Effects of joint orientation and spacing on the boreability of jointed rock mass using tunnel boring machines

Rock mass boreability plays a key role in the performance of tunnel boring machines (TBMs). The angle and orientation of the plane of joint are two factors which affect the boreability of the jointed rock masses. This paper is aimed at describing the combined effects of joint orientation and spacing on the boreability and presenting a new model and classification system to predict the boreability of the jointed rock masses, focusing on field studies. In this regard, a database of three water conveyance projects including the Queens Tunnel in USA, Kerman Tunnel in Iran, and Maroshi-Ruparel Tunnel in India has been employed. A total fracturing factor (presented by Bruland) has been applied as well. The results showed that the increase in the total fracturing factor will result in higher boreability. A model was developed for predicting the boreability of the jointed rock mass based on the total fracturing factor, and the convergence coefficient of this model was 0.7. The volumetric joint count classification system was also developed to analyze the relationship between the total fracturing factor and the volumetric joint count. In order to predict the boreability of the jointed rock mass in the Zagros Tunnel in Iran, the boreability was classified in three categories. The result showed a good agreement between the predicted and actual boreability of the jointed rock mass. © 2021, Saudi Society for Geosciences.