Geotechnical risk assessment based approach for rock TBM selection in difficult ground conditions

There are many potential sources of geotechnical risk in mechanized rock tunnelling. Problems such as encountering fault zones with running and water bearing gouge, tunnel walls instabilities in running or blocky grounds, hard and abrasive rock sections and convergent tunnel sections are principal causes in geotechnical risk occurrence. On the other hand, the performance of each TBM encountering such conditions will be different. Therefore, using different TBMs will have variable risk levels. This paper is to discuss rock TBM selection based on geotechnical risk minimization. So, a new approach was proposed based on decision analysis using decision tree. Based on the newly proposed approach, the most appropriate TBM is one that has the minimum risk level either before or after hazards mitigation measures. To be able to check the performance of this approach in practice, selection of machine for Nosoud water transfer tunnel has been evaluated. A shielded TBM (either single or double shield one) was proposed for the tunnel based on the newly proposed method. However, a double shield TBM was selected because of its more flexibility in difficult ground conditions in comparison with single shield TBM and limitation of project construction duration. The machine performance during tunnelling period verifies the success of excavation using selected TBM. (C) 2007 Elsevier Ltd. All rights reserved.