BEHAVIOR OF PINDOS FLYSCH DURING TUNNELLING THROUGH A THRUST ZONE. DEFORMATION AND ROCK BEHAVIOR. EXPIERENCE FROM THE ACHELOOS RIVER DIVERSION TUNNEL TO THESSALY.

Abstract

The Acheloos diversion project consists of a series of reservoirs and a diversion tunnel. The tunnel has a designed internal diameter of 6m and a total length of ca. 17 400 m. Through the tunnel waters from the upper Achellos will be transferred to Thessaly. The project area belongs entirely to the Pindos zone. To the west (Mouzaki - Drakotrypa area) limestone, Jurassic chert and transitional strata (limestone and siltstone interchanges) overthrust sandstone and siltstone of the Pindos Flysch association (Paleogene). The thrust plane is well exposed and its geometrical features are clearly defined on the surface. Within silt- and sand- stones of the flysch, the developement of shear zones borders the thrust plane. Tunneling through limestone and chert advanced without specific problems. Tunneling through the flysch sequence slowed down the advancing rate. This was partially due to the composition and structure of the formation. The stand-up time was reduced due to compositional changes and the throughout development of shear zones. Heavy support measures were insalled and immediate monitoring began in the area where the thrust zone was developed. Data analysis and its results show that the rock formation remained into a dynamically active status. Four months after the excavation forces acting at the tunnel perimeter, exceeded the support measures bearing abilty causing tunnel radial convergence and the development of damages became visible. In this paper we describe tunneling conditions, the geology and the response of formations during excavation, as this was interpreted by monitoring data. We describe the damages caused as well as the counter measures applied in order to control and terminate the tunnel convergence.