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INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. 7 /B /2 Some effects on natural slope stability induced by the 1980 Italian earthquake* Conséquences du seisme de l'Irpinia de 1980 sur la stabilité des talus naturels B. D'ELIA, Professor of Geotechnical Eng., University of Roma, Italy F. ESU, Professor of Geotechnical Eng., University of Roma, Italy A. PELLEGRINO, Professor of Geotechnical Eng., University of Naples, Italy T. S. PESCATORE, Professor of Geology, University of Naples, Italy SYNOPSIS. On November 23rd, 1900, a strong earthqua ke in Irpinia triggered numerous landslides and déf ormât ^ onal phenomena.in natural slopes composed of rocks, clays or structurally complex formations such as a re widespread alo ng the Apennine chain. With the exception of rock f alls, the movements involved large ancient, slide b odies and because they have a low margin of safety these formations a re periodically mobilized by the earthquakes that s uccessively affect the area. This paper deals with three typical cases , concerning two large landslides and the deformati on of a hill, wich involve stiff overconsolidated clays or struct urally complex formations where clay is the predomi nant component. The main features of the movements and the results of geotechnical surveys and direct surface observat ions emphasi ze the complexity of the phenomena which can beonl y explai ned on the basis of hypotheses concerning the stres s fields and the if evolution in time. I NTRODUCTI ON GEOLOGI CAL SETTI NG AND SEI SMI CI TY OF THE AREA Earthquakes are known to be a triggering cause of l andsli^ The earthquake of the 23rd November, 1980, (M = 6.5 ) with des. In clayey or detr.ital soils, where liquefacli cn must intensity VI-X MSK scale involved a broad stretch o f the Sou be totally ruled out, the failure mechanisms causin g the thern Apennines, over a 100 km long in the NW-SE di rection landslides are far from being understood. and about 50-60 km wide in the SW-NE direction; thi s is a The phenomena observed for these soils vary widely bot h high seismic risk area where earthquakes having int ensity in terms of type and magnitude of ground movements. X-XI degrees have occurred frequently over the last 500 The literature is rather poor in papers dealing wit h ea£ years (1456, 1688, 1694, 1702, 1732, 1930). The mai n shock th quake induced slides in clayey soils; for Italy, some of the 1980 earthquake lasted about 50 s for accele rations historical data are available on slope movements in clayey > 0.05 g, and about 80 s for accelerations > 0.01 g , and soils (Cotecchia et al.,1969). However during this centii was followed by numerous aftershocks. ry, even though very severe earthquakes have occurr ed A simplified sketch of the geolithologic and tecton ic cha (Messina, 1906; Avezzano, 1915; Irpinia, 1930 and 1 962) racteristics of the region is shown in Fig. 1. landslides in clay slopes have neither been detecte d nor The southern Apennine consists of three main zones in sue studied. cession from W to E : On November 23rd, 1980, the earthquake in Irpinia t rigg^ The Apennine Chain is formed of various "nappes" red off numerous landslides and deformation phenome na in with highly differentiated lithostratigraphic natural slopes formed of clays or structurally comp lex so composition. The main geological units (Mesozoic- ils where clay is the predominating component. Cenozoic) consist of monotonous sequences of car Given the general lack of knowledge of such phenome na,in bonatic rocks, 4000-5000 m in thickness, and of the framework of the effects induced by the 1980 ea rthqu sequences of "structurally complex formations", ake, three typical cases were selected as a pilot s tudy: over 1000 m thick. The tectonic events that o£ two large landslides and the deformation of a hill consi ^ curred inside these allochthonous units induced sting of varicoloured clays, covered by a thick con gl om£ very high fracturing in the rocks and minute fla rate slab. king in the clays. To the north-west of the cha The study approach focused mainly on surveys and di rect in there are areas of tectonic sinking, where surface observations. Though there are some uncerta inties volcanic apparatuses developed. due to the lack of data on the situation prior to t he ear^ thquake, this study does give some indications for explai The Bradano Foredeep is filled by terrigenous se^ ning the complex processes triggered inside natural slo­ quences (Plio-Pleistocene) where clayey and cla_ pes by seismic actions. yey-sandy soils prevail. These are authoctonous * The research has also seen the participation of: S.Di Nocera, G.Iaccarino, F.Rippa for the geologica l aspects; I.Cotti, G.Federico, G.Fenelli, M.Paparo Filomarino, L.Picar elli, G.Tancredi for the geotechnical aspects; G. La Barbera, C. Nuti for the dynamic aspects. 