## 4 Validation and Analysis
### Data Preparation
#### 4.1.2 Parameter Selection  
Equation (3) shows that cutting ability evaluation is related to the operational and geological parameters. Among the operational parameters, there is a certain relationship between some parameters. Merging parameters with high correlation can help remove redundant parameters and reduce the computational cost of data mining. The simplest way to determine whether these parameters are interrelated is to calculate the Pearson correlation coefficient \(\rho\) among the operational parameters, as shown in Equation (21):  
\[\rho(X,Y)=\frac{COV(X,Y)}{\sigma_{X}\sigma_{Y}} \tag{21}\]  
where \(\rho(X,Y)\) is the Pearson correlation coefficient between variable \(X\) and \(Y\), \(COV(X,Y)\) is the covariance of variable \(X\) and \(Y\), while \(\sigma_{X}\) and \(\sigma_{Y}\) are the standard deviations of \(X\) and \(Y\), respectively.  
Figure 6: TBM advance rate during operation.  
The major TBM cutterhead and driven system parameters monitored and recorded include advance rate (\(V\)), thrust (\(F\)), cutterhead torque (\(T\)), cutterhead rotational speed (\(RSP\)), advance mileage (AM), advance displacement (AD), penetration rate (\(p\)), drive motor current (I), drive motor torque (MT), and drive motor frequency (MF). These 10 parameters were randomly sampled from the trusting data extracted in Section 4.1.1 and used for correlation analysis. The correlation analysis results of 160,000 sets of data are shown in Table 2.  
Generally, when \(|\rho|\geq 0.8\), the two parameters are considered to be strongly correlated. As shown in Table 2, the following parameters are strongly correlated: advance rate (\(V\)) and penetration rate (\(p\)); drive motor current (I), torque (MT), and cutterhead torque (\(T\)); cutterhead rotational speed (\(RSP\)) and drive motor frequency (MF). Therefore, some of them can be removed to reduce the model's complexity. Eventually, the following eight parameters were chosen as input variables:  
1. (Rock type: Unlike subway tunnel construction, hard rock tunnel construction may encounter more flexible geological conditions. Generally, different types of rock have different value ranges of petrophysical parameters.
2. Uniaxial compressive strength: The measurement of the strength characteristics of rock materials, which is widely used to represent geological conditions in previous research.
3. Advance rate: The derivative of advance displacement, which is related to the TBM forward velocity.
4. Trust: The pressure of the thrust cylinders, which provides the major power of driving TBM forward.
5. Cutterhead rotational speed: The angular velocity of cutterhead rotating with TBM spindle, which is related to the relative velocity of cutter to rock.
6. Advance mileage: The displacement distance of TBM during the whole tunneling, which is related to the rolling distances of disc cutters.
7. Advance displacement: When TBM works in the gripping-thrusting-regripping procedure, the propulsion cylinders reach out and retract cyclically. Advance displacement is the stroke of propulsion cylinders.
8. Drive motor current: The electric current of the drive motor operating at constant power mode, which shows the working power of the drive motors.