Safety Thickness Design of Horizontal Pillars under the Synergistic Effect of Upper and Lower Backfill

Horizontal pillars are the main load-bearing elements in underground stopes. The stability of pillars directly influences the overall safety of mines, and their thickness directly affects the ore loss and the economy of mining. However, traditional design methods have shortcomings, such as shape simplification and the consideration of too few factors, which are not compatible with horizontal pillars in the filling method. An innovative method to accurately determine the safety thickness of irregular pillars under the filling condition was proposed in this study. First, the formula for calculating overlying backfill load to mine steeply inclined ore bodies was derived by analyzing the condition of irregular horizontal pillars. Support from the lower backfill body was considered further. A 3D numerical model of horizontal pillars that uses contact element was presented to study the effect of the maximum principal stress and the deflection on the pillar thickness. Finally, based on the maximal tension stress theory, the critical safety thickness with different safety factors was calculated, and the FLAC3D numerical simulation method was used to calculate the stability of mined-out areas on large-scale reserving horizontal pillars with safety thickness. Results show that the maximum tensile stress does not exceed the ultimate tensile strength of horizontal pillars under the critical safety thickness whose value obtained by the new method is smaller than the value calculated through the traditional design methods. In addition, deflections are restricted to an acceptable range. The pillars and stopes remain stable and massive destruction is not found. The validity and security of the safety thickness formed in the new design method were confirmed through the simulation experiment, which provide some reference and experience for the design of the safety thickness of horizontal pillars in similar mine sites by using the filling method.