Slope stability and retaining walls: when the ground will not hold itself

The mountain roads of Albania show the problem every winter: a slope that has stood for years moves after a few days of rain. The landslide seems sudden, but almost never is. It is the result of an equilibrium that was always fragile and that water upset.

What the factor of safety measures

Stability analysis compares the forces that hold the soil mass in place with those that push it to slide. The ratio between them is the factor of safety. A value below one means movement; a value just above one means that any small change — a long rain, an excavation at the toe of the slope — is enough to cross the limit. That is why design does not simply aim to exceed one, but to leave a margin.

Water is the decisive factor

A rise in the groundwater level reduces the effective pressure between grains and, with it, the shear strength. This is why most landslides occur after periods of rainfall. Consequently, drainage — drains, weep holes, upper ditches — is often the most effective and cheapest measure. A well-calculated wall without drainage acts as a dam and adds precisely the force it was meant to hold back.

When a wall and when a slope

If space allows, flattening the slope is the most durable and most economical solution. A retaining wall comes in when space is lacking or when the elevation difference is large. In that case four things must be checked together: sliding, overturning, the bearing capacity of the soil beneath the base and the strength of the concrete section itself.

How AlbGIS helps

• Slope Stability — factor of safety using the Simplified Bishop and Fellenius methods, with automatic search for the critical circle and sensitivity analysis.
• Retaining Wall — verification using the Rankine method for cantilever, T, L and gravity walls, with pressure diagrams.
• Soil Bearing Capacity — a check according to Terzaghi for the wall foundation, with a safety factor.