In hard rock TBM tunneling, disc cutter wear is caused not only by initial fragmentation of a rock face but also by fragmentation of residual rock pieces and blocks between a cutterhead and a rock face. These phenomena are considered as primary fragmentation and the secondary one, respectively. In order to appraise the total amount of cutter wear, the several indices have been proposed based on laboratory tests. It focused only on primary fragmentation. However, there has not been any studied on appraisal of cutter consumption and cutter life in TBM applications in consideration with the secondary fragmentation. In this paper, first, the equations to estimate the amount of cutter wear caused by the primary and secondary fragmentations were proposed. Also, the ratio of cutter wear by secondary fragmentation to the total amount of wear was defined by using these equations. Secondly, they were applied to two tunnels drilled by medium-scale TBMs through sedimentary rocks and granitic formation. The total amount of cutter wear was compared with the estimation given by the client and the appraisal of the cutter consumption was discussed based on the ratio of cutter wear by secondary fragmentation.
Use of a TBM in excavation of underground openings has the great advantages in terms of minimizing loosening of rock and advance rate in comparison with drill-and-blast method. For more than 40 years in Japan, it has often been used for construction of headrace and aqueduct tunnels in pumped-storage hydroelectric power stations and for excavation of a pilot drift prior to full excavation of a highway tunnel of 200 m2 in cross-sectional area. And, these days, it has been recognized as one of the expected excavation methods in construction of disposal galleries in the project of geological disposal of high-level radioactive waste.
Rock Fragmentation in excavation with a TBM should be divided into primary fragmentation and secondary one. The primary fragmentation is to fragment virgin rock surface and the secondary fragmentation is to fragment rock pieces and blocks between a face and a cutterhead. In the earlier studies, the indices to estimate the amount of cutter consumption and cutter life have been proposed based on theoretical approach and laboratory tests of rock pieces. For example, there are F-Value by Shimazek & Knatz (1970), Total Hardness by Tarkoy (1975), Cutter Life Index by the Norwegian Institute of Technology (1988), and Modified F-Value by Ewendt (1989). These are expressed by experimental formulae based on tensile strength, content rate of high abrasive minerals and hardness values such as rebound, scratch, indentation, and abrasion. And, they have been applied to a lot of applications and the relations to the amount of cutter consumption and cutter life have been investigated. These studies, however, focused only on the primary fragmentation as the cause of cutter wear in TBM excavation.
The secondary fragmentation should mainly be caused by joints in rock. The secondary fragmentation causes the significant damage against a cutterhead and scrapers, and the excessive wear of cutter.