CSLB C-27 Landscaping ContractorSite Preparation and DrainageHard
A C-27 contractor is preparing a subgrade for a new gravel pathway that will experience moderate foot traffic. To ensure long-term stability and prevent settling, what is the generally recommended minimum compaction level for the subgrade in terms of Modified Proctor density?
- A95% Modified Proctor
- B80% Modified Proctor
- C85% Modified Proctor
- D90% Modified Proctor
Show answer & explanationAnswer & explanation
Correct answer: A. 95% Modified Proctor
For subgrades supporting pathways, patios, or other hardscapes, especially those experiencing foot traffic, a minimum compaction level of 95% Modified Proctor density is generally recommended. This ensures sufficient bearing capacity, minimizes settlement, and provides a stable base for the overlying materials, preventing future dips and uneven surfaces.
Why the other options are wrong
- B. 80% Modified Proctor is typically too low for any structural subgrade and would lead to significant settlement.
- C. 85% Modified Proctor is also generally insufficient for hardscape subgrades and would likely result in long-term settlement.
- D. 90% Modified Proctor might be acceptable for some non-critical landscape areas, but 95% is the standard for reliable hardscape bases.
Subgrade Compaction (Hardscape)
The process of densifying the existing ground (subgrade) beneath a hardscape element to achieve a specific density, typically measured by Modified Proctor.
- Essential for hardscape stability.
- Prevents future settlement and uneven surfaces.
- Minimum 95% Modified Proctor density is standard.
- Achieved through mechanical means (e.g., plate compactor).
Memory trick: Ninety-Five's the Number, No Niggles, No Nosedives.