How to Analyze Mine Slopes

The face angle — also called the batter angle — is the inclination of a single cut face, from its toe to its crest. It is the angle the excavator or the blast produces, and it is the one most often measured in the field.

The bench face angle is measured from the toe of one face to the toe of the face above. Because it includes the berm, it is always shallower than the face angle. It is the angle that describes the repeating unit of the wall.

The overall slope angle runs from the toe of the lowest bench to the crest of the highest, in one straight line. It is the angle the geotechnical study constrains, because it governs the stability of the wall as a whole rather than of any single face. Steepening it recovers waste; getting it wrong is the failure mode that matters.

All three come from the same source: the surveyed surface. From a DEM, slope is computed per cell as the steepest gradient at that point, which gives a continuous slope map — useful for seeing where a wall is steeper than it should be, but it is a per-cell figure, not a wall angle.

The wall angles are geometric, and they need the toe and crest lines. Taking a section perpendicular to the wall and measuring toe-to-crest on that section is the direct method, and it is what a design check reduces to.

The distinction matters because the two answer different questions. A slope raster tells you where to look. A section through the toe and crest tells you what the angle actually is at that point.

Vegetation and equipment. If the analysis runs on a surface model rather than bare ground, a line of trees along the crest raises the crest elevation and flattens every angle measured through it. Cleaning the surface first is not cosmetic; it changes the answer.

Grid resolution. A coarse DEM smooths the break at the toe and the crest, so the measured face angle comes out shallower than reality. The steeper the face, the worse the effect.

One section is not the wall. An angle measured at one station describes that station. Walls curve, faces are irregular, and the section that shows the problem is rarely the one drawn first — the check has to be repeated along the wall.

Design versus as-built. The angle in the design and the angle in the ground diverge as soon as mining starts. The useful comparison is between them, not either one on its own.

A geometric check confirms whether the wall matches the design: is the overall angle within its limit, is the berm wide enough for machine access, is the last bench a full bench or a partial one, does the face angle hold along the wall.

It does not tell you whether the wall is stable. Stability depends on the rock mass, the joint sets, groundwater and blast damage — none of which are in the surface. Slope stability analysis is a separate geotechnical discipline, and a wall that passes every geometric check can still be unstable for reasons the geometry cannot show.

Treating a geometric check as a stability result is the most consequential mistake in this whole area. The check tells you the wall is built as designed; whether the design was right is a different question and belongs to a geotechnical engineer.