Copyright © IMCET 2019 International Mining Congress and Exhibition of Turkey. All rights reserved.In the last decade, popularity of lignite reserves has tremendously increased as an indigenous alternative to fulfill the growing energy demand of Turkey. Most of the proven reserves are situated in shallow depths and extracted by surface mining. Lignite beds and host rock are prone to show weak rock properties from geomechanical aspect. Rock mass failure or structurally controlled mechanisms may drive any potential slope instability. Developing practical tools for preliminary slope design can be useful for a safe mining operation. This study proposes a modified methodology to predict a safe overall slope angle for weak rock slopes of lignite mines. Rock mass rating (RMR) and slope mass rating (SMR) systems can be remarkably misleading for weak rocks. Fuzzy modification improved these mechanisms. Moreover, structurally controlled instabilities were better estimated. Later, a popular slope performance chart was modified by the fuzzy approach to improve the rock mass failure analysis. Finally, a Turkish lignite mine slope failure was presented. The developed methodology was validated on the case study.