IL RUOLO DELLA GEOMORFOLOGIA NELLA TUTELA DELLA TORRE PENDENTE DI VERNAZZANO (UMBRIA)

Abstract

C. Cattuto, E. Gambini & L. Gregori, The role of geomorphology in the conservation of the leaning Tower of Vernazzano
(Umbria). (IT ISSN 0394-3356, 2007).
In the estern side of the Trasimeno Lake, near the confluence between Tegone and Rio Streams, lie the ruins of a medieval castle of
which only the guard tower remains almost intact. Because of its visible inclination this mighty architectural feature is known as the leaning Tower of Vernazzano.
The castle premises (houses, churches, walls, etc.) have been progressively affected and destroyed by the landslide processes associated with the deep and rapid erosion of the Tegone and Rio streams within the thick layer of “Arenarie del Trasimeno” sandstone, surfacing as highly fractured layers and beds of sandstone and calcarenites.
The fast evolution of the slopes has reduced the large foundation plane of the Castle to a small surface, delimited by a high vertical
escarpment continuously reduced by rockfalls along its whole edge.
On top of the above surface activity, deeper and more complex landslide processes are present which – together with the rotational
displacements in the rock – have been affecting the Nort-Eastern part of the relief supporting the leaning Tower.
Downstream of this structure, even though at a significant distance, lies a disordered pile of stones, eradicated trees, and layers and
blocks of rock sloping down into the bed of the Rio torrent. The movement of this aggregate doesn’t occur as a whole, but happens
through rockfalls and landslides occurring at different moments and locations. This alternates with periods of comparative stability, to
the point that a path leading to the Tower lies over the pile, even though it often gets interrupted or dragged downstream.
This landslide event appeared localized and ascribable to the stream erosion at the base of a detritic cone. The geomorphologic study
of the area, instead, has shown that the landslide is the surface manifestation of deeper and more complex landslides that can be
regarded as rotational displacements in rock. Such displacements have eventually reached the foundation zone of the leaning Tower,
triggering planar sliding phenomena which led the Tower to lose its verticality, leaning in the directions of the strata.
As a first-aid measure to prevent the collapse of the Tower, an anchorage system consisting of a steel cage and tie rods was deployed,
but in the future it will be necessary to work on the (still unknown) foundation plane of the Tower, on draining water from the entire surface and on reducing the erosion at the base of the slope.