Engineering Geology for Society and Territory – Volume 2Landslide Processes

Giorgio LollinoDaniele GiordanGiovanni Battista CrostaJordi CorominasRafig AzzamJanusz WasowskiNicola Sciarra Editors

Giorgio Lollino • Daniele GiordanGiovanni Battista CrostaJordi Corominas • Rafig AzzamJanusz Wasowski • Nicola SciarraEditors

Engineering Geologyfor Society and Territory –

Volume 2

Landslide Processes

123

32New Interpretation of Lemeglio CoastalLandslide (Liguria, Italy) Based on Field Surveyand Integrated Monitoring Activities

F. Faccini, L. Crispini, L. Federico, A. Robbiano, and A. Roccati

AbstractThe Lemeglio landslide has been known since the end of nineteenth century because itrepresents a well-preserved coastal landslide in the Mediterranean environment. The landslideaffects engineering structures: several drillings have been performed that allowed establishingthe thickness and the bedrock nature; moreover they have been equipped with geotechnicaland hydrogeological monitoring tools. It represents an active complex landslide, with a meanrate of movement up to 3 cm/y, whereas rapid phenomena like rockfalls occur at the landslidescarps. Recently, further information on ground movements has been collected through theradar interferometry technique PSinSAR. This monitoring activity, together with field surveys,allowed us to make a new delimitation of this large slope instability phenomenon. It nowaffects also the Lemeglio village and the north catchment of the hystorically-known area. Onthe basis of the collected data we can ascribe all the coastal area to a rock block slide orrotational sagging.

KeywordsCoastal landslide � Slope inclinometers � PSInSAR™ technique � Liguria

32.1 Introduction

In this work the results of geomorphological and engineer-ing-geological studies conducted along the eastern Liguriancoast between Moneglia and Deiva Marina are presented(Fig. 32.1). The studied area is historically affected byinstability phenomena, which cause damages on buildings,

roads and tunnel railway: in this sector the Lemeglio land-slide is well known in scientific literature from the end ofnineteenth century, mainly due to the railway construction(Almagià 1907).

In fact, compared to the other Ligurian coastal landslides,Lemeglio’s one—the largest in Eastern Liguria—has pre-served almost intact its geomorphological features. Amongnatural causal factors, we can recognize the ground condi-tions (stratigraphical and geo-structural features, contrast inpermeability), the geomorphological processes (tectonicuplift, wave erosion of the slope toe) and physical processes(intense, short period rainfall and prolonged high precipita-tion). Nevertheless, landslide hazard is also strongly relatedto man-made processes, and namely to the railway tunnelingduring the second half of nineteenth century and to theremoval of rock blocks along the coastline.

Activities for this study started from bibliographic sour-ces. Several data derived from drilling activities, geotech-nical and hydrogeological monitoring, radar interferometrytechnique, and original geologic and geomorphologic

F. Faccini (&) � L. Crispini � L. FedericoDiSTAV, University of Genova, corso Europa 26, 16132 Genoa,Italye-mail: faccini@unige.it

A. Robbianoc.so Garibaldi 58, 16043 Chiavari, GE, Italye-mail: geotecam@libero.it

A. RoccatiNational Research Council (CNR), Research Institute for Geo-hydrological Protection (IRPI), Via Madonna Alta, 126, 06128,Perugia, Italye-mail: anna.roccati@irpi.cnr.it

G. Lollino et al. (eds.), Engineering Geology for Society and Territory – Volume 2,DOI: 10.1007/978-3-319-09057-3_32, © Springer International Publishing Switzerland 2015

227

survey, provide an integrated analysis of the area; the resultsdo not completely agree with the more recent studies andsuggest a new interpretation of the phenomena.

32.2 Field Survey

The geology of the area is characterized by two Formations(ISPRA 2012): the “Scisti Zonati” (SZO, weak siltstone andclayey shales with sandstone layers) and the overlying“Arenarie del Gottero” (GOT, sandstones with thin inter-layers of shales). The Scisti Zonati show pervasive tectonicdeformations; main folds have NE-SW-trending axes, nor-mal to the coastline; the sandstones bedding mainly dipssouthwards with various orientation (Fig. 32.1c).

NW-SE striking normal faults are clearly identified on thesea bottom, but can be recognized on the mainland onlythrough geomorphological elements (Fanucci and Nosengo

1979). Another normal faults system, striking orthogonal tothe coastline, is present and often triggers large massmovements.

