Engineering geology aspects for design and classification - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 80

2.5.5 Classification for excavation and support
2.5.5.1 General and objective for mechanised tunnelling
The classification systems known previously were almost exclusively produced for drill
and blast tunnelling. Classification provides the basis for payment and schedule. For a
TBM drive, cuttability is also a decisive factor, because the time to bore a stroke increases
in less cuttable rock and more time is thus available for temporary support.
In addition to rock mass classification as a grading of stability, mostly through the required
extent of support measures, cuttability should be integrated in a suitable manner in order
to satisfy the basic objective. If the determination of these matters is omitted from the
contract, diverging opinions between the contract parties are inevitable.
2.5.5.2 Classification systems and investigation of suitability for tunnel
boring machines
Classification according to rock mass properties. Terzaghi, Stini and later Lauffer,
Packer and Rabcewicz all developed rock quality classes based on the rock mass prop-
erties or also the behaviour of the rock mass. All grade the rock mass between “stable”
and “squeezing” with various numbers of intermediate classes. In particular, Bieniawski
created the RMR System (Rock Mass Rating System) and Barton the Q System (Quality
System) from theoretical knowledge of rock mechanics [142]. Both systems are based on
quantitative rock mass parameters without consideration of the cuttability for TBM drives.
Suitability of the RMR System (Rock Mass Rating System) . The procedure for the RMR
System and its strengths and weaknesses are dealt with under 2.4.2.4.
In the absence of such a system for TBM drives, the RMR System can also produce usable
results for TBM drives, provided the conditions are also considered critically.
This rock mass classification was developed from and for of conventionally driven tun-
nels. For TBM drives, the tearing of the rock outside the intended contour due to blasting
is not applicable. The positive influence of the use of mechanised tunnelling is described
according to Alber et al. by the following formula [5]:
RMR TBM = 0,84 RMR D+B + 21 (20 < RMR D+B < 80)
RMR TBM represents the RMR value for TBM drives and RMR D+B , that for drill and blast.
In a rock mass of poor to very poor quality, the difference between conventional and
mechanised tunnel drives is less significant.
Suitability of the Q System (Quality System) . The Q System described in Section 2.4.2.3
was further developed for TBM drives by Grimstad and Barton [13, 83]. The intention was
to create a Q TBM Index based on the Q Index.
This rock mass classification for TBM as a combination of a forecast for temporary sup-
port with a forecast for the cuttability and material wear may be usable in some cases. In
general, the cuttability and disc cutter state cannot be aligned with the rock classification.
Barton attempted to estimate the penetration rate PR and the advance rate AR of a TBM
drive using Q , Q TBM and additional values. In addition, the Cutter Life Index ( CLI ) as
a wear parameter with a scale of values limited to a maximum (Fig. 2-26) is not very