Highwall stability assessment in strip mining operations: The case of complex multilayer deposit with thick overburden

Strip mining is a cost-effective and high-productivity method that is widely used for the exploitation of stratiform deposits. The assessment of highwall stability is a fundamental requirement that conditions the technical feasibility of this method and determines its operational limits. This paper presents a stability assessment of a complex multilayer phosphate deposit in Morocco, characterised by a tight alternation of phosphate ore seams and waste interlayers combined with progressively increasing overburden thickness. In fact, the stratigraphic analysis of the exploited deposit revealed a progressive increase in overburden thickness in the direction of mining advance, defining the complex geological framework within which the simulation scenarios were built. To evaluate highwall stability under these conditions, a limit equilibrium approach was adopted. The results show that overburden thickening leads to a progressive reduction of the factor of safety which defines a technical limit for conventional strip mining. Another finding is a clear shift in the controlling factor of stability beyond 20 m, where the critical slip surface relocates within the overburden layer. These findings provide a practical basis for the anticipation of geotechnical constraints and the adaptation of mining operations before they reach critical configurations.