Geotechnical Study Area G22 Grottammare Landslide, Adriatic Coast, Central Italy
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Geotechnical Study Area G22 Grottammare landslide, Adriatic coast, central Italy GEOTECHNICAL STUDY AREA G22 GROTTAMMARE LANDSLIDE, ADRIATIC COAST, CENTRAL ITALY Plate G22 Panoramic view of Grottammare slope profile, Adriatic coast, Italy 1. INTRODUCTION The town of Grottammare (Province of Ascoli Piceno, central Italy) is a well known example of a coastal landslide system (Plate G22). Cancelli et al. (1984a; b) investigated this landslide among those found along the Italian Adriatic coast between Pesaro and Vasto: “the hypothesised landslide model is a composite one, with movement taking place along translational slip surfaces and a progressive failure mechanism within the overconsolidated clays cropping out at the foot of the cliff”. Antonini et al. (1993) considered this mass movement a ‘roto-translational slide’: “landslides with movement along a complex rotational or translational surface of rupture, slightly undulated at places, corresponding to lithological, structural or tectonic discontinuities”, and identified the landslide crowning and accumulation zone. 2. GEOLOGY AND GEOMORPHOLOGY Geological and geomorphological investigations carried out at a 1:5000 scale, also with the help of aerial photographs (both on a small and large scale; the latter being particularly useful considering the remarkable extent of the phenomenon), revealed the presence of another four large coastal landslides, Figures G22.1 and G22.2. These movements, extending over a surface of 20 to 80 hectares, are classified as complex landslides, that is characterised by a 1 Geotechnical Study Area G22 Grottammare landslide, Adriatic coast, central Italy main deep roto-translational slide affecting the bedrock clays, with rock and debris falls from the steepest and highest portion of the scarp, which is made up of sand and conglomerate; superficial slides affect the detrital cover, see Figures G22.8 and G22.10. Deep-seated movements have caused the formation of headlands which have stretched out from the coastline for several hundred metres. Sea currents easily eroded them within a few decades, redistributing the material along the coast. These mass movements are quite similar to the coastal landslides affecting south-eastern England (Miramar landslide, Bromhead 1986; Folkestone Warren, Bromhead and Hutchinson 1991). Figures G22.3 and G22.4 show historical maps of the Grottammare coastline, and development of the town. In the area of Colle delle Quaglie (Figure G22.5 and G22.9) these headlands are documented on maps going back to the 18th century (Various Authors 1989). The large boulders, made up of conglomerates and well-cemented sandstones which are found emerging from the sea or submerged near the coast, are the remains of landslide bodies dismantled by wave action rather than blocks which have fallen from the top scarps, as inferred by some authors. Even in the area underlying the ancient hamlet of ‘Vecchio Incasato’ two large rock blocks were found (mass of several thousand of cubic metres); they were dismantled in the last century following the construction of the Adriatic railway (‘Sasso Piccuto’ and ‘Sasso San Nicola’). The presence of landslides is historically documented in the Colle delle Quaglie area; they resumed their activity several times and are regarded as deep-seated movements of the kind previously described (Mascaretti 1851; Paoli 1848; Perozzi 1928; Silvestro 1996). See also Figure G22.6. Deep-seated movements are totally congruent with the models proposed by Bromhead (1979) which take into account the importance of the stratigraphic attitude and slope hydraulic conditions. According to the same author, in the case examined here the presence of more resistant and better drained materials at the top of the cliff (gravels and sands overlying clays) allows deep rotational landslides to take place involving the basal clays. 3. CURRENT SITUATION As regards the state of activity, these processes should be considered as dormant, according to the definition reported in the UNESCO International Landslide Glossary (R.I.G. 1995): “dormant landslides which may be reactivated by their original causes”. The definition “abandoned slopes” reported by Esu and Grisolia (1991) for these coastal slopes (owing to the regression of the coastline and the presence of protection works and transport infrastructure), does not exclude the reactivation of slope movements. This may occur as a consequence of changes of hydraulic conditions within landslide bodies, that is following slow re-equilibrium of pore-water pressures and consequent decrease of shear strength in soils, and also due to creep, softening and weathering processes (Lanzo 1991; Bromhead and Dixon 1984; Bromhead 