A skin of soil sliding on rock or on a stiffer layer beneath. Because the slide is long compared with its depth, the ends do not matter and the whole problem collapses to one slice — which is why this is the one slope calculation worth being able to do in your head.
| FS | what it means |
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A factor of safety is not a probability. It is the ratio of what resists to what drives, computed from parameters that are themselves uncertain — and the friction angle is usually known to within a few degrees at best. An FS of 1.3 built on a guessed φ is worth less than an FS of 1.1 built on a measured one.
A rock slope sliding on one discontinuity — a bedding plane, a joint, a fault. It only happens when the plane daylights in the face and dips less steeply than the face but more steeply than its own friction angle, and the panel checks those conditions before it computes anything, because a factor of safety for a failure that cannot occur is worse than useless.
In soil the slip is curved, so the mass is cut into slices and every slice is weighed. Two methods are run and both are shown — Fellenius, which ignores the forces between slices and is therefore always conservative, and Bishop, which does not and is closer to the truth. The search finds the worst circle, not a circle you chose.
Slopes that stood for a century fail in a wet week. Water does two things at once — it adds weight, and it takes away the friction that was holding the grains together — and the second is much the larger effect. This works out how much water it would take.
| what you see | what it means |
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It assumes the ground is one material with one strength, that the slip surface is the shape it was told, and that the water pressure is what you typed. Real failures are usually about the thing that was not in the model — a clay seam, a relict shear surface, an old landslide reactivating, a leaking drain. A calculation cannot find those; walking the ground and reading the history can.
And it computes drained stability throughout, with c′ and φ′. A rapid load on clay is an undrained problem with φ = 0 and a quite different answer.
Whether a wedge of rock can slide at all, from the orientations of the joints, is the Markland test in Rock mass and stability. Bearing capacity and settlement are Foundations and tunnels, and the rainfall that puts the water there is in Groundwater flow.