Slope Stability Calculator

In the shear-strength term, (γz cos²β − u) × tanφ, pore pressure u is subtracted from the total normal stress before friction is applied — this is the effective-stress principle, where only the stress actually carried by the soil skeleton (not the water) contributes to frictional resistance. A rainstorm or rising water table raises u without changing the soil's cohesion or friction angle, which directly lowers the calculated shear strength and factor of safety in this formula, matching how real slopes often fail after heavy rain.

The calculator searches angle by angle, starting from your entered slope angle and stepping upward, until it finds the angle where the factor of safety would drop to exactly 1.0 given your soil's cohesion, friction angle, unit weight, and pore pressure. That angle represents the steepest grade the slope could theoretically reach before failing outright, which is useful for judging how much safety margin your current slope has, or how much flatter an unstable slope would need to be regraded.

A factor of safety of exactly 1.0 means the slope is at the theoretical brink of failure, with shear strength exactly equal to driving stress — leaving no margin for uncertainty in soil properties, unexpected groundwater rise, or seismic loading. Common geotechnical practice targets 1.5 for permanent slopes to build in that margin, while temporary construction slopes (open for a shorter, more controlled period) are sometimes designed to a lower threshold like 1.3.

The infinite slope method assumes failure occurs along a shallow plane parallel to the ground surface, with the slope's length large enough relative to the slip depth that the edges don't affect the result — that assumption breaks down for deep-seated, curved (rotational) failure surfaces, which don't run parallel to the ground and involve soil mass rotating rather than sliding on a flat plane. Slopes suspected of deep or rotational failure modes need a full limit-equilibrium method like Bishop's or Spencer's, which model a curved slip surface directly.