Geomechanical characterization of rock mass rating and numerical modeling for underground mining excavation design

Abstract

The objective of the study is the geomechanical characterization of the rock mass rating RMR system and numerical modeling for mining underground excavation design of the Djebel El Ouahch tunnel, in Constantine (Algeria).The geological and geotechnical character- ization of the rock mass is important for the design of underground mining excavations. In this article, we present the results of the RMR characterization of the rock mass and the numerical modeling by the finite element method (FEM), under the conditions of the Djebel El Ouahch tunnel, Constantine (Algeria).The RMR system is a useful tool for characterization of the rock mass quality and establishing the appropriate support system. For poor rock (Class IV), the excavation should be top heading and bench 1.0 m – 1.5 m advance in top heading. Support should be installed concurrently with excavation, 10 m from face. Rock bolting should be systematic with 4 m – 5 m long, spaced 1.5 m – 1.5 m in the crown and walls with wire mesh, Shotcrete of 100 m -150 mm in the crown and 100 mm in sides. The steel sets should be light to medium ribs spaced 1.5 m only when required. The rock mass consists of generally poor rocks with average stand- up time of 10 hours for 2.5m span with mass cohesion ranges between 100 kPa – 200 kPa and rock mass friction angle ranges from 15° to 35°. The FEM project due to its precision calculates the safety factor and evaluates the principal deformations and displacements of the rocks mass .The originality of this work lies in the use of two different approaches , the RMR system and numerical method (FEM) for analyzing the quality and evaluation of the deformations and displace- ments of rock mass .This method has become a very common practice in underground mining excavation design.This study illustrates that the results obtained by RMR of the argillite rock mass in the case is 28.00 ,ranging from 21.0 to 40.0 classified as Class IV (Poor Rock), while the results of FEM reveal that in accordance with the poor quality of the rocks, large deformations and displacements were observed around the underground mining excavation, which can be at the origin of the ruptures. The value of the safety factor of the order of 0.95 to 1.24 shows the instability of the excavation, and the appearance of very considerable hazard zones in the argillite layer.