It is very common in practice to hear the terms "lateritic soils" and "saprolite". What is not common is that the difference between them is really understood, at least for a great majority of students and professionals linked to Geotechnical Engineering. So let's take the opportunity to clarify the matter.
Lateritic soils are those materials that have been strongly affected by seepage, and subjected to severe chemical weathering. They usually have red, brown and yellow colors. Due to the action of rainfall, normally the more soluble constituents of the soil (such as kaolinite and silica) are washed away, leaving a material rich in iron and aluminum on the surface. These soils are frequently found in the most superficial zone, and are characterized because they have lost the original structure of the rock and present a mineral composition different from that of the original rock.
On the other hand, the saprolite is made up of materials that present a lower degree of weathering, and therefore maintain in their structure some features of the original rock. Although its texture is that of a soil, its behavior is highly influenced by the presence of discontinuities. This type of soil is usually found below lateritic soils.
The distinction between both materials is not simple in the field, although the main feature that needs to be considered during the geological evaluation of a slope in residual materials is that lateritic soils are always found above the saprolite.
And why is it so important to distinguish them? For the simple reason that the vast majority of measurements made on active landslides, have shown that the failure surfaces are usually located in the saprolite. But this is a subject we will discuss in the next post.