ABSTRACT
The geomaterial in engineering design rarely shows uniform distribution of its properties and behavior. Geotechnical investigation does not detect all possible design parameters accurately and some uncertainties always remain. In such conditions, preliminary design with simplifying assumptions, and detailed design using observational methods during construction, are recommended. Back analyses are very powerful tools for interpreting the results of field measurements. They should be used not only to determine material properties but also to generate a mechanical model of soils and rocks. A brief review of back analysis procedures is presented, including comparisons, problems, recent advances and further development. A classification of different back analysis methods, considering those deterministic and non-deterministic aspects applicable in geotechnical engineering problems, is proposed. The application of observational methods to a large urban tunneling project is illustrated as a case study. The importance of geotechnical/structural/geodetic instrumentation as a practical engineering tool for systematic monitoring of tunnels and buildings in urbanized areas is shown, together with details of how the Niayesh tunnel monitoring plan, taking into consideration all requirements, was created and implemented. Based on the monitoring results, probable alerts, possible countermeasures and several design optimizations were identified. In sequential excavation methods, for reasons of safety and cost, it is essential to fully understand the influence on tunneling performance of both a given excavation sequence, and the trailing distance and face-advancing sequences of different excavation stages in soft ground urban tunneling. Different excavation sequences employing the side drift method were planned and modeled using a three-dimensional finite element method and the optimal excavation sequence was selected. Finally, the trailing distance between different excavation stages was analyzed numerically and the optimal distance for minimum surface settlement was determined.
