The angle of internal friction of soil mass is the angle whose

ANS:A - tangent is equal to the rate of the maximum resistance to sliding on any internal inclined plane to the normal pressure acting on the plane

The angle of internal friction (ϕ) is a fundamental parameter used in soil mechanics to characterize the shear strength properties of soils. It represents the angle at which a soil mass can resist shear stresses before it starts to undergo internal failure and undergoes sliding along an inclined plane. To understand the concept more thoroughly:

1. Internal Inclined Plane: When an external force is applied to a soil mass, it induces shear stresses within the soil. These shear stresses act on internal inclined planes within the soil mass.
2. Maximum Resistance to Sliding: The soil's ability to resist sliding along these internal inclined planes is quantified by its shear strength. The maximum resistance to sliding refers to the maximum shear stress that the soil can withstand before sliding occurs.
3. Normal Pressure: The normal pressure acting on the plane refers to the perpendicular pressure exerted by the soil mass on the inclined plane. This pressure acts perpendicular to the plane's surface.
4. Angle of Internal Friction: The angle of internal friction (ϕ) is defined as the angle between the inclined plane and the normal to the plane, at which the soil mass just begins to slide due to shear stresses. Mathematically, the tangent of this angle is equal to the ratio of the maximum resistance to sliding on any internal inclined plane to the normal pressure acting on the plane.

So, when we say "the angle of internal friction is the angle whose tangent is equal to the ratio of the maximum resistance to sliding on any internal inclined plane to the normal pressure acting on the plane," we are essentially describing the fundamental definition of the angle of internal friction in soil mechanics. This angle is crucial in analyzing the stability of soil slopes, determining bearing capacities of foundations, and designing earth-retaining structures, among other geotechnical engineering applications.