Behavior of backfill material made of high-water material and recycled concrete lumps for highwall mining

The highwall mining technique is the family member of green mining system, for which the development of the cost-effective backfill material is always the critical issues to be accounted for. In the present research, an innovative backfill material made of the high-water material and recycled concrete lumps was introduced. A total of 42 specimens with variable configurations were prepared and tested to evaluate the effects of the water-to-powder of the high-water material and particle size of concrete lumps on the compressive behaviour of the developed backfill material. Both the acoustic emission (AE) and the digital image correlation (DIC) instruments were applied to investigate the progressive failure modes of tested specimens in parallel. Test results revealed that the high-water material cemented recycled concrete present an ideal ductility when it is subjected to the uni-axial compressive loading, which is different from normal backfill materials featured with brittleness in natural. It also suggested that the effect of the water-to-power ratio of the high water material is much more obvious than that of the lumps size on the enhancement of deformation ability. In terms of the peak strength, these specimens with the 20 mm to 35 mm lumps is higher than that of its counterparts, which suggested that the lump size is one of the most important factor during the design of the proposed backfill material. Moreover, the peak strength of specimens with the small water-to-powder ratio (i.e., 0.75) surpasses those with ratios of 1.0 and 1.25 by 20.4 % and 29.5 %, respectively. Although the specimens with smaller water-to-powder ratio of high water material exhibited the higher compressive strength, it should be noted that the reduced flowability of which will affect the casting process in turn. Research outcomes obtained from this study is expected to facilitate the wide application of the highwall mining.