  In submerged sandy slopes, soil is frequently eroded as a combination of two
main mechanisms: breaching, which refers to the retrogressive failure of a
steep slope forming a turbidity current, and, instantaneous sliding wedges,
known as shear failure, that also contribute to shape the morphology of the
soil deposit. Although there are several modes of failures, in this paper we
investigate breaching and shear failures of granular columns using the
two-fluid approach. The numerical model is first applied to simulate small
scale granular column collapses with different initial volume fractions to
study the role of the initial conditions on the main flow dynamics. For loosely
packed granular columns, the porous medium initially contracts and the
resulting positive pore pressure leads to a rapid collapse. Whereas in
initially dense-packing columns, the porous medium initially dilates and
negative pore pressure is generated stabilizing the granular column, which
results in a slow collapse. The proposed numerical approach shows good
agreement with the experimental data in terms of morphology and excess of pore
pressure. Numerical results are extended to a large-scale application known as
the breaching process. This phenomenon may occur naturally at coasts or on
dykes and levees in rivers but it can also be triggered by humans during
dredging operations. The results indicate that the two-phase flow model
correctly predicts the dilative behavior and, the subsequent turbidity
currents, associated to the breaching process.
