Free Online Slope Stability Risk Calculator: Probability of Failure

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The infinite slope model runs 10,000 times with strength, weight, and pore pressure drawn from their ranges. The headline is not the factor of safety but the probability it falls below 1: a slope at FoS 1.3 with scattered soil data can still fail one year in twenty, and this is the tool that says so.

A factor of safety with a probability

A single factor of safety hides the question that matters: given how scattered the soil data really is, how often does the slope dip below 1? The tool above answers it. Cohesion, friction angle, unit weight, and pore pressure each get a range, the infinite slope model runs 10,000 times, and the output is a distribution of safety factors with the failure probability read straight off it. A slope can carry a respectable P50 of 1.28 and still hold a measurable chance of failure in a wet year, and that sentence is worth more than any single number.

Histogram of Monte Carlo factor of safety outcomes for a soil slope with the FoS equal to one line marked, beside a tornado chart ranking friction, cohesion, and pore pressure
Ten thousand versions of the slope, and the only question that matters: how often does it dip below 1?

The pore pressure ratio is where the weather lives: widen its range for a slope with poor drainage and watch the failure probability respond.

What the numbers mean

  • Probability of failure below about 1 percent is broadly consistent with routine slopes; above a few percent deserves drainage, flattening, or better data.
  • The tornado usually ranks friction angle and pore pressure at the top, which is why lab shear tests and piezometers earn their cost.
  • The infinite slope model suits shallow planar sliding in long slopes; deep rotational failures belong to the Bishop tool this one links to.
  • Ranges narrower than your data justifies produce confidence you have not earned; the tool is a mirror, not a shield.

What this tool is for

Screening cut slopes and natural hillsides probabilistically, explaining to a client why FoS 1.3 is not always enough, and connecting soil variability to risk in the classroom.

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