What is the definition of extremely gravely soil?

Extremely gravely soil is defined as soil that contains 60% or more rock fragments by volume, but it cannot exceed 90% rock fragments. This definition aligns with the characteristics of soil that has a high concentration of gravel, which impacts its drainage and suitability for certain applications, such as septic systems. Such a high percentage of rock fragments indicates that the soil's ability to retain moisture and nutrients is limited, making it less ideal for growing plants, but also affecting how septic systems function due to the limited capacity for effluent absorption and filtration.

Understanding the composition of extremely gravely soil is crucial for septic installers because it informs decisions about site suitability, design requirements, and system performance. In contrast, the other options describe soils with varying fragment percentages, none of which meet the criteria for being classified as extremely gravely, thus leading to different implications for soil management and septic system installation.