CSLB C-27 Landscaping ContractorSite Preparation and DrainageMedium
A C-27 contractor is preparing a site for a new patio installation. The existing subgrade has variable soil types, including areas of loose fill and compacted clay. To ensure a stable and long-lasting foundation for the patio, what is the most critical step the contractor should take regarding the subgrade before laying the base material?
- AAdd a 2-inch layer of sand to equalize the surface and provide a cushion for the base material.
- BInstall a perforated drainage pipe around the perimeter to collect and divert subsurface water.
- CExcavate the subgrade to a uniform depth and then compact it to a minimum of 95% Modified Proctor density.
- DApply a geotextile fabric over the entire area to prevent weed growth and improve drainage.
Show answer & explanationAnswer & explanation
Correct answer: C. Excavate the subgrade to a uniform depth and then compact it to a minimum of 95% Modified Proctor density.
Achieving a uniformly compacted subgrade to a specified density (e.g., 95% Modified Proctor) is paramount for the stability and longevity of any hardscape. This prevents differential settlement and provides a solid foundation for the base material.
Why the other options are wrong
- A. Adding sand without proper subgrade compaction will not solve the underlying stability issues and can even lead to further settlement if the subgrade is not firm.
- B. Drainage pipes address water issues but do not fix an inherently unstable subgrade due to variable compaction or loose fill.
- D. Geotextile fabric can help with stabilization and weed control but does not address the fundamental issue of an unstable or non-uniform subgrade.
Subgrade Compaction for Patios
The process of densifying the natural soil (subgrade) beneath a patio or hardscape to achieve uniform stability and prevent settlement.
- Crucial for long-term hardscape stability.
- Aims for uniform density, often 95% Modified Proctor.
- Prevents differential settlement and cracking.
Memory trick: COMPACT the subgrade, or your patio will CRACK.