CSLB General Building (B)Site Work, Foundations and ConcreteHard
A contractor is constructing a concrete retaining wall that will hold back saturated backfill soil. To prevent excessive hydrostatic pressure from building up behind the wall, which drainage feature should be incorporated at the base of the wall?
- AA vapor barrier placed directly against the back face of the wall
- BA thicker layer of compacted clay placed immediately behind the wall
- CAn additional layer of rebar placed closer to the backfill side
- DA perforated drain pipe embedded in gravel with weep holes through the wall
Show answer & explanationAnswer & explanation
Correct answer: D. A perforated drain pipe embedded in gravel with weep holes through the wall
A perforated drain pipe surrounded by free-draining gravel, combined with weep holes through the wall, channels groundwater away and relieves hydrostatic pressure that would otherwise increase lateral loads on the retaining wall. This is standard practice per retaining wall design details.
Why the other options are wrong
- A. A vapor barrier would trap water behind the wall, worsening hydrostatic pressure.
- B. Compacted clay is relatively impermeable and would trap water, increasing pressure.
- C. Additional rebar addresses structural strength, not water pressure relief.
Retaining Wall Drainage
A system of perforated pipe, gravel backfill, and weep holes designed to relieve hydrostatic pressure behind a retaining wall by channeling groundwater away.
- Free-draining gravel wrapped in filter fabric surrounds the perforated pipe
- Weep holes allow water to exit through the wall face as backup drainage
- Without drainage, hydrostatic pressure can significantly increase lateral loads and cause wall failure
Memory trick: Give water a way out, or the wall will pay the pressure.