scarps and widespread open cracks along the slopes. Debris flow types are concentrated on the steep slopes, where eluvial-colluvial deposits and a “soil” mantle, due to weathering processes, occur. During the geomorphological field survey a distinction was also made between the preserved landslide deposits and the deposits partly washed away by erosional processes (Fig. 2). 4. Methodology Previous studies have mainly addressed some geotechnical topic for weathered rock masses involved in landslides in different areas of the Serre Massif, such as, in the Allaro basin (Calcaterra et al., 1993; Calcaterra and Parise, 2005; Pellegrino et al., 2008) or in the M.te Granieri (Genevois and Prestininzi 1979; Pellegrino and Prestininzi,

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Fig. 2. Fabrizia landslide-inventory map.

2007; Bretschneider et al., 2013). Our research takes a step forward compared to previous studies, because it is aimed to define a complete geological model in the area surrounding Fabrizia. The research is based on several and detailed field investigations, geognostic surveys, geotechnical laboratory tests and petrographic analyses. The geological investigation (Fig. 3A) consisted of the following: a) 9 seismic refraction traverses for about linear 1000 m investigated, b) 16 mechanical drillings and relative Rock Quality Designations (RQD), Standard Penetration Tests (SPT) and Lugeon tests (P = 10 atm), c) a collection of 14 undisturbed samples for laboratory tests and petrographic analyses. In addition, a field investigation based on qualitative and quantitative criteria was carried out in order to characterize the weathering degree of the outcropping granitoid rocks. Methodologies and relative results are described below. 4.1. Field investigation The weathering profiles have been studied following the methodology proposed by many authors (Gullà and Matano, 1997; Borrelli et al., 2012, 2014; Perri et al., 2012, 2015) and tested in several weathered profiles of Calabria, in agreement with the well-known classification schemes used for weathered rock masses (Dearman, 1976; I.A.E.G.,

1981; G.C.O., 1988; G.S.L., 1995). Thus, the weathering degree of a rock is divided into six different classes (I to VI) from the fresh/unweathered bedrock to completely weathered rock/residual soils. The methodology is based on visual descriptions, following qualitative (e.g., progressive change of rock color, hammer tests or hand scraping on rock masses) and quantitative (e.g., Schmidt Hammer tests) simple index tests (Table 1). Thus, the observed weathering profiles have been classified as rock masses varying from completely weathered rocks and thin residual soil layers (along the gentler slopes and toward the top of the slope) to highly weathered rocks with limited corestones of moderately weathered rocks (along the incised torrents and steep slopes). Tests by Schmidt Hammer gave respectively rebound values (R-value) ranging from 0 to 10 (ascribed to weathering classes V–VI) to 10–43 (ascribed to weathering classes IV–V with corestones of the class III). In particular, the Schmidt Hammer test gives R-values ranging from 10 to 22 for the rock masses ascribed to the weathering classes IV– V that enclose rare and limited rock volumes belonging to the classes IV (R-value: 20–30) and III (R-value: 31–43). The residual soil levels (class VI) characterize only some portions of the top slopes, where the Schmidt Hammer values are rejected. The Schmidt Hammer test was performed by standard L type Schmidt hammer, following standard methods described by ASTM D5873 (2013). No correction was made


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Fig. 3. A) map of the geognostic investigation; B) litho-mechanical map based on qualitative and quantitative criteria, where: 1) granitoids from highly to completely weathered with limited volumes of moderately weathered granitoids, Schimdt Hammer values from 10 to 43 (classes IV–V with small volumes of the class III); 2) granitoids from completely weathered to residual soils, Schimdt Hammer values from 0 to 10 (classes V–VI); 3) eluvial-colluvial deposits.

for the inclination of the Schmidt hammer because this correction becomes negligible when large numbers of measurements with significant scatter are collected (e.g., Snyder et al., 2003; Duvall et al., 2004). In order to obtain the main joint characteristics in each weathering zone a geomechanic survey has been performed. Therefore, in the class VI the parent rock is completely decomposed to soil, consequently, there is no rock texture or mass preserved and the joints are absent. In the classes V–VI the joints are typically short, no water is present and few joint openings are visible in the typically rounded exposures. Generally, joints are mostly filled with weathered material. In the classes IV–V centimetric apertures characterize the main joints, which are

commonly filled with weathered products or soil and marked by iron staining. Sometimes, clay materials fill the wide apertures and joint edges are distinctly rounded; no water is present into the joints. In the class III the joints have the following characteristics: closely spaced, surficial, very closed apertures, no filling is generally present, the edges remain fairly sharp and slightly wavy rather than straight; sometimes the joint surfaces are wet. The geomechanic survey allowed to observe as the joint spacing has a tendency to increase from moderately weathered zone to completely weathered zone. The orientation of the observed discontinuities appears to be consistent with the local tectonic deformation, as follow: NE-SW, NW-SE and subordinately E-W.

