Stabilization measures for shallow tunnels with ongoing translational movements due to slope instability

Egnatia highway links Igoumenitsa, the biggest port of North-West Greece, with Kipi, the eastern border of the country. Tunnels, 75 in total, comprise an important portion of Egnatia highway. S3 is a shallow twin bore slope tunnel with average length of 230 m and width of about 12 m located in the Northern Greece, in the middle part of Egnatia highway. The tunnel passes along a hill slope characterized of mild inclination approximately 20 degrees parallel to the tunnel axis, with geological formations of poor quality which consist of phyllites, with a high degree of weathering, that are converted locally to soil like material. Small to medium sized fragments of stronger rock are present. The overall mass has been extensively relaxed with open structural discontinuities in places. During the construction of the S3 tunnel, much higher displacements than those initially expected, cracks and local failures on the temporary support shell as well as on the ground surface upstream of the tunnel, were observed. Geotechnical monitoring indicated that both tunnel bores were moving downhill. This translational movement continued despite the fact that the construction process had already been suspended. In this paper the design methodology for stabilization measures, as well as the construction sequence of these measures is described. A detailed multi-staged numerical back-analysis based on actual geotechnical measurements data and crack observations on the support shell and on the ground surface, was performed, in order to determine the possible sliding shear zone in the vicinity of the tunnel. Shear strength properties of the rockmass elements that failed in plastic yield were properly reduced, until the calculated support shell displacements reached the actual measured displacements. The finite difference code FLAC 2D was used for the simulation of the excavation and support process of the tunnel. The multi-staged back-analysis, consists of 36 simulation steps, including full excavation and primary support of the two bores of the tunnel, application of the stabilization measures and investigation of the seismic effect in the tunnel. The Mohr-Coulomb plasticity failure criterion was used for the simulation of the rockmass. The effectiveness of different types of stabilization measures was investigated by using excessive multi-staged Finite Difference Method (FDM) analysis. The most effective combination of stabilization measures was selected in order to prevent additional ground movements and to stabilize the tunnel. Stabilization measures consist of reinforced concrete piles, pre-stressed permanent anchors and consolidation cement grouting applied on the periphery of the tunnels as well as on the intermediate pillar. The effect of potential seismic loading was numerically investigated. (A). "Reprinted with permission from Elsevier". For the covering abstract see ITRD E124500.