Landslides (2006) 3:252–259DOI 10.1007/s10346-006-0046-zReceived: 13 June 2005Accepted: 26 April 2006Published online: 20 June 2006© Springer-Verlag 20066

Alessandro Chelli . Giuseppe Mandrone . Giovanni Truffelli

Field investigations and monitoring as tools for modellingthe Rossena castle landslide (Northern Appennines, Italy)

Abstract In the evening of February 28, 2004, a landslide tookplace in the village of Rossena (Northern Apennines, Italy), built atthe base of a crag shaped in a basalt mass and wrapped in highlydeformed formation of clay and shale with blocks. The failuredamaged some houses, roads and fields but, fortunately, themedieval Rossena Castle, lying on the crag, was not involved at all.The goal of the study was to attain a technical and geological modelof the slope to generate a landslide risk zonation, for regularity anddevelopment planning, so that the most correct action plans couldbe proposed. A detailed geological and geomorphological surveyallowed for distinguishing the different gravitative landform of thisarea. It was very helpful to plan direct and indirect investigation,including borehole drillings, samplings, seismic (tomography), andelectrical surveys. A monitoring system was built up immediatelyafter the event (three wire extensometers and one inclinometer),then progressively substituted by a more complete one (twotiltmeters, two jointmeters, four inclinometers, two incrementalextensometers, and two piezometers). The phenomenon can bedivided in different parts. The central sector of the slope isinterested by compound slides likely affecting the bedrock and canbe considered, at present, the ‘engine’ of the whole instabilityframework. Indeed, as a consequence, in the upper portion of theslope the huge blocks in which the outer part of the crag isdisjointed experienced vertical displacements and, locally, top-plings. Finally, the lowest sector is affected by slow movements,probably connected to bedrock creep or rock flow, while the toe,really at the foot of the slope, by shallow landslides. This instabilityframework is the result of a complex evolution, starting almostmore than 9,000 years ago, as testified from a radiocarbon dating.In more recent time (19th century), the Rossena landslide was alsotriggered by an earthquake that induced the partially breaking upof the crag, causing rock falls and cracks in the ground.

Keywords Complex landslide . Monitoring . Conceptualmodelling . Northern Apennines . Italy

IntroductionThe territory of the Emilia Romagna (Italy), pertaining to theNorthern Apennines, is affected by a large number of landslides ofdifferent types and sizes linked, mainly, to the geological featuresof the bedrock. The complex tectonic history of the chaindetermined the superimposition of many stratigraphic unitscontaining heterogeneous and/or chaotic rock masses. Theseformations are characterized by poor geomechanical propertiesthat cause the strong tendency for slope instability (Bertolini andPellegrini 2001).

In the evening of February 28, 2004, the village of Rossena(Reggio Emilia Province), at about 425 m a.s.l. in the Po Plane sideof the Northern Apennines, was involved in a landslide during

rainfall on a consistent snow cover (about 30–50 cm). Upslopefrom the village, on a 50- to 70-m high crag, a massive castle stands.Fortunately, it was not involved in the landslide, but the roads,some houses of the village, and the downslope fields weredamaged.

The castle is the best preserved stronghold of the Longobardtimes, enlarged and reinforced in the 10th century and partiallyrebuilt by Bonifacio, the father of Matilda of Canossa (the Vice-Queen of Italy and probably the most important woman in theMiddle Ages), as a defensive structure guarding the Enza Valley(Coratza et al. 2004).

The aim of the study is to find an engineering-geological modelof the gravitative phenomena observed in the area surroundingRossena. As a result of these studies, it will be possible toadequately design mitigation and remedial action plans.

Geological and geomorphological featuresThe Northern Apennines are a fold-and-thrust belt of complexorigin and evolution, starting in the Late Cretaceous as aconsequence of the closure of the “Ligurian-Piedmontese” oceanicbasin. In the Rossena area (Fig. 1), two main rock units outcrop(Papani et al. 2002):

1. Mainly dismembered and tectonically fatigued clay and shaleformations characterized by stratal disruption, lack of internalcoherence, and destruction of the original stratigraphic se-quence. They are the result of the disruption of a sedimentarypile lying on the subducting plate of the Cretaceous to Eocenetrench fore-arc system, which generated the Ligurian accre-tionary wedge.

