Premise

The inhabited center of Craco, a town of ancient medieval origin, was totally destroyed and abandoned due to the inexorable progress of a landslide, which developed in its south-western part between 1959 and 1972.

A detailed study, carried out later through geological and geomorphological surveys, geotechnical analyzes and stability checks, highlights the main stages of the progressive destruction of the country, accelerated by the construction of expensive consolidation works, carried out with a lack of prior geological evaluation of the phenomenon to be countered. .

The village of Craco is located on a series of hilly undulations that reaches the maximum elevation of 390 meters above sea level and separates the valleys of the Bruscata and Salandrella streams.

The upper part of the relief is composed of conglomerates of the Lower Pliocene in transgressive support on varicoloured clays of the Cretaceous-Eocene age.

This most ancient succession is in tectonic contact with Pliocene blue clays emerging in the medium-low part of the slope.

Sezione

Three main landslides affect the SW flank of the relief. These are three rototranslational slides with detachment edges set in the conglomerate plate, sliding surfaces developed mainly in varicolorous clays and foot areas falling in correspondence of the fault escarpment that separates the Pliocene blue clays from the varicolor clays.

Of these three landslides, with very similar mechanisms, the one in the central position was instrumental in the destruction of the historic center of Craco.

The morphological evolution of this landslide is only partially known over the last century.

Landslide History

The effects of the landslides of the terrible earthquake of 1688, with macroseismic intensity of the 10th degree of the Mercalli scale and epicenter in the Craco-Pisticci area are not known.

However, it is very probable that the extent of the damage assessed was largely determined by the activation or reactivation of numerous landslides, including those of Craco.

The first certain news of landslide activity dates back to 1888 when, to safeguard the current SS 103, an arched bridge was built with seabed structures resting at a depth of 18 meters. The chronicles report a drop of 20 cm, which occurred immediately after the construction of the work.

There is no news of the landslide activity until 1931 when a worsening of the landslide movement caused extensive fractures to the structure carried out and reinforcement and defense operations were completed.

In 1954, after a long period of apparent inactivity of the landslide, a football field was built at the landslide terrace downstream of the state road.

This initiative entailed leveling operations and dangerous artificial filling of depressed areas.

In November 1959 an exceptional rainfall, exceeding 400 mm, measured on five consecutive days in the nearby Pisticci rain station, caused the total reactivation of the landslide movement. This caused the destruction of the football field, a new location of the road bridge and the first cracks in the houses upstream of it.

Starting from this episode, exacerbations of the movement were repeated in December 1963 and in January 1965, when the situation in the inhabited center began to become critical.

In fact, the 1888 bridge, which was consolidated several times, underwent a one-meter shift with a contemporary drop of two meters and damage which rendered it unusable.

Many houses upstream of the structure had to be abandoned due to the strong landslide activity between 1963 and 1965.

The growing dramatic situation provoked on one hand a partial decree of transfer of the country, on the other the substantial financing for the construction of new stabilization works. On the basis of geognostic surveys, including 10 continuous coring surveys, of the useless depth between 8 and 23 meters, a project of total demolition of the old retaining wall and the construction of a new wall, consisting of a platform of reinforced concrete 4 meters wide and 60 meters moon, based on side-by-side reinforced poles with a diameter of 800 mm. The poles were arranged with a length such as to overcome the conglomerate and degraded parts and to be registered in the varicoloured clays, it is supposed on the basis of the stratigraphy of the surveys.

The geological model used to design this work is unknown.

Immediately after the construction of the new retaining wall, in April 1971, there was an almost total collapse of the inhabited area, with the opening of large detachments, reaching the highest portions of the inhabited area.

The new retaining wall underwent a drop of 2 meters and a block translation of more than 3 meters, while a drop of 15 meters of the ground downstream caused the structure to be stripped.

The studies carried out after the event allow to make some considerations on its main evolutionary phases.

The construction of the bridge in 1888 did not cause any particular alterations, probably due to the relatively light and at the same time permeable arch structure: this seems to be confirmed by the very reduced landslide activity until 1950.

After this long period of quiet, the construction of the football field and the consequent bridging operations were particularly harmful for the progressive weakening of the slope. These operations combined with the exceptional rains of 1959 caused the development of intense landslide activity between 1959 and 1965.

Source: “The defense of historic centers threatened by landslides: Craco an experience not to be repeated“, Mario del Prete, Unibas.

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