Stability Analysis of Pillars in an Inclined Airleg Stope Under Static Loading Conditions

Abstract

Understanding the failure mechanism and stability of pillars is necessary for their efficient extraction in Airleg stopes (ALS). By substituting standing support structures that can sustain the weight of the roof, pillars can be removed effectively and safely. Most pillar extraction, however, is carried out in flat dipping ALS, which neglects the horizontal stress influence on pillar stability which raises the risk of sliding along planes. To take pillar inclination into account, the tributary area method was extended by Pariseau in 1982. However, the pillar strength and friction at the pillar-floor and pillar-roof interfaces were not considered in this extended approach, which instead used the horizontal and vertical stress ratio. As a result, this study presents an empirical solution that takes into consideration these parameters and it is verified in a finite volume numerical simulation. To accomplish this, a stability analysis of the efficient inclined pillar extraction is carried out to recover 100% of the original pillars and leave standing support structures as pillar replacements. The solution and numerical simulation are then employed to ascertain the stress component’s direction and magnitude acting on an inclined pillar. It is drawn from the solution and numerical results that the stress load on an individual pillar exhibited a 150 MPa maximum stress. Furthermore, a laboratory test of 150 mm diameter and 370 mm length Pine props are conducted to determine their ultimate and critical buckling load (${P}_{cr}$). It is drawn from the results that the props can handle a 700 kN (71 tons) load with an average ${P}_{cr}$ of 35.3 MPa. Implying that 6.0 props can handle the presented solution and numerically estimated stress load on an individual inclined ALS pillar. Therefore, the appropriate integrated support system requires coupling the 6.0 Pine prop’s support structure and GEWI thread bars installed in a 2.0 m × 2.0 m and 1.5 m × 1.5 m pattern, respectively.

Appendix A

The ${L}_{B}$ of the GEWI bolt is attained by computing the difference in volume of the borehole and thread bar per 600 mm (Eq. 32 and Figs. 22 and

Fig. 23

GEWI bolt installation and encapsulation

23).

$$V_{Grout} = (\pi r^{2} {\text{h}})_{Borehole} - (\pi r^{2} {\text{h}})_{Bolt}$$

(32)

${L}_{B}$ = 912 mm. (See Tables 12, 13, 14 and 15)