Abandoned quarries become favourable sites for uncontrolled waste dumps, causing major safety hazards because of potential mass movements. An old quarry, near the town of Kozani, is filled with 772.000 m3 Construction and Demolition Wastes (CDW), containing a variety of materials (aggregates, wood, metals, plastics, bricks, soil etc.), in a wide range of fragment sizes, from clay to boulders. The size of particles and voids influence the behaviour of the filled slope, affecting both shear strength and drainage characteristics. Major factors that should be considered in a stability analysis for CDW, include waste material and soil cover composition, overall degree of compaction, moisture contented, permeability, pore pressure and shear strength parameters. Shear strength parameters were chosen by bibliographic references based on specialized field tests of similar materials. The analyzed profiles computed slip zones of low thickness and range. Therefore, successive back analysis was conducted to the sites of mapped cracks, aimed to determinate the mechanical properties of the slope during the failure. Since, friction angle and bulk density were retained for back analysis, the cohesion was examined. Although deposits are composed of loose heterogeneous materials, which have not been condensed, cohesion exists owing to the "interlocking" of the varying particle size of materials.