Abstract: The surface trace length or the calculated value is often used to represent the ductility of the structural surface in the analysis of rock mass structure. However, the problems such as excessive cutting of structural surfaces or incomplete identification of structures may exist. In this paper, based on the measured trace length l, the probability of discontinuity persistence is analyzed. The probabilistic calculation formula for the formation of discontinuity is derived, and the classification of combination of structural body under different persistence is set up. According to the underground mining project of the zinc polymetallic ore body at the Tongkeng mine, the 3# ore block in the middle section of +255 m is selected as the test area, and the combination relationships of structural body with various possibilities in the mining and pillar area are studied. The results show that the calculated maximum probabilistic interval of the persistence is in the range of 0.5l and 0.8l. The structure combinations can be divided into three types, i.e., minimum, maximum, and variable combinations. There are different structure combinations in the mining and the pillar areas. It is suggested that the area of D6 structural body should be supported after excavation in time. In order to derive the true value of the combinations of structural body more accurately, various monitoring methods of rock mass structure should be supplemented in the project as far as possible. The study results can provide a reference for the analysis of the combination relationship of rock mass structures.

Key words: rock mass structure, persistence of discontinuity, trace length, combination of structural bodies, ore block