  Very soft grain assemblies have unique shape-changing capabilities that allow
them to be compressed far beyond the rigid jammed state by filling void spaces
more effectively. However, accurately following the formation of these systems
by monitoring the creation of new contacts, the changes in grain shape, and
measuring grain-scale stresses is challenging. We developed an experimental
method overtaking these challenges and connecting their microscale behavior to
their macroscopic response. By tracking the local strain energy during
compression, we reveal a transition from granular-like to continuous-like
material. Mean contact geometry is shown to vary linearly with the packing
fraction, which is supported by a mean field approximation. We also validate a
theoretical framework which describes the compaction from a local view. Our
experimental framework provides insights into the granular micro-mechanisms and
opens new perspectives for rheological analysis of highly deformable grain
assemblies in various fields ranging from biology to engineering.
