In the context of backfilling a trench that measures 3 feet by 4 feet, which method is not a viable compaction approach?

In backfilling a trench that measures 3 feet by 4 feet, the use of a heavy roller compactor, mechanical vibratory plate, or water jet method can all contribute effectively to compaction. These methods provide adequate force or vibration to effectively compact soil in confined spaces, such as the given trench size.

The mechanical vibratory plate is specifically designed for small areas and can easily enhance compaction in narrower spaces by applying vibration that rearranges soil particles into a denser configuration. Heavy roller compactors are also useful for larger areas but can be adapted for smaller spaces provided enough clearance is available. The water jet method, while not traditional, can effectively reduce air voids and assist in settling soil when used correctly, especially in certain types of soil.

Spread dumping, on the other hand, is not a viable compaction approach in this context. It typically involves placing soil in loose layers without applying any immediate force to compact those layers. This method would not provide the necessary density required for backfill in a trench application, potentially leading to future settling and instability. Effective compaction is critical to ensure the backfill remains stable and supports any structures above it.