2. A more or less fractured and weathered ophilolitic sequenceconsisting of slices, lacking of metamorphism, of oceanic crust.

The chaotic clay complex (1) is made up of at least threedifferent rock types Cretaceous in age with similar geotechnicalbehavior: 1) a sequence made of rock blocks in a pelitic matrix(“limestone in shale”), 2) a scaly varicolored clay formation, and 3)a mélange of clay and pieces of rocks of different nature (blocks ofbasalt, limestone, or flysch ranging in size from a few centimetersto many meters). As most of the chaotic complex, it ischaracterized by poor technical properties. According to theMarinos and Hoek (2001) classification for heterogeneous rockmasses, first applied in this area by Mandrone (2004), they rangefrom flysch types F to H, showing a Geological Strength Index (G.S.I.) usually lower than 20 (Hoek et al. 1992; Hoek and Brown 1997).Moreover, at the base of the sliding surface, it is possible to assessthe following properties: mi=6, intact uniaxial compressivestrength=3 MPa. In terms of Mohr–Coulomb criterion, the best-

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fitting curve at a depth of about 20 m lead for this chaotic claycomplex to a cohesion of about 0.04 MPa and a friction angle of nomore then 18°. Anyway, they may be still weakened by a significantdegree where free drainage is not present because they are subjectto changes in moisture content.

The ophiolitic masses are dismembered allochthonous bodiespackaged within the chaotic complex. There are many of thesemasses in the area, ranging in size from a few to many hundreds ofmeters, made up, especially, of massive basalts, serpentine and

pillow lavas. It is very common to find large outcrops of brecciastoo, sometimes polygenic and more or less re-cemented by calcite.Ophiolitic masses usually have good geomechanical properties,and they outcrop in crags that show a high degree of joints andfaults, according to main tectonic stress fields. Commonly,fractures are very open and infilled with clay and pieces of rocksfallen inside. This is a near-surface effect that did not penetrate therock mass at depth. As a result, the ophiolitic complex can varyfrom a massive or blocky behavior, when big blocks are

Fig. 1 Geological–geomorphologicsketch of the Rossena landslide, high-lighted in pale yellow. The location ofthe vertical electrical sounding E4 isalso shown

Fig. 2 Geological-geomorphologiccross-section (for location, see Fig. 1) ofthe Rossena landslide. Keys: 1) basaltand basaltic breccias, 2) chaotic claycomplexes, 3) chaotic clay complexesinvolved in the landslide, 4) chaoticclay complexes involved in the rockflow, 5) slope deposits, 6) slidingsurfaces, 7) ancient landslide depositswith organic layers, 8) boreholes.Letters and arrows at the top show thefour different parts of the landslide

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interlocked, to situations in which blocks are heavily broken intoangular rock pieces so G.S.I. goes from more than 70 to 25–30(Hoek and Brown 1997), where the jointing pattern is difficult topredict.

The morphology of the area is characterized by the ophioliticmasses that stand out on the ground as eroded remnant becausethe erosive processes removed the soft surrounding pelitic rocks.The most prominent body is the crag where the Rossena castlestands. A scree slope surrounds it, which is made mainly by clastsof centimeters size with a silty-clayey reddish matrix overlayingancient landslide deposits and the chaotic clay complex of thebedrock (Fig. 2). Moreover, the slope shows an irregular profilecharacterized by concave areas and scarps highlighting that themorphology is mainly due to instability processes. Where thepelitic rocks outcrop, linear erosive processes, like rill erosion,caused the development of badlands. In addition, the presence ofalluvial deposits between the landslide tongue and the streamprevented the undercutting of the displaced material.

Materials and methodsA detailed geomorphologic and geologic survey allowed fordistinguishing the different landslides that shape the north-easternslope of the Rossena area. The Global Positioning System (GPS)device was used to map in detail the boundaries of the landslidebodies and to place the morphologic indicators of the differentmovements. In particular, the portions of the landslides whereextensional movements exist are distinguished from those wherethere are compressive evidences.

Direct and indirect methods of investigation (Fig. 3) were usedto study the subsurface characteristics of the Rossena landslide toimprove the knowledge and to gain data suitable for modeling.Investigations were conducted on the upper part of the slope, nearthe road, the castle, and the village. The program of siteinvestigation included borehole drillings, samplings, and seismicsurveys (tomography). Later, electrical surveys (vertical sounding)were carried out in the central and lower part of the slope.

