Underground mining operations produce substantial amounts of waste rocks in order to access ore bodies. These waste rocks are typically hoisted to surface and stored on ground surface as waste rock piles. This manner of waste rocks management requires energy consumption for transporting the waste rocks from underground to ground surfaces. The environmental impact of mine wastes on ground surface is another problem of this practice. When part of these materials are used as rockfills for filling underground openings, additional energy consumption and cost expenses are necessary during the operations of block crushing, grading and re-transportation. An alternative practice is to directly pour the waste rocks produced during underground development work in underground mine stopes being filled with paste backfill. This leads to a considerable reduction of cost and consumption of energy. However, the mixing behavior of waste rocks poured in paste backfill has never been studied. If the cohesionless waste rocks are not fully mixed with the cemented paste backfill, the fill mass may fail and collapse upon a side-exposure during the excavation of an adjacent secondary stope, resulting in significant ore dilution, ore loss, equipment damage, or even personal injury. It is thus critical to have a good understanding of the mixing behavior of waste rocks poured in paste backfill. To this end, a series of physical model tests have been performed in laboratory. In this paper, a part of the results will be presented and discussed.