Special Design Methods for Construction of Tunnel Portals in Areas with Extremely Steep Morphology

摘要

This paper describes the design methods and the construction techniques applied at two (2) tunnel sites in Greece, where the highway alignment runs almost parallel to very high and extremely steep mountainous cliffs and slopes in a seaside area. The project belongs to a three-lane national highway tunnelling system, approximately 50 km outside Athens. The rock mass consists of slightly to intensively cataclastic limestones, conglomerates and tectonic breccia. The tunnel portals are located almost nearby the existing national highway, connecting Athens to Peloponnese. Certain safety issues had to be applied with regard to the evidence of rockfalls and the structural stability considerations of the existing mountain slopes, due to tunneling under low side cover conditions at cross-sections up to 240 m~2 (approximate height 14 m and width 19 m). Special design issues had to be applied due to the presence of an active seismic extended fault zone passing through the tunnels. Any effort of performing earthmoving works and open-cut excavations was impossible due to the steep morphology of the ground surface in combination with the existing environmental restrictions. Prototype and innovative design methods for special construction of reinforced concrete structures (RCS and RCSP) for forming adequate side cover feature for the tunnels were employed. Specific designs for filling the internal tunnel area with special box shaped polysterine material were adopted. Extended numerical modelling and significant detailization of the construction sequence were developed in order to secure the tunnel advance, the close monitoring and control of all movements of the existing structures and the surrounding rock mass of the tunnels, as well as the deformation response of the concrete structures due to tunnelling.