  The constitutive relations of a dense granular flow composed of
self-propelling frictional hard particles are investigated by means of DEM
numerical simulations. We show that the rheology, which relates the dynamical
friction $\mu$ and the volume fraction $\phi$ to the inertial number $I$,
depends on a dimensionless number $\mathcal{A}$, which compares the active
force to the confining pressure. Two liquid/solid transitions -- in the Maxwell
rigidity sense -- are observed. As soon as the activity is turned on, the
packing becomes an `active solid' with a mean number of particle contacts
larger than the isostatic value. The quasi-static values of $\mu$ and $\phi$
decrease with $\mathcal{A}$. At a finite value of the activity $\mathcal{A}_t$,
corresponding to the isostatic condition, a second `active rigidity transition'
is observed beyond which the quasi-static values of the friction vanishes and
the rheology becomes Newtonian. For $\mathcal{A}>\mathcal{A}_t$, we provide
evidence for a highly intermittent dynamics of this 'active fluid'.
