This chapter is concerned with the resistance of a soil to failure in shear, a knowledge of which is required in analysis of the stability of soil masses. If at a point on any plane within a soil mass the shear stress becomes equal to the shear strength of the soil then failure will occur at that point. Originally, prior to the postulation of the principle of effective stress, the shear strength (τ f) of a soil at a point on a particular plane was expressed by Coulomb as a linear function of the normal stress at failure (σ f) on the plane at the same point: () τ f = c + σ f tan ϕ https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429209307/b43df8c6-a58e-4416-be47-a3da8aac7534/content/eq276.tif"/>