In open-pit mining, the design of a slope is rarely a “one-size-fits-all” calculation. Instead, engineers partition the pit into geotechnical domains and account for structural controls to ensure the walls remain stable while maximizing ore recovery.
Geotechnical domains: creating the “Design Map”
A geotechnical domain is a volume of rock that shares similar geological, geomechanical, and hydrogeological characteristics. Instead of treating the entire mine as a single block, engineers divide it into these domains to apply specific design parameters [1].
Domains allow for different slope angles in different parts of the pit [2]. For example, a domain consisting of fresh, hard granite can support a much steeper slope than a domain of weathered, clay-rich schist. They help predict how a slope might fail. In a domain of highly fractured rock (low RMR/GSI), a circular failure (like a soil landslide) is more likely. In a domain with massive rock and distinct joints, wedge or planar failures are the primary concern.
Mines can drastically reduce the “stripping ratio”, the amount of waste rock removed to get the ore and save millions of dollars in excavation expenses by steepening slopes in competent rock through precise domain definition.
Structural controls: the “Skeleton” of stability
Structural controls refer to the physical discontinuities in the rock mass, such as faults, joints, bedding planes, and foliation [3]. These represent planes of weakness that dominate the stability of the rock [3].
Structural sets often dictate the height of a bench and the width of a “catch berm.” If a major joint set is prone to small-scale toppling, berms must be wide enough to catch falling debris before it hits workers below. Major structures like faults can act as “release surfaces.” Even if the rock mass is strong, a single large fault dipping into the pit can trigger a massive, deep-seated slope failure.
Rock is often “anisotropic,” meaning its strength varies depending on the direction of stress [3]. Structural mapping allows designers to account for this by orienting the pit walls to avoid unfavorable intersections with these weak planes.
Ignoring structural variability can result in unduly optimistic slope angles, unanticipated failures, and costly redesigns. Detailed structural mapping, orientated core logging, and photogrammetry are all necessary methods for detecting important discontinuities and determining domain boundaries.
In modern design, these two factors are combined into a 3D geotechnical model. This model tracks the “interplay” between the two: a domain might have high rock strength, but if a major structural fault cuts through it, the design must be adjusted (flattened) to account for that specific structural risk.
Reference
[1] M. Bester et al., “A risk-based methodology to improve the definition of geotechnical sectors in slope design,” Journal of the Southern African Institute of Mining and Metallurgy, vol. 119, no. 12, pp. 1027–1038, Dec. 2019, doi: 10.17159/2411-9717/685/2019.
[2] V. Vergara, O. Cabello, and V. Pérez, GEOTECHNICAL DESIGN STRATEGIES IN OPEN PIT MINES WITH THE PRESENCE OF OLD UNDERGROUND EXCAVATIONS. 2022.
[3] M. Alm and Y. Sabry, “Analysis of geotechnical characteristics of open pit rock masses implementing structural discontinuities in the limit equilibrium method,” Discovery Nature, vol. 2, pp. 1–12, Dec. 2025, doi: 10.54905/disssi.v2i4.e15dn3149.
