# Effect of Rock Abrasiveness on Wear of Shield Tunnelling in Bukit Timah Granite
## 1 Introduction  
The abrasiveness of rocks significantly influences the wear endured by excavation tools in tunnel boring machines (TBMs). Particularly in slurry shield TBMs, wear occurs not only in the excavation tools but also in the discharge pipes and pumps of slurry transport systems. A planned or unplanned cutterhead intervention is necessary for assessing the condition of the cutterhead in TBMs, and in order to replace or repair worn or broken excavation tools. Furthermore, the regular maintenance of slurry pipes, including processes such as pipe thickness measurement, rotation, and replacement, is required to prolong the life cycle of slurry pipes. The consumption of disc cutters, slurry pipes, and pumps has a significant impact on the maintenance of TBMs and on overall construction costs and time in general. Wear involves the gradual removal of material from a solid surface via abrasion, corrosion, or a combination of both. Abrasive wear is the most common type of wear that occurs in disc cutters, where a harder material abrades a softer material. Erosive wear typically occurs in slurry transport systems, where the material is slowly eroded from a surface owing to the effect of small particles on the surface. Wear prediction models for disc cutters are based on cutter ring wear due to the abrasiveness of rocks. The interaction between the disc cutter rings and the surrounding rock during excavation involvesa complex tribology system. The Colorado School of Mines (CSM) [1], Norwegian University of Science and Technology (NTNU) [2], and Gehring models [3] are the most common models used to estimate cutter wear. The CSM model is based on disc cutter life, defined using the Cerchar Abrasivity Index (_CAI_) [4]. The _CAI_ is the most popularly used method for describing the abrasiveness of rocks. The rock specimen is firmly placed in a clamp, and a steel pin with a Rockwell hardness scale C (HRC) of 55 is mounted on the rock surface under a static force of 70 N. The rock surface is slowly scratched to a length of 10 mm. This method is repeated at least five times in directions parallel and perpendicular to the original scratching direction on the rock surface using a new or resharpened steel pin tip each time. The _CAI_ is then determined by measuring the diameter of the worn flat pin at an accuracy of 0.01 mm, and calculated by multiplying this diameter in units of 0.01 mm by 10. The Cutter Life Index (_CLI_) is considered regarding the NTNU model to evaluate the disc cutter life through various special tests proposed by NTNU. The _CLI_ is evaluated using Siever's J-value (_SJ_) and abrasion value steel (_AVS_). _SJ_ represents the rock surface hardness and is calculated as the average of a drill hole depth in 1/10 mm following 200 turns of an 8.5 mm drill bit. _AVS_ is the weight loss measured in milligrams of a test part of steel from a new cutter ring after 1 min, i.e., 20 revolutions. The _CLI_ can be expressed using _SJ_ and _AVS_, as follows [5]:  
\[CLI=13.84\Big{(}\frac{SJ}{AVS}\Big{)}^{0.3847}, \tag{1}\]  
The [PERSON] model exploits the relationship between the _CAI_ and the weight loss due to wear of a disc cutter. The specific wear on a disc cutter, in mg steel per meter rolled, is expressed as follows [3]:  
\[v_{s}=0.74 CAI^{1.93}, \tag{2}\]  
where \(v_{s}\) is the specific disc cutter wear in mg/m. The wear of cutting tools, including disc cutters, can be measured using the wear coefficient. The wear coefficient (_wc_) is defined as the ratio of the radial abrasion of the disc cutter ring (_a_), in mm, to the travelled distance of the disc cutter ring (_r_), in km, as follows:  
\[wc\left[\text{mm}/\text{km}\right]=\frac{a\left[\text{mm}\right]}{r\left[ \text{km}\right]}, \tag{3}\]  
A higher wear coefficient indicates a faster decrease in the disc cutter lifespan, and the wear coefficient is closely associated with rock abrasiveness. Few studies have investigated the wear coefficients of disc cutters.