Wide trenches should be avoided for small diameter pipe, particularly in which soil type?

• A. Silty
• B. Sandy
• C. Hard Pan
• D. Hard Clay

Answer: (D)

Keeping trench widths close to the pipe size helps control where soil movement can occur and makes bedding, backfill, and compaction more reliable. In cohesive soils such as hard clay, wide trenches create a large mass of soil that can lose strength or shift with moisture changes, increasing the risk of wall failure, slurry formation, and movement around the pipe. A narrow trench minimizes these risks by reducing the amount of unstable soil around the pipe and making proper backfill compaction easier, which helps prevent settlement or pipe displacement. So, wide trenches are especially problematic in hard clay for small-diameter pipe.