Final Answer:
If a trench is over-excavated, clean soil should be placed in the trench at 12 inches (305 mm) intervals and compacted to form a firm bed. True.
Step-by-step explanation:
In construction and excavation practices, over-excavation can occur, leading to an uneven trench bottom. To address this, the practice of placing clean soil at intervals of 12 inches and compacting it is indeed true. This process ensures the creation of a stable and firm bed at the bottom of the trench, which is essential for various construction projects.
Proper compaction is critical to prevent settling and instability, ensuring the safety and durability of structures. This approach aligns with industry best practices for trenching and excavation safety, emphasizing the importance of maintaining stable conditions.