The planes that exist in soil mass is ______________

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Option A: Principal plane

Explanation & Concepts:
In a soil mass, at any point, there exist specific planes on which the shear stress is zero. These planes, where only normal stresses (principal stresses) act, are defined as principal planes.

Working Principle: According to the principle of stress transformation in geotechnics, any stress state can be represented by a Mohr's circle. The points where the Mohr's circle intersects the horizontal axis ($ au = 0$) correspond to the major and minor principal stresses acting on mutually perpendicular principal planes. These planes are fundamental for analyzing the failure criterion of soil (e.g., Mohr-Coulomb failure envelope).

Key Formulas: $	au = \frac{(\sigma_1 - \sigma_3)}{2} \sin(2	heta)$ — Shear stress on a plane inclined at $	heta$; $\sigma_n = \frac{\sigma_1 + \sigma_3}{2} + \frac{\sigma_1 - \sigma_3}{2} \cos(2	heta)$ — Normal stress on a plane inclined at $	heta$

Key Points: Principal planes are always mutually perpendicular.; On principal planes, the shear stress component is strictly zero.; The major principal plane is the plane on which the maximum principal stress acts.; The minor principal plane is the plane on which the minimum principal stress acts.

Advantages: Simplifies stress analysis for soil stability.; Essential for calculating shear strength parameters $c$ and $\phi$.

Disadvantages: Does not account for non-homogeneous soil layers directly.; Assumes soil as a continuum, which may not hold for fragmented rock masses.

Applications: Slope stability analysis.; Bearing capacity calculations for foundations.; Earth pressure determination for retaining walls.

Additional Info: Option B (Principal stress) is a physical quantity (a force per unit area), not a geometrical plane.. Option C (Stress plane) is a vague term not recognized in standard soil mechanics terminology.. For soil, $\sigma_1$ is typically the vertical stress and $\sigma_3$ is the horizontal confining stress in triaxial conditions.

Answer: A is correct — Principal planes are the specific planes within a soil mass where the shear stress is zero and the normal stress reaches its maximum or minimum value.

Exam Tip: Always remember that the angle between the major and minor principal planes is exactly $90^\circ$.

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Option B: Principal stress

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Option C: Stress plane

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Option D: None of the mentioned

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