# Rock Mass Structure Classification of Caves Based on the 3D Rock Block Index
## 5 Classification of Rock Mass Structure Based on 3D Rock Block Index
### Classification of Rock Mass Structure  
Rock mass quality classification is an indispensable link in rock mass engineering. With regard to the research performed on rock mass quality classification, domestic and foreign experts as well as scholars have conducted a substantial amount of work. Now, it has formed a relatively perfect system. Since the 1970s, different rock mass quality classification methods, such as RMR, SMR, RMS classification, and [PERSON]'s GSI classification, have been proposed successively for the classification system of rock mass quality in hydropower engineering [34; 35; 36; 37]. The classification of rock mass structure types is the basis of rock mass quality classification evaluation. It can comprehensively reflect geological characteristics, structural plane properties, rock strength, rock deformation characteristics, and other semi-quantitative indexes. [PERSON] [38] conducted a detailed analysis of the mechanical properties of rock mass through experiments and classified the rock mass into four types of structures: loose structure, cataclastic structure, secondary block structure, and blocky structure. According to the classification standard of rock mass structure in the national standard \"Code for Geological Investigation of Water Conservancy and Hydropower Engineering\" (GB50287-99) and the definition of RBI, [PERSON] proposed the classification standard of rock mass structure based on RBI [19; 39]. According to the RBI index, rock structures can be divided into six types (see Table 6). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Classification** & **RBI Value** & \begin{tabular}{c} **Structural Characteristics of** \\ **Rock Mass** \\ \end{tabular}  &  
The 3D rock block index is used as a quantitative evaluation index to determine the rock mass structure of the flat cave, and based on the classification criteria of the RBI index in Table 6, the rock mass structure of each statistically homogeneous section of the long exploratory cave CPD1 is classified. The classification results for rock mass structure are shown in Table 5. In order to verify the reasonableness of the above rock mass structure classification results, a flat cave seismic wave physical exploration test was carried out on the long exploration cave CPD1. The measured seismic wave depth of the flat cave is 0-800 m. The surveying line is arranged on the left wall of the flat cave, 1 m high from the bottom of the cave. Based on the first clear P-wave record and the continuous S-wave record of the same phase axis, \(t_{P}(\mathrm{x})\) and \(t_{s}(\mathrm{x})\) of the initial arrival phase of the P-wave and S-wave at each detection point are obtained from the original seismic record. Then, the \(\theta_{P}(\mathrm{x})\) and \(\theta_{s}(\mathrm{x})\) curves are calculated. Then they are used to calculate the longitudinal and transverse wave velocities in the rock, as follows:  
\[v_{P}=2\frac{\Delta x}{\Delta\theta_{P}} \tag{9}\]  
\[v_{s}=2\frac{\Delta x}{\Delta\theta_{s}} \tag{10}\]  
where \(v_{P}\) is the longitudinal wave velocity of rock mass; \(v_{s}\) is the transverse wave velocity of the rock mass; \(\Delta x\) is the distance difference on the time distance curve; \(\Delta\theta_{P}\) is the time difference on the \(\theta_{P}(\mathrm{x})\) curve; \(\Delta\theta_{s}\) is the time difference on the \(\theta_{s}(\mathrm{x})\) curve; and the unit for each of the above parameters is m/s. According to the measured longitudinal and transverse wave velocities of the flat cave rock mass, the integrity coefficient of the rock mass is calculated via Formula (11). The seismic wave test results are shown in Table 7.  
\[k_{v}=\frac{v_{P}}{v_{P}} \tag{11}\]  
where \(v_{P}\) is the longitudinal wave velocity of rock mass; \(v_{P}\) is the acoustic longitudinal velocity of the intact and fresh rock mass. According to the previous and current test results, the \(v_{P}\) value of fresh rock mass is 6000 m/s.