Due to uncertainties in the ground model and spread of the geotechnical properties, the prediction of the exact system behaviour of underground structures in many cases is difficult. In order to circumvent this problem the so-called observational approach is applied in many cases. Monitoring plays an important role to verify or falsify the assumptions made during the design, to adjust excavation and support measures to the actual conditions and to assess stability of the system. For the management of the remaining residual risk a so-called geotechnical safety management plan is implemented. Recently the OeGG published a handbook summarizing the state of the art in tunnel monitoring and geotechnical safety management.
The rapid development of monitoring techniques and methods to evaluate and interpret monitoring data has led to a significant decrease of risks during tunnel construction. For many tunnels, the monitoring of absolute displacements has become standard practice. Not always, the potential of the method is properly exploited. Recently the OeGG (2014) published a handbook summarizing the state of the art in tunnel monitoring and geotechnical safety management. Some important items are briefly introduced.
Common to all applications are following targets:
Verification of assumptions made during design,
Adjustment of excavation and support to comply with actual conditions and fulfill the requirements,
Assessment of stability of the system.
Of particular importance for shallow tunnels is:
Continuous control of deformations of tunnel, surface and influenced structures and buildings,