Table 1 Classification and description of the weathering classes observed in the studied area, following the methods proposed by many authors (e.g., Dearman, 1976; I.A.E.G., 1981; G.C.O., 1988; G.S.L., 1995; Gullà and Matano, 1997; Borrelli et al., 2012, 2014; Perri et al., 2012, 2015). Weathering class III - moderately weathered rock

IV - highly weathered rock

V - completely weathered rock

VI - residual soil

e.g., Gullà and Matano, 1997; Borrelli et al., 2012, 2014; Perri et al., 2012, 2015

e.g., Dearman, 1976; I.A.E.G., 1981; G.C.O., 1988; G.S.L., 1995

Less than 35% of the rock material is decomposed and or The rocks locally show a pervasive change of the original color; original texture and disintegrated to a soil; color yellowish brown; fresh or discolored microstructure of the fresh rocks are preserved; the tip of geologist's hammer slightly rock is present either as a continuous framework or as scratches the rock mass and large pieces are hardly broken if the rock is struck by head of corestones; Plagioclase feldspar party decompose to gritty small hammer; NSchmidt values range from 25 to 40. pieces; NSchmidt rebound value 25–50. The rocks show a complete change of the original color, with reddish-yellow coatings on More than 35% of the rock material is fractured surfaces; original texture and microstructure of the fresh rock are still decomposed and or disintegrated to a soil; color yellowish brown preserved; these rocks make an intermediate dull sound when it is struck by hammer and to yellowish orange/brown; fresh or discolored rock is present are hardly indented by the tip of geologist's hammer; the rock mass does not undergo a either as a continuous framework or as corestones; Plagioclase clear strain by geologist's hammer and large pieces are easily broken if they are struck by feldspar powdery to gritty; NSchmidt rebound value 15–30. hammer; NSchmidt values range from 10 to 25. The rocks show a complete change of the original color, with reddish-yellow coatings on All rock material is decomposed and/or disintegrated to soil; color yellowish brown to reddish brown; the original mass structure is fractured surfaces; original texture and microstructure of the fresh rock are present in still largely intact; Plagioclase feldspar powdery to soft, very relicts these rocks are sometimes characterized by a soil like behavior and are easily easily grooved by pin; Orthoclase feldspar gritty, less easily indented by the tip of geologist's hammer;; large pieces of these rocks can be broken by grooved; zero rebound from Schmidt hammer. hand; NSchmidt values range from 0 to 15. The rock mainly consists of residual, detrital–colluvial soils with limited portions of All rock material is to soil; the mass structure and material fabric completely weathered rocks; the rocks show a complete change of the original color, with are destroyed; color dark reddish brown; there is a large change pervasive fractures and discontinuity surfaces where a widespread iron-stained and in volume, but the soil has not been significantly transported; clay-rich pedogenic matrix is observed; original texture of the fresh rock are completely Feldspar completely destroyed; Quartz only remaining primary destroyed. These rocks are characterized by a soil-like behavior; large pieces can be easily mineral; grains reduced in size compared with fresh condition. broken by hand

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A litho-mechanical map of weathered granitoids on the cut slope (Fig. 3B), based on the above mentioned qualitative and quantitative criteria, shows the main weathering classes ranging from the class VVI (completely weathered rock and thin residual soil layers) to the class IV-V (from highly to completely weathered rock) with limited volumes of class III. The weathering profiles observed are generally simple (sensu Brand and Phillipson, 1985) and characterized by a progressive weathering increase toward the top of the slopes, with minor corestones (Fig. 4).

4.2. Drilling surveys Sixteen boreholes were drilled up to 35 m depth within the granitoid basement. The weathering profiles of the borehole samples have been studied following the methodology in Table 1. In addition, to evaluate the geotechnical properties into various weathering classes, standard penetration tests (SPT) and rock quality designation (RQD) were carried out during the drilling. The SPT and RQD tests were performed following standard methods described by ASTM D1586 (2011) and ASTM D6032 (2008), respectively. Lugeon tests (P = 10 atm) were made to measure the water absorption capacity. The continuous sampling from the drilled boreholes shown, beneath a thin cover of eluvial-colluvial deposits, a first horizon of weathered granitoid in a sandy-gravelly soil with interbedded silty matrix and locally clay layers. The borehole samples can be broken by hand and are characterized by a soil like behavior. Sometimes and only in the shallowest boreholes levels, a thin residual soil horizon was found, in which the samples can be easily broken or crumbled by hand. The first horizon of weathered granitoid was classified as completely weathered granitoid with residual soils levels (class V-VI) (Figs 5A and B). The standard penetration test (SPT) was used to evaluate the in situ state of the soil. SPT-N values have suggested very soft soil only for the shallower layers (10 b NSPT b 20), changing to a higher density soil (maximum value NSPT = 49) with increasing depth. RQD test was not applicable in this weathered horizon. Lugeon tests have shown a decrease of water absorption with depth, ranging from 29.3 Lugeon Units/value (UL) to 10.1 UL. In accordance with the correlation proposed by Drogue and Puech (1969) the Lugeon values (UL) correspond respectively to permeability coefficients (k) ranging from 2.9 10−6 m/s to 1 10−6 m/s. This first weathered horizon, found along all 16 boreholes,

