What characterizes unsaturated and saturated soil conditions?

Unsaturated soil is characterized by the presence of both air and water within its pore spaces. This condition occurs typically in the upper layers of soil where the water content is below the soil's field capacity, meaning there are still voids filled with air. In contrast, saturated soil is a condition where all the pore spaces are filled with water, leaving no space for air. This difference in water content affects a number of soil properties, including permeability and strength.

Understanding the contrast between these two conditions is crucial in geotechnical engineering, particularly regarding stability issues, moisture retention capabilities, and overall soil behavior under various loading conditions. The statement that saturated soil is fully filled with water while unsaturated soil contains both components accurately captures this dynamic.