What factor could cause significant issues in foundation stability?

Organic materials in soil are a critical factor affecting foundation stability. When present in significant amounts, organic materials such as decomposed plant matter can lead to unpredictable changes in soil behavior. As these materials break down, they can cause reductions in soil strength and bearing capacity. This decomposition often results in volume changes, which can create voids or lead to uneven settling as the organic matter decays. Consequently, structures built on such soils may experience movement, which can result in cracking and other forms of structural damage.

In contrast, while soil shrinkage, moisture content, and excessive compaction are all relevant to foundation stability, they generally result in more predictable behaviors compared to the unpredictable nature of organic materials. Soil shrinkage and moisture variations can often be managed through proper drainage and soil treatment techniques, while excessive compaction can be adjusted by modifying building practices. However, the instability caused by organic materials is often more difficult to address once construction is underway, making their presence particularly concerning for foundation integrity.