The co-mixing of tailings and waste rock involves the intimate blending of tailings and waste rock to create a new material (Co-Mix) with superior physical and hydraulic properties. The new material has a low hydraulic conductivity, low compressibility, high water retaining capacity and high shear strength. Co-mixed tailings and waste rock restrict oxygen entry and water seepage to minimise acid generation and metal leaching. When compacted, Co-Mix materials have low volume change characteristics with respect to total stress and matric suction together with high shear strength. Co-mixed tailings and waste rock may be used to construct cover systems, internal barriers and seals, confinement structures or a full depth waste repository. The theoretical properties of Co-Mix blends are investigated. The results of a laboratory program to characterise the physical and hydraulic properties of co-mixed tailings and waste rock are presented. Various ratios of tailings were blended with different waste rock materials. Hydraulic conductivity tests were conducted at confining pressures ranging between 0 and 400 kPa. The saturated hydraulic conductivity for the Co-Mix materials was found to vary between 4_x000D_
10-5 m/s and 5_x000D_
10-9 m/s depending on blend ratio, waste rock type and confining pressure. These results together with other physical properties show excellent hydraulic and strength characteristics can be achieved for Co-Mix materials.