# Effect of Rock Abrasiveness on Wear of Shield Tunnelling in Bukit Timah Granite
## 3 Results  
### Wear of Disc Cutters  
Figure 7 shows the number of disc cutters that were replaced in this study. A total of 833 disc cutters were replaced for TBM No. 1, which comprised 50 double-disc cutters and 783 single-disc cutters. In TBM No. 2, 952 disc cutters were replaced, comprising 144 double-disc cutters and 808 single-disc cutters. The number of double-disc cutters replaced for TBM No. 2 was approximately 2.9 times greater than the number of double-disc cutters replaced for TBM No. 1. This discrepancy in frequency of replacement is because there were more soil and mixed zones in the area where TBM No. 2 was excavated, resulting in frequent replacement due to increased frequency of abnormal wear. There were two main reasons for the replacement of the disc cutters. One reason was the replacement due to normal wear, which was the case when the entire cutter ring gradually wore  
Figure 5: Disc cutter configurations of cutterhead. Figure 6: Thickness measurement of slurry discharge pipes: (**a**) pipe thickness measurement; (**b**) measurement point. out and surpassed the allowable wear limit, following which it was replaced. The other reason was replacement because of abnormal wear, including flat wear, partial wear, cutter ring crack, bearing failure, and seal failure. It was discovered that 310 disc cutters were changed owing to abnormal wear for TBM No. 1. In TBM No. 2, 357 disc cutters were replaced owing to abnormal wear. The replacement of the disc cutters was related to the disc cutter position and installation radius. As shown in Figure 7, the number of disc cutter replacements increased with the installation radius. As the installation radius increased, the rolling distance of the disc cutter increased, and the disc cutter rotational speed also increased, such that the rate of disc cutter wear increased and thus warranted more frequent replacement. In particular, the gage cutters had an allowable wear limit of 15 mm, and were replaced more frequently than other cutters. Among the total replacement of disc cutters for TBM No. 1, the replacement rate due to abnormal wear was 69% for the centre cutters, 41% for the face cutters, 25% for the transition cutters, and 14% for the gage cutters. Similarly, for TBM No. 2, the replacement rate owing to abnormal wear was 82% for the centre cutters, 46% for the face cutters, 21% for the transition cutters, and 7% for the gage cutters. It is evident that the occurrence of abnormal wear was related to the disc cutter position and installation radius. On the one hand, it appeared that more abnormal wear occurred in the centre cutters because of the smaller rotation radius; the rotation was not smooth, owing to friction in the tangent direction of the centre cutters. On the  
Figure 7: Number of disc cutters changed: (**a**) TBM No. 1; (**b**) TBM No. 2.  
other hand, the rotation radius was relatively large for the transition of gage cutters, and the rotation of the disc cutter was smooth, so it appeared that more normal wear occurred. Figure 8 shows the relationship between the wear coefficient and the disc cutter position. The centre cutters show the highest wear coefficient values. The wear coefficient gradually decreases with the increase in disc cutter installation radius, and it then tends to increase again for the transition cutters. Over cutters were rarely used; hence, limited data were available for their analysis. The reason for the centre cutter showing a high wear coefficient value was similar to the reason for the high abnormal wear. In the centre cutters, friction in the tangent direction of the cutters increased because the rotation radius was small, and the wear coefficient increased accordingly, owing to eccentric load and a reduction in cutter rotation. Additionally, when the curved section was excavated, the outer cutters were more in contact with the ground; this phenomenon also occurred when the straight section was excavated, and the alignment was maintained while the machine was continuously steered.