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 sitesrnin Greece, where the highway alignment runs almost parallel to very high and extremely steeprnmountainous cliffs and slopes in a seaside area. The project belongs to a three-lane national highwayrntunnelling system, approximately 50 km outside Athens. The rock mass consists of slightly tornintensively cataclastic limestones, conglomerates and tectonic breccia. The tunnel portals are locatedrnalmost nearby the existing national highway, connecting Athens to Peloponnese. Certain safety issuesrnhad to be applied with regard to the evidence of rockfalls and the structural stability considerations ofrnthe existing mountain slopes, due to tunneling under low side cover conditions at cross-sections up torn240 m~2 (approximate height 14 m and width 19 m).rnSpecial design issues had to be applied due to the presence of an active seismic extended fault zonernpassing through the tunnels. Any effort of performing earthmoving works and open-cut excavationsrnwas impossible due to the steep morphology of the ground surface in combination with the existingrnenvironmental restrictions. Prototype and innovative design methods for special construction ofrnreinforced concrete structures (RCS and RCSP) for forming adequate side cover feature for the tunnelsrnwere employed. Specific designs for filling the internal tunnel area with special box shapedrnpolysterine material were adopted.rnExtended numerical modelling and significant detailization of the construction sequence wererndeveloped in order to secure the tunnel advance, the close monitoring and control of all movements ofrnthe existing structures and the surrounding rock mass of the tunnels, as well as the deformationrnresponse of the concrete structures due to tunnelling.