The stability formula developed by Van der Meer is used for the design of different kind of rock slopes. In the formula is among a number of other parameters also the permeability of the structure represented. A more permeable structure has the ability to dissipate more water and therefore more energy, this results into a lower required weight of the armour layer. This coefficient, described as the Notional Permeability P, has been determined for three different types of structures. A homogeneous structure, a structure with a permeable core and an impermeable structure. In practice structures are being build who deviate from these standard situations. Therefore there is a demand for values of P about structures other than the known standard situations. In this thesis P values are found by means of physical scale model tests. First of all two reference structures were tested. The permeable and the impermeable structure with known values of P= 0.5 and respectively P=0.1. The values found in this study are almost equal to the values above. The new structure has an impermeable core covered with a thick filter layer. On top of that an under layer is placed and finally there is a double armour layer.