Final answer:
Larger particle soils like sandy soils achieve a steeper slope when poured into a pile, but both clay and sandy soils can have reducing slope stability when saturated due to water reducing internal friction and cohesiveness.
Step-by-step explanation:
The material that is likely to achieve the greatest slope when poured into a pile before saturation is one with larger particle sizes, such as sandy soil, since it typically has larger pore spaces and a more stable structure. When a material like soil becomes saturated with water, the slope stability can deteriorate, potentially leading to mass wasting events like debris flows. The primary reason is that water reduces the friction between soil particles which is critical to the material's strength. This effect is more pronounced in clay soils which have smaller particle sizes and therefore smaller pore spaces that retain more water. Upon saturation, the increased water in these fine spaces adds pressure and reduces cohesiveness, making the slope more prone to movement.
Conversely, sandy soils have larger pore spaces which allow water to drain more readily, resulting in less water retention and less of an impact on the structural integrity when saturated. Yet, both types of soil can experience a reduction in slope stability when saturated, as even larger particles can be affected by water reducing internal friction.