  Granular materials -- aggregates of many discrete, disconnected solid
particles -- are ubiquitous in natural and industrial settings. Predictive
models for their behavior have wide ranging applications, e.g. in defense,
mining, construction, pharmaceuticals, and the exploration of planetary
surfaces. In many of these applications, granular materials mix and interact
with liquids and gases, changing their effective behavior in non-intuitive
ways. Although such materials have been studied for more than a century, a
unified description of their behaviors remains elusive.
  In this work, we develop a model for granular materials and mixtures that is
usable under particularly challenging conditions: high-velocity impact events.
This model combines descriptions for the many deformation mechanisms that are
activated during impact -- particle fracture and breakage; pore collapse and
dilation; shock loading; and pore fluid coupling -- within a thermo-mechanical
framework based on poromechanics and mixture theory. This approach allows for
simultaneous modeling of the granular material and the pore fluid, and includes
both their independent motions and their complex interactions. A general form
of the model is presented alongside its specific application to two types of
sands that have been studied in the literature. The model predictions are shown
to closely match experimental observation of these materials through several
GPa stresses, and simulations are shown to capture the different dynamic
responses of dry and fully-saturated sand to projectile impacts at 1.3 km/s.
