CSLB C-27 Landscaping ContractorSite Preparation and DrainageHard
A landscape contractor is installing a retaining wall on a sloped property to create a level planting bed. The wall will be 4 feet tall and constructed from segmental concrete blocks. Which of the following is the most critical factor to consider during the excavation and preparation of the wall's foundation to ensure its long-term stability and prevent failure?
- AExcavating a trench for the footing that extends below the frost line for the region.
- BProviding adequate drainage behind the wall with granular backfill and a perforated pipe.
- CCompacting the subgrade beneath the footing to a minimum of 98% Standard Proctor density.
- DEnsuring the base course of blocks is laid on a perfectly level sand setting bed.
Show answer & explanationAnswer & explanation
Correct answer: C. Compacting the subgrade beneath the footing to a minimum of 98% Standard Proctor density.
While all options are important, proper compaction of the subgrade beneath the footing is the single most critical factor for the long-term stability of a retaining wall. An inadequately compacted subgrade will lead to differential settlement, causing the wall to lean, crack, or fail, regardless of other design elements.
Why the other options are wrong
- A. Extending below the frost line is important in regions with freezing temperatures to prevent frost heave, but California generally has mild winters where this is less critical than subgrade compaction for overall stability.
- B. Drainage behind the wall is crucial for reducing hydrostatic pressure, but if the foundation itself settles, the wall will still fail.
- D. A level setting bed is important for initial construction, but a settling subgrade will undermine this regardless.
Retaining Wall Foundation Compaction
The process of densifying the soil beneath a retaining wall's footing to achieve uniform support and prevent settlement, which is vital for structural integrity.
- Subgrade compaction is paramount for retaining wall stability.
- Prevents differential settlement and wall failure.
- Often requires 95-98% Standard or Modified Proctor density.
Memory trick: A strong foundation is the HEART of a stable wall.