  We show that deterministic systems with strong nonlinearities seem to be more
appropriate to model sandpiles than stochastic systems or deterministic systems
in which discontinuities are the only nonlinearity. In particular, we are able
to reproduce the breakdown of Self-Organized Criticality found in two well
known experiments, that is, a centrally fueled pile [Held et al. Phys. Rev.
Lett. 65 (1990) 1120] and sand in a rotating tray [Bretz et al. Phys. Rev.
Lett. 69 (1992) 2431]. By varying the parameters of the model we recover
Self-Organized Criticality, in agreement with other experiments and other
models. We show that chaos plays a fundamental role in the dynamics of the
system.
