Rock Mass Stability Analysis at an Underground Opening: The Case of Lega Dembi Gold Mine, Shakiso, South Ethiopia

Abstract

The stability of rock mass is critical in an underground excavation because it ensures the safety of the working environment and the successful ore exploration. Numerical Rocscience softwares enable rapid assessments of the support requirements for underground excavations at any stage of a project, even when geotechnical data is limited. The underground excavation frequently relies on Numerical analysis and other methods such as rock mass classification methods, which provide a quick way of assessing the rock mass quality and support requirements. Instabilities in rock slopes are a major geological hazard that frequently causes the failure of various civil engineering structures such as open-pit mines and tunnels. The current research was conducted at the Lega Dembi underground gold mine, specifically in Shakiso, South Ethiopia. The tunnel length, width and height are 1.5km, 5m and 5.5m respectively. It used for ore transportation and frequently impacted by rock slope instabilities. As a result, the purpose of this research is to identify and analyze critical rock slope sections along this tunnel using the Finite Element Method (FEM), kinematic analysis, Rocdata, unwedge, and Rocsupport methods. The main lithologies along the tunnel line are (1) Metagranodiorite (2) Amphibolite (3) Quartz Mica Schist and (4) Graphitic Quartz Mica Schist. On the specimen obtained from core and surface outcrop the following tests are carried; dry and saturated density, slake durability and point load tests are carried out. Dry and saturated test results range from 24 to 25KN/m3 and 26-27KN/m3 respectively. Graphitic-quartz mica schist has a lower strength value than the other rocks and amphibolite is the very competent geological unit. From laboratory strength tests and other Strength tests design parameters determined from rock lab Software. Based on the geological and Tunnel geometry information, the kinematic analysis shows two dominant failures mode that are planar and wedge failures that had occurred at sakaro underground opening. The support system also design for the access tunnel using bolt only and bolt with shotcrete. The results shows the use of a bolt with a diameter of 17mm, spacing 0.25 and 1m length and a factor of safety 2.44 which is acceptable from economical and safety point of view. This factory of safety is at which point the tunnel would be stable, and this is accomplished with numerical modelling of underground rock mass stability, including an investigation of the effectiveness of rock supports.