13
At shallow depths (100 -266 m), the average reduc-
tion in pillar strength remains relatively low, ranging from
4.11% to 10.04%. This phase, termed as the “Transition
Phase,” indicates that the effects of dirt band thickness are
less impactful at lower depths where the confining stress
from overburden is limited. At these depths, the pillar is
better able to withstand the weakening influence of the
dirt bands due to lower stress concentrations. Additionally,
the roof, middle, and floor placements of dirt bands con-
tribute differently to stability, with floor-positioned bands
generally resulting in slightly higher stability than roof-
positioned ones, as roof-based dirt bands experience more
concentrated stress, potentially leading to localized failures.
As depth increases beyond 266 m, the strength reduction
becomes more pronounced, marking the beginning of
the “Enhanced Reduction Phase.” At these greater depths
(300–500 m), where the overburden stress is significantly
higher, the reduction in pillar strength ranges from 6.08%
to 11.18% at 300 m depth and 5.74% to 10.56% at 500 m
depth, depending on the thickness of the dirt band. The
data suggest that at greater depths, thicker dirt bands create
larger zones of weakness within the pillar structure, ampli-
fying the stress concentration and reducing the overall load-
bearing capacity of the pillar.
This trend shows how important depth is in the failure
of pillars with dirt bands. As depth goes up, the pressure
around coal pillars increases, making them more vulner-
able to weaknesses like dirt bands. As the depth of the
pillar increases, its axial burden increases, affecting the
weak portions of the pillar which in turn, makes the pil-
lars with greater thickness of the dirt bands, unstable. It
must be noted that, the 0.5 m thick dirt bands positioned
at depths of more than 300 m are the most weakened which
implies that the thicker bands weaken the pillars in areas
of higher stress. This is in conformity with the principles
of rock mechanics that suggests that weak layers or dirt
bands located at areas of high stress are most likely to fail
due to shear stress and will thus develop cracks along those
layers. The orientation of dirt bands in the coal pillar is a
crucial factor in determining its stability. Stress distribu-
tions in these dirt bands will also differ as the roof-interface
dirt bands will have developed cracks and a higher point
of failure because they will have received vertical and shear
loads while the dirt bands at floor-interface will have been
minimally exposed. This distribution is especially impor-
tant where cover is deep and stress is distributed unevenly
and thus weaknesses at the roof are exacerbated.
Generally, these results demonstrate how important
it is to assess dirt band properties while analysing pillar
stability in underground mining operations. Conventional
empirical models used to estimate pillar strength might not
sufficiently account for these geological discontinuities,
which could result in an overestimation of stability in situa-
tions with significant weak strata. The study’s findings high-
light how, especially in the case of deep mining operations,
a cooperative strategy using numerical models to incorpo-
rate the thickness and spatial arrangement of dirt bands
can greatly increase the accuracy of stability predictions. By
using such advanced modelling approaches, design criteria
could be improved and the chance of unanticipated failures
linked to unidentified geological discontinuities inside coal
pillars decreased.
CONCLUSIONS
This study explores how dirt bands impact the stability of
coal pillars, examining how band thickness, location, and
mining depth influence structural integrity. Results show
that thicker dirt bands lead to a significant reduction in pil-
lar strength, especially at greater depths where a 0.5 m band
can reduce strength by up to 10.56% at 500 m compared
to just 4.11% at 100 m. The location of dirt bands also
matters: those near the roof interface cause greater instabil-
ity due to concentrated stress, while centrally placed dirt
bands allow for more uniform stress distribution, enhanc-
ing stability. Additionally, deeper mining depths amplify
these effects, with substantial strength reductions observed
at depths beyond 300 meters.
The findings suggest that traditional empirical models,
which typically exclude such geological discontinuities, may
overestimate pillar stability in deeper mines. Incorporating
factors like dirt band thickness, position, and depth into
predictive models could improve accuracy, offering a safer
foundation for mining design in high-stress conditions.
This study highlights the value of numerical modelling
in producing precise stability assessments, advocating for
updated models to address geological factors overlooked in
conventional approaches, ultimately advancing safer min-
ing practices.
ACKNOWLEDGMENT
The authors are obliged to the Director of NIT Rourkela,
for their permission to publish this paper. The cooperation
provided by the mine management of GDK‑11 Incline
mine, SCCL during the field study is thankfully acknowl-
edged. The views expressed in the paper are those of the
authors, and not necessarily of the institute to which they
belong.
