Large-size lump (e.g., coal reject or waste rocks) is one of the most popular backfill materials for underground mines attributed to its cost-effective and easy-to-obtain. The massive void between lumps, however, does generally affect the overall performance in eliminating the ground subsidence. This paper presents a conceptual backfill material, termed as lump-grout (LG) material, the attractive feature of which is that the voids between lumps are totally filled up with high-water content cementitious grout (CG) material. To explore the mechanical behaviour of LG material, a total of 36 LG specimens were prepared and tested under the uniaxial compression and the tensile loading. Other 20 CG specimens were tested in parallel for comparison. Key configurations investigated in the present research covered the particle size of lumps and the water-to-powder ratio of CG material. Test results indicated that the stress–strain behaviour of LG specimen is predominately affected by the streng