# Effect of Rock Abrasiveness on Wear of Shield Tunnelling in Bukit Timah Granite
## 4 Discussion
### Wear Rate of Slurry Discharge Pipes  
The wear rates of the slurry discharge pipes for weathering grade G(V) of Bukit Timah granite and the mixed zone were 1.5 times higher than those for the weathering grade G(I) to G(IV) of Bukit Timah granite. The most common mechanism of slurry pipe wear was erosive wear, which was influenced by the slurry transport speed, particle size, and shape of the slurry. In weathering grade G(V) of Bukit Timah granite, most of the abrasive minerals, including quartz, feldspar, and mica, still existed in the ground [15]. This is demonstrated in the equivalent quartz content values shown in Table 1. The equivalent quartz content, regardless of the weathering grade, was not significantly different. Abrasive minerals were often contained in the mixed zone, and these minerals were transported to the bottom of the pipes to accelerate the wearing process. Most of the excavated materials for the weathering grades G(I) to G(IV) of Bukit Timah granite were, however, held in rock chip shapes, and only a small portion of the pipe was in contact with these abrasive minerals. The chances of interaction with the pipe were also lower for abrasive minerals in weather grades G(I) to G(IV) than in weathering grade G(V) of Bukit Timah granite and the mixed zone. The slurry discharge speed also varied with the weathering grade of Bukit Timah granite. The average rate of the slurry discharge for grades G(I) to G(IV) was 3.61 m/s, and that for G(V) and the mixed zone was 3.72 m/s. The rate of slurry discharge was higher for grades G(V) and the mixed zone than for grades G(I) to G(IV), explaining the increased wear rate of slurry discharge pipes for G(V) grade and the mixed zone. The slurry discharge speed at the project site was increased from 3 to 4 m/s, and the relationship between the slurry discharge velocity and the wear rate is presented in Figure 12. The wear rate of the slurry discharge pipes appeared to be higher in proportion to the slurry discharge velocity. An empirical relationship between the slurry discharge velocity and the wear rate is expressed as follows:  
\[wr\;[\text{mm/m}]=10^{-17.88+4.402V} \tag{11}\]  
where \(V\) is the slurry discharge velocity in m/s. Figure 12: Relationship between the slurry discharge velocity and the wear rate. Relationship between the Wear Coefficient of Disc Cutters and the Wear Rate of Slurry Discharge Pipes  
The correlation between the wear coefficient of disc cutters and the wear rate of slurry discharge pipes differed from that with the weathering grade of Bukit Timah granite. With an increased degree of weathering, the wear coefficients of the disc cutters decreased. In comparison, the wear rates of slurry discharge pipes in G(I) to G(IV) were nearly similar or not significantly different. Figure 13 shows the relationship between the wear coefficients of disc cutters and the wear rates of slurry discharge pipes. Although there was reasonably wide variability, a linear relationship between the wear coefficient and the wear rate in G(I) to G(IV) exists, which was similar to full face rock. However, when the wear coefficient increased, the wear rate appeared to decrease in G(V) weathering grade ground and increase in the mixed ground, which included soils. The ground components of the G(I) to G(IV) weathering grade were mostly rocks, and the wear coefficient of the disc cutter was higher than those of G(V) and the mixed zone when excavating in this section. In addition, most of the muck was transported in rock chip forms. Therefore, the wear coefficient and the wear rate are linearly correlated. In contrast, most of the G(V) and mixed zone ground were mainly composed of residual soils. In soils, the wear coefficient of disc cutters was relatively low, but the wear rate of slurry discharge pipes was rather high. In the rocks, including the mixed zone, the wear coefficient of disc cutters had a relatively high value, and the wear rate of the slurry discharge pipes increased proportionally.