First, a simple system of monitoring (three wire extensometersand one inclinometer) was built up immediately after the event inFebruary 2004. It was progressively substituted in the followingmonths by a more complex one in which two tiltmeters, two

jointmeters, four inclinometers, two incremental extensometers,and two piezometers play an important role. Except inclinometersand extensometers, other equipment is fitted with data loggercollecting at least two measurements per day with the purpose offinding correlations between movements and rainfall measured intwo rain gauges located within a range of 10 km.

The first system of monitoring allows us to identify the rates ofmovement and, coupled with boreholes results, to determineapproximately the depths of the sliding surfaces so that the secondone was adequately planned in terms of selection of the specifictypes of devices, definition of locations and depths of theinstrumentation and, finally, definition of data acquisitiontechniques. The purpose of the monitoring is: 1) to determine theshape of the sliding masses, in particular of the ophiolitic crag onwhich the castle stands, 2) to check any movement in the village, 3)to identify the depth of the sliding masses in the upper part of thelandslide, and 4) to determine the absolute lateral and verticalmovements, and the rate of movement (velocity) of the differentparts of the complex landslide.

Results

Ground investigationIn the landslide of Rossena, many different types of phenomenaaffecting different parts of the slope were observed (Figs. 1 and 2,sectors A, B, C, and D).

At the top of the hill, the crag (Sector A) is crossed by families offractures, which cut the basaltic mass in different directions. Thethree main discontinuity systems dip more than 70° with dipdirections of 5, 70, and 120°, respectively. They cause the disjointingof the rock mass in huge blocks that, especially in the outer part ofthe crag, experienced differential movements. During the event ofFebruary 28, 2004, some blocks collapsed, with about 1 m of ver-tical offset, just below the wall of the old building, whiletopplings involved some outer blocks. Small rock falls and debrisflows occurred, respectively, in front of the crag and in the coversthat surround it, which were made of coarse debris in a clayeymatrix. A mainly rotational landslide, probably affecting also thebedrock below the crag, involved the oldest part of the village.Successively, several fractures—with offset between 1 and 2 m—

Fig. 3 Location of the boreholes ofthe geophysical surveys and of themonitoring system; red line show thelimits of the Rossena landslide. Keys: Bboring (used as I inclinometer, Eextensometer, P piezometer), S seismicsurvey, E vertical electrical soundings(see also Fig. 1), C tiltimeter, W wireextensometer, and J joint meter

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and many small landslides occurred in this part of the slope,enlarging the crown and the flanks of the landslide andcontributing to widening of the damaged area.

The top of Sector B, at about 420 m a.s.l., is characterized by asharp scarp (about 1–2 m height). In this portion of the slope, thetype of movements can be referred to as a compound landslide(Skempton and Hutchinson 1969): in fact they change, turningfrom mainly rotational into earth slide (in the upper part) to earthflow (in the lower one). Moreover, several surface discharges ofgroundwater appear near the border between sectors B and C. Theylikely mark the boundary between an upper layer made by materialderiving from the wasting of the basaltic crag, in which a waterseepage can develop, and a lower one made up by a mainly clay-rich material, less permeable, deriving from the varicolored clayformation (both levels were involved in the landslide).

Sector C is characterized by another concave surface whereimportant water impoundment appears. It is due to the mainly claycolluviummantling the bedrock, almost outcropping as testified bythe abrupt change of acclivity downslope. However, the state ofpreservation of the rocks is very poor, and the scaly varicoloredclay are very loose and weathered if compared with other outcropsin the same stratigraphic position.

In the steeper portion of the clayey slope (sector D), many seriesof fractures appear in the ground, some of them showingdecimetric offset, cutting the slope from the center towards east.On the basis of their distribution, these fractures seem to be theexpression of an initial failure rather than the movement of a well-defined landslide body.

An extended, well-stratified, and preserved outcrop of the scalyvaricolored clay formation is in correspondence with the foot ofthe slope, at an elevation of about 300 m. It seems to contrast withthe development of the landslide, playing the role of a buttress. Infact, in the passage between sectors C and D, the landslide isdivided into two parts. The first one, on the right side of thebedrock outcrop, is affected by many fractures and secondarygravitative phenomena, while in the second one, on the oppositeside of the same outcrop, an earth flow with no evidence of recentmovement can be observed.

