## 3 Rock Engineering Characteristics and Rock Mass Classification
### Basic Quality (BQ) Classification of the Rocks Surrounding the Tunnel  
A surrounding rock classification for the explored tunnel was implemented in accordance with relevant regulations in the Chinese Specifications for Design of Highway Tunnels (JTG C20-2011). Initially, a rock hardness classification was carried out depending on the mechanical properties of different rocks in the tunnel site, and the \(BQ\) value was calculated according to the rock mass integrity coefficient \(Kv\) determined using borehole acoustic logging and the development degree of rock mass fractures. Meanwhile, the surrounding rocks were preliminarily graded. Afterward, the detailed grading of surrounding rocks was carried out depending on the tunnel overburden and the criticality of groundwater to surrounding rocks by taking into account the influence of the surrounding rock's geological structure, the development degree of the surrounding rock joints, and the impact of weathering characteristics of various surrounding rocks on their strength. The rocks surrounding the tunnel were classified into five types according to the standard value \(Rc\) for a saturated rock's compressive strength in the tunnel site as follows: extremely soft rock, soft rock, slightly soft  
Figure 3: Cavern karst phenomena: (**a**) calcium crystal precipitation and (**b**) cavern wall dripping. rock, slightly hard rock, and hard rock. Field sampling and laboratory tests of rock masses from the exploration area revealed that the saturated compressive strength of limestone in the tunnel site was 35.6 MPa, which is classified as slightly hard. By consulting Clause 3.6.3 of the Specifications for Design of Highway Tunnels (JTG C20-2011) and Clause 4.2.2 of the Standards for Engineering Classification of Rock Masses (GB/T 50218-2014), the computational formula was derived as:  
\[BQ=90+3 Rc+250 Kv, \tag{1}\]  
where \(Rc=90 Kv+30\) if \(Rc>90 Kv+30\); and \(Kv=0.04 Rc+0.4\) if \(Kv>0.04 Rc+0.4\). In the case of complicated rock engineering conditions, correcting the results of a primary rock mass quality classification is necessary. The correction of rock mass classification in the studied project primarily involved the groundwater effect correction coefficient \(K_{1}\). Table 2 lists the correction results. There was no weak structural plane in the moderately weathered stratum of the tunnel site, nor the presence of high geostress. The effect correction coefficient K\({}_{2}\) for main weak structural plane occurrence was set to be 0, and the effect correction coefficient K\({}_{3}\) for initial stress state was also set to be 0. In Table 3, the final correction results of the surrounding rock classification are presented. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**System**} & \multirow{2}{*}{**Lithology**} & \multirow{2}{*}{**Rock Sample No.**} & **Basic Quality Index** & **Groundwater Effluent Status** & **Effect Correction Coefficient K\({}_{1}\)** \\ \hline \multirow{3}{*}{\(\backslash\)} & & K0+000–K0+018 (18) & & \\ \cline{3-4}  & & & & This section was located in the karst fissure & \\ \multirow{3}{*}{\(\backslash\)} & & K0+018–K0+023 (6) & 349.5 &
\begin{tabular}{c} This section was located in the karst fissure \\ zone developed along the rock stratum. Water \\ seepage was found during the investigation. In \\ the rainy season, the groundwater is \\ discharged in drips. \\ \hline \multirow{3}{*}{\(\backslash\)} & & & The surrounding rocks in this section are \\ \multirow{3}{*}{\(\backslash\)} & & & moderately weathered limestone, where there \\ \cline{3-4}  & & & is scarce groundwater. Local dampening was \\ \cline{3-4}  & & & observed during the investigation. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Groundwater effect correction factor \(K_{1}\).