With flow, seepage changes pore pressure, therefore effective stress. Strength and permeability are affected by the level of effective stress. In Figures a and b below, two Mohr-Coloumb failure envelopes are shown. Assume that Figure b is used to represent the stress conditions at which a block were to slip along an inclined plane. Estimate the angle of inclination at which the block would slide down the plane.
Point of tangency represents shear and normal stresses on the failure plane at failure.
This FS is for given failure surface only. Suppose the slope in the figure above were subjected to a period of heavy rain. Rainfall could increase degree of saturation and therefore the total unit weight. Rainfall could increase the water content of the soil, decreasing the cohesion c, or undrained strength. Or, rainfall could increase the pore pressure which would decrease the effective stress and the strength.