Adding a shear key to a rigid retaining is a measure to mainly tackle which form of possible failure?

Question

BlackTom AI · Accepted Answer

A shear key is intended to increase the lateral resistance of the wall by developing additional passive earth pressure beneath the base, so it primarily combats sliding.

1) Relevant concepts:
- Modes of retaining-wall failure: sliding (lateral displacement along base), overturning (rotation about toe), bearing-capacity failure (excess base pressure causing soil shear), global instability (mass movement of the whole ground), and structural failure (insufficient wall strength).
- Resisting forces against sliding include base friction (W · tan δ), cohesion (if any), and passive earth pressure mobilized in front of the wall or under a shear key. A shear key mobilizes additional passive resistance by inserting a recess or projection into the foundation soil.

2) Step-by-step reasoning:
- Driving force (H) = lateral earth thrust (active pressure integrated over wall height).
- Resisting shear (R) = base friction + any passive force from the key + cohesion contribution.
- Factor of safety against sliding FS = R / H. Adding a shear key increases the passive component Pp, so R increases and FS increases.
- The key works by engaging soil in front of/under the key so that when the wall tries to move, the soil must be sheared or compressed — resisting movement directly.

3) Intermediate calculations (conceptual):
- Passive force from a rectangular key of depth d and unit width b can be approximated by integrating p = Kp γ z from z = 0 to d:
  Pp ≈ 0.5 · Kp · γ · d^2 · b.
- This shows Pp grows with d^2, so a deeper key markedly increases resistance to sliding and FS.

4) Why the correct answer is correct:
- Because the shear key directly increases lateral resistance (passive earth pressure) and base shear capacity, its main role is to prevent lateral translation (sliding) of the wall.

5) Why other options are incorrect:
- Overturning: better addressed by increasing base width, adding weight, or toe/buttress — a shear key has limited effect on resisting overturning moment.
- Bearing-capacity failure: resolved by increasing footing area, deeper foundations or improving soil, not primarily by a shear key.
- Global instability: requires slope stabilization or overall ground reinforcement, not just a local key.
- Structural failure: concerns wall material/section design, not primarily mitigated by a shear key.

6) Conclusion:
The shear key’s action increases passive resistance to lateral movement, so the correct answer is Sliding, which matches the provided answer.

Adding a shear key to a rigid retaining is a measure to mainly tackle which form of possible failure?单项选择题

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