We carried out a geomorphological survey in an areaincluded inside a triangle, whose vertices are the lower partof Fosso del Mandola catchment, Punta Rospo and MtCrocetta—wider than the area previously studied. This hasallowed to highlight several landforms and processes mainlydue to gravity and running waters. The sea wave action hasbeen equally influential on the coastal erosion as shown alsoby the frequency of sea storms from SW (Fig. 32.1c).

The main landslide, well-identified for almost 40 years,lies in the central portion of the slope, between Mt Crocetta,Lemeglio village and Punta Rospo, between 200 m a.s.l. andmean sea level. The main landslide scarp is clearly visible inthe upper part of the slope, along the western side of MtCrocetta; from this scarp (at about 400 m a.s.l.) and fromother minor scarps rockfalls occur, that represent an hazard

Fig. 32.1 a Location area; b Neotectonic sketch map (Landslide limitsby basin master plan); c Geological and geomorphological map: 1Landslide; 2 “Arenarie del Gottero” formation (GOT); 3 “Scisti Zonati”formation (SZO); 4 Bedding; 5 Swampy deposits; 6 Scarps; 7Counterscarps; 8 Geomechanical analysis station. Stereograms 1–4

(right) show the Markland’s test (friction angle of joint planes ϕ = 30�).The rose diagram shows the sea wave direction and the related heightat La Spezia Buoy (1989–2007 period, APAT data): a 0.25–1 m; b 1–2 m; c 2–3 m; d >3 m. A-A trace of the simplified geological section ofFig. 32.2

228 F. Faccini et al.

for buildings and communication routes. A simplified rockslope failures evaluation was conducted through the Mark-land’s test (Fig. 32.1c).

The landslide shows two portions with different features:an upper part (between 170 and 130 m a.s.l.) almost flat(named “Acquario”), and a lower one, by the sea, very steep(about 50 %). In the northern sector, in the Fosso del Mandolacatchment, other smaller landslides have been identified.

In themain landslide body several drillingswere performedand allowed to detect a thickness of the debris cover rangingfrom 15 m to more than 50 m in the flat area (Fig. 32.2a).Standing on the Geological Strength Index classification forheterogeneous rock masses (Marinos and Hoek 2001), theArenarie delGottero could be considered as aB (40–45),whilethe Scisti Zonati may be evaluated as a E (25–30).

In order to obtain geotechnical and hydrogeological data,the drillings were set up with inclinometric cases and piez-ometers (Figs. 32.2b and 32.3a).

In the studied area several morpho-tectonic evidenceseither related to the watersheds, slopes, water courses andgeneral characters have been recognized.

32.3 Monitoring Activities

With the exception of rockfalls (only from active rockyscarps), active deformations are detected in the wholelandslide body, with changing velocities. Larger displace-ments are concentrated in the lower part, close to the sea,even if the Lemeglio village is affected by significant dam-ages on buildings.

The southwestern portion, where buildings date back tothe seventies, shows a rate of west-directed displacement upto 3.3 cm/y and a sliding surface between 8 m (I16) and

21 m (I10) deep (Figs. 32.2b and 32.3a): several inclino-metric cases became useless because of excessive deforma-tion in less than 2 years. Along the counterscarp, at about125 m, the sliding surface is around 48 m deep (I8), whereasthe flat portion records lower displacement values.

On the whole, rates resulting from inclinometric moni-toring range 1 and 3 cm/y, which indicates extremely andvery slow movement. The landslide shows an intermittentmovement and is active during the rainy periods, when watertable causes pore pressure increase and effective shearstrength reduction in the debris properties. Indeed, inside thelandslide there is permanent aquifer, which has a levelranging from 7 m (P2, P4) to 29 m (P6, P7) below groundlevel related to rainfalls (Fig. 32.3a).

Satellite monitoring by means of PSInSARTM technique(Regione Liguria 2012) are based on ERS and ENVISAT sat-ellites, both in ascending and descending geometries. The dis-tribution of PS data for ERS descending acquisition geometry(T437 La Spezia) in the period 1992–2000 shows mean annualrates rangingbetween−2 and−8mm,with increasing rates fromthe uppermost portion of the landslide body towards the toe(Acquario locality). The distribution of PS data for ENVISATdescending acquisition geometry (T258 Genova) in the period2004–2010 shows displacements comparable to those citedbefore andmean annual rates ranging between −2 and −16mm,with a similar trend of increasingvelocities inAcquario locality,where some targets record rates up to and higher than 23mm/y.On the whole wemonitored more than one hundred targets: theidentified “anomalous areas”, characterized by both significantand homogeneous mean rate values, are mainly located in thesouthwestern sector of the landslide, in agreement with the in-clinometric data, and in the uppermost sector, farther theLemeglio village, in the Fosso del Mandola catchment(Fig. 32.3b).

