## 1 Introduction  
With the development of traffic engineering, an increasing number of tunnels are being constructed in areas with poor geological conditions [1, 2]. Classification of surrounding rock is the basic work of tunnel engineering, and an accurate surrounding rock classification can provide a reliable basis for tunnel design and construction. Tunnel surrounding rock refers to the rock mass within the range of stress redistribution generated around the tunnel excavation or the rock mass affecting its stability after tunnel excavation [3]. The concept of surrounding rock classification was first proposed by [PERSON], a European scholar, in the 1870s, when he systematically classified limestone. In essence, the surrounding rock classification is used to grade and evaluate the stability of the surrounding rock within the influence range of tunnel excavation. By the end of the 18 th century, [PERSON] qualitatively divided rocks into five categories: crumbly rock, soft rock, fractured rock, subband rock, and hard rock [4], making the concept of surrounding rock classification gradually known to the engineering community. The surrounding rock classification method has been considered by scholars, who have proposed hundreds of classification methods. In the early 20 th century, Platt grading appeared, and in 1946, the Terzagi classification appeared [5]. The surrounding rock classification method considers the factors of the support design and ground stress. Since the 1950s, there has been a classification method for evaluating the stability of engineering rock masses and their corresponding support forms. In 1969, a classification method based on the rock quality index (RQD) was developed. In 1973, [PERSON] established a geomechanical rock mass rating (RMR) classification method based on jointed rock masses [6]. In 1974, [PERSON] classified rocks using the rock quality coefficient Q [7; 8]. The mine rock mass classification (MRMR) system was introduced in 1975 [9] and has been modified and expanded many times [10; 11]. It has been widely used in coal mining, especially in Australia. The application research of engineering rock classification was developed in the 1960s in China [12]. From 1954 to 1972, the Platt classification standard with the rock hardness coefficient as the index was mainly used. This classification method had a single index and great limitations, but it was economical and reasonable and was widely used at that time. After 1972, based on a large number of underground constructions and a rich understanding and experience of underground engineering rock mass, it became a multi-index, multi-type, and substantial classification method. With the application of acoustic technology in Japan's national railway, in 1981, the acoustic surrounding rock classification method for underground caverns was proposed [12]. Since the 1980s, a large number of classification methods have emerged. In 1984, the dynamic classification method of surrounding rock stability was proposed. In 1985, the 89,003 unit of the Chinese People's Liberation Army proposed the classification of tunnel engineering surrounding rock and its application in the covering design. In 1985, the Southwest Research Institute of the Scientific Research Institute of the Ministry of Railways proposed rock mass classification of railway tunnel engineering. In 1988, the geological study of large underground caverns of hydropower stations and the classification of surrounding rock were proposed. However, there was no unified rock classification standard. From 1989 to 1991 [13], the Standard for Engineering Classification of Rock Masses (GB 50218-94) was compiled [14], which stipulated that the classification of surrounding rock should be based on the BG index of rock hardness and rock mass integrity. The index was modified by considering the groundwater state, main structural plane occurrence, and initial ground stress.