has a variable thickness ranging from 3 m (borehole S11) to 22 m (borehole S3) (Fig. 6). A second horizon of weathered granitoid in sandy matrix was classified as completely weathered rock with corestones of highly weathered rock (class IV-V) (Fig. 5A and B), in which large pieces of samples (b 6– 7 cm) are easily broken by hammer or sometimes by hand. SPT-N measured values suggest dense soils (NSPT N 40), changing to very dense soils with depth where SPT-N values ranging from around 70 to reject (with N-values higher than 100); RQD index is still not applicable. Lugeon tests have shown values ranging from 17.5 UL to 7.1 UL, which correspond respectively to k = 1.7 10−6 m/s and k = 7 10−7 m/s. The thickness of this weathered horizon is variable, ranging from 2 m (borehole S7) to 17 m (boreholes S1 and S14) and it is found between 5 m (borehole S11) and 29 m (borehole S14) of depth (Fig. 6). A third weathered granitoid horizon was found at variable depths in all boreholes (Fig. 6). This horizon is characterized by samples that can be easily broken by a hammer or sometimes by hand in pieces of 7– 8 cm in size; sometimes they preserve the form of core barrel with samples longer than 10–12 cm and breakable only by hammer. The SPT test does not result applicable (NSPT = reject) and RQD index ranges from not applicable to maximum 50% (Fig. 6). Water absorption tests provided results ranging from 10.3 UL to 5.1 UL corresponding, respectively, to k = 1.0 10−6 m/s and k = 5 10−7 m/s. This weathered horizon was classified as highly weathered granitoid (class IV) (Fig. 5C and D). In almost all drillings, it was drilled up to end borehole. Only along borehole S16 it was found as a clear thick corestone inside classes IV–V (Fig. 6). Finally, a fourth weathered granitoid horizon was found only in boreholes S16, S13 and S12, at depths of 19–21 m from ground level down to the borehole end. The drilled samples keep the shape of core barrel with length higher than 15–20 cm; the samples cannot be broken by hand and they are hardly broken by head of hammer. The tests made during the drilling suggest: SPT test was not applicable (NSPT = reject), whereas the RQD index ranges from 48% to maximum 80% (Fig. 6). The weathered horizon was classified as moderately weathered granitoid (class III) (Fig. 5E), where the water absorption tests gave results ranging from 8.7 UL to 4.9 UL corresponding, respectively, to k = 8.6 10−7 m/s and k = 4.8 10−7 m/s. Geotechnical data, detected in the different weathering classes (from classes V–VI to III), are summarized in Table 2. As regards to hydrogeological features, the water-table has been found in all 16 boreholes at variable depths ranging from 5 m

Fig. 4. Weathering profiles of representative granitoid cut slopes of the studied area (see Fig. 3B for the location).


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Fig. 5. Example of some borehole samples: A and B) samples from the classes V–VI to the class IV–V; C and D) class IV samples; and 5E) class III samples.

(boreholes S16 and S12) to 17 m (borehole S5) from ground level and mostly into the weathered classes IV and V (Fig. 6). Lugeon test was performed in 15 boreholes (from S2 to S16) showing a gradual decrease in permeability with depth (Fig. 7) and with decreasing of weathering grades. In fact, as the weathering increases, the rock masses become more porous, highly fractured and more permeable. Lugeon values highlighted a permeability increase from the class III (k = 8.6 10−7– 4.8 10−7 m/s), characterized by mixed permeability deriving from porosity and fissures, to the classs V–VI (k = 2.9 10−6–1 10−6 m/s) characterized mainly by porosity permeability.

4.3. Laboratory tests During the drilling of 16 boreholes, 8 undisturbed samples of weathered granitoids were collected from each weathering state (Fig. 8). All samples were carefully taken from borehole cores and transported to the laboratory to measure some geotechnical parameters. The main limitation to evaluate the geotechnical properties of the weathered granitoids is that few tests are suitable across the entire range of the weathered rock. The analyzed samples, ranging from the class III to the class V-VI, are characterized by both a soil-like material behavior

Fig. 6. Weathering grades of granitoid rocks found into 16 boreholes and geotechnical data obtained by tests during drilling.

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Table 2 Geotechnical data of weathered granitoids, obtained by boreholes and laboratory tests and by seismic refraction profiles in which: Rj: reject with NSPT value higher than 100; U.L. = Lugeon Units (P = 10 atm); k = permeability coefficient determined from Lugeon tests; Ø’ = effective internal friction angle; c’ = effective cohesion; γ = bulk density, (*) value derived from literature (Calcaterra and Parise, 2005); UCS = uniaxial compressive strength (determined by the methodology of Deere and Miller, 1966); Vp: longitudinal waves velocity; Vs: measured transverse waves velocity; E: Young's modulus; S: shear modulus; P: Poisson's ratio;

Description Weathering class Schmidt Hammer NSPT Rock quality designation (RQD) Lugeon test (U.L. = L/min/m) and Permeability (k = m/s) Laboratory tests UCS (MPa) (average) Vp (m/s) Vs (m/s) E (GPa) S (GPa) P

