  We study some integral model of P.E.L. Shimura varieties of type A for
ramified primes. Precisely, we look at the Pappas-Rapoport model (or splitting
model) of some unitary Shimura varieties for which there is ramification in the
degree 2 CM extension. We show that the model isn't smooth, but that it is
normal with Cohen-Macaulay special fiber. We moreover study its special fiber
by introducing a combinatorial stratification for which we can compute the
closure relations. Even if there are "extra" components in special fiber, we
prove that those do not contribute to mod p modular forms in regular degree. We
also study the interaction of the stratification with the natural
stratification given by the vanishing of some partial Hasse invariants, in the
case of signature (1,n-1).
