Earth Pressure with Cohesion – Effect of c on Active and Passive States In classical Rankine/Coulomb earth pressure theory for cohesive backfill (c ≠ 0), how does soil cohesion affect the magnitudes of active earth pressure and passive earth resistance?

Introduction / Context:
Earth pressure theories predict lateral stresses on retaining structures. When soils possess apparent cohesion c (in addition to friction), the lateral pressure expressions include cohesive terms that alter both active and passive magnitudes. Understanding the direction of this influence is vital for safe and economical wall design.

Given Data / Assumptions:

• Backfill has shear strength parameters c (cohesion) and φ (friction angle).
• Rankine framework for simplicity; similar trends apply in Coulomb theory.
• Unit weight γ and wall height H are otherwise unchanged.

Concept / Approach:

For cohesive-frictional soils, the active pressure at depth z is reduced by a cohesion term, while the passive pressure is enhanced by a cohesion term. In Rankine form for a vertical wall with horizontal backfill, the resultant forces include ± 2c√K terms (per unit height) multiplied by H, where the sign is negative for active (reducing it) and positive for passive (increasing it).

Step-by-Step Solution:

Verification / Alternative check:

Mohr–Coulomb envelopes show that cohesion shifts the failure line upward. In the active case, less lateral stress is needed to reach failure (so pressure reduces). In the passive case, higher lateral stress can be mobilized before failure (so resistance increases).

Why Other Options Are Wrong:

• Decreasing both or increasing both: Contradict the opposite-sense effect of c on active vs passive.
• Increasing active and decreasing passive: Opposite to the effect predicted by the equations.

Common Pitfalls:

Using short-term undrained c values in long-term drained analyses; neglecting tension cut-off in the upper zone of active pressure where cohesion may predict unrealistic tensile stresses.

Final Answer:

Decreases active pressure and increases passive resistance