It is common knowledge that TBM are remarkable machines. It nevertheless takes mental effort to accept that they can have world record 1 day, 1 week and 1 month tunnel advance as high as 172m, 703m and 2163m. However, the best monthly project averages for the usually smaller 3m to 6m diameter machines which have delivered these incredible records are ‘only’ 1.1 to 1.3 km, not anywhere near 2km. In other words, tunnel length and time take a toll, due both to geology, hydrogeology, and machine-related delays. Unfortunately, the other side of the coin has seen more than a few TBM that remain buried in mountains forever (needing drill-and-blast completion), or they are delayed for many months or even years. This huge range of performance demands a lot from models of TBM prognosis: the range of tunnel advance may vary over four orders of magnitude from 0.001m/hr (i.e. stuck in fault zone) to an occasional 10m/hr penetration rate. Because of the huge range of an empirical parameter QTBM, this range is possible to encompass, and is founded on the 0.001–1000 Q-value range, plus particular emphasis on comparing cutter thrust and rock mass strength, 1 to 100MPa.


TBM prognosis models must be capable of explaining less than 1m/hr penetration rate (PR) in hard massive rock, perhaps short-term 10m/hr penetration rate (PR) in softer jointed rock, but only 0.01 m/hr average advance rate (AR) when severely delayed in fault zones. The implied eighty meters during one year, struggling to get through a faulted zone is clearly close to the limit of acceptance. Values an order of magnitude lower than this mean virtual burial. So we see in these figures a ten thousand-fold variation (fastest 10m/hr, slowest 0.001m/hr) that needs a geo-technical, quantifiable explanation.

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