## 3 Middle Rock Pillar Classification Method  
### Middle Rock Pillar Classification Index System  
It is unreasonable for the tunnel surrounding rock classification index system to only consider the classification method of a single index or a small number of indices and the classification method of all indices without distinguishing different levels. The early-stage classification only considered the rock's compressive strength. The RQD classification only considered the integrity of the rock mass. Although the single index was convenient, overemphasis on the determining role of the single index was not suitable for complex rock masses. Some classification methods considered comprehensive factors, such as uniaxial compressive strength, elastic modulus, joint occurrence, weathering coefficient, integrity coefficient, ground stress state, groundwater state, and other factors. However, it was difficult to simultaneously obtain many indices during operation. Some indices had repeated influences on the rock mass, and there was a mutual restriction relationship. Therefore, the following points were considered in the establishment of the index system of the middle rock pillar to ensure that the indicators are not constrained by each other:  
1. We determined the primary and secondary relationships of each index and classified them into levels. The main index is the strength and stability of various middle rock pillars, which is a common factor in various rock masses and belongs to the basic index. Secondary indices varied according to different engineering design types, occurrence conditions, and site conditions, which belong to the auxiliary index. The basic quality and foundation frame were first determined by the basic index, and then the basic quality was modified according to the auxiliary index.
2. Each classification index should be independent of the others, and the influence of a single index should correspond to the properties of a single aspect of the rock mass. For example, the degree of weathering of a rock mass cannot be used as a basic index because it affects both the strength and integrity of the rock mass.
3. Through a combination of qualitative and quantitative descriptions, an index system was established to improve the accuracy of classification through a comprehensive evaluation.  
Based on the sorting of the three grading factors that affected the middle rock pillar, a classification index system of the middle rock pillar was constructed from the two dimensions of the basic and auxiliary indices, as shown in Figure 2. The integrity and hardness indices of the middle rock pillar were classified as basic indices because these two indices are common properties of the rock mass and play a fundamental and decisive role in influencing the quality of the middle rock pillar. At the same time, both indices can be quantitatively evaluated, and the rock mass quality can be more accurately evaluated based on quantifiable indices. Different from the rock quality factor RQD selected in RMR and Q classification methods, the integrity index chooses the rock mass integrity coefficient \(K_{\rm v}\). The rock uniaxial compressive strength \(R_{c}\), instead of the rock point load strength \(I_{s}\), was selected as a hard indicator, in contrast to the RMR and Q classification methods. Because \(R_{c}\) and \(I_{s}\) were both applicable, their operation of deviation was small and quantitatively accurate.  
After establishing the middle rock pillar classification index system, it was necessary to further refine each index, and evaluate the basic and auxiliary indices that affect the middle rock pillar quality through the scoring method.