When placing backfill, a backfill material with a moisture content of what should be on the dry side of optimum?

• A. 0%
• B. 1%
• C. 2%
• D. 3%

Answer: (C)

The key idea is how moisture content affects how well soil compacts. The maximum dry density occurs at the optimum moisture content, so to achieve good compaction you often keep the backfill slightly drier than that point. Being on the dry side by about two percentage points gives enough moisture for particle interlock and compaction without the problems that excess water causes, like pumping and loss of density. If it’s drier than that, compaction becomes less effective; if it’s too wet, the soil can become overly plastic and rut or pump. So about two percent below the optimum moisture content is a practical target.