This paper describes the studies carried out for the geological and geothecnical characterisation of the rock mass and the solutions adopted in the different kinds of works. It points out the most important characteristics of the works. It also refers to the difficulties found during the execution of these studies and to the different solutions adopted due to the high difficulty of access. A more detailed analysis is made of the Encumeada tunnel including some finite element calculations performed. Lastly, it analyses the advantages and the limitations of the solutions that have been adopted in the project and indicates some recommendations.
The sustained development of Madeira Island has been, from among other factors, related to the rational development of its water resources. These are the more important and vital as the underground resources are scarce and limited. It is mainly along the northern coast and in the zones with the highest orography of the island that the rainfall is more significant and is almost continuous all over the year, which makes it possible to feed the water supply systems of the levadas. These levadas systems began to be constructed in the XV century and are closely related with the progressive peopling and farming of the island. The levadas consist of small weirs built on the water lines and of a set of canals, with small inclination and reduced size, which by-pass the topographic obstacles and lead intake water to the population located at lower levels. They are located at different levels and form more than 1 400km of total length (Fig. 1). With the demographic growth of the island the pressure on the available water resources has significantly increased. In order to cope with that increase in consumption, the company IGA that manages these resources, has began to implement an integrated development programme, which relies on the increase in the intake, storage and transport capacities of those levadas.
Table 1 presents the main characteristics of those works (Rosa et al., 2002). These are generally works with a comparatively reduced cross-section, which usually occupy a strip with about 1.5m width, but with significant linear development. The intake structures are located on the water lines, corresponding to small weirs of cyclopean concrete that can be overtopped, which create small water mirrors upstream and which supply laterally the levadas canals. The canal is usually integrated in the body of those structures, which permits its longitudinal continuity. The open canals are made of simple concrete and are usually 0.6m wide and 0.4 to 0.6m high. Close to those canals, there is usually a pedestrian path being, which is the case of the final sections of the Faja do Rodrigues levada towards Ribeira do Inferno, since these were excavated in vertical scarps (Fig. 3). The tunnel stretches have usually an uneven section with 1.6m width and 1.8m height, and have no covering. They only have a primary support in a few highly localised zones.