Question

Presence of tail-water in a gravity dam ____________________

(a) increases the principal stress and decreases the shear stress

(b) increases both the principal stress and the shear stress

(c) decreases the principal stress and increases the shear stress

(d) decreases both the principal stress and the shear stress

I had been asked this question in an online quiz.

My query is from Design of Gravity Dams in section Gravity Dam Design and Construction of Irrigation Engineering

Answer 1

The correct answer is (d) decreases both the principal stress and the shear stress

To elaborate: The principal stress is given by the formula –

P = Pv sec(Φ)^2 – p’ tan(Φ)^2 where Pv is the intensity of vertical pressure and p’ is the tail-water pressure

The shear stress on the horizontal plane near the toe is given by –

S = (Pv – p’) tan(Φ)

From both the equations, it is clear that the tail-water pressure is opposite in nature and it reduces the value of principal stress and shear stress.