At shallow depths (100 -266 m), the average reduc-
tion in pillar strength remains relatively low, ranging from
4.11% to 10.04%. This phase, termed as the “Transition
Phase,” indicates that the effects of dirt band thickness are
less impactful at lower depths where the confining stress
from overburden is limited. At these depths, the pillar is
better able to withstand the weakening influence of the
dirt bands due to lower stress concentrations. Additionally,
the roof, middle, and floor placements of dirt bands con-
tribute differently to stability, with floor-positioned bands
generally resulting in slightly higher stability than roof-
positioned ones, as roof-based dirt bands experience more
concentrated stress, potentially leading to localized failures.
As depth increases beyond 266 m, the strength reduction
becomes more pronounced, marking the beginning of
the “Enhanced Reduction Phase.” At these greater depths
(300–500 m), where the overburden stress is significantly
higher, the reduction in pillar strength ranges from 6.08%
to 11.18% at 300 m depth and 5.74% to 10.56% at 500 m
depth, depending on the thickness of the dirt band. The
data suggest that at greater depths, thicker dirt bands create
larger zones of weakness within the pillar structure, ampli-
fying the stress concentration and reducing the overall load-
bearing capacity of the pillar.
This trend shows how important depth is in the failure
of pillars with dirt bands. As depth goes up, the pressure
around coal pillars increases, making them more vulner-
able to weaknesses like dirt bands. As the depth of the
pillar increases, its axial burden increases, affecting the
weak portions of the pillar which in turn, makes the pil-
lars with greater thickness of the dirt bands, unstable. It
must be noted that, the 0.5 m thick dirt bands positioned
at depths of more than 300 m are the most weakened which
implies that the thicker bands weaken the pillars in areas
of higher stress. This is in conformity with the principles
of rock mechanics that suggests that weak layers or dirt
bands located at areas of high stress are most likely to fail
due to shear stress and will thus develop cracks along those
layers. The orientation of dirt bands in the coal pillar is a
crucial factor in determining its stability. Stress distribu-
tions in these dirt bands will also differ as the roof-interface
dirt bands will have developed cracks and a higher point
of failure because they will have received vertical and shear
loads while the dirt bands at floor-interface will have been
minimally exposed. This distribution is especially impor-
tant where cover is deep and stress is distributed unevenly
and thus weaknesses at the roof are exacerbated.
Generally, these results demonstrate how important
it is to assess dirt band properties while analysing pillar
stability in underground mining operations. Conventional
empirical models used to estimate pillar strength might not
sufficiently account for these geological discontinuities,
which could result in an overestimation of stability in situa-
tions with significant weak strata. The study’s findings high-
light how, especially in the case of deep mining operations,
a cooperative strategy using numerical models to incorpo-
rate the thickness and spatial arrangement of dirt bands
can greatly increase the accuracy of stability predictions. By
using such advanced modelling approaches, design criteria
could be improved and the chance of unanticipated failures
linked to unidentified geological discontinuities inside coal
pillars decreased.
CONCLUSIONS
This study explores how dirt bands impact the stability of
coal pillars, examining how band thickness, location, and
mining depth influence structural integrity. Results show
that thicker dirt bands lead to a significant reduction in pil-
lar strength, especially at greater depths where a 0.5 m band
can reduce strength by up to 10.56% at 500 m compared
to just 4.11% at 100 m. The location of dirt bands also
matters: those near the roof interface cause greater instabil-
ity due to concentrated stress, while centrally placed dirt
bands allow for more uniform stress distribution, enhanc-
ing stability. Additionally, deeper mining depths amplify
these effects, with substantial strength reductions observed
at depths beyond 300 meters.
The findings suggest that traditional empirical models,
which typically exclude such geological discontinuities, may
overestimate pillar stability in deeper mines. Incorporating
factors like dirt band thickness, position, and depth into
predictive models could improve accuracy, offering a safer
foundation for mining design in high-stress conditions.
This study highlights the value of numerical modelling
in producing precise stability assessments, advocating for
updated models to address geological factors overlooked in
conventional approaches, ultimately advancing safer min-
ing practices.
ACKNOWLEDGMENT
The authors are obliged to the Director of NIT Rourkela,
for their permission to publish this paper. The cooperation
provided by the mine management of GDK‑11 Incline
mine, SCCL during the field study is thankfully acknowl-
edged. The views expressed in the paper are those of the
authors, and not necessarily of the institute to which they
belong.