A discussion on hard rock TBM cutter wear and cutterhead intervention interval length evaluation

Cutter wear is an important issue for hard rock TBMs as it is directly related to the time and cost of tunneling. It may even become more important for a closed faced TBM tunneling when the planning for a costly intervention becomes very crucial. The major goal of this study is to provide more accurate empirical methods for the cutter wear and optimum intervention interval evaluation. This needs abrasivity index or wear index measurement from various tunnels in different rock types as well as the actual cutter consumption information. To achieve this goal, two methods are adopted. The first method entails compiling TBM performance data and cerchar abrasivity index (CAI) information from various past case histories and data analysis to propose a new empirical formula for the specific cutter weight loss. The second method involves abrasivity measurement using a new proposed laboratory test for a set of available samples from some recently completed tunnels and data analysis to provide an alternative empirical method for specific cutter weight loss prediction. The new abrasion test (NAT), proposed in this paper, is aimed to improve the process of abrasivity measurement using a constant cross section small disc to partially mimic the real disc cutter interaction with the tunnel face. The results show that both of the achieved new empirical formulas provide a better prediction as compared to the well-known Gehring model. They also provide information to predict optimum cutterhead intervention interval length and the number of worn-out cutters. A full-scale test is also conducted to check the validity of the new formulas. In the final section of this paper, the method to evaluate the optimum intervention interval length and the number of worn-out cutters is described and its prediction is evaluated for a tunnel project in Singapore. The evaluation of the new methods proposed in this paper shows a very good agreement with the actual cutter consumption and intervention interval length. The outcomes of this paper can be very useful in both planning and construction stages of a TBM tunneling project where the intervention interval length and cutter wear become critical decision making factors for scheduling and cost evaluation and for reducing the risks of unwanted severe damage on the cutters.