What thickness can modern-day precast manholes have?

Modern-day precast manholes can indeed have a thickness ranging from 8 inches to 2 feet. This range of thickness is designed to accommodate various applications, structural load requirements, and soil conditions. Manholes must be durable and able to support the weight of traffic above, as well as withstand environmental factors. The specified thickness ensures that the manhole provides sufficient strength and long-term performance.

The options reflecting thinner ranges, such as 4 inches to 12 inches and 6 inches to 18 inches, do not cover the broader needs for heavier usage or specific engineering requirements. Similarly, the range of 10 inches to 3 feet exceeds typical design specifications for precast manholes in standard applications, as they often do not require such excessive thickness. Thus, the range established in the correct answer accurately reflects the construction practices for modern precast manholes.