ABSTRACT
Costanera Norte is an urban expressway concessionary in Santiago de Chile connecting its Western and Eastern sides. To enhance this connectivity several measures were necessary, including four new tunnels. One of these tunnels, is ‘Tunnel Lo Saldes’, a three-lane one that has a total length of 65 m, and an excavation rectangular section 14 x 7.2m. It has been excavated in anthropic materials, colluvial deposits, and hard tuffs. But the main singularity of the tunnel comes from its extremely low overburden, 2m to the surface, were an existing operative motorway is located. This low overburden has demanded a special construction method, based on two lateral side-drifts and a central one, a pre-grouting sequence and forepoling, and in some cases, a prior grouting of the ground and/or a face reinforcement with fibre-glass bolts. The support consists in wide flange steel ribs spaced 0.75m and 60 cm of shotcrete Sh35.
