Rock mass classification is one of the most effective method for the stability evaluation of rock mass. Based on the classification standard in China, uniaxial compressive strength and rock mass integrity index are introduced to build a new engineering rock mass classification standard. A huge data added to the former data base, combining with the engineering experience over 20 years, and the particularity of underground engineering are taken into account to modify the basic quality (BQ) index. The new cla ssification method is assessed by comparing with existing BQ method. The influence of integrity of rock mass becomes significant while the effect of uniaxial compressive strength decreases in our proposed equation. Two sub-classification is added in class III and IV with five existing classes remains the same. Adding the sub-classification of weak rock mass can effectively improves the evaluation precision of rock stability.
Engineering classification of rock mass is the basic research topic and method in rock mechanics which is of great importance in engineering practice. The classification of rock mass is not only the preliminary condition for engineering construction survey, design, and preparation of quotas, but also the necessary conditions for the economically selection of projects, feasibility assessment, rock reinforcement design and safety construction. The engineering rock mass classification method can be divided into two categories, that is, general classification and specific classification. The general classification method is applicable to all types of rock masses, and is not used for specific projects. The specific classification method is designed for various types of engineering, such as rock mass classification for the tunnel slope rock foundations.
BQ classification refers to the "Standard for Engineering Rock Mass Grading" (Lin 1996; Bieniawski et al. 1796; Hougton et al. 1976; Wu 2012; Lin 1999), which is jointly drafted by the Ministry of Water Resources, the Ministry of Construction, the Ministry of Railways and other departments. The BQ classification provides a unified grading method applicable to various rock mass projects. It is a combination of multi-factor and multi-indicator, as well as qualitative and quantitative analysis, which ranks engineering rock mass in two steps. The first step is to classify the qualitative and quantitative properties of the rock mass by the rock hardness and integrity. The rock mass quality grade is divided into five grades according to the quality and stability of rock mass. The second step is the engineering rock mass classification. Other important factors affecting the basic quality of the engineering rock mass (correction factors) are considered according to various engineering characteristics. The BQ value is modified and then the specific engineering rock mass grading is determined.
