In trench backfill compaction practices, if native soil is used and it is relatively dense, compaction should be performed on the backfill that is located?

• A. bottom backfill
• B. middle backfill
• C. upper backfill
• D. sides backfill

Answer: (C)

When the trench bottom native soil is already relatively dense, the firm base reduces the risk of settlement from that layer, so the main concern becomes the material that sits above it. Compaction is concentrated in the upper backfill because that is the portion directly supporting surface loads and most susceptible to settlement over time. The dense bottom native soil already provides a stable foundation, and over-compacting it could disturb the bedding or pipe. Therefore, you focus compaction on the backfill located on top of the dense native layer—the upper backfill.