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Ground Control Analysis of
Highwall Mining Layouts
When designing a highwall mining layout, the mining engineer must specify 1) web pillar width, 2) number of web pillars between barrier pillars and 3) barrier pillar width. The design parameters are determined by the highwall miner hole width, the mining height and the overburden depth. In addition, the mine planner must estimate the pillar strength, the applied stress on pillars and the pillar stability factor. Coal Pillar Strength Numerous empirical formulas are available to predict coal pillar strength; however, the Mark-Bieniawski formula applies best for web pillars, which are very long, narrow rectangular pillars. For long pillars whose length is much greater than their width, the Mark-Bieniawski formula (11) reduces to SP = SI [ 0.64 + 0.54 W / H ] (1) Where: Sp = web or barrier pillar strength SI = in situ coal strength W = web or barrier pillar width H = mining height In situ coal strength is normally taken as 6.2 MPa (900 psi). Mining height can be equal to the seam thickness, but it may be greater if some rock is mined with the coal. Coal Pillar Stress Tributary area method is useful to estimate vertical stress on web and barrier pillars. Average vertical stress on a web pillar is SWP = SV (WWP + WE) / WWP (2) Where: SV = in situ vertical stress WWP = web pillar width WE = highwall miner hole width. The highwall mining equipment dictates the hole width which varies from 2.7 to 3.6 m (9 to 12 ft). In situ vertical stress depends on the overlying rock density and overburden depth. Vertical stress gradient is typically 0.025 MPa/m (1.1 psi/ft). Overburden depth may be taken as the maximum overburden depth on a highwall mining web pillar, which is very conservative, or alternatively as a high average value computed as DDesign = 0.75 * DMAX + 0.25 * DMIN

Where: DMAX = maximum overburden depth DMIN = minimum overburden depth. Finally, the stability factor for web pillars against strength failure is simply SFWP = web pillar strength / web pillar stress (SWP) (4) For design purposes, the stability factor for web pillars typically ranges from 1.3 to 1.6. Based on data in MSHA highwall mining ground control plans, studies (1) found that stability factor for web pillars in practice ranged from 1.3 to 1.6 in about 30% of the plans and exceeded 1.6 in 45%. These stability factor estimates from the ground control plans were based on the information provided, and their adequacy is not implied. This survey also found that the width-to-height (W/H) ratio of web pillars exceeded 1.0 in 75% of the cases examined. In general, keeping the web pillar W/H ratio above 1 is desirable to maintain better web pillar integrity. If the number of web pillars in a panel is selected as “N”, then the panel width is given by WPN = N (WWP + WE) + WE (5) Neglecting the stress carried by the web pillars (i.e. assuming that they have all failed), the average vertical stress on a barrier pillar is SBP = SV (WPN + WBP) / WBP (6) Where: WPN = panel width WBP = barrier pillar width Similarly, the stability factor for barrier pillars against strength failure is simply SFBP = barrier pillar strength / barrier pillar stress (SBP) (7) Because the stress carried by web pillars within a panel is neglected, the stability factor for barrier pillars can be as low as 1. Studies (1) found that the width of barrier pillars exceeded 5 m (16 ft) in more than half the cases examined and more important, the W/H ratio for barrier pillars exceeded 3 in 2/3 of the cases. Barrier pillars with a W/H ratio greater than 3 are superior for sound geomechanics reasons. The ARMPS program (11) applies similar relations to the above for estimating the stability factor of web and barrier pillar combinations. When using ARMPS to analyze highwall mining layouts, the mining engineer should consider all the web pillars plus one barrier pillar in the analysis. The loading condition is normally
development loading (option 1); however, if old underground workings are nearby, alternative loading conditions such as a front gob (option 2) may be necessary.
Web and Barrier Pillar Design Charts and Design Examples
The above equations for web and barrier pillar analysis can be implemented into a spreadsheet (9) or programmable calculator. In lieu of either, figures 4 and 5 are design charts for web pillars while figure 6 provides design guidance for barrier pillars. Figure 4 applies to a 2.7-m-wide (9ft) highwall miner hole, while figure 5 applies to a 3.6-m-wide (12 ft) hole. In figures 4 and 5, options a and b apply to stability factors of 1.3 and 1.6, respectively. In figure 6, options a, b and c apply to panel widths of 30.5, 61 and 122 m (100, 200 and 400 ft), respectively. Note that this design chart assumes a barrier pillar stability factor of 1.0 and it neglects any load carrying capacity of the web pillars within a panel. Compared to ARMPS, these charts always give wider web and panel widths and are therefore conservative.
0 1 2 3 4 5 6 20 40 60 80 100 120 140 160 180 200 Depth of cover - m Web pillar width - m Mining height = 0.61 m Mining height = 1.22 m Mining height = 2.44 m Mining height = 3.66 m Figure 4A. Suggested web pillar width with stability factor of 1.3, coal strength of 6.2 MPa and 2.75-m-wide

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