Geomechanical characterization, 3-D optical monitoring and numerical modeling in Kirkgecit-1 tunnel, Turkey

• Geomechanical characterization as backbone of tunnel design approaches
• Displacement magnitudes and angles predicting relative changes in stiffness
• Enhancing value of monitoring as predictive tool by decreasing intervals
• Numerical modeling as independent frame for data analysis and support decision

The geomechanical characterization of a twin-tube tunnel during exploration and excavation stages is presented and used as a base (a) to interpret the monitored long-term displacements, (b) to numerically investigate if the displacement pattern at a given section could be predicted using an equivalent continuum model and (c) to assess adequacy of the NATM recommended excavation method and support systems. The pre-excavation rock mass classification by RMR and Q systems utilizing logs of two exploration boreholes lead to reasonably similar predictions of the as-built rock mass quality.The resultant displacement magnitudes and angles are interpreted with reference to the as-built geological cross-sections of the tunnel. All displacement patterns monitored at five posts in each section are consistently effective in providing advance warning for the presence of zones of contrasting conditions despite very low magnitudes of displacements. Numerical simulations predict no imminent danger of stress-induced instability and that the twin tubes could stand unsupported. A light support system as recommended by NATM (and similarly by RMR and Q systems) is necessary to prevent structural instability, loosening and overbreaks. The numerical analysis using the core-softening approach also exemplifies the sensitivity of tunnel convergence behavior to moving position of the excavation face, and consequently supports the combined use of the displacement vector magnitudes and angles to predict the presence and to appreciate the magnitude of changes in rock mass stiffness ahead of the excavation.