8
variation in strength values (e.g., 1.40 MPa at 100 m and
0.15 MPa at 300 m). This alignment validates the numerical
modelling results with Sheorey’s empirical approach, sug-
gesting that the simulation accurately captures the strength
behaviour of coal pillars in this depth range. However, at
depths exceeding 300 m (specifically 400 m and 500 m),
negative values for the variation in strength (–1.47 MPa
at 400 m and –3.3 MPa at 500 m) indicate discrepancies
between the two methods. Specifically, Sheorey’s empirical
approach overestimates the pillar strength at higher depths
of cover (Figure 8).
This result suggests that the empirical approach may be
less reliable for deep cover conditions, potentially because it
does not fully account for the complex stress redistribution
and material behaviour at such depths. This finding empha-
sizes the importance of numerical modelling for accurately
assessing coal pillar strength in high-depth scenarios, where
empirical methods may have limitations.
INFLUENCE OF DIRT BANDS OF
VARYING THICKNESS ON THE
STABILITY OF COAL PILLARS UNDER
VARYING COVER DEPTHS
In the present investigation, varying thicknesses of dirt bands
(0.2, 0.3, 0.4, and 0.5 m) have been incorporated into the
modelled coal pillar (Figure 9) to assess their repercussions
on pillar stability across diverse depths. These dirt bands
are strategically located at significant positions within the
pillar, encompassing the roof-interface (Figure 9(a)), floor-
interface (Figure 9(c)), and the central region of the pillar
body (Figure 9(b)), to elucidate how each specific placement
influences the pillar’s load-bearing capacity. The evaluation
of pillar strength is conducted across a spectrum of cover
depths (100, 200, 266, 300, 400, and 500 m), thereby facili-
tating a thorough examination of depth-dependent stability.
The comprehensive outcomes of this analysis, which takes
into account the effects of both dirt band thickness and their
specific placement within the pillar at varying depths, are
elaborated upon in the following sections.
Table 3. Results of numerical simulation study for the estimation of pillar strength at different depths of cover
Depth
(m)
Pillar Strength Estimated
Using Numerical
Simulation
(MPa)
Pillar Strength Estimated
Using Sheorey’s Empirical
Approach
(MPa)
Variation in
Strength
(MPa) Remarks
100 13.8 12.4 1.40 Below 300 m, there is good agreement
between both methods, while at greater
depths, negative values indicate that
Sheorey’s approach overestimates pillar
strength.
200 15.2 14.63 0.57
266 16.6 16.09 0.51
300 17 16.85 0.15
400 17.6 19.07 –1.47
500 18 21.30 –3.3
Sheorey’s empirical approach
Numerical simulation
Overestimation of strength by
Sheorey’s empirical approach
Figure 8. Comparison of coal pillar strength estimates from Sheorey’s empirical approach and
numerical simulation across different depths of cover
variation in strength values (e.g., 1.40 MPa at 100 m and
0.15 MPa at 300 m). This alignment validates the numerical
modelling results with Sheorey’s empirical approach, sug-
gesting that the simulation accurately captures the strength
behaviour of coal pillars in this depth range. However, at
depths exceeding 300 m (specifically 400 m and 500 m),
negative values for the variation in strength (–1.47 MPa
at 400 m and –3.3 MPa at 500 m) indicate discrepancies
between the two methods. Specifically, Sheorey’s empirical
approach overestimates the pillar strength at higher depths
of cover (Figure 8).
This result suggests that the empirical approach may be
less reliable for deep cover conditions, potentially because it
does not fully account for the complex stress redistribution
and material behaviour at such depths. This finding empha-
sizes the importance of numerical modelling for accurately
assessing coal pillar strength in high-depth scenarios, where
empirical methods may have limitations.
INFLUENCE OF DIRT BANDS OF
VARYING THICKNESS ON THE
STABILITY OF COAL PILLARS UNDER
VARYING COVER DEPTHS
In the present investigation, varying thicknesses of dirt bands
(0.2, 0.3, 0.4, and 0.5 m) have been incorporated into the
modelled coal pillar (Figure 9) to assess their repercussions
on pillar stability across diverse depths. These dirt bands
are strategically located at significant positions within the
pillar, encompassing the roof-interface (Figure 9(a)), floor-
interface (Figure 9(c)), and the central region of the pillar
body (Figure 9(b)), to elucidate how each specific placement
influences the pillar’s load-bearing capacity. The evaluation
of pillar strength is conducted across a spectrum of cover
depths (100, 200, 266, 300, 400, and 500 m), thereby facili-
tating a thorough examination of depth-dependent stability.
The comprehensive outcomes of this analysis, which takes
into account the effects of both dirt band thickness and their
specific placement within the pillar at varying depths, are
elaborated upon in the following sections.
Table 3. Results of numerical simulation study for the estimation of pillar strength at different depths of cover
Depth
(m)
Pillar Strength Estimated
Using Numerical
Simulation
(MPa)
Pillar Strength Estimated
Using Sheorey’s Empirical
Approach
(MPa)
Variation in
Strength
(MPa) Remarks
100 13.8 12.4 1.40 Below 300 m, there is good agreement
between both methods, while at greater
depths, negative values indicate that
Sheorey’s approach overestimates pillar
strength.
200 15.2 14.63 0.57
266 16.6 16.09 0.51
300 17 16.85 0.15
400 17.6 19.07 –1.47
500 18 21.30 –3.3
Sheorey’s empirical approach
Numerical simulation
Overestimation of strength by
Sheorey’s empirical approach
Figure 8. Comparison of coal pillar strength estimates from Sheorey’s empirical approach and
numerical simulation across different depths of cover