There are many examples of TBM tunnels through mountains,or in mountainous terrain,which have suffered the ultimate fate of abandonment,due to insufficient pre-investigation.Depth-of-drilling limitations are inevitable when depths approach or even exceed 1 or 2 km.Uncertainties about the geology,hydro-geology,rock stresses and rock strengths go hand-in-hand with deep or ultra-deep tunnels.Unfortunately,unexpected conditions tend to have a much bigger impact on TBM projects than on drill-and-blast projects.There are two obvious reasons.Firstly the circular excavation maximizes the tangential stress,making the relation to rock strength a higher source of potential risk.Secondly,the TBM may have been progressing fast enough to make probe-drilling seem to be unnecessary.If the stress-to-strength ratio becomes too high,or if faulted rock with high water pressure is unexpectedly encountered,the "unexpected events" may have a remarkable delaying effect on TBM.A simple equation explains this phenomenon,via the adverse local Q-value that links directly to utilization.One may witness dramatic reductions in utilization,meaning ultra-steep deceleration-of-the-TBM gradients in a log-log plot of advance rate versus time.Some delays can be avoided or reduced with new TBM designs,where belief in the need for probe-drilling and sometimes also pre-injection,have been fully appreciated.Drill-and-blast tunneling,inevitably involving numerous "probe-holes" prior to each advance,should be used instead,if investigations have been too limited.TBM should be used where there is lower cover and where more is known about the rock and structural conditions.The advantages of the superior speed of TBM may then be fully realized.Choosing TBM because a tunnel is very long increases risk due to the law of deceleration with increased length,especially if there is limited pre-investigation because of tunnel depth.
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