In order to improve the road accesses to Funchal city in Madeira Island, due to its increasing development, an accessibility plan was implemented involving the construction of new roads, through the whole island. The restraints related to morphology, the highly heterogeneous geological character of the volcanic formations and the high urban occupancy rate of the soil, led to significant civil works associated with the lay-out of these new accesses, amongst which a significant number of tunnels, some built in urban areas.
The Santa Cruz tunnels are integrated in the Funchal-Airport Expressway and are located West and East of the Santa Cruz village. These are tunnels with small length (135 m each), large span (8,5 m between walls) and low rock mass cover (20 m), located on highly heterogeneous volcanic formations, where disagregated breccious formations prevail. The minimum effective width between walls of the cross-section, as shown in Figure 1, was established to permit the inclusion of four tracks, two in each direction and a central barrier. The adopted cross-section resulted from severe lay-out restrictions in the crossing of the anta Cruz village, which did not allow the use of the currently adopted solution of two parallel tunnels. The inner section of the tunnels has 149 m2 and the total maximum excavation section had 182 m2. Figure 1 also shows the comparison between the section of the Santa Cruz tunnels with the CREL (Lisbon External Regional Circular) tunnels that also have four tracks (Brisa, 1995) and with one of two-lane tunnels currently used in Madeira island. During the design of t e tunnels cross-section geometry, an attempt was made to ensure the minimum gauge and the good performance of the surrounding rock mass after the excavation, by limiting decompression deformations and stress concentration on localised zones of the section. The roof has a thickness ranging from 0,40 m at the top and 0,60 m in the connection to the walls, which present thickness, ranging from 0,60 m to 0,85 m, being braced at the invert level by footings connected by a slab, with thickness ranging from 0,85 m to 0,30 m, respectively. The semi-elliptic configuration of the roof and the upper zone of the walls allowed the use of the same curved framework for casting the lining of tunnels throughout their entire length. The Santa Cruz tunnels were built based on the observational method, which permitted to save both in costs as in construction time and, simultaneously, ensure safety at work. Therefore, a complementary survey programme was established, consisting on boreholes and on cartography of the excavation fronts. Furthermore, an observation plan was defined, in which the reading and analysis of the instrumentation results in effective time has played an extremely important role in the design adjusting to the effective conditions of the rock mass.
The lenticular and layered structure of these formations, as well as the sometimes abrupt lithologic variations, provide the rock mass with a fairly heterogeneous character.