In the ends of the decade of eighty in "Rua da Rocha dos Campos, Água d'' Alto, Vila Franca do Campo", Island of S. Miguel, they occurred several overthrown of slopes, turning very unstable an area of old urban occupation. The studies developed in the sequence of those events, had as objective to find a solution that stabilized the cliff assuring the conservation of the existent main road in the South coast of the island. That road locates about 60 meters of distance of the face of a sharp cliff with about 80 meters height. The approved project consists on a structure composed by a rock embankment to protect the bottom of the cliff from sea abrasion, a support wall in gabions and an embankment with pumice and other pyroclastic material (bagacinas). In the present work we pretend to show the characteristics of the materials used in the execution of the protection and how its application in work was controlled.
Instability of coastal cliffs, especially in volcanic islands, it is a complex problem. That instability is natural and it is due to geologic features, namely thick deposits of pyroclastic material of projection and flow or alternation of pyroclastic deposits and lava flows, which they originate structural heterogeneities that in any of the cases give place to processes of differential erosion. Those processes associated to regimes of intense precipitations it provokes frequent mass movements. Besides, in the islands of Azores, the urban occupation locates close to the coastline. Because of that, almost in each new rupture of coastal slope, appear new residences in situation of serious risk. Thus, lots of times works of coastal protection and slopes stability are inevitable to preserve the existence of some communities and even to assure its survival.
The place of the intervention is a small marine bay with a beach, defined by cliffs with heights of the order of the 80 meters. In geological terms the cliffs are constituted by pyroclastic deposits standing over lava flows. In the bottom of the cliff, especially on the nascent side, they are visible rocky blooming of trachyte flows, and over these there are a thick pyroclastic deposit of trachyte pumice that includes pyroclastic flow, pumice-fall, mudflow deposits and trachyte flows interbedded with lacustrine deposits.
Since the works would be executed in the bottom of the cliff, with access just only going on foot or with track machines, and the material would be spilled down from the top of the cliff, it was necessary to evaluate the risks associated to the intervention, particularly those ones that could affect the workers during the execution of the work. The main appraised risks are link with the probability of mass movements as sliding or overthrown in the cliff, what took to the preparation of an evacuation plan foreseeing the escape possibility and evacuation by the sea.