What type of soil is generally considered less stable for excavations?

Clayey soils are typically considered less stable for excavations due to their unique physical properties. These soils have a high plasticity and can retain water, causing them to expand when wet and shrink when dry. This behavior can lead to significant changes in volume, which compromises the structural integrity of excavations. Additionally, when saturated, clayey soils can become slippery and lose their strength, making it difficult to maintain safe slopes during excavations. The cohesive nature of clay can also lead to difficulties in handling and removing the material compared to other soil types, which can impact excavation efficiency and safety.