# Effect of Rock Abrasiveness on Wear of Shield Tunnelling in Bukit Timah Granite
## 2 Materials and Methods  
Figure 2 illustrates the geological conditions along the tunnel route, wherein Bukit Timah granite from fresh weathering grade G(I) to residual soil G(VI), a mixed face of soil and rock, and alluvial deposits of the Kallang formation exist. The Bukit Timah granite was generally composed of granitic rocks and various minority lithologies, including dolerite, diorite, basalt, dacite, and rhyolite. The depth of the tunnel alignment varied from 32.5 to 65 m. The tunnel centreline level was up to 62 to 65 m below  
ground level. The water table along the tunnel route was generally less than 5 m below ground level, indicating that the water pressure at tunnel level was very high, typically above 4 bar and exceeding 6 bar along some alignment sections. Table 1 lists a summary of the geomechanical characteristics of the Bukit Timah granite located at the project site. The weathering classification of the Bukit Timah granite, in accordance with BS 5930 [14], is shown in Table 2.  
A 5.29 km long, single-tube tunnel was excavated using two slurry shield TBMs 6.9 m in diameter. A slurry shield TBM can be used to control and prevent the inflow of groundwater through the excavation face when appropriate pressure is applied. The slurry shield TBM was equipped with a 19 inch disc cutter, and equipped with a total of 48 cutter rings and a cutter spacing of 85 mm. Table 3 lists the TBM specifications, and Figure 3 shows photographs of the TBMs used for the project. The degree of disc cutter wear was measured during the cutterhead intervention. A total of 126 cutterhead interventions were conducted for TBM No. 1, which was used to excavate 2.64 km. For TBM No. 2, which was used to excavate 2.65 km, 125 cutterhead interventions were carried out. Figure 4 shows the measurement of disc cutter wear during the cutterhead interventions. The degree of wear was measured using a special ruler. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Weathering**} & \multicolumn{2}{c}{\({}^{1}\)**UCS (MPa)**} & \multicolumn{2}{c}{\({}^{2}\)**BTS (MPa)**} & \multicolumn{2}{c}{\({}^{3}\)**KQC (\%)**} & \multicolumn{2}{c}{_CAI_} \\ \cline{2-10}  & **Range** & **Average** & **Range** & **Average** & **Range** & **Average** & **Range** & **Average** \\ \hline G(I) & 17.0–161.1 & 91.1 & 4–18 & 11.2 & 26.2–57.2 & 46.9 & 1.70–6.23 & 3.82 \\ G(I) & 10.9–144.5 & 71.1 & 2–19 & 10.4 & 22.0–54.6 & 44.5 & 1.30–6.44 & 3.77 \\ G(III) & 1.1–109.8 & 57.2 & 2–12 & 6.1 & 332–63.2 & 48.6 & 1.30–6.41 & 3.58 \\ G(IV) & 3.1–64.3 & 24.2 & 0–14 & 5.4 & 40.8–61.6 & 54.3 & 0.28–5.95 & 3.23 \\ Mixed & NA & NA & NA & NA & 22.0–57.2 & 46.1 & 0.28–6.44 & 3.61 \\ \hline \hline \end{tabular} \({}^{1}\) UCS: uniaxial compressive strength, \({}^{2}\) BTS: Brazilian tensile strength, \({}^{3}\) EQC: equivalent quartz content.  
\end{table} 
Table 1: Geomechanical characteristics of Bukit Timah granite. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Geo Notation** & **Classification** & **Characteristics** \\ \hline G(I) & Fresh & Unchanged from the original state. Intact strength unaffected by weathering. Not easily broken using a hammer. \\ G(II) & Slightly weathered & Slight discolouration, and slight weakening. Fresh rock colours generally retained but stained near joint surfaces. \\ G(III) & Moderately weathered & No easily broken using a hammer. \\ G(V) & Highly weathered & Considerably weakened. Cannot be broken by hand. Essily broken using a hammer. Stained throughout. \\ G(V) & Completely weathered & Core can be broken by hand. Completely discoloured. \\ G(V) & Completely weathered & Considerably weakened. Can be crumbled by hand. \\ G(VI) & Residual soil & Original rock structure completely degraded to a soil. Can be crumbled by hand. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Weathering classification for Bukit Timah granite. Figure 2: Geological conditions along the tunnel alignment.