Moreover, outside the landslide area, on the left side of it, afracture crosses the slope starting from the elevation of 315 m andreaching 290 m a.s.l., where it disappears. It shows variable widths,from 40 to 200 cm, and variable vertical offsets from 10 to 30 dm. Itseems as if it is the morphological expression of a translationallandslide, laterally linked to the main phenomenon.

Subsurface characterizationThe site investigation included boreholes drilling, sampling, andgeophysical survey. Moreover, laboratory analysis studied theparticle size and Atterberg limits of the displaced material and ofthe bedrock. Also an organic layer was dated with 14C technique.

DrillingsNine boreholes (only six with core sampling) were drilled aroundthe crag and immediately downslope from the village. Theirlocations are shown in Fig. 3 and along the cross-section of Fig. 2,while in Table 1 the most important technical characteristics andmeasurements are summarized.

Sub-horizontal borehole B5 shows that the ophiolitic cragconsists of pillow basalt affected by many cataclastic zones inwhich tectonic breccias are very common. It confirms that theexternal part of the rock mass is fractured and the joints can bevery open (up to decimeters), so they likely conduct a large amountof water at the base of the crag.

The ophiolitic body is embedded within the scaly varicoloredclay (as testified by B2) and the passage between these twolithologies is characterized by a very weathered and jointed rockmass so that especially the more external parts can be consideredas made up of big blocks that can locally move each other.

In Sector B, the sliding surface reaches the maximum depth(probably more than 40 m), as shown by B3 and B4. In particular,B4 highlights that different older landslide deposits were involvedin the recent failure, sometimes separated by organic layers (onewas found at about 15 m deep).

The boreholes in the oldest part of the village (B6 and B7)revealed that, also in this area, the sliding surface is quite deep (20–30 m). Moreover, they highlight that the bedrock on which the oldvillage was found is made not by scaly varicolored clay as in a largepart of the landslide but by a mélange of clay and blocks ofdifferent nature.

Geophysical surveySeismic and resistivity measurements of the slope were investi-gated to determine the subsurface structures and to correlate thesedata with those form the borings.

Seven seismic tomographies (S) allow determining the subsur-face geometry of the landslide and the surrounding areas (Fig. 3).The average thickness of the landslide is between 20 and 30 m, butlocally the sliding surfaces can reach more than 40 m. In detail, thesliding surface probably corresponds to the velocity of about2.2 km/s (Fig. 4); a lower velocity is recorded above this surface,showing multiple landslide units linking each other both crosswise

Table 1 Boreholes technical characteristics and results (locations are shown in Fig. 3)

ID Depth (m) Sliding surf. depth (m) Bedrock AnnotationsB1 25 – – No core samplingB2 40 12 Varicoloured clay Basalts from 22 m of depthB3 30 20 Varicoloured clay Slope deposit until 10 m, than dismembered pieces of scaly varicoloured clayB4 25 Not found – At 15 m, organic layer, age of 8,960 b.p. (14 C)B5 40 Basalts and basaltic breccias Horizontal drilling with decimetric empty spaces at 10, 15, and 18 mB6 30 Not found – At 17 m, serpentine block of about 2 mB7 30 22 Basaltic breccias Found also blocks of limestone and serpentineB8 30 – – No core samplingB9 30 – – No core sampling

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(seismic surveys S5 and S6) and lengthwise (S1, S2, S3, S4, and S7) ofthe head of the landslide. Sometimes, inside the landslide bodythere are some high velocity portions, like in S1. They are probablysome big blocks fallen or toppled from the ophiolitic crag, thenwrapped up in the landslide.

Four vertical electrical soundings (Figs. 1 and 3) investigated thecentral part of the landslide but, unfortunately, they show no largecontrasts in resistivity, so it is difficult to define sharp boundaries.In general, they agree with other data in the higher part of the slope(electrical surveys E1 and E2), while E3 is characterized by clearsuperficial reflectors (4–5 m) only, probably the interface betweenthe looser external part of the bedrock and the scaly varicoloredclay of the bedrock. The only one that investigates the lateralenlargement on the left side of the main landslide (E4), at about300 m a.s.l., shows a sharp change in resistivity at about 20 m indepth.

