## 1 Introduction  
The choice of excavation method in any tunnel construction project directly affects the project cost. The worldwide use of a shield tunnel boring machine (TBM) in tunnel excavation is continuously increasing not only for the construction of subway, railway, roadway and hydropower tunnels but also for utility tunnel construction (electricity power lines, sewage, gas pipelines etc.). Mostly, these utility tunnels have smaller diameters than the conventional tunnels. The trend of tunnel construction in urban and subsea areas for electricity transmission is increasing these days due to the expansion of underground power lines business. This trend is leading to an increasing need for tunnel extension, averaging around 10 km per year in South Korea. For the use of electricity transmission lines, tunnels are usually constructed with the help of shield TBMs which have the potential to create a section of about 7 m\({}^{2}\) area (around 3 m diameter tunnels). The KEPCO (Korea Electric Power Corporation) research institute persistently investigates the performance of shield TBM in order to analyze the efficiency and safety of the utility tunnels construction for electricity transmission purposes. The performance of TBM is a key factor in determining the project duration and cost, which can be further subdivided in two parts. Firstly, the efficiency of excavation in the rock mass based on cutterhead design and the penetration of disc cutters in the rock mass caused by the thrust force need to be assessed. Secondly, the ability to rotate the cutterhead, already penetrated in rock, needs to be determined through the advance rate (excavation speed) of the machine. This study focuses in particular on the derivation of an estimation method of torque based on the performance of cutterhead drive. Numerous empirical and theoretical estimation methods for the TBM performance were developed in the past few decades [1; 2]. Among these methods, the NTNU (Norwegian University of Science and Technology) approach [3] and CSM (Colorado School of Mines) approach [4] are recognized worldwide and these models were modified afterwards by various researchers for developing more precise performance prediction models, owing to the immensely increasing trend of using TBMs in the tunnel construction industry. Besides these two models, empirical methods like QTBM [5; 6], RME [7; 8] and the computer aided approach [9; 10] are also used which rely on well documented case histories. The literature reveals that the performance of the machine in TBM tunneling depends on (1) geological and geotechnical parameters in the excavation environment, (2) cutterhead design parameters and (3) machine operating parameters [11; 12]. The first parameter includes rock strength, jointing conditions and drillability. The cutterhead parameters include diameter, spacing, number and geometry of the disc cutters (with or without scrapers). The main TBM operational parameters include rotational speed of the cutterhead, torque, thrust force and power supply. A previous research work indicates that the cutterhead penetration is a major function of torque, however, the rotational speed plays a minor role in the performance [13]. For the TBM performance estimation in all the aforementioned approaches, the three parameters were combined in different ways based on the experimental work and/or field observation. The intact rock strength plays a major role in the TBM performance along with joints (spacing and RQD) [14]. The TBM performance estimation approaches developed so far are mostly based on the field data [15]. However, these field-related geotechnical datasets are mostly based on the exploration results obtained from bore holes drilled along the tunnel route during the project design phase. Relying completely on the borehole data is mostly misleading, especially in cases where the spacing between the boreholes are significantly large.