What does the classification SP represent in soil types?

The classification SP refers to "Poorly graded clean sand; sand-gravel mixes." In soil classification systems, particularly the Unified Soil Classification System (USCS), SP specifically describes soils that consist primarily of well-draining, coarse-grained materials with little to no fines. This classification indicates that the particles are relatively uniform in size, lacking a range of particle sizes that would help fill voids and create a dense packing.

In practical terms, this means that poorly graded sand allows for rapid drainage and has low cohesion. Such soils are significant in construction and geotechnical engineering because they may not support vertical loads effectively and are more susceptible to shifting or erosion under certain conditions. Therefore, understanding the characteristics of SP can aid in assessing the stability and behavior of soil in various engineering applications.