Identification of non-pyramidal key blocks in jointed rock masses for tunnel excavation

摘要

Key block methods based on ubiquitous approaches have been widely used over the past 30 years to perform rapid analyses of rock mass stability. Although these methods have meant a great step forward, they only consider pyramidal key blocks. This paper provides a new geometrical method for identifying both pyramidal and non-pyramidal key blocks in discontinuous rock masses for tunnel excavation. Using a ubiquitous approach, the non-pyramidal key blocks are determined by geometrical dislocation in relation to the advance in the direction of the tunnel axis. The blocks are parameterized with the advance and the block with the maximum volume is calculated. We point out that non-pyramidal pentahedral blocks larger than the pyramidal tetrahedral may be found depending on discontinuity planes. These pentahedral blocks will never be obtained by truncation of a tetrahedral block. The concepts introduced in this study have been implemented in the ASTUR computer program. Using this program, we select the orientation in which a tunnel must be excavated so as to generate the minimum number of large volume key blocks. We conclude that identification of pyramidal and non-pyramidal key block are necessary when studying tunnel stability. It is not enough to analyze pyramidal blocks as is proposed in most of the ubiquitous approaches used nowadays.