0

10

20

30

40

50

0 10 20 30 40 50

dept

h (m

)

cumulative displacement (mm)

I16

I11

I1

I8

I10

(a) (b)400

300

200

100

0

Fig. 32.2 a Simplified geological section A-A (see Figs. 32.1 and 32.3); WT = mean water table; SS = sliding surface; GOT = Arenarie delGottero; SZO = “Scisti Zonati”; b Inclinometric cumulative displacements in the June 2009–July 2011 period

32 New Interpretation of Lemeglio Coastal Landslide (Liguria, Italy) 229

The most significant displacement rate from PSinSARdata can be linked to the degree of damage on buildings(Frattini et al. 2013).

We therefore hypothesize than the coastal landslideperimeter is larger than previously known: according to ourdata, the Lemeglio landslide actually encompasses the ridgethat hosts the old village (where no bedrock crops out andmany buildings are damaged) and—to the north—the leftside of Fosso del Mandola catchment, almost up to theconfluence with the Bisagno creek (Fig. 32.4b).

In agreement with the presence of NW-SE striking (aboutN130) neotectonic faults with normal kinematics and othermorpho-tectonic elements, we can ascribe a part of the largescale landslide to a rock block slide or a rotational sagging(Hutchinson 1988).

32.4 Conclusions

All the surveys carried out in this study allowed to obtain acharacterization of the Lemeglio landslide, and the moni-toring results have led to the landslide dynamics assessment,related to the water table variations. The sea wave actioncontributes to weaken the stability of the slope close to theseaside that’s also exposed to the marine erosion.

Fig. 32.3 a Lemeglio landslide monitoring instrumentations (I = incli-nometer, P = piezometer), mean displacement cumulate vectors andwater table range levels (m below g.l.). The dashed line indicates the

landslide limit referred to Fig. 32.1c. b The PSinSAR target andanomalous area based on ERS and ENVISAT satellites (mm/y)

(a) (b)

Fig. 32.4 a Lemeglio area in a 1815–1823 map (Stati Sardi diTerraferma); b shaded relief map based on the DEM performed withLidar survey with a 1 point/m resolution: the dashed line indicates thelandslide limit by basin master plan, the dash-point line indicates thenew delimitation proposed in this study

230 F. Faccini et al.

The Lemeglio slope instabilities include different phe-nomena, from extremely slow deformations to rockfalls.Large landslides like the one we studied can evolve intorapid mass movements, threatening life and impactingengineering structures. The geotechnical and hydrogeologi-cal monitoring is therefore essential; we moreover hope for acoordinated action aimed at arranging a continuous dataacquisition and at defining early warning thresholds.

In view of the fragile geologic-tectonic setting of the area,of the slope topography, of the climatic features and of theman-made activity, we believe that a plan of structuralactions to protect the landslide toe from sea wave storms andto restore the surface hydrographic network is essential.Indeed, the comparison from the historical and the actualmaps highlighted the disappearance of some streams alongthe landslide and the morphological changes of the seasideslope in the last 200 years (Fig. 32.4), whereas hystoricalsources testify to the removal of rock blocks along thecoastline to build breakwater piers.

References

Almagià R (1907) Studi geografici sulle frane in Italia. L’Appenninosettentrionale e il Preappennino Tosco-Romano. Memorie dellaSocietà Geografica Italiana, vol XIII, 343 p

Fanucci F, Nosengo S (1979) Rapporti tra neotettonica e fenomenimorfogenetici del versante marittimo dell’Appennino ligure e delmargine continentale. Bollettino della Società Geologica Italiana96:41–51

Frattini PR, Crosta GB, Allievi J (2013) Damage to buildings in largeslope rock instabilities monitored with the PSInSAR™ technique.Remote Sens 5:4753–4773

Hutchinson JN (1988) General report. Morphological and geotechnicalparameters of landslides in relation to geology and hydrogeology.In: Proceedings of 5th international symposium on landslides,Lausanne, vol 1, pp 3–35

Istituto Superiore per la Protezione e la Ricerca Ambientale andRegione Liguria (2012) Carta Geologica d’Italia alla scala 1:50.000,Foglio 232 Sestri Levante

Marinos P, Hoek E (2001) Estimating the geotechnical properties ofheterogeneous rock masses such as flysch. Bull Eng Geol Environ(IAEG) 60:85–92

32 New Interpretation of Lemeglio Coastal Landslide (Liguria, Italy) 231