Monitoring of tunnel behaviour through cataclastic rocks

Abstract

Field instrumentation was used in the construction of two twin tunnels for the new Egnatia Motorway in Greece. This paper describes the displacement monitoring results taken during tunnelling through heavily fragmented limestone varying from a block size >10 cm down to sandy gravel and sandy silt or gouge. The primary lining displacements monitored by surveying methods appear to be directly related to the detailed geological subdivisions of the limestone observed at the tunnel face. Maximum values of settlement were recorded during tunnelling through the gouge and reached 80 mm, while horizontal movements up to 40 mm were recorded associated with outward deflection of the support frames. The development of settlement with time shows that settlement of the tunnel crown and sides increased sharply while the face of the tunnel was within a tunnel diameter of the station point under consideration and reached equilibrium when the excavation face was at least 3 to 4 tunnel diameters beyond it. Surface settlements above the axis of the tunnel were first observed when the face of the tunnel was directly beneath the station. At the entrance portals of the tunnels the limestone overlies a thin sedimentary clay formation and gypsum. Horizontal movements along the interface between the limestone and the underlying clay sequence as well as within a clay/gypsum layer at the portals were observed using a standard inclinometer.