1986). Reactivations of deep movements, with regression of the landslide crown up to some tens of metres, are historically documented along the coastline north of the town of Grottammare, between the S. Lucia and Acquarossa streams. The landslide affecting Grottammare is rather old, since it first took place before the medieval settlement of ‘Vecchio Incasato’, predecessor of the town. A detailed structural analysis of the crack patterns on buildings clearly shows the presence of slow but extremely deep movements. More superficial landslide movements seem to have affected the ancient wall structures in a more localised way. Rock falls affecting the top conglomerates were in any case the main cause for the abandonment of the ‘Vecchio Incasato’ settlement from the end of the 18th century, with the transfer of the inhabited centre to the underlying coastal plain. With respect to the authors previously quoted (Cancelli et al. 1984a, b; Antonini et al. 1993), a greater complexity and extension has been recognised for the large Grottammare landslide, which shows a strongly asymmetrical body (3-D). Geomorphological investigations have identified the presence of a trench (or graben) which 2 Geotechnical Study Area G22 Grottammare landslide, Adriatic coast, central Italy superficially assumes the shape of an elongated valley, with concave transverse profile. This phenomenon is explainable by assuming a deep-seated model of movement with a translational slip surface. The bedrock clays underlying the terraced alluvial deposits of the 2nd order seem to have shifted in block to the east. To the north, where the top of the slope attains higher altitudes (120m asl), with the sandy- conglomerate deposits superimposed on the clayey bedrock, the failure model is ascribable to a roto-translational slide, with several landslide levels sloping towards the sea. In this area (‘Vecchio Incasato’) the slope profile is similar to that at Colle delle Quaglie. Besides morphological evidence, other clues have been found. At the foot of the slope, downstream of the church of St. Augustin, basal clays outcrop with dip upstream of about 30- 35°. Some deep boreholes (65 to 100 m), carried out within the framework of the consolidation works at ‘Vecchio Incasato’, have identified a stratigraphic attitude with dismembered bedrock blocks progressively lowered and tilted towards the sea. This landslide seems to be rather ancient, as witnessed by the presence of marine erosion of dormant scarps ascribable to a period ranging from Roman times (3rd century B.C.) to the Middle Ages (1200 to 1300), at least according to the evolution model of the Marche’s coastline during the recent Holocene reconstructed by Coltorti (1991). In that period an “active cliff” was present along this coast and, at the mouth of the River Tesino, “the sea stretched inland about 750 m from the present coastline”. The cliff’s coastal margin was therefore formed rather recently, presumably starting from 1400 (Buli 1944), following a massive increase of solid load carried by the rivers. This was caused by intense inland deforestation and other anthropogenic activities. Environmental changes have also caused the formation of the 4th-order river terrace, which is perfectly correlated with the coastal plain (Biondi and Coltorti 1982). Evidence of this recent advancement of the coastline is also witnessed by an inlet downstream from ‘Vecchio Incasato’ which, according to historians (Fabbioni 1997), was an active harbour from Roman times up to the 16th century. More recently, the progradation of the coastal plain is also witnessed by the available maps, which were drawn during the past two centuries. 4. LESSONS LEARNT The geomorphological and geotechnical investigations still in progress aim to prove the validity of the failure model proposed and the possibility of its reactivation. The results of the research should lead to the identification of the highest landslide-risk areas. The aim therefore, following the geomorphological mapping and interpretation, is to produce planning guidance. The town of Grottammare has a long record of instability which has been recorded in books and papers for many centuries. It demonstrates the valuable contribution historical information can provide to landslide investigation. A series of early maps and engravings have illustrated how the toes of past landslides have extended out seaward of the coastline before being removed and re-distributed by coastal processes. Historical information of this kind is just one important element of landslide investigation. The town of Grottammare has been developed over an ancient landslide complex with the old Medieval town located on the steep upper slopes and the more recent development including the main town and residential areas (including high-rise buildings) on the lower tier