Establishment of classification formula of rock mass around road tunnel in metamorphic schist

Metamorphic schist, because of its low strength and instability under external forces, is often considered as soft rock category. The practice of multiple projects shows that the factors influencing the stability of the tunnel surrounding rock in metamorphic schist are different, and their mechanism of action are also distinct. But, they were not considered fully in the various classification systems on surrounding rock mass in China and abroad. This paper, on the basis of China's Code for Design of Road Tunnel, considering the differences of influencing the failure mechanism of the rock mass around a tunnel in metamorphic schist, establishes a "Triple Index System" suiting for the tunnels in metamorphic schist which is consisting of the basic rating index, the modified rating index and the additional rating index, and also proposes a BQSM formula and two limit formulas of the basic quality index of rock mass, which are obtained from the statistic regression of lots of field samples, and creates the BQSM formula and classification model for the classification of rock mass around a tunnel in metamorphic schist, which have been repeatedly used and corrected in the field. Application of the formula to the field classification of the 12 road tunnels along the Shiyan to Man Chuanguan Expressway shows that the result of its calculation has a higher coincidence-rate in comparison with the actual classification in the field. It proves that the formula is reasonable and feasible. The study results in this paper have provided a new approach and ideas for the classification of rock mass around a tunnel in metamorphic schist area or in stratified weak rock mass. © 2013, EJGE.