Which class of soils is considered unsuitable for standard systems based on texture?

D soils, classified as Class 4 soils, are deemed unsuitable for standard systems primarily due to their physical properties, particularly texture. Class 4 soils are typically characterized by high clay content, poor drainage, and potentially a high water table or bedrock. These characteristics impede the percolation of wastewater, which is a critical factor for the effective operation of standard systems, such as septic systems. Poor drainage can lead to system failure, which can cause environmental contamination and public health issues.

Conversely, Class 1a, Class 2, and Class 3 soils possess properties that allow for better drainage and percolation, making them more appropriate for standard systems. Class 1a soils are the best in terms of texture and drainage, while Class 2 and Class 3 soils may still provide adequate conditions for effective waste treatment but may need specific design considerations. This distinction highlights why Class 4 soils are specifically labeled as unsuitable for these applications.