Laboratory testsGrain size analyses and Atterberg limits of five samples (Fig. 5)from boreholes B3 and B4 show that they can all be classified asorganic clay, generally with a very high degree of plasticity, exceptthe sample from borehole B4, at 23.8 m of depth, which is a clayey-sandy gravel. Moreover, these analyses show that the base of thelandslide (sample B3 at 11 m), made up of varicolored clay, is a weakrock that, if weathered, behaves like a soil containing over 60% ofclay. The weathering of this bedrock produced a terrain (B3 at 3 m)slightly coarser, showing about the same amount of clay, a little lesssilt and a little more sand and gravel.

The material of the main landslide body is composed of well-graded grains, a little finer in the first 15 m (B4 at 10.5 m) and a littlecoarser at the bottom (gravels reach about 50% in the B4 at 23.8 m).Between these two levels, in borehole B4, a layer containing organicmatter was found; a sample of it was dated, giving an age of 14C8,960±60 yr B.P. (Beta–197895). Moreover, the fabric of this level isvery similar to that of the colluvium mantling the slope, leading usto believe that it is of a similar origin in a nearly superficialenvironment.

Monitoring systemImmediately operative a few days after the initial landslide, theresults (Table 2) from the wire crack meters (Fig. 3) showed that thecrag was substantially stable. Some small movements wererecorded contemporarily by the three instruments in the followingmonths. Figure 6 shows an example of the record of the wire crackmeter compared with daily rainfalls. It is possible to distinguish theinstrumental drift (larger for longer instruments) and the variationdue to rainfalls (peaks of movement in correspondence of theevents of the March 25, April 9 and 19, and May 4).

The second system of monitoring, still in use, is verydifferentiated and investigates (Fig. 3) a large part of the upperportion of the landslide. The best results (Table 2) were obtainedwith inclinometers. They show that the movements continued formonths but with very small displacements (no more than 1–2 cm).Results from the inclinometer I3 are shown in Fig. 6. They highlightcoherence in measurements, a sharp sliding surface at about 22 mof depth and probably a secondary surface at about 8 m.

Fig. 4 Seismic cross-section (profiles 1and 2) along the main body of theRossena landslide; keys: red lines failuresurfaces, arrows direction of move-ments, B3 and B4 boreholes

Fig. 5 Grain size analysis and Atter-berg limits (plasticity chart) of fivesamples from boreholes B3 and B4(numbers show depth in meters fromsurface). Keys: WL liquid limit, IPplasticity index

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Vertical and horizontal incremental extensometers show similartrends, like tiltmeters and jointmeters. In particular, incrementalextensometer E5, investigating in horizontal the crag, shows thatthe outer part of the ophiolitic mass is still slowly moving alongjoints, usually very open and often with no filling of fine material.On the other hand, electrical piezometers show that, in thelandslide body, water levels are quite regular, due to the lowhydraulic conductivity of these terrains. Seasonal variations (1 yearof measurements available only) do not exceed a couple of meters.

AnalysisGeomorphological mapping, subsurface investigation, and mon-itoring system have provided information to create an engineering-geological model of the Rossena landslide. Many types oflandslides, with different styles, distributions, and states of activity,contribute to the evolution of a complex picture (Figs. 1 and 2).

The main movement is the complex landslide starting at about420 m a.s.l. (sector B). It is a compound slide in which the surface ofrupture has a very steep main scarp that flattens with depth,

reaching almost 40 m deep. The last part of the landslide masks,with superficial flows (max 10 m thick), the lower phenomenonwhose presence is suggested by the flat area at the elevation ofabout 360 m. This latter section (sector C) is characterized by acompound slide also affecting part of the bedrock. This fact issuggested by the state of preservation of the rocks outcropping inthe steeper area on the right side of the landslide site (downhillto the largest flat area), which appear worse compared with thoseof the same type at the foot of the slope (sector D).