1 9 4 3 7 /B /2 highly overconsolidated marine clays with a high Ca C 0 3 c o ntent ("blue clays") and by conglomerates. The blue clays are characterized by synsedimentary discontinuities and by joints that are generally closed. The conglomera tes a re composed of round-shaped gravels and silty sands . Due to variations in degree of cementation their mechan ical properties range from those of soft rocks to those of in­ coherent deposits. The structurally complex formations are classed as follows (AGI, 1979): - arenaceous-pelitic and calcareous-pelitic formati ­ ons; the structure, orderly in some places and dis­ arranged in others, is formed of alternating layers of weak to hard rock and highly indurated clay;the lapideous component is always fractured; the clay­ ey component has a typical scaly or fragmented st Fig.l Geolithologic scheme of southern Italy. rueture; 1. Volcanic and sedimentary deposits (Quaternary); 2. Plio-Pleibtocoiiic deposits of the Bradano Fored£ - mainly pelitic formations with a typical scaly st ­ ep? ructure, with a small amount of rock fragments (li^ 3. Terrigenous deposits of the Apennine Chain mestones, sandstones, marls). (Structurally complex formations); The mechanical behaviour of such formatiors depends on the 4. Carbonatic rocks of the Apennine Chain (a) and o f ir macrostructural features, on the rock-clay ratio , and the Apulian Foreland (b) (3 and 4: Mesozoic and on the mechanical characteristics of the pelitic co mpone­ Cenozoic); nt. 5. Eastern boundary of the allocthonous units; The clayey fractions of the two lithologic types, h avi ng 6. Main Plio-Pleistocenic faults; different composition and structure have some commo n pro­ 7. Epicentres of major earthquakes. perties (Pellegrino and Picarelli, 1964): a high co nsisten cy depending on high preconsolidation loads and dia genetic bonds; the presence of a closely-spaced network of discon­ tinuities bounding fragments ranging from a millime ter to soils that were involved only in tectonic uphe^ a few centimeters; a marked physical and mechanical weak aval movement s. ness depending on the structure and index propertie s. - The Apulian Foreland is the most stable area in the Southern Apennine and it consists of a ca£ bonatic rock sequences (Meso-Cenozoic), some 5000 m thick; these sequences are practically LANDSLI DES AND GROUND DEFORMATI ON PHENOMENA TRI GGER ED BY underformed. THE 1980 EARTHQUAKE The mostseismically active zone is the Apennine Cha in,that The earthquake triggered numerous huge flows, slide s and over the last two million years underwent phases of interi complex slides (largest volume > 10 x 10^ m^) in na tural se upheaval at a rate of 1-1.5 mm/year. Such upheav al oc­ slopes, and ground deformation phenomena. Rock fall s were curred through two main sets of faults having a NW- SE and uncommon and of limited extent (Fig.2 and Table I). NE- SW trend. Most of the mobilized slides were old, dormant or a ctive Besides the lithological characteristics, the slidi ng pro slides. In most cases, there was some delay (a few hours neness of hill-slopes is due also to the structures indu­ to a few days) between the main shock and the slidi ngs( Co ced by the tectonic events. tecchia, 1982; D' Elia,1983). Some of these slides h ad been A determining event in controlling slope stability was the set in motion also by the 1930 earthquake. Wurmian glacial phase that reached its peak 18,00(> years a Amongst the many slides that have occurred, two tha t are go, and that depressed the sea level down to 100 ra bel ow quite similar in terms of their geometry and kinema tics the present levels, thus bringing about a deepening of the have been studied; they are also quite characterist ic and main talwegs and instability conditions of the slop es. representative of the earthquake induced instabilit y phe­ The slides alternated periods of quiescence and per iods of nomena . regional and local mobilization. A hill, on top of which rises an ancient small town (Bi- saccia), was chosen to study the deformation proces ses. This case, because of the hill shape and subsoil pr ofile, represents a situation that is rather common throug hout GEOTECHNI CAL CHARACTERI STI CS OF THE EPI CENTRAL AREA central and southern Italy where many old towns wer e built on hilltops.

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