Completely weathered granitoid with residual soils levels

From highly to completely weathered granitoid

V–VI 0–10 b49 0 UL = 29.3–10.1 k = 2.9 10−6–1 10−6

IV–V 10–22 40-Rj 0 UL = 17.5–7.1 k = 1.7 10−6–7 10–7

IV 20–30 Rj 0–50% UL = 10.3–5.1 k = 1 10−6–5.0 10–7

III 31–43 Rj 48–80% UL = 8.7–4.9 k = 8.6 10−7–4.8 10–7

Ø′ = 31°–33° c′ = 28.44–13,34 kPa γ =18.63–18.82 kN/m3 Not applicable

Ø′ = 36° c′ = 7.84 kPa γ = 19.12 kN/m3 14.72–23.51 (18.61)

Ø′ = 37°-39° c′ = 7.16–5.88 kPa γ = 24.61–24.81 kN/m3 27.31–45.22 (35.15)

Not applicable γ = 25.2 kN/m3 (*)

430–680 207–327 0172–0431 0.064–0.160 0.35

(classes V–VI, IV–V and partly IV) and a rock-like material behavior (classes III and partly IV). In particular, the weathering class IV shows a soil-like fraction in a smaller percentage and a rock-like behavior in higher percentage of the samples. However, in agreement with Deere (1964) all samples belonging from III to V–VI weathering class, were classified as poor rocks. A direct shear test was used to analyze only the samples with a soil-like material behavior in order to determine values of effective internal friction angle (Ø′) and effective cohesion (c ′). Uniaxial compressive strength (UCS) was estimated for rock-like behavior as discussed in the following paragraph. The Casagrande shear box was used testing the samples under consolidated drained conditions with a shearing rate of 0.61 mm/min−1, as suggested by ASTM D3080M (2011). The direct shear data are summarized in Table 2 and

Highly weathered granitoid

810–1220 433–704 0.926–2.356 0.356–0.941 0.30–0.25

Moderately weathered granitoid

49.05–90.9 (66.78) 1720–1810 1022–1045 4.960–5.187 1.984–2.074 0.25

graphs of measured parameters versus depth are plotted in Fig. 9. The latter shows gradually increasing values of internal friction angle with the depth ranging from 31° to 33° in completely weathered granitoid with residual soils levels (classes V–VI), to 37°–39° in highly weathered granitoid (class IV). The cohesion, instead, decreases gradually with increasing depth. Similar trends are described by many authors (e.g. Baudracco et al., 1982; Christaras, 1991; Chigira et al., 2002; Moon and Jayawardane, 2004; Calcaterra and Parise, 2010) recognizing the increase of weathered state as cause of loss of rock strength and increase of clay. The latter is mainly due to the conversion of primary minerals to secondary clay minerals (e.g. Baynes and Dearman 1978; Baudracco et al., 1982; Christaras, 1991; Chigira et al., 2002; Moon and Jayawardane, 2004; Calcaterra and Parise, 2010). Bulk density (γ) was determined only for the samples belonging to the classes IV, IV–V and V–VI, following the standard methods described by ASTM C29M (2009). The bulk density value of class III was derived from literature (Calcaterra and Parise, 2005). 4.4. Uniaxial compressive strength (UCS)

Fig. 7. Permeability variation with the depth, based on Lugeon tests.

Several authors studied the relationships between the Schmidt hammer data and the UCS values (e.g., Deere and Miller, 1966; Shorey et al., 1984; Katz et al., 2000; Kahraman, 2001; Aydin and Basu, 2005; Kiliç and Teymen, 2008; Nazir et al., 2013 and many others). The UCS is the most useful index to assess the intact strength of the rock, which decreases with increasing of the weathering grade (e.g., Dearman et al., 1978; Lee and De Freitas, 1989; Zhao et al., 1994). In this paper, unconfined compressive strength (UCS) was calculated by the most used methodology of Deere and Miller (1966), that correlates Schmidt hammer rebound values with unconfined compressive strength. The UCS was performed to characterize the geotechnical features of rock masses belonging from III to IV–V weathering class. The UCS value for weathering classes V–VI was not considered, because the most Schmidt hammer values are very low or equal to 0. According to Shorey et al. (1984) and Li et al. (2000) the Schmidt hammer should be not used for rocks with UCS values less than 10 MPa, because no significant rebound should occur due to the strong plasticity. The UCS data are summarized in Table 2. In agreement with the geotechnical classification proposed by Deere and Miller (1966), the average UCS values of III, IV and IV–V classes fall in medium strength class, low strength class and very low strength class, respectively. These data are consistent with the UCS range values in the different


