Laboratory tests were conducted to investigate the uniaxial compressive strength (UCS) of two types of potential infill materials for standing supports. While one type of infill material was made from coal reject fines and a cementitious grout, the other was a mixture of tailings and a cementitious grout. 81 cylindrical specimens with a 50 mm diameter and 100 mm height were prepared and tested. The effect of various water-to-grout (w/g) ratios and grout-to-coal reject fines/tailings mix ratios on the UCS of the infills were investigated. Test results indicated that the strength of both infills was adversely affected by the w/g ratio. In addition, when the volume ratio of the coal reject fines in the infill was not greater than 50%, the strength of the infill was similar to that of the control group specimens. Interestingly, almost all the infills made of tailings and grout had a greater UCS when compared with the control group. The infill made from 50% tailings and 50% grout with the w/g ratio of 1.2 achieved the highest strength enhancement ratio, being 1.92 times the UCS of the control group.