  We study fluid-induced deformation and fracture of cohesive granular media,
and apply photoporomechanics to uncover the underpinning grain-scale mechanics.
We fabricate photoelastic spherical particles of diameter d=2mm, and make a
monolayer granular pack with tunable intergranular cohesion in a circular
Hele-Shaw cell that is initially filled with viscous silicone oil. We inject
water into the oil-filled photoelastic granular pack, varying the injection
flow rate, defending-fluid viscosity, and intergranular cohesion. We find two
different modes of fluid invasion: viscous fingering, and fracturing with
leak-off of the injection fluid. We directly visualize the evolving effective
stress field through the particles' photoelastic response, and discover a hoop
effective stress region behind the water invasion front, where we observe
tensile force chains in the circumferential direction. Outside the invasion
front, we observe compressive force chains aligning in the radial direction. We
conceptualize the system's behavior by means of a two-phase poroelastic
continuum model. The model captures granular pack dilation and compaction with
the boundary delineated by the invasion front, which explains the observed
distinct alignments of the force chains. Finally, we rationalize the crossover
from viscous fingering to fracturing by comparing the competing forces behind
the process: viscous force from fluid injection that drives fractures, and
intergranular cohesion and friction that resist fractures.
