CSLB General Building (B)Site Work, Foundations and ConcreteMedium
A soils report requires foundation backfill to be compacted to 90 percent relative compaction using mechanical tampers. To achieve uniform compaction throughout the fill depth, the contractor should place the backfill material in:
- ASaturated lifts of any thickness to eliminate the need for mechanical compaction
- BOne thick lift up to 4 feet deep, compacted only at the top surface
- CThin lifts of approximately 6 to 8 inches, each compacted before placing the next
- DLoose lifts of 3 feet, relying on the weight of the fill itself for compaction
Show answer & explanationAnswer & explanation
Correct answer: C. Thin lifts of approximately 6 to 8 inches, each compacted before placing the next
Backfill must be placed in thin lifts, typically 6 to 8 inches loose thickness, with each lift compacted before the next is placed. This ensures compaction energy reaches the full depth of each lift and achieves the specified relative compaction throughout the fill, rather than only near the surface.
Why the other options are wrong
- A. Saturating soil does not substitute for mechanical compaction and can weaken bearing capacity.
- B. A 4-foot lift is far too thick for compaction equipment to reach the bottom effectively.
- D. Fill weight alone does not achieve the required relative compaction specified in soils reports.
Backfill Compaction Lifts
The practice of placing backfill in thin layers (commonly 6-8 inches) and compacting each layer before adding the next to achieve uniform density.
- Typical lift thickness is 6 to 8 inches loose
- Each lift must be compacted before adding the next
- Prevents under-compacted zones deep within the fill
Memory trick: 'Layer by layer, tamp before you stack' ensures full-depth compaction.