According to the simulation test results, the spatial distribution of joints significantly affects the properties of rock mass. However, in the design and construction of rock mass engineering, the geometric parameters of joints are often ignored. For the design and construction of tunnel engineering, the surrounding rock classification is more important, and the results of the surrounding rock classification affect the tunnel blasting design, excavation method, and supporting parameters. To achieve safer and more economical tunnel engineering, the surrounding rock grading is optimized in joint surrounding rock tunnel engineering.
Many surrounding rock classification methods have been proposed rapidly and accurately understand the quality of rock mass and guide the design and construction of rock mass engineering. Among these the BQ surrounding rock classification method is the most widely used in tunnel engineering in China. According to the National Engineering Rock Mass Classification Standard (GB/T50218-2014), the evaluation result of the rock mass quality is reflected in the BQ. The classification process is divided into two steps. Initially, the BQ value is calculated according to the preliminary survey report or field test, and then the BQ value is obtained by modifying the BQ according to the field geological conditions during the engineering process. The calculation formula are shown in Formula (2) and Formula (3) as follows:
$$BQ=100+3{R}_{c}+250{K}_{v}$$
2
In the formula, \({ R}_{c}\)-- saturated UCS of rock (MPa);
\({K}_{v}\) -- integrity coefficient;
$$\left[BQ\right]=BQ-100({K}_{1}+{K}_{2}+{K}_{3})$$
3
Where \({K}_{1}\)-- groundwater influence coefficient;
\({K}_{2}\) -- influence coefficient of occurrence of main control structural plane;
\({K}_{3}\) -- influence coefficient of ground stress.
According to Equations (2) and (3), the BQ classification method is based on rock strength and rock integrity, supplemented by various geological conditions for modification. However, the impact of the joint geometric parameters is not considered. The strength of the jointed rock mass significantly influences the stability after the excavation of the chamber. Therefore, based on the BQ classification, the influence of the joint dip angle and joint spacing on the UCS of the rock mass is considered and optimized.
According to these research results, when the joint spacing is small and the physical and chemical parameters of the joint are greater than N and 3, respectively, the UCS of the jointed rock mass does not change with the joint spacing. At this time, with an increase in β, the UCS of rock mass exhibits a linear decreasing trend. The UCS of the rock mass when β is equal to 45° is considered as the average value of the rock mass UCS, and the UCS of rock mass when β is equal to 0° and 90° is changed by + 7% and − 6%, respectively. Then, when the physical and chemical parameter N is greater than 3, the joint geometric parameter correction of the basic quality index of the rock mass is represented by \(\text{B}{\text{Q}}^{{\prime }}\) and is calculated as follows:
$$B{Q}^{{\prime }}=\left(-0.00144\beta +1.068\right)BQ$$
4
where: \(BQ{\prime }\)-- Joint geometric parameter correction of BQ;
\(\beta\) -- joint dip angle (\(^\circ\)).
According to these research results, when the joint physical and chemical parameters are greater than N and less than 3, the UCS of the rock mass initially decreases and then increases with an increase in β. When N = 2.14, the variation curve of the UCS of the rock mass with β was linearly fitted, and the relationship between the UCS and β of the rock mass was obtained as follows:
\(\sigma =26.027-0.1132\beta\) (\(\beta \le 45^\circ\)) (5)
\(\sigma =18.698+0.03207\beta\) (\(\beta \ge 45^\circ\)) (6)
After integrating equations (5) and (6), the average UCS is 22.17 MPa, and the corresponding β values are 34.1° and 108.3°. When the physical and chemical parameter N is less than three, the joint geometric parameter correction of BQ is \(\text{B}{\text{Q}}^{{\prime }}\) and calculated as follows:
\(B{Q}^{{\prime }}=\left(-0.00508\beta +1.174\right)BQ\) (\(\beta \le 45^\circ\)) (7)
\(B{Q}^{{\prime }}=\left(0.00147\beta +0.843\right)BQ\) (\(\beta \ge 45^\circ\)) (8)
In summary, considering the influence of the joint geometric parameters on the USC of a rock mass, the BQ is optimized. The calculation formula is as follows:
$$N\ge 3 B{Q}^{{\prime }}=\left(-0.00144\beta +1.068\right)BQ$$
$$N<3 B{Q}^{{\prime }}=\left(-0.00508\beta +1.174\right)BQ (\beta \le 45^\circ )$$
$$B{Q}^{{\prime }}=\left(0.00147\beta +0.843\right)BQ (\beta \ge 45^\circ )$$