# Suitability Evaluation of the Lining Form Based on Combination Weighting-Set Pair Analysis
## 2 The Suitability Level of lining Form  
The established suitability evaluation model in this study can not only utilize the results of current drilling exploration, reduce the workload, and improve the efficiency of design scheme, but also considers the decision of expert evaluation. The five evaluation indices are related to the minimum overburden thickness, the minimum in situ stress, the hydraulic fracturing pressure, the hydraulic gradient, and the surrounding rock classification, which can be recorded as \(A\), \(B\), \(C\), \(D\), and \(E\), respectively. The suitability of the reinforced concrete lining form is divided into five evaluation levels: extremely suitable (I), more suitable (II), basically suitable (III), relatively unsuitable (IV), and extremely unsuitable (V). The relationship between indices and evaluation level of the lining is shown in Table 1.  
(a) Evaluation index \(A\) is selected with reference to the minimum overburden criterion. It is calculated by Equation (1). This index determines the suitability level of the safety degree of reinforced concrete linings by the ratio of the minimum overburden thickness of rock mass at the design location of high-pressure pipelines to the hydrostatic pressure. In the Norwegian criterion, the safety degree value is in the range of 1.05-1.1. However, the rock weathering in China is relatively deep. Professor [PERSON]'s suggestion is mostly adopted in the practical engineering applications and the evaluation level is more suitable when the index value is in the range of 1.3-1.5 [14]. \[A\geq\frac{r_{R}cos\,\alpha_{CR}}{\gamma_{w}h_{\mathrm{s}}} \tag{1}\]  
where \(C_{RM}\) is the minimum overburden thickness of rock mass (m); \(h_{\mathrm{s}}\) is the hydrostatic pressure head at the selected position of the high-pressure pipeline (m); \(\gamma_{w}\) is the weight of water, which is taken as 10 kN/m\({}^{3}\); \(\gamma_{R}\) is the weight of rock mass (kN/m\({}^{3}\)); and \(\alpha\) is the slope angle of the mountain body at the selected position of the high-pressure pipeline. (b) Evaluation index \(B\) is selected according to the hydraulic fracturing criterion, which is used mainly to check the permeability of rock mass and fracture and determine whether they meet the requirements of permeability stability. The index is calculated by comparing the normal stress \(\sigma_{n}\) on the joint or fracture surface with the hydrostatic pressure of high-pressure pipelines, which could help to determine the suitability level of the safety degree of reinforced concrete linings. When Equation (2) is satisfied, tensile stress does not occur on the joint or fracture surface of the surrounding rock under water pressure, and hydraulic fracturing does not occur on the surrounding rock. Generally, the index should have a certain safety margin. According to the engineering experience, it is more suitable for the value range of 1.2-1.3 [15]. \[B\geq\frac{\sigma_{n}}{\gamma_{w}h_{\mathrm{s}}} \tag{2}\]  
(c) Evaluation index \(C\) is selected according to the minimum principal stress criterion. This criterion is actually an engineering generalization of the hydraulic fracturing criterion. In engineering applications, the normal stress \(\sigma_{n}\) is difficult to obtain on the joints or fracture surfaces of high-pressure pipelines, and therefore it is replaced by the minimum principal stress \(\sigma_{min}\). The index uses the ratio of minimum principal stress \(\sigma_{min}\) to the hydrostatic pressure in high-pressure pipelines to determine the suitability level of safety degree, taking the same range of values as the evaluation indicator B. It is obtained by Equation (3).