Derive an expression for the factor of safety FS for a planar block slide with a tension crack behind the crest. The slope height is H and has a face angle β measured from the horizontal to the slope face; the failure surface is inclined at α degrees to the horizontal. Tension crack depth is h. Only the force of gravity acts on the slide mass; specific weight is γ. A Mohr–Coulomb failure criterion applies to the failure surface (cohesion is c, angle of internal friction is φ). Note: The slide mass extends b units into the plane of the page. Show schematically a plot of FS (y-axis, range from 0.0 to 4.0) as a function of the reciprocal of the slope height H−1 (x-axis, range from 0.0 to 0.001) and discuss briefly the form and significance of the plot.