ABSTRACT
This paper presents the concept, the design challenges and the risk mitigation measures of underground construction of a metro station directly underneath unexpectedly encountered archaeological findings. The construction of the station under consideration was planned to be executed with the so-called top to down construction method (cover and cut) with the use of permanent diaphragm walls and horizontal slabs supported directly by the walls. During the excavation after construction of the diaphragm walls, the lower part of a Byzantine basilica Church with an inlay mosaic floor was discovered, underneath the top slab, 5 m below the ground level. The Contractor was instructed to change the design and the construction plan in order to secure and support the significant archaeological findings. An array of six intersected tunnels of diameter about 2.5 m and length of 21.75 m was proposed to underpin the findings. The sequence included three primary and three secondary tunnels, excavated conventionally, lined with shotcrete and filled with reinforced concrete to form a permanent horizontal slab about 1.8 m below the foundation level of the findings. Afterwards the excavation continued unhindered underneath the slab up to the final level and the original construction concept applied.
