Abstract: Backfilling of open pit tailings to underground mining effectively improves the stability of open pit slopes and
solves a series of environmental problems caused by tailings stockpiling,However,the backfilling tailings have adverse effects
on the stability of boundary pillars in underground mining In this thesis,aiming at the problem of determining the thickness of
boundary pillar under the condition of tailings backfilling,a scientific method for determining the thickness of boundary pillar
combining theoretical calculation and numerical simulation is proposed. In this method,the initial value of the thickness of the
boundary pillar is realized by modifying the calculation formula of K. B. Rubeneit and taking into account the two factors of
slope angle and tailings backfilling height. Based on the results obtained from the modified calculation formula,combined with
the Rhion-FALC3D coupling numerical simulation method,a three-dimensional numerical model is established. By analyzing the
stress,displacement and plastic zone variation of boundary pillars with different thicknesses in the underground mining process,
the further optimization of the boundary pillar thickness value is realized. Taking a deep open-pit iron mine as an example,the
proposed method is applied and verified. The results show that:when the thickness of the boundary pillar increases gradually
from 25 m to 40 m,the stress and displacement of the bottom plate of the boundary pillar show a nearly linear decrease,and the
area of the plastic zone within the boundary pillar gradually decreases until it disappears completely. When the thickness of the
pillar exceeds 30 m,the plastic zone no longer penetrates the pillar,and the maximum tensile stress inside the pillar is less than
the tensile strength of the rock. Therefore,the optimal thickness of the pillar should be 30 m. The research results can provide
reference for the development of the backfilling to underground mining plan for this kind of open pit.