Wide trenching should be avoided in which soil type when installing small-diameter pipes?

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

Answer: (B)

Wide trenching raises the exposure of the trench walls, and in cohesive soils like hard clay, that becomes a problem. Clay can be very strong in compression, but its strength changes with moisture and it has a tendency to shrink, crack, and swell. When a trench is widened, newly exposed clay faces dry out or wet, lose shear strength, and may crack or move, increasing the risk of wall instability or even collapse. For small-diameter pipes, minimizing trench width helps reduce the potential for wall movement and settlement around the pipe as the surrounding clay changes moisture. In contrast, sands are granular and drain quickly, making wide openings less troublesome from the stability standpoint, and hard pan presents digging challenges rather than wall-collapse behavior driven by moisture changes.