At the University of Napoli Federico II a comprehensive research programme was started to study soil nailed structures in the soft pyroclastic silty sands (pozzolana) widely spread in the subsoil of Napoli. The program includes tests on nails instrumented with strain gauges in a large pullout apparatus and 3D numerical analyses. In the paper, the preliminary results obtained with the latter will be presented and compared with some of the experimental pullout test results. Installation of driven nails induces a stress disturbance at the soil nail interface, which is essentially a reduction of normal stresses and a tendency of the stress state to become isotropic. It is argued that numerical testing can be usefully adopted to understand the true soil nail interface failure mechanism and to highlight the link between it and the overall pullout strength, instead of being used just to fit the experimental pullout curves.