## 1 Introduction  
In mechanical excavation projects for civil and mining applications, it is important to select appropriate excavation methods to evaluate cutting performances and costs. It is well established that the cutting performance and construction cost in mechanical excavation are affected by the rock properties. Among the different rock properties, rock brittleness is known to be an important factor affecting the characteristics of rock cutting [1, 2, 3, 4, 5]. Although rock brittleness is an important property that determines the failure characteristics of rocks upon loading and unloading conditions [5], there has been no universal agreement on the measurement and definition of rock brittleness. In many previous studies, numerous concepts of rock brittleness indices have been proposed and discussed based on different parameters, such as strength parameters [1, 6, 7, 8, 9, 10], stress-strain (SS) curves [5, 7], and force-penetration curves from indentation tests [3, 4], etc. Of the brittleness indices, strength-based concepts expressed by the mathematical relationship between the uniaxial compressive strength (UCS) and the Brazilian tensile strength (BTS) of rock are generally adopted to measure the rock brittleness; these are summarized in Table 1. The other brittleness index concepts have been well-summarized in previous studies [5, 6]. To evaluate the cutting performance and efficiency of mechanized rock excavation, the specific energy (SE) has been widely used. This is defined as the energy required to excavate a unit volume of rock mass; thus, it is a representative index for identifyingthe cutting efficiency of a cutting tool and mechanical excavators under given cutting conditions [11]. The optimum cutting condition (s/p ratio) is usually expressed by the ratio of cutter spacing (s) to penetration depth (p), and it provides minimal specific energy under a given rock type. Meanwhile, SE under the optimum cutting condition can be used to simply estimate the excavation rate of a TBM as the following equation:  
\[\text{ICR}=\text{k}\times\frac{\text{P}}{\text{SE}} \tag{1}\]  
where ICR is the instantaneous cutting rate (m\({}^{3}\)/h), k is the energy transfer ratio, which is typically between 0.7 and 0.8, P is the power consumed during the excavation, and SE is the specific energy under the optimum cutting condition, which is obtained from the rock cutting tests or empirical equations. Therefore, identifying the optimum specific energy for a given rock formation and cutting condition is one of the most important tasks in the preliminary design phase of TBM tunneling. Furthermore, cutter wear is important for the estimation of the excavation performance of the mechanical excavator. The cutter wear is difficult to predict because many influencing factors involve the wear process. It has been shown that the cutter wear is affected by several factors, such as the operational parameters of the machine (e.g., thrust, torque, and RPM), geological properties of intact rock and rock mass (e.g., mechanical properties, mineralogical properties, abrasivity, and discontinuities), and features of cutting tools (e.g., diameter, tip angle, installation angle, and material). Previous extensive studies have investigated the effect of these influencing factors on the cutter wear by using several approaches, such as empirical, numerical, and soft-computing methods [12; 13; 14; 15; 16; 17; 18]. Of the rock properties, the rock abrasivity is an important property that is directly related to cutter wear and consumption during tunnel excavation work. There are many suggested methods to determine the abrasiveness of rock, with the Cechar abrasivity index (CAI) test being one of the most common methods used. The CAI is known as a good indicator for cutter wear, and it is used to understand the effect of geomechanical parameters (i.e., mineral composition, rock hardness, grain size, rock strength, and brittleness) on the cutter wear [19; 20; 21; 22; 23; 24].