RQD could influence the wear of disc cutters in TBM. Most prediction models for TBM tool life is based on the abrasivity of the rock and empirical relations to tool life. NTNU has several prediction models for hard rock TBM tunnelling, and in the final revision of the model Javier Macias included the thrust of the cutters as an input to the tool life. In fractured rock the thrust level of hard rock TBMs is normally reduced compared to "unfractured" rock. Thus, the fracturing is indirectly a part of the tool life prediction in the NTNU model. I am sure that Javier Macias can ellaborate more.

Hello Pål Drevland Jakobsen and Gabriel Lehmann

The RQD indicates/evaluates the rock mass degree of fracturing (the lower the RQD, the larger the degree of fracturing). Fractured rock masses promote greater penetration rates and thus effectively prolong cutter life (in meters per cutter and cubic meters per cutter).

But, in fractured rock masses, or in situations where extremely good rock chipping occurs, the cutters will be exposed to large instantaneous loads (3.5 – 10 times higher than a nominal/average cutter load – Entacher et al., 2013). Under such conditions, a cutter ring exhibits a tendency to chip along its edge and extensive ring chipping and high cutter thrust may result in bevel edge wear, loosened rings and blocked bearings. Others: additional loads will result in higher abrasion on protruding cutters when the difference in diameter between adjacent cutters is too large as a result of deficient wear height control. Heavy vibration of the cutterhead results in high lateral forces on the cutters, which in turn causes additional abrasion.

Due to the aforementioned effect of fractures, higher levels of cutter consumption, measured in hours, will be expected in fractured rock mass (medium/low RQD). This effect will be more dominant in rocks exhibiting low rock drillability/high strength and high abrasivity. If the degree of fracturing is very/extremely high (i.e. very low RQD near 0), the effect (in h/c) will be neglected – rock mass totally fractured and extremely high rock mass boreability.

There is more information (including a graph indicating this effect) in my PhD thesis (Thesis Hard Rock Tunnel Boring: Performance Predictions and Cutter ...

Gabriel Lehmann , I am open and glad for further discussing the topic.

Kind regards,

Javier

RQD is a parameter what we derive to aid our understanding of rock behaviour. Abrasivity is governed by the inherent mineralogy of the rock , strength and presence of joints, when all other machine parameters are ideal. There may be cases where high RQD has correlated with high abrasivity, it need not have to be the case always.e.g thick bedded shales may give high RQD but low abrasivity.

For wear of TBM disc cutters why you have chosen only RQD as the governing factor?

Gabriel! and NTNU Norwegian/Spanish colleagues! One answer that I have not yet seen as strong as expected is the following. If one has the case of ultra-high RQD, low Jn (few joint sets) specifically combined with high UCS and high quartz content/abrasivity, then a high cutter thrust F will be needed to try to over-come SIGMA - which is a Qtbm estimate of rock mass strength (typical range 1 to 100MPa - we can never know this value - but an estimate is needed). We may then encounter the situation that thrust can be increased (i.e. mean F of 28, 30 even more than 30 tons) yet the penetration rate PR (and of course AR) reduce. There are many earlier examples of this, and they were contrary to popular modelling results. When the above combine to force PR below 2m/hr. even down to 1m/hr it is clear that the TBM will be operating a lot of its e.g. 20 hours with increasingly worn cutters, and 'too many' (maybe 10?) have to be changed in the maintenance shift - so better half way through the day as well when high UCS, high q%, high RQD and low Jn all combine to make Q, Qtbm (and RMR) too high.

Dear Nick ( Nick Ryland Barton),

Hope you are doing well! great to having interesting discussions with you!

I will try to answer and maybe make clearer the rock mass quality implications in cutter life.

As you have stated, in the case of ultra-high RQD, few or no joint sets. (Jn low) resulting in a high Q value, if combined with high UCS (e.g. 200-300 MPa), a very high cutter thrust (up to the recommended limit 31 tons even over limit!) will be needed and yet penetration rate and advance rate significantly reduced.

In addition, if the rock mass presents extremely high abrasivity (e.g. CLI=5 and abrasive minerals content 70%), the cutter life will be significantly reduced (e.g. less than 1 cutter per meter when low penetration rate resulting in ).

The NTNU model uses time dependant for cutter life which is related to rolling distance.

So, from the “abrasivity” parameter (CLI) is estimated a basic cutter ring life in hours (for 432 and 483 mm cutter diameter as the most common in hard rock TBMs).

Then, the basic life is corrected (due to the normalization of the model) by:

- Abrasive minerals content (rock);

- TBM diameter (TBM specification);

- Number of cutters (TBM specification);

- Cutterhead velocity (TBM operation);

- Gross cutter thrust (for high rock abrasivity) (TBM operation).

The result (dividing by the total number of cutters of the cutterhead) is the cutter life in hours per cutter (h/c).

Then, multiplying by the net penetration rate (m/h) is obtained the cutter life in meters per cutter (m/c). An finally multiplying by the cross section, the cubic meters per cutter (m3/c).

So, when high QTBM values (High RMR, Low kekv) and therefore extremely low penetration (m/h), the cutter life (in m/c or m3/c) is significantly reduced when comparing with the same rock (same abrasivity) and obviously machine) but with lower QTBM values (higher penetration m/h) resulting in higher cutter life (in m/c and m3/c).

To summarize, the cutters when high QTBM (high Q-High UCS among others) resulting in low penetration (m/h) will need longer rolling distance to accomplish a certain meters of tunnel than a more fractured rock (lower QTBM) and therefore lower cutter thrust and higher penetration (m/h), even if they encountered same abrasivity conditions (CLI, abrasive minerals content).

Finally, these cutter life variations will have a great impact on advance rate (e.g. m/day), specially for low penetration rates/high QTBM values. Even worse when combined tough boring conditions and 432 mm cutter diameter as Guadarrama project; so, easily more than 3-4 cutters per stroke for non -fractured rock and high UCS, very abrasive rock (very high QTBM and low penetration).

Attached I summarize an hypothetical example (“rough” numbers).

Important to note that the cutter life in h/c only varies between the different QTBM(RMR, kekv) cases, by the variation of cutter thrust (only for highly abrasive rock).

Hope that it will make clearer rock mass quality and penetration (m/h) implications in cutter life and finally answer your question (or continue the discussion the next time we meet!).

Kind regards,

Javier