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 m² (approximate height 14 m and width 19 m).