  Mechanical densification of granular bodies is a process in which a loose
material becomes increasingly cohesive as the applied pressure increases. A
constitutive description of this process faces the formidable problem that
granular and dense materials have completely different mechanical behaviours
(nonlinear elastic properties, yield limit, plastic flow and hardening laws),
which must both be, in a sense, included in the formulation. A treatment of
this problem is provided here, so that a new phenomenological, elastoplastic
constitutive model is formulated, calibrated by experimental data, implemented
and tested, that is capable of describing the transition between granular and
fully dense states of a given material. The formulation involves a novel use of
elastoplastic coupling to describe the dependence of cohesion and elastic
properties on the plastic strain. The treatment falls within small strain
theory, which is thought to be appropriate in several situations; however, a
generalization of the model to large strain is provided in Part II of this
paper.
