The DMT Material Index ID is an index helpful for identifying the soil composition (sand, silt, clay). Even a non experienced operator, after a few minutes, notes that in sand the two readings po and p1 are distant (e.g. 5 and 20 bar), while in clay they are close (e.g. 5 and 6 bar).
Obviously ID is not the equivalent of a sieve analysis. The sieve analysis defines soil composition based on grain size, ID based on the mechanical response in terms of po and p1. Note also that a mixture of sand and clay would probably be “wrongly” interpreted by ID as a silt. On the other hand such mixture will probably behave mechanically as a silt. Since the engineer is often interested in the grain size distribution not “per se”, but indirectly, just to infer from the grain size the mechanical properties, perhaps, in some cases, it could be more expressive, for association with the mechanical behavior (a sort of Soil Behavior Type Index), the soil description based on ID than the description based on the sieve analysis.
ID is a sort of ratio stiffness to strength, i.e. a mechanically connected index, hence it is indicative of aspects of the soil mechanical behavior rather than of the grain size, though the two things are to some extent connected.
It is noted that ID is just an auxiliary information provided by DMT. The most useful design parameter provided by DMT is, by far, the operative modulus M.
A case in which occasionally clay can be interpreted by ID as granular material is shallow desiccated clay, which gets fractured by the penetration and look or behave as a granular material. This can happen near surface, above water table.