The main instability in sectors B–C determined the massmovements in both the highest and the lowest part of the slope.The huge blocks (sector A), in which the outer part of the Rossenacrag is disjointed, experienced vertical displacements and, locally,topplings. Probably, these movements are related to a rotationallandslide, affecting both the crag and the chaotic clay complex andare the cause of most of the damage to the village. The instability ofthe central part of the slope also determined the development ofrelatively small superficial landslides at the foot of the slope (sectorD), which probably contributes to the widespread instability,

Table 2 Landslide monitoring system working from 2004: technical notes and results

Instrument I.D. and location Main characteristics Period of observation Measurement (depth and entity)Inclinometer I3 in B3 L=30 m From 12 Jul ≈20 m; Δ=15 mm

I7 in B7 L=30 m From 22 Oct ≈13 and ≈27 m; Δ=12 mmI8 in B8 L=30 m From 01 Dec ≈9 m; Δ=2.5 mmI9 in B9 L=30 m From 01 Dec ≈3 and≈17 m; Δ=2.5 mm

Incremental extensometers E5 in B5 L=35 m, horizontal From 12 Jul Between 12,5 and 20.5 m, Δ 5 mmE7 in B7 L=30 m, vertical From 12 Jul Between 0 and 28 m, Δ 1 mm

Jointmeters J1 E From 15 Aug −0,7 mm (compression)J2s E From 15 Aug No data

Elettrical piezometers P1 in E, L=25 m, open pipe From 30 Jul Average −1,87; min. −2,42; max. −1,27 mP2 in E, L=25 m, open pipe From 30 Jul Av.−10,67;min.−11,00; max.−9.99 m

Tiltmeters C1 E From 09 Aug X=0.010°; Y=0.026°C2 E From 09 Aug X=−0.458°; Y=0.193°

Wire crackmeters W1 L≈30 08 Mar–10 May 16.03, 27.03, 09.04, 18.04W2 L≈40 08 Mar–10 May 15.03, 28.03, 09.04, 20.04W3 L≈35 08 Mar–10 May 15.03, 23.03, 27.03, 09.04, 20.04

L length, E electrical, M mechanical, Δ max. movement. For tiltmeters max., angular deviations are shown while for wire crackmeters only the date in which clear displacement wererecorded are shown

Fig. 6 Examples from monitoring re-sults: a movement from wire extens-ometers (dotted lines highlight theinstrumental drift) compared withrainfall; b movements in inclinometerin B3

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generating a self-feeding mechanism that progressively led to theevolution of the lower part of the slope. In fact, it is possible thatsector D is affected by a slow movement of external part ofbedrock, probably connected to a bedrock creep or a rock flow(Varnes 1978; Clerici et al. 2002).

As a matter of fact, the landslides of sectors B and C can beconsidered as the ‘engine’ of the whole instability framework. Atpresent, the landslide of sector B shows the most evident signals ofmovement, and they tend to develop at the expense of the lowerones. In any case, the geometrical relationships between thelandslides of sectors B and C are unclear and it is difficult todefinitely state which was the first one to move.

This complex situation is the result of an evolution startingmore than 10,000 years ago. In fact, the texture of the layer datedwith radiocarbon methods accounts for the presence of a colluvialdeposit in which organic matter was accumulated, whcih separatedat least two different landslide events. Moreover, the texture of thedeeper landslide deposit shows the presence of a coarser grain size,connected with intense physical weathering of the crag, probablydue to different and more severe climatic conditions. Indeed, latePleistocene landslide events were found in the Northern Apen-nines, interesting phenomena which are still active (Tellini andChelli 2003). In more recent time, according to historical research,the Rossena landslide was triggered onMarch 13, 1832 by a 7–8 MCSdegree earthquake, with its epicenter about 20 km away (Colla1832). This quake induced the breaking up of the crag causing rockfalls and cracks in the ground.

DiscussionWhen in a landslide a rigid block overlies materials with plasticbehavior, it is usually identified in literature as a lateral spread(Fig. 7). They are defined as an extension of cohesive soil or rockmass combined with a general subsidence of the fractured massinto a softer underlying material (Cruden and Varnes 1996).Examples of this landslide type are very common, as reported inthe Northern Apennines (Cancelli and Pellegrini 1987) and in manyother mountain areas (Rohn et al 2002; Vlcko 2004; Baron et al2005). In these cases, the rock mass is not laterally confined and theongoing extension leads to a widening of the fractures, partly filledwith scree from neighbouring lithologies or, in some cases, bymaterial squeezed out from the substratum made up of clay orshale.

