Which type of clay is generally known for having a high plasticity index?

Fat clay is recognized for its high plasticity index, a measure that indicates how much water a clay can retain while maintaining its plastic properties. This type of clay, often referred to in geotechnical engineering, contains a higher proportion of fine particles and can undergo significant changes in volume and shape in response to moisture content. These characteristics contribute to a greater ability to deform without cracking, making fat clay particularly useful in applications requiring flexibility, such as in embankments or earthen structures.

In contrast, low plasticity clay typically retains less water and exhibits lower deformation capabilities, resulting in a low plasticity index. Lean clay contains fewer fine particles and has a plasticity index that falls between that of fat clay and low plasticity clay, but it does not reach the same levels of plasticity as fat clay. Sand, being a granular material, lacks the cohesive properties associated with clay, exhibiting no plasticity at all; thus, it cannot hold moisture in the same way. Overall, fat clay's distinctive characteristics in terms of water retention and plasticity make it the correct answer for this question.