Which of the following is not acceptable material for the base of a boiler?

Compacted earth is not an acceptable material for the base of a boiler due to its inability to provide the necessary structural support and load-bearing capacity required for such heavy equipment. A boiler exerts significant weight and operational forces, and relying on compacted earth may lead to shifting, settling, or erosion over time, compromising the stability and safety of the boiler system.

In contrast, materials such as concrete, steel, and gravel are commonly used for boiler bases due to their strength and durability. Concrete provides a solid and stable foundation, steel can offer additional structural support, and gravel assists with drainage and preventing water pooling, which can also be detrimental to boiler operation. Thus, while these materials ensure a robust foundation that aligns with industry standards, compacted earth does not meet those essential criteria for safety and reliability.