Các tính năng cần thiết của NMT (Barton et al, 1992). . 1. Các tính năng Các lĩnh vực ứng dụng thông thường: overbreak cho đá nối, kết thúc khó khăn hơn về quy mô (qc = 3 để 300 MPa) Clay khu mang, căng thẳng slabbing (Q = 0,001 đến 10 hoặc nhiều hơn). | Norwegian method of tunnelling 135 Table Essential features of NMT Barton et al. 1992 . . Features 1. Areas of usual application Jointed rock giving overbreak harder end of scale qc 3 to 300 MPa Clay bearing zones stress slabbing Q to 10 or more 2. Usual methods of excavation Drill and blast hard rock TBM hand excavation in clay zones 3. Temporary rock reinforcement and permanent tunnel support may be any of the following CCA S fr RRS B B S fr B S B S fr S sb NONE Temporary reinforcement forms part of permanent support Mesh reinforced shotcrete not used Dry process shotcrete not used Steel sets or lattice girders not used RRS and S fr are used in clay zones and in weak squeezing rock masses Contractor chooses temporary support Owner consultant chooses permanent support Final concrete lining are less frequently used . B S fr is usually the final support 4. Rock mass characterization for Predicting rock mass quality Predicting support needs Updating both during tunnelling monitoring in critical cases only 5. The NMT gives low costs and Rapid advance rates in drill and blast tunnels Improved safety Improved environment Notations CCA cast concrete arches S fr steel fiber reinforced shotcrete RRS reinforced ribs of shotcrete B systematic bolting S conventional shotcrete sb spot bolts NONE no support needed. DESIGN OF SUPPORTS The Q-value is related to the tunnel support requirements with the equivalent dimensions of the excavation. The relationship between Q and the equivalent dimension of an excavation determines the appropriate support measures as depicted in Fig. . Barton et al. 1974 have identified 38 support categories Fig. and specified permanent supports for these categories. The bolt length l which is not specified in the support details 136 Tunnelling in weak rocks can be determined in terms of excavation width B in meters using the following equations of Barton et al. 1974 . l 2 B ESR m for pre-tensioned rock bolts in roof