In the Rossena landslide, the rigid rock mass is wrapped inplastic lithologies. In fact, the ophiolitic body is found in boreholeB2 under 10 m of varicolored clay, i.e., it has deep roots below theground. This fact implies that the rigid mass is laterally confined so

that horizontal displacement cannot develop. In these cases, thefailure of the rock blocks evolves as topplings, fallings, and mainlyvertical collapses.

Indeed, the Rossena landslide is not triggered by undrainedloading in the water-saturated mudrock substratum as in the outerpart of the typical lateral spread, but the ‘engine’ of the instabilityare the complex landslides in the central part of the slope and thecrag was involved as a consequence of the first movements.

ConclusionsThe landslide of Rossena affects a hilly area where mainlydismembered and tectonically fatigued clay and shale outcrops.The morphology is characterized by ophiolitic rock masses thatstand out on the ground. They are surrounded and wrapped in theabove-mentioned plastic lithologies.

This area was already affected in the past by similarphenomena. In fact, a landslide body more than 9,000 years agowas found, thanks to a radiocarbon dating performed on anorganic deposit found in a borehole about 15 m deep. Moreover,historical information highlights the seismically induced breakingup of the crag, triggered on March 13, 1832 by a 7–8 MCS-degreeearthquake.

The Rossena landslide can be described as a composite andcomplex phenomenon, multiple in its central part, confined andenlarging (Cruden and Varnes 1996). The total length of thelandslide is about 1,000 m with 220 m of difference in elevation; itsmean width is about 250 m, while its depth spans between 20 and30 m. It covers an area of approximately 300,000 m2, with a volumeof about 7,000,000 m3. The study outlines a complex framework inwhich the different portions of the landslide are interconnectedamong them. Furthermore, the behavior of the rock masses of thecrag is not trivial, considering that the presence of rigid ophioliticbody wrapped in plastic clayey rocks is not an uncommonsituation in the Northern Apennines.

The precise cause of the initiation of February 28, 2004landsliding remains unknown, but probably the melting of thesnow cover can be regarded as the main triggering factor. However,the following factors may have influenced the stability of the slope:1) The bedrock made of weak rock, 2) the presence of thick multi-layered ancient landslide deposits favoring the creation of slippingsurfaces, and 3) scree deposits from the crag on the ground surfaceacting as water reservoir.

In conclusion, site investigation and monitoring results high-light that the inner part of the crag is stable so also the ancientcastle is probably safe, at the moment. On the contrary, the villageis in an area at risk, still affected by small movements and

Fig. 7 Idealized block diagrams of atypical lateral spread (a) compared tothe situation observed in the RossenaLandslide (b). In the first case, acohesive soil or rock mass (1) overliessofter materials (3) and a scree deposit(4) is forming at the border of the rigidblocks; in the second one, rigid rockmasses (2) are wrapped in plasticlithologies and laterally confined sothat horizontal displacement cannotdevelop. Arrows show main directionsof movements

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potentially involved in further re-activations. The engineeringgeological model proposed, reflecting ground and subsurfaceobservations coupled with monitoring results, revealed itself veryuseful to project remedial works required to stabilize the slope andto minimize the risk for the village and the castle.

AcknowledgementsG. Truffelli designed the subsurface investigations and themonitoring systems, while A. Chelli and G. Mandrone areresponsible for the interpretation of the data, for the results, andfor the analysis. Grants: Fil UniPr 2004 (Head: S. Perego), MIUR-COFIN 2002 (Head C. Tellini) and UniTo 60% 2006 (Head: M.Fornaro).

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A. ChelliDip. Scienze della Terra, Università di Parma,Parco Area delle Scienze 157/a,43100 Parma, Italy

G. Mandrone ())Dip. Scienze della Terra, Università di Torino,Via Valperga Caluso 35,10125 Torino, Italye-mail: giuseppe.mandrone@unipr.itTel.: +39-011-6705113Fax: +39-011-6705146

G. TruffelliRegione Emilia Romagna, Servizio Tecnico dei Bacini Secchia e Sinistra Enza,Via Emilia S. Stefano, 2542100 Reggio Emilia, Italy

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