Retaining wall design — identify the incorrect statement (choose the wrong requirement) Which of the following statements about retaining wall safety and drainage is incorrect? Select the one that does not reflect good engineering practice.

Introduction / Context:
Retaining walls stabilize soil at grade separations. Their design must ensure equilibrium against external actions and include provisions for drainage to reduce pore water pressures behind the wall. A single incorrect detail can critically undermine performance. This item tests recognition of a commonly misstated drainage provision.

Given Data / Assumptions:

• Conventional gravity or cantilever reinforced concrete retaining wall.
• Backfill subject to rainfall and infiltration; hydrostatic pressure must be relieved.
• Standard stability checks include overturning, sliding, and bearing capacity.

Concept / Approach:

Correct design requires: sufficient weight or structural resistance to lateral earth pressure, sliding safety using base friction/keys, and bearing pressures within allowable limits including eccentricity checks. For drainage, weep holes (and drainage layers or pipes) are placed near the base region of the retained soil, not near the top, so that water can exit where hydrostatic pressures accumulate. A graded filter or geotextile prevents soil loss through weep holes.

Step-by-Step Solution:

Verification / Alternative check:

Typical details show a drainage blanket (free-draining gravel) behind the wall with a perforated collector near the base and weep holes exiting through the stem at low elevations. Water naturally accumulates low in the backfill because of gravity, so outlets must be provided there.

Why Other Options Are Wrong:

• (a)–(d) are correct and essential checks: resisting earth pressure, preventing overturning/sliding, and limiting bearing pressure are core requirements.

Common Pitfalls:

• Omitting filters on weep holes causing soil piping and staining.
• Ignoring surcharge and seismic increments in stability calculations.

Final Answer:

Weep holes for drainage should be provided near the top of the retaining wall to drain the retained earth.