We study the transition from cohesive to noncohesive granular states of
synthetic rocks under oedometric loading, combining simultaneous measurements
of ultrasound velocity and acoustic emissions. Our samples are agglomerates
made of glass beads bonded with a few percent of cement, either ductile or
brittle. These cemented granular samples exhibit an inelastic compaction beyond
certain axial stresses likely due to the formation of compaction bands, which
is accompanied by a significant decrease of compressional wave velocity. Upon
subsequent cyclic unloading and reloading with constant consolidation stress,
we found the mechanical and acoustic responses similar to those in noncohesive
granular materials, which can be interpreted within the effective medium theory
based on the Digby bonding model. Moreover, this model allows P-wave velocity
measured at vanishing pressure to be interpreted as an indicator of the
debonding on the scale of grain contact. During the inelastic compaction,
stick-slip like stress drops were observed in brittle cement-bonded granular
samples accompanied by the instantaneous decrease of the P-wave velocity and
acoustic emissions which display an Omori-like law for foreshocks, i.e.,
precursors. By contrast, mechanical responses of ductile cement-bonded granular
samples are smooth (without visible stick-slip like stress drops) and mostly
aseismic. By applying a cyclic loading and unloading with increasing
consolidation stress, we observed a Kaiser-like memory effect in the brittle
cement-bonded sample in the weakly damaged state which tends to disappear when
the bonds are mostly broken in the non-cohesive granular state after
large-amplitude loading. Our study shows that the macroscopic ductile and
brittle behavior of cemented granular media is controlled by the local
processes on the scale of the bonds between grains.Comment: 22 pages, 15 figure