TBM vs D&B - A difficult choice in mountain terrain - Some geotechnical guidelines

Tunnelling at depth through mountainous terrain poses difficult challenges not just for tunnel boring machine (TBM) drivages but also for application of traditional drill and blast (D&B) methods. Geological problems, when-encountered at significant depth, can prove disastrous, depending on stress state, rock competence and groundwater inflows. Extremes of ground conditions present major contrasts to tunnelling and often demand use of flexible rock engineering solutions. This need is seen as being at variance with the constraints imposed-by the rigidity of TBM design. Traditionally, TBM’s have largely been utilized on projects where the risks associated with deep tunnelling are low. This risk-aversion attitude has generally been due to the perceived adverse consequences of entrapping or damaging a TBM. Geotechnical guidelines are presented here for aiding the choice of tunnelling method and for guiding the selection of appropriate equipment for dealing with the wide range of varying ground support requirements.