The Infinite Slope

On a plane parallel to a long slope the shear is fixed by the weight, τ = γz sin α cos α. Friction can resist tan φ′ times the effective normal stress. Raise the water table and the water takes part of the squeeze: the grains are pressed together less, and friction resists less.

What you are seeing

• A slice of the slope. The soil down to the slip plane (dashed brown), with a slice and its weight W. The water between the water table and the slip plane is shaded blue, with arrows showing the flow. Water flows parallel to the surface, so the equipotentials (dotted) are perpendicular to it. The standpipe at the slip plane shows the pore pressure there as a head, u/γ_w.
• Factor of safety. FS against slope angle: dry (blue), with the water table at the surface (dark blue), and at the current water table (thick orange). The dot is the slope now, red if it fails. The dashed line marks the steepest slope that stands.

Assumptions

The slope is infinitely long, with seepage parallel to its surface. The unit weight is taken the same above and below the water table.