This paper studies thoroughly and deeply the results of about one hundred triaxial compression tests on thirty types of rockfill
materials. The materials are categorized in accordance with their particles shape (angular / rounded) and gradation
characteristics. The main tool of the study is the Hyperbolic Model developed by Duncan and Chang. The focus of the study is
on the variations of deformation modulus of the materials (Ei and Et) with confining stress (&sigma3). Features of the mechanical
behavior of the rockfill materials, as compared with the general behavior of soils, are highlighted through the exponent
parameter (n) of the Hyperbolic Model. It is shown that high confining stresses may have adverse effects on the deformation
modulus of the rockfill materials and make them softer. The particle breakage phenomenon which happens during compression
and shearing is found as the main factor responsible for the above effects and, in general, responsible for controlling the
behavior of the materials. For the rockfill materials of this study, two correlations for estimating the initial elasticity modulus (Ei)
and the internal friction angle (&phi) in terms of particles shape, confining pressure (&sigma3), and coefficient of uniformity (Cu) are
suggested.