## 2 Methodology and Strategy
### Strategies for Mixed Ground  
Given that the research on the operating parameters of EPB shield tunneling is mainly concentrated in uniform stratum conditions nowadays, the research on the operating parameters of EPB shield tunneling under the mixed ground condition is relatively backward. However, since mixed ground conditions are a kind of common special geological condition in shield tunnel construction and they appear in a large number of sections in this research--which means there could be more than one geological parameter at the same position during the excavation process--it is necessary to make a reasonable selection of operating parameters of EPB shield tunneling in the mixed ground condition. In the previous literature, [PERSON] et al. [5] mainly analyzed the advance rate data in the mixed ground and compared it with other data in single geological conditions. [PERSON] et al. [6] proposed a network prediction method using PSO combined with LSTM, and applied it to construction projects involving mixed ground geology. For mixed ground, they tended to use the thickness of each layer as a comprehensive method of weighting, but the final effect of this model was not particularly ideal. Therefore, there are prominent problems in this research of how to select more suitable parameters to predict the advance rate of the mixed ground, or how to deal with the geological parameters of the mixed ground to make them become more suitable input parameters for model prediction. In order to guarantee the accuracy of the prediction model in a single stratum as well as make the prediction model adapt to the situation of mixed ground, the following countermeasures are proposed in this paper, and they are carried out in field verification in Section 4. Most of the tunnel faces during tunneling are composed of two kinds of strata, but a small part is composed of three or more kinds of strata. In order to simplify the calculation and optimize the parameters, this paper will select two dominant strata as upper layer and lower layer data respectively. Strategy A: Divide each type of geological parameters into two columns. One column represents the rock (soil) geological parameters of the upper part of the mixed ground, and the other represents the lower part geological parameters. Figure 3 shows the parameters of each part. The whole circle represents the face of the shield excavated by the shield tunneling, and the object on the left represents the side view of the cutter head. \(S_{u}\) and \(S_{d}\) represent the area of upper strata and lower strata in excavation face separately. Figure 3: Diagram of mixed ground. (\(\gamma\), \(K_{0}\), and \(\delta\) are the geological parameters that will be introduced later. The dotted line represents the central axis of the shield tunneling). Strategy B: On the basis of Strategy A, the composite ratio parameter \(p\) is added to represent the area proportion of the upper soil layer in the whole tunnel face. It can be expressed in Equation (10):  
\[p=\frac{S_{u}}{S_{a}} \tag{10}\]  
where \(S_{u}\) represents the area of upper strata in excavation face, \(S_{a}\) represents total area of excavation face. Strategy C: A comprehensive geological index is proposed to represent an overall excavation difficulty of the excavation face. The comprehensive geological index is defined as Equation (11):  
\[\omega_{i}=p\times\theta_{i1}+(1-p)\times\theta_{i2} \tag{11}\]  
where \(\omega_{i}\) represents comprehensive geological indicators, \(\theta_{i1}\) is the geological parameters of the upper strata, and \(\theta_{i2}\) is the geological parameters of the lower strata, \(\theta_{i}\in\{\gamma,\,K_{0},\,\delta\}\). Strategy D: Calculate the comprehensive geological index with the thickness of the formation as the weight, which is also a comprehensive geological index that was proposed by [PERSON] et al.