Simply flooding the backfilled or partially backfilled trench will result in good compaction only when ________.

• A. granular material with many fines
• B. granular material with few fines
• C. clayey material
• D. silt-rich material

Answer: (B)

Flooding helps compact backfill when the material is clean granular with few fines. Water allows grains to rearrange and air to escape, letting the coarse particles settle into a denser, more stable packing. Low fines mean the material drains well after the flood, so pore pressures don’t keep the fill from densifying and the density gains are achievable with the moisture present. If there are many fines, or the material is clayey or silt-rich, water can't drain effectively, capillary suction and cohesion resist rearrangement, and the fill won’t achieve good compaction—in fact, it can become muddy and less dense.