What type of soil should not be used for backfill due to its expansion characteristics?

Clay soil is not suitable for use as backfill because of its expansion and contraction properties. When clay is wet, it can absorb water and swell, greatly increasing in volume. Conversely, when it dries out, clay shrinks, leading to significant changes in ground stability and potentially causing damage to structures. This characteristic makes clay soil unpredictable and problematic for backfilling, especially in areas where moisture levels fluctuate significantly.

Sand soil, loose gravel, and organic soil lack these expansive qualities, making them more suitable for backfilling. Sand and gravel provide good drainage and stability, while organic soil, often too loose and rich in decomposing materials, can decompose over time, leading to settling issues. Therefore, understanding the properties of different soil types is crucial for ensuring structural integrity when backfilling.