TUNNEL MONITORING DURING THE EXCAVATION PHASE : 3-D KINEMATIC ANALYSIS BASED ON GEODETIC DATA

Tunnel excavation produces a re-distribution of stresses in the rock and a tendency for closure of the void produced – a tendency counteracted by the tunnel support shell. In some tunnels a very stiff support shell not permitting any ground deformation is selected. In other tunnels, excavated on the basis of the popular so-called “NATM” a quite different approach is adopted: A controlled deformation of the ground is permitted (i.e. limited closure of the rate of approximately 1%) and this gives the opportunity to the stresses to be partly released and a less stiff and less-expensive support system to be used. In this case, an accurate, systematic and continuous monitoring of the tunnel deformation is absolutely necessary during the excavation. In the last years, the introduction of modern total station instruments provided a simple, low cost and functional way to record the absolute 3-D displacements of a large number of controlled points (usually 3-5 points every approximately 10-20m). Such data from a number of tunnels were used to study: i) the distribution of deformation in the various tunnel sections, ii) the variation of maximum section closure along the tunnel axis, iii) the time-history of deformation and iv) the relationship between closures and ring closure delays.