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Petrographic analyses have been carried out on all selected samples (from 9 to 14 in Fig.8) of the three boreholes. All the studied samples show a granular framework microfabric (Baynes and Dearman, 1978), with Ip and Xd values that ranges as a result of the sample weathering stages (Table 3). In particular the sample of the S3 borehole is mainly composed of a soil-like material characterized by sand and gravel grain-size fractions. The samples of the S12 and S16 boreholes are characterized by a generally clay-poor, granular framework-microfabric with interlocking granular aggregate enclosing decomposed minerals. The transition from the fresher class (III) to the more weathered classes (V–VI) mainly consists of the sericitization of plagioclase along preferential lines and pitting on mineral surfaces, neoformation of unidentified clay minerals within both K-feldspar and plagioclase, and partial clay neoformation and exfoliation of biotite along lamellae and rims. Transformation of biotite into Fe-oxides is frequently observed in the more weathered samples (Fig. 10). Furthermore, some intragranular and intergranular small cracks (Fig. 10) tend to expand and propagate from the class III to the classes V–VI (e.g., Borrelli et al., 2012, 2014; Perri et al., 2015, 2016). The grain-size analyses confirm that the dominant fractions for the more weathered granitoid samples (classes IV–V and V–VI) are sand and gravel with a minor percentage of silt–clay (Fig. 11). Fig. 8. Location of some undisturbed samples for geotechnical tests and petrographic analyses.

weathering grades determined by Genevois and Prestininzi (1979) in the Serre Massif, as well as by Santi (2006) and Zhao et al. (1994) in other weathered granite complexes. 4.5. Petrographic and grain-size analyses Several samples collected from three boreholes (S3, S12 and S16, Fig. 8), representative of the weathering profiles of the studied area, were made into thin-sections for microfabric and petrographic analyses. The increase of the weathering process has been characterized using petrographic indexes for quantifying the changes in the intrinsic properties of rocks from different points of view, some of which can be related to the engineering properties of the weathered rocks. The most commonly used are the decomposition index (Xd; Lumb, 1962), indicating the extent to which the rock microfabric and composition are affected by weathering, and the micropetrographic index (Ip; Irfan and Dearman, 1978), defined as a ratio among primary and accessory unaltered minerals and secondary minerals together with microcracks and voids (e.g., Borrelli et al., 2012, 2014). Furthermore, the grain-size analyses were carried out following standard methods described by ASTM E112 (2013) on three selected borehole samples (9, 10 and 12 in Fig. 8).

4.6. Seismic refraction It is well known that the seismic refraction prospecting is a suitable method to detect the upper bedrock, to measure the thicknesses of weathered layers and to evaluate the mechanical properties in rock masses (e.g. Lee and De Freitas, 1990; NRC, 2000; Stokoe and Santamarina, 2000 and references therein). The method is based on the analysis and interpretation of the first arrivals of critically refracted waves in order to provide a two-dimensional seismic P-wave velocity (VP) model. Generally, increasing Vp implies an improvement of the mechanical and geotechnical properties due to lower weathering and jointing state inside the rock masses (e.g., Palmstrom, 1996). Therefore, a seismic refraction survey was carried out to measure the thicknesses of weathered layers and relative mechanical properties along the south-western boundary of the Fabrizia village. The survey consisted of 9 profiles resulting in a total of 1000 m linear (Fig. 3A) and passing through most of the 16 boreholes to incorporate well log information as constraints for the model velocity inferred by the seismic data interpretation. Seismic data were acquired by an ELGI ESS 01-24/48M seismograph and 24 Geospace (14 Hz) geophones with a spacing of 5 m. Seismic energy at the shot points was delivered by explosive charges. All data were processed by standard Time Delay and GRM methods (Winsism 8.0 by Geophysical Instrument Supply Co.), providing useful

Fig. 9. Internal friction angle and cohesion trend with depth of some selected granitoid samples (see Fig. 8 for the location) obtained by a direct shear test.

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Table 3 Characterization of studied granitoids from boreholes S12 and S16, in terms of decomposition index (Xd; Lumb, 1962), microfabric, micropetrographic index (Ip; Irfan and Dearman, 1978) and weathering stages. Borehole

Sample

Depth (m)

Decomposition index (Xd)

S3 S12 S12 S16 S16 S16

9 10 11 12 13 14

6m 14 m 23 m 17 m 25 m 30 m

0.41 0.39 0.35 -

information about depth and thickness of the main seismic discontinuities as well as the elastic constants of the rock masses investigated. The VP model, obtained combining the nine seismic profiles, shows on the left side of the section a gradual increase of velocity with depths, ranging from 430 m/s at the surface to 1810 m/s at − 35 m (Fig. 12). Three horizons, with gradual increase in VP, have been recognized in the velocity model. The relative discontinuities have been constrained also by means of well data discussed in the previous section. The data comparison between seismic results and well information shows differences in depth discontinuities lower than ± 2 m. VP ranging from 430 m/s to 680 m/s correspond to strongly weathered masses with weak geotechnical properties, which have been ascribed to the weathering classes V–VI. A significant VP increase to 810–1220 m/s indicates a less weathered state of the rock masses. The visual analysis of borehole samples together with geotechnical tests leads to the conclusion that this horizon consists of weathered granitoid belonging to the class IV-V. Finally, a further VP increase to values ranging from 1720 to 1810 m/s highlights rocks with better mechanical properties. This horizon only occurs in the left side of the velocity model, at depths of about 20 m, therefore it has been compared only with data from boreholes S16, S13 and S12 (Fig. 6). The latter indicates a decrease of jointing state (RQD = 48–80%) and weathering degree, confirming this granitoid as belonging to the class III.

