We propose a dilatant, two-layer debris flow model validated by full scale density/saturation measurements obtained from the Swiss Illgraben test site. Like many existing models we suppose the debris flow consists of a matrix of solid particles (rocks, boulders) that is surrounded by muddy fluid. However, we split the muddy fluid into two fractions. One part, the inter-granular fluid, is bonded to the solid matrix and fills the void space between the solid particles. The combination of solid material and inter-granular fluid forms the first layer of the debris flow. The second part of the muddy fluid is not bonded to the solid matrix and can move independently from the first layer. This free fluid forms the second layer of the debris flow. During flow the rocky particulate material is sheared which induces dilatant motions that change the solid/fluid concentration of the first layer and then his density. As suggested by real data of Illgraben, the rheology used is not constant and uniform but a function of the flow composition/ density. The model is then compared to real debris flow data of Illgraben and tested on a real event in Ritigraben for which erosion data are available.