When would subsurface material have high porosity and low permeability?

• A. When material is heavily compacted
• B. When the material has many unconnected pores
• C. When the material is saturated with water
• D. When the material is composed of clay

Answer: (B)

The scenario where subsurface material exhibits high porosity and low permeability occurs when the material has many unconnected pores. High porosity refers to the amount of void space in a material, indicating that there are numerous spaces or pores available. However, if these pores are unconnected, water and other fluids cannot easily flow through the material, resulting in low permeability. This characteristic is very common in certain types of soils and sediments, particularly those that have fine particles and a granular structure where the voids are not linked.

In contrast, other conditions mentioned would lead to different properties. For example, heavily compacted materials typically result in reduced porosity and can also affect permeability negatively. Saturated materials may have high porosity, but if the pores are not connected, permeability remains low. Materials composed of clay often have both high porosity and low permeability due to their fine particle sizes, but the correct answer specifically addresses the condition of having unconnected pores, making it distinct from the general characteristics of clay.