100 km of tunnels were classified with the RMR system and analyzed for rock mass strength and rock mass stability. Empirical relations between TBM penetration and utilization as a function of rock mass strength and rock mass stability are presented.
100 km de tunnels ont ete classifie avec Ie système RMR et ont ete analyse sur resistance et stabilite de la masse rocheuse. Des relations empiriques de la penetration et de l'utilisation de machines à forer sur la base de resistance et stabilite des masses rocheuses sont presentees.
100 Tunnel-km wurden nach dem RMR System klassifziert und auf Gebirgsfestigkeit und Gebirgsstabilitat untersucht. Empirische Beziehungen zwischen TBM-Penetration und Ausnutzungsgrad als Funktion von Gebirgsfestigkeit und Gebirgsstabilitat werden vorgestellt..
The two basic tasks in TBM tunneling, as for any underground excavation, are (i) to excavate and remove the rock and (ii) to maintain tunnel stability until the final lining, if necessary, is installed. Application of rock engineering for TBM drives should be directed towards solving these tasks which define the TBM performance and finally the project delivery time. Performance prediction of TBM drives requires the estimation of both TBM penetration and utilization. The rate of penetration (m/h), defined as the distance mined divided by TBM operating time, depends on the TBM design (thrust, torque, disc size and spacing) as well as the rock mass properties that with stand penetration. TBM utilization is defined as the percentge of shift time during which mining occurs and depends on equipment and non-equipment downtimes (Mcfeat-Smith and Tarkoy, 1979). Maintenance of equipment and tunnel are typically scheduled on a regular basis and are mainly fixed quantities. Two major factors in estimating utilization rates are (i) the stability of the rock mass and (ii) the proper support systems as incorporated in the TBM. Figure 1 shows schematically two pairs of interactions between TBM and rock mass that influence performance data. Rock mass strength and disc thrust determine the TBM penetration while rock mass behavior often requires support installation and influences so TBM utilization.
The prediction of TBM penetration is typically based on extensive laboratory testing on intact rock. Uniaxial compressive strength, hardness, abrasivity and similar intact material properties have been used for this purpose. Unfortunately, very few rock masses correspond to laboratory conditions. Realistic test procedures on intact rock properties were developed at the Norwegian Institute of Technology (NIT, 1988) and the Colorado School of Mines (Ozdemir, 1995), but only the Norwegians take the effect of discontinuities on penetration into account. Generally, the presence of discontinuities increases TBM penetration. In rock engineering terms, the presence of discontinuities reduces the rock mass strength to a fraction of the intact rock strength, and for a given TBM, penetration should rather be measured against rock mass strength than against intact rock strength. Several methods to estimate the rock mass strength exist, one being the use of rock mass classifications (e.g. RMR system) and failure criterion. The RMR system (Bieniawski, 1989), which was developed for drill-and-blast (D&B) excavations, was used to quantify the rock mass quality along the TBM drives. Since the TBM excavation preserves the rock mass quality better than the D&B method, the RMR values derived from drillcores or D&B adits may be increased.