Abstract: Particle is one of a large number of media forms and is widely existed in the nature. Particles are widely used as engineering materials, such as dam rockfills, railway ballasts, and foundation backfills. The macro-mechanical behavior of granular media is closely related to its micro-structural evolution. It involves the overlapping effects of factors such as porosity ratio, particle morphology, particle fragmentation, and anisotropy. Analyzing the independent influence of each factor reveals that the macro-mechanical behavior of granular media is an important way of mechanism. In the investigation, the DRS-I type ultra-high-pressure direct shear test system was used to perform direct shear tests on steel balls with three different particle sizes under normal pressures ranging from 0.4 to 12 MPa to avoid the overlapping effects of particle morphology, particle fragmentation and anisotropy. The correlation of peak strength, residual strength and porosity evolution was analyzed separately, in order to provide a reference for revealing the meso-mechanical mechanism of granular media. The results show that: 1) the spherical granular media prepared by the falling rain method generally exhibits shear softening characteristics, and there are obvious peak stresses and residual stresses during the shearing process; 2) the peak intensity shows a certain degree of decrease as the confining pressure increases the small trend corresponds to the extreme value of the change slope of the porosity ratio; 3) the residual strength is basically not affected by the particle size and the confining pressure, corresponding to the stable value of the porosity ratio, that is, the critical state; A good description of the correlation between the strength characteristics of the granular media and the volume change characteristics.

Key words: granular media, softening, shear dilatancy, direct shear