Microfabric

Micropetrographic index (Ip)

Granular-framework Granular-framework Granular-framework Granular-framework Granular-framework Granular-framework

1.1 3.1 4.5 3.5 4.6 4.9

Weathering stage V–VI IV–V III IV–V III III

Measures of seismic wave velocities, both Vp and Vs, with the corresponding elastic parameters (Young's modulus, shear modulus and Poisson's ratio) of the relative weathering classes, are summarized in Table 2. Only Poisson's ratio increases with weathering state due to a greater amount of clay in more weathered classes. This trend is in agreement with that described by Viana Da Fonseca et al. (2006) in weathered granite rocks. 5. Discussion Direct and derived results, which allowed distinguishing the different weathering-grade horizons, were based on a large amount of information derived from the following: a) semi-quantitative characterization (visual observation and Schmidt hammer test) of the granitoid rocks; b) data obtained during drilling of 16 boreholes (visual observation on borehole samples, NSPT tests, RQD tests, Lugeon tests); c) seismic refraction investigations; d) laboratory tests and minero-petrographic analyses on some borehole samples; and e) unconfined compressive strength (UCS). Borehole stratigraphy coupled with geotechnical tests have revealed a sequence of weathered rocks occasionally argillified, with a poor rock mass quality and intense jointing. The weathered horizons were classified as follows: classes V–VI, classes IV–V, class IV and class III (Fig. 6).

Fig. 10. Photomicrographs of weathering stages for the studied granitoid (crossed polarized light). (A) moderately weathered sample (class III); (B) highly weathered sample (class IV); (C) highly to completely weathered sample characterized by abundant clay minerals and cracks (classes IV–V); (D) completely weathered sample characterized by cracks and Fe-oxides (classes V–VI). Plg, plagioclase; Bt, biotite; Qtz, quartz; CM, clay minerals; Fe-ox, Fe-oxides.


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Fig. 11. Granulometric analyses of 9, 10 and 12 granitoid samples (see Fig. 8 for the location).

An aquifer was recognized in weathered materials (classes IV and V) characterized by a soil like behavior, as confirmed by SPT values, where Lugeon tests gave a relative high value of permeability. The permeability values decrease gradually from V–VI to III weathering classes, which show a transition from a soil-like to a rock-like behavior, where the SPT test gradually becomes not applicable and RQD index increases. The purpose of the geophysical investigations was to collect data for determining depth, thickness and mechanical features of the weathered material. The seismic refraction survey shows the presence of only three horizons: classes V–VI (VP = 430–680 m/s), classes IV–V (VP = 810– 1220 m/s) and class III (VP = 1720–1810 m/s) (Table 2 and Fig. 12). Therefore, the seismic velocity values of the second horizon were considered representative for the both classes IV–V and IV, as detected in the drilled boreholes. The lack of the class IV horizon in VP model is probably due to a gradual change from the V to the IV class, without a well-defined discontinuity but with presence of highly weathered granitoid corestones (class IV) within a completely weathered granitoid matrix. However, the seismic velocities values and the geotechnical data detected into each weathering class (Table 2), are consistent with the classifications suggested by many authors for weathered granites in different geological settings (e.g. Dearman et al., 1978; Irfan and Dearman, 1978; Krank and Watters, 1983; Lee and De Freitas, 1989; Santi, 2006). The different weathering stages detected by geognostic surveys and field investigations were also confirmed by petrographic analyses on several samples. The petrographic analyses show a transition from the

fresher class (III) to the more weathered classes (V-VI) mainly consisting of the sericitization of plagioclase, neoformation of unidentified clay minerals and partial clay neoformation. The latter was also confirmed by laboratory tests that show a rise of cohesion and a decrease of internal friction angle values with increasing of the weathering grades. Uniaxial compressive strength data have further confirmed a loss of rock strength from the III to V–VI weathering class, allowing theirclassification as poor/very poor rocks in agreement with Deere (1964). The set of geotechnical data provided results summarized in Fig. 13, that highlights a significant weakness increase detected with an improve of the weathering grade. The geological, geotechnical, petrographic and seismic data, have allowed the reconstruction of a reliable geological model (Fig. 14). The model, passing through most of the 16 boreholes and all seismic lineaments, shows four different weathering horizons with a decrease of the weathering-grade with depth. In particular, the classes V–VI shows a behavior typical of soils, characterized by a sand–gravel fraction with minor clay mainly derived by the conversion of primary minerals to secondary clay minerals. This weathered horizon was found at shallow depth and is characterized by a variable thickness ranging from a few meters up to about 22 m. The class IV-V was found between 6 m and about 25 m of depth, while the class IV was generally found at variable depths ranging from 10 m up to 35 m. Finally, the class III was found at a depth greater than 20 m and only in a portion of the investigated area. The geological model shows also how the reduction of the

Fig. 12. P-wave velocity model and color velocity scale associated, derived from the union of the 9 seismic lineaments (indicated with green lines) located in Fig. 3A.

