Abstract: During the two-step mining process in the high stage, the cementation quality of one step was of great significance to the safe recovery of the two-step pillar. In the Lilou iron mine, the difference strength between the in situ samples and the surface test blocks is analyzed based on the strength detection and scanning election microscope(SEM) experiments. The in situ strength increment of high stage cemented backfill is "hump" morphological distribution. It is divided into the upper part (-330 - -310 m), the middle part (-360 - -330 m) and the lower segment (-390 - -360 m). The porosity of 3 peaks and 4 valley values of 1:4 filling stope is extracted by the SEM pore characterization technique; and the nonlinear negative correlation between the strength increment and the porosity is explained, in accordance with the exponent function of y = 29.281e–0.032x. Combining engineering arrangement, the mechanism of deadweight pressure and drainage arrangement on the curing strength of high stage cemented backfill is revealed from the coupling macroscopic(exposing and in situ coring) and micro-pore structure development characteristics. Then the ratio parameters of filling slurry in the high stage stope are adjusted, which in the middle part is reduced from 1:6 to 1:8 and the heights of -325, -350 m and -375 m are reduced from 1:4 to 1:6, to ensure the safe and efficient mining.

Key words: high stage cemented backfill, space variation law, scanning electron microscopy, macro-micro coupling, engineering optimization