This paper investigates the underlying physics governing the explosion driven expansion and fragmentation of spherical beds packed of partially saturated sand with varying mass fractions of interstitial oil. The breakup onset of the sand shells is characterized by the formation of fragments/agglomerates consisting of a large number of constituent grains, which in later time present themselves as prolific and regular jetting streams. Test data show a postponed jetting formation when sand shells are subject to the explosion with a higher detonation velocity, meanwhile a reduced jet mass scale is observed. A kinetic energy driven breakup model is proposed based on the instability criterion involving the opposing forces of stabilizing inertial pressures and destabilizing viscous resistance. This analytical model is capable of predicting the onset of granular material fragmentation as well as the characteristic fragment size, which is consistent with the experimental results.