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Fig. 13. Decay of geotechnical properties of rock mass in relation to the increase of weathering grade.

piezometric level, observed at some boreholes (Fig. 6), occurs in normal faults detected through the field survey. Therefore, faults and relative fracture zones would seem to form preferential underground drainage zones. Faults and joints have been considered as one of the main causes in weathering rock masses and sometimes they control the mechanical denudation and geomorphic development (e.g., Pachauri and Pant, 1992; Jacobson et al., 2003; Liu et al., 2007). The geomechanic survey has revealed the presence of strongly weathered materials and fracture zones in high fault concentration areas, where the joint spacing increases from a moderately weathered zone to a completely weathered zone. The latter, happens because the increase of weathering destroys the structural features of rock masses and, thus, the joint spacing has a tendency to increase (e.g., Ehlen, 1999, 2002; Komoo, 1998). In the geological model (Fig. 14) a higher thickness of strongly weathered materials (classes V–VI) appear in correspondence of boreholes S3 and S2 due to a presence of a fault concentration zone. This geological condition worsened the geomechanical properties of rock mass and consequently the weathering is enhanced by a high degree of fracturing. The reconstruction of the geological model (Fig. 14) and the geotechnical properties detected for each weathering degree (Table 2) represent an important support for the study of the landslide processes. Indeed, the characterization of weathering profiles is frequently used by engineering geological researchers to evaluate the predisposing factors relative to different landslide types (e.g., Calcaterra et al., 1996a, 2004; Borrelli et al., 2007). In the studied area a high number of active and dormant mass movements have been identified, partially related to the rejuvenation of the Molini Basin due to the neotectonic activity (Ietto and Ietto, 2004). On the other hand, the presence of more weathered rocks with weak geotechnical properties at shallower depths

represents the main factor favoring the instability phenomena. Considering the landslide typology (Varnes, 1978), a transition from rockfall to rotational/translational slides and, finally, to debris flow phenomena has been recognized as the geotechnical features decreases from III to V-VI weathering class. The geological sections in Fig. 15 show the correlation between the different weathering classes and the landslide types recorded in the studied area. In particular, rockfall phenomena originate along step slopes and are controlled by the characteristics of the main discontinuities. This instability phenomenon occurs very rarely, involving only limited volumes of class III locally found as corestones within classes IV–V. Translational slides occur mainly between different weathering degrees (classes IV and V) and involve a mixture of soil and highly weathered rocks, characterized by weak geotechnical properties. Rotational slides dominate when the mass movements originate at the top of the slopes, generally involving all weathering grades varying from classes V–VI to IV. Debris flow-type instabilities dominate only in the weaker rock masses (e.g., class V–VI) and eluvial-colluvial terranes. They are mainly triggered during intense and extensive rainfall by sporadic increases of loads and negative pore pressure due to saturated conditions at shallow depth (Costa, 1984; Johnson and Sitar, 1990). These conditions reduce further the shear strength and weaken the coherence of the parent material (Palacios et al., 2003). In such conditions, the eluvial-colluvial deposits and more weathered layers tend to collapse moving along the entire slopes. This process commonly starts as soil slip (detachment along planar surface) and then evolves into debris flows (Campbell, 1974; Ellen, 1988; Fannin and Rollerson, 1993; Dietrich and Sitar, 1997) following the mechanism of “impulsive loading” described by Bovis and Dagg (1992). During the down-valley fast movement, the debris flow phenomena are able to erode and transport further weak materials available along the slopes, with high hazard condition for inhabited areas. In the Serre Massif this instability type, generally, occurs on slopes with gradients of 25–45° with sliding surfaces at an average depth of about 1.5–2 m (Calcaterra and Parise, 2005). In the studied area, evidence of debris flow phenomena were detected along slopes with a gradient of 20–40° and only in the more weathered layers (classes V–VI) or in eluvial-colluvial covers with landslide surfaces at depth lower than 2 m. Deeper landslides (depth N 2 m) are rotational and translational slides with failure surfaces occurring mainly in the band of transition between the classes IV and V, sometimes, involving also the shallowest part of the class IV (Fig. 15). This transitional zone, made up of an irregular mixture of soil and highly weathered rock, supports also an active water-bearing stratum. To this regard many authors (e.g. Terzaghi and Peck, 1967; Deere and Patton, 1971; Rogers and Selby, 1980; Hencher and Mc Nicholl, 1995; Zhang et al., 2000; Sidle and Chigira, 2004; Jiao et al., 2005) studied the potential detrimental influence of water-bearing stratum into weathered rock masses affected by instability phenomena. They hypothesized that landslides, in most cases, are induced by an excess pore pressure that

Fig. 14. Geological model based on geophysical, geological and geotechnical data.


