Slope stability analysis

Natural hillsides, cuttings, embankments and dams all stand only because the soil's shear strength along any possible slip surface exceeds the shear stress driving movement. Slope stability analysis finds the weakest such surface and expresses safety as a factor of safety: available strength divided by the strength needed to just hold the slope.

For an infinite slope with seepage parallel to the surface, a closed-form expression shows that a dry cohesionless slope is stable up to its angle of friction, while full seepage roughly halves the safe slope. For finite slopes, circular slip surfaces are analysed with the method of slices: the soil above the trial circle is cut into vertical slices, and forces or moments on each slice are summed. The ordinary (Fellenius) method is simple but conservative; Bishop's simplified method includes inter-slice normal forces and is the usual hand method; Janbu, Spencer and Morgenstern-Price handle non-circular surfaces and satisfy more equilibrium conditions. Software searches thousands of surfaces to find the minimum factor.

Water is the main trigger of slope failures in India, especially in monsoon: rising pore pressure cuts effective stress and strength. Remedies include drainage (surface drains, horizontal drains), flattening or benching, toe buttresses, retaining structures, soil nails, anchors and vegetation. Monitoring with inclinometers and piezometers checks behaviour over time.