Natural Slope of Cohesionless Soil – Names for the Limiting Plane at Angle φ A plane inclined at an angle equal to the soil’s angle of internal friction φ to the horizontal is the inclination at which loose cohesionless soil can stand without lateral support. This plane is commonly called:

Introduction / Context:
In stockpiles and embankments of cohesionless soil, the steepest stable slope under self-weight is characterized by the angle of repose, very close to the angle of internal friction φ for clean, dry sand. The corresponding plane at angle φ to the horizontal is referred to by several synonymous terms in practice.

Given Data / Assumptions:

• Cohesionless soil, dry or drained condition.
• Stability governed by frictional resistance.
• Small-scale surface effects (fabric, particle shape) may cause minor deviations.

Concept / Approach:

The limiting natural slope occurs when the downslope component of weight is balanced by frictional resistance with no lateral support. In routine usage, the terms 'natural slope line', 'repose line', and 'φ line' all describe this limiting plane. While precise definitions may vary across texts, they point to the same physical concept: the slope at or near the friction angle.

Step-by-Step Solution:

Verification / Alternative check:

Photography of dry sand heaps shows faces at a nearly constant angle, and shear box tests quantify φ to which the repose angle is closely related.

Why Other Options Are Wrong:

'None of these' is incorrect because the listed terms are acceptable synonyms for the concept.

Common Pitfalls:

Confusing angle of repose with compacted slope design angles that include stability factors and drainage considerations.

Final Answer:

All of the above