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From the tunnelling perspective, the Himalayas arguably pose the most challenging ground conditions almost anywhere in the world. One of the prime reasons for this is that they are the youngest of the mountain chains. They are demonstrably rising faster than anywhere else. Their composition is also younger generally, and in consequence less well consolidated than all of the other older fold belts. This is consistent with the fact that they constitute one of the most active of the plate margin zones, rising at a rate that is almost double that of the Andes, which, in turn is almost three times that of the Alps. Almost nowhere else, on a world scale, except around the Pacific Ring of Fire, is even on the same active “stress” scale. As insitu stress levels are to a large extent geologic age dependent the younger the mountain belts the more imbalanced is the stress state. Even though impressively rugged, the Canadian Rockies and West Coast Ranges of North America, the Urals of Central Russia, or the coastal ranges of Norway are largely benign from a stress imbalance perspective. As a result, stress conditions (magnitude and variation) can potentially be more extreme and adverse on a Himalayan tunnelling project than even has been encountered in some of the worst sub-mountain tunnel drives (including the Olmos and Yacambu Tunnels of Peru and Colombia respectively, which are landmark projects from the bursting and squeezing perspective). On a ranking scale, these Andean tunnels traversed much worse ground conditions and arguably met greater geotechnical challenges than were encountered anywhere along the 50+ km length of the Lötchberg and Alp Base deep tunnel drives, echoing the fact that the level of active out-of-balance deviatoric stress state experienced by these tunnels is likely mountain range dependent. One can thus postulate a tunnelling difficulty ranking scale for the mountain  chains of the world, viz., – #1 the Himalayas, arguably the most difficult and challenging, through #2 the Andes and #3 the Alps, through to #4 the least difficult of the main chains, the Rockies and the Western Cordillera, with #5, 6, 7 corresponding to older and older mountain cores – with the Archean Canadian and Scandinavian, Algonquin and Adinondak age mountain belts being almost totally benign stress-wise. This is not to say that there aren’t adverse faults and challenging zones of poor ground in even these old mountain range areas. The dominant difference relates to stress state. 
Hydrovision India 2011, SESSION 5c: (Risk Management in Tunnelling)
Golder Associates, Toronto, Canada
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