313

tests and petrographic analyses on some undisturbed borehole samples. The following observations and conclusions were reached: 1. Field investigations indicate that the weathered profiles are mainly characterized by rocky masses varying from completely weathered rocks and thin residual soil layers (along the gentler slopes and toward the top of the slope) to highly weathered rocks with limited corestones of moderately weathered rocks (along the incised torrents and steep slopes). 2. The geotechnical data, obtained from borehole tests coupled with seismic refraction surveys and laboratory tests on some granitoid samples, have allowed us to recognize different weathered horizons useful in the reconstruction of the reliable geological model. In particular, the model suggests the presence of four different weathered horizons: a first horizon of completely weathered granitoid in sandygravelly soil with interbedded silty matrix layers (class V-VI); a second horizon of completely weathered granitoid in sandy matrix with corestones of highly weathered granitoid (classes IV–V); a third horizon of highly weathered granitoid (class IV); a fourth horizon of moderately weathered granitoid (class III) at depths greater of 20 m. 3. For each detected weathering grade the main geotechnical properties have been estimated, which show a significant decay of quality rock mass with increasing of the weathering grade. 4. Petrographic and grain-size analyses carried out on selected samples collected from three boreholes (S3, S12 and S16) confirmed the transition from the fresher classes (III) to the more weathered classes (V–VI) characterized by an increase of the content in the silty-clay fraction.

Fig. 15. Geological sections with hypothetic landslide surfaces.

changes the effective stress state of the weathered rocks mainly during strong rainfall. Therefore, in the study area the possible increase of water-table, occurring during very rainy seasons, may involve the more weathered shallow horizons already characterized by weak geotechnical properties. This phenomena may decreases the effective stress state in the shallow weathered rock masses and, thus, increasing the landslide probabilities. To this regard, the widespread instability events that occurred in the Serre Massif during the strong rainfalls of 1972 and 1973 (Giangrossi, 1973; Petrucci et al., 1996) are a clear example. In that period the area surrounding Fabrizia was characterized by activation or reactivation of several landslides (Ietto et al., 2012), causing high hazard condition in the whole Fabrizia inhabited perimeter.

6. Concluding remarks The Calabria region is characterized by widespread landslides due to the combination of its tectonics context and the weathering processes involving both igneous and metamorphic rocks. In particular, the slopes of the surroundings of the village of Fabrizia and the neighboring valley of the Allaro River have a long history of landslide. The aims of this paper were to study and characterize the weathering features of the granitoid rocks involved in mass movements and, thus, describe the geomorphological evolution of a south-central sector of the Calabria-Peloritani Arc. In detail, the present paper shows a multidisciplinary study of the weathering processes of granitoid rocks involved in landslide phenomena occurring close to Fabrizia village (Serre Massif, southern Calabria). The study was based on field investigations and geognostic surveys composed of seismic analysis, in situ tests, geotechnical laboratory

Thus, the relationship between the intensity of weathering processes of the granitoids and the landslide types recorded in the studied area, indicate the following: the rocks of the class III are mainly involved in rockfalls; translational slides occur between different weathering degrees (classes IV and V) and rotational slides dominate at the top of the slopes and generally involve all the weathering grades (from class IV to classes V–VI). The shallow horizons composed of thin eluvial-colluvial deposits and more weathered granitoids (classes V–VI) may generate dangerous debris flows or debris avalanches during intense and prolonged rainfalls. This approach allows us to study and characterize the granitoid rocks involved in landslides in the Fabrizia surroundings (Serre Massif, southern Calabria). The weathering processes are closely related to the geodynamic and tectonic evolution of this area. Thus, the methodology used in this work may be considered as a key-case applicable to other weathered context in the Mediterranean area where weathered granitoid complexes widely outcrop. Acknowledgements This research was carried out within the MIUR-ex 60% Project (Resp. F. Ietto) and the public funding (Fabrizia Municipal Committee resolution n° 34-11/04/2001). The authors are indebted to the Editor Markus Egli, Robert Cullers and an anonymous referee for their reviews, discussions and useful suggestions on the manuscript. References Alcalá, F.J., Guerrera, F., Martín-Martín, M., Raffaelli, G., Serrano, F., 2013. Geodynamic implications derived from Numidian-like distal turbidites deposited along the InternalExternal Domain Boundary of the Betic Cordillera (S Spain). Terra Nova 26, 119–129. Angì, G., Cirrincione, R., Fazio, E., Fiannacca, P., Ortolano, G., Pezzino, A., 2010. Metamorphic evolution of preserved Hercynian crustal section in the Serre Massif (Calabria– Peloritani Orogen, southern Italy). Lithos 115, 237–262. Apollaro, C., Accornero, M., Marini, L., Barca, D., De Rosa, R., 2009. The impact of dolomite and plagioclase weathering on the chemistry of shallow groundwaters circulating in a granodiorite-dominated catchment of the Sila Massif (Calabria, Southern Italy). Appl. Geochem. 24, 957–979. ASTM International Standard, 2008. Standard Test Method for Determing Rock Quality Designation (RQD) of Rock Core (D66032) pp. 1–5.

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