Limited information on the ground structure and properties, as well as simplifications in the design lead to uncertainties in assessing ground and systems behaviors. Considering all expected and unexpected conditions in the a-priori design is acceptable only in special cases. For safe and economical tunnel construction the residual risk needs to be managed by appropriate monitoring and a safety management system.
The paper addresses requirements for the geotechnical safety management, as well as state-of the-art monitoring and data evaluation techniques in the context of the observational approach. Experience with safety management systems over the last two decades shows that the risk can be significantly reduced. Case histories are used to demonstrate the potential of proper monitoring and safety management.
Uncertainty is unavoidable in underground projects. It originates from the impossibility to completely investigate the geological and geotechnical conditions, even if considerable effort has been made during the preparation phase. Upscaling of small scale laboratory test results to representative rock mass volumes and simplifications in our models and analyses further decrease the accuracy of the prediction of the ground and system behaviors.
To allow for safe and economical construction of underground structures, sound preparation, as well as accompanying measures during construction are essential.
When applying the observational approach, following key issues shall be addressed in accordance with Eurocode EC 7 (ӦNORM EN 1997–1):
Design concept for determining excavation and support
Determination of safety relevant parameters, including definition of expected behavior and criteria for the assessment of the system stability on the basis of the expected ground conditions
Monitoring concept, allowing a continuous comparison of expected and observed behavior, including all organizational and technical requirements
Management concept for cases where ground conditions and/or system behavior deviate from that predicted, both for favorable and unfavorable deviations.
During construction attention should be paid to following issues:
Appropriate organization of the site, and employment of qualified personnel
Technical provisions for high quality observation of the system behavior
Efficient information flow and reporting