EFFECT OF UNDERLYING GOAVES ON SUBSIDENCE WHILE
WORKING A LONGWALL FACE AT SHALOW DEPTH
– A CASE STUDY @ PVK NO.5 INCLINE
M.S.Venkata Ramaiah Lolla Sudhakar M.Venkat Ramana Rao
Dy.GM., 5Gr.of Mines Dy.Manager (Project Planning) Under Manager, PVK
The Singareni Collieries Company Limited, Kothagudem.
Subsidence is considered as one of the major problems in underground mining
particularly at shallow depth. Effect of subsidence with longwall method of working is more
critically viewed as it lands to problems of spontaneous heating in goaf, strata control
problem on the high capital intensive longwall face equipment and land degradation due to
formation of wider cracks on surface with wider span of extraction added to it in subsided
PVK is one of the highly mechanized mines of Singareni Collieries located in
Kothagudem Area, where extraction with Chinese Longwall equipment was started in the
year 1995 and is being continued. Till now 9 Longwall panels were completed successfully in
the top most seam (Queen seam) of the mine..
The present 10th Longwall panel is LW.P.No.1 (10th Longwall panel of the mine),
which is located in rise side of the property is at a minimum depth cover of 48m. Original
longwall panel No.1 was planned with 150m face length but with the experience of 2.54m of
maximum subsidence in adjacent dip side panel No. 2 with about 98m of depth, The panel
No.1 was divided in mid length of the property into 2 longwall panels of 60m face length
each viz. 1A & 1. Extraction in the dip side panel i.e. Panel No.1A is completed and the
extraction in Panel no.1 is started on 02-02-2004 and as on date the face retreated by 430m.
A review of subsidence over the already worked out longwall panels shows that
subsidence is not uniform, further the susbsidence is high because of the presence of
underlying goaves. The crack pattern and the degree of cumulative subsidence indicate that
the underlying goaves of King seam with their maximum span has imposed certain effect on
The maximum subsidence observed in Panel no.1A was 1.4m and in Panel no.1 was
2.07m till 10-04-2004 with 60m face length. The estimated subsidence based on the study of
subsidence profiles of already worked out panels was around 2.2m with 150m face length.
But with the reduction in face length, the observed subsidence was less than the estimated.
The present paper deals with the effect of under lying goaves in increasing the
amount of subsidence, deviation of actual value with the predicted with the change in length
of the face of a longwall panel lying at a shallow depth cover, measurement of strain and
practices continued during operation.
Padmavathikhani No.5 Incline of Kothagudem area mainly consists of three seams,
the details of all the seams and partition between them is furnished below.
SEAM DESCRIPTION THICKNESS (M)
Worked in middle section
leaving coal & shale in roof 2.14 to 10.2
Sandstone, shale, shaley
PARTING 42.0 to 44.0
coal with thin coal bands
KING SEAM Worked in two sections 3.57 to 9.45
PARTING Sand stone 5.00 to 6.00
Developed & worked upto
BOTTOM SEAM 43L due to seam is thinned 0.30 to 4.62
out from 43L to 1.5m
Top seam is extracted mainly with Longwall technology using Chinese equipment.
The area in King seam below the worked out Longwalls and present Longwall of Top seam is
already developed and depillared in two sections before the commencement of Longwall in
Top seam. In King seam, Bottom section being stowed and top section is being caved with
Bord & pillar technology. The parting of 45m between Top seam and King seam is being
disturbed and effected by subsidence due to the extraction in King seam, which is evident
from the incident that, there is an occurrence of symptoms of spontaneous heating and
leakage of black damp from King seam goaf into Top seam in Longwall Panel no.2. The Top
seam working plan is shown in PLAN-1.
The topmost raise side panel i.e. Panel no.1 is left in early days for introduction of
Continuous miner and extraction by Wonga-valli method. But it could not be materialized
due to various other reasons. Hence, it is proposed to extract the Panel no.1 with Longwall
technology in the year 2003. Having experienced 2.54m of maximum subsidence in adjacent
dip side panel (Panel no.2), started estimating the maximum subsidence for Panel no.1 with a
face length of 150m.
From the observation of subsidence profiles of already worked out longwall panels,
lot of points were derived and used in predicting the amount of subsidence for panel no.1
which is at a shallow depth cover of 48m. With 150m face length the subsidence estimated
was more than 3m, DGMS persons were not satisfied and refused to give permission for
extraction of this longwall panel. Hence to reduce the maximum subsidence it is decided to
split the entire property in the mid length into two halves of 60m face length each with a
barrier of 22.5m thickness between them.
This paper places the observations made from the subsidence profiles of already
worked out panels, effect of under lying goaves and their usage in predicting the subsidence
profiles of Panel no.1A & 1 and also the comparison between the predicted and actual
profiles of Panel no.1A, in which extraction is completed on 08-11-2003.
DEVELOPMENT OF EMPHERICAL RELATION FOR SUBSIDENCE FROM THE
SUBSIDENCE PROFILES OF EARLIER PANELS:
The observed surface profiles of worked out panels (shown in Annexure-I) clearly
indicating that the subsidence profiles are asymmetrical to their central axis as the angle of
draw and subsidence are generally more on the starting side of the panel than the finishing
side. This is because of the fact that the energy released in the first break will be higher than
the energy released in the subsequent break1. Also the subsidence value is more than the
normal, whenever any goaf is encountered in below seam beneath the panel.
All the subsidence profiles of longwall panels of this mine are more or less following
the empherical equation given below. (The details of deriving of this equation are presented
in “Geomechanics and ground control” seminar conducted by CMRI in 2003)2.
S = 0.1508 (W/H)2-0.8248(W/H)-0.5292 ---------- (Eq. 1)
S = Maximum subsidence, m
W = Width of the panel, m
H = Depth of the panel, m
The subsidence profiles obtained for different panels using the above formula are
deviating from the continuity whenever any goaf encounters below the panel. The study has
been done in co-relating the amount of goaf and the amount of increase in subsidence. The
empherical equation for addition factor (named as Goaf factor) to be added to the subsidence
value, predicted with eq.1 is given below.
Gf = 0.0001(P)2-0.0128(P)-0.3265 --------- (Eq. 2)
Gf = Goaf Factor.
P = Percentage of Goaf area in below seam present beneath the
longwall panel of top seam at a particular point.
Subsidence = Eq.1 +Eq.2
Subsidence = [0.1508(W/H)2-0.8248(W/H)-0.5292]+[0.0001(P)2-0.0128(P)-0.3265]
The corrected profiles after applying the goaf factor and the original profiles of
worked out panels are given in Annexure-II.
PANEL NO.1A – PREDICTED Vs ACTUAL SUBSIDENCE:
Some of the details of the Panel no.1A are given below.
Panel Length : 520 m
Face Length : 62.5 m
Depth : 54 m(min.), 96 m(max.)
Panel started on : 10-7-2003
Extraction completed on : 08-11-2003
Max. Subsidence : 1.46 m
SUBSIDENCE PROFILE OF PANEL NO.1A ( AS ON 27-11-2003)
6 5 4 3 2 1 0
Actual Subsidence Profile
Profile without goaf correction
Predicted Profile after Goaf correction
From the graph it is clear that, the subsidence profile is changing its angle at certain
points where there is a goaf in King seam below the panel of Top seam which is evident from
the PLAN-2. The estimated maximum subsidence value is around 2.2m, but the actual
amount observed was 1.46m. The difference may be due to the following reasons.
1. Reduction in Face length from 150m to 60m
2. Faster rate of extraction (@ 6m/day)
3. Time lapse for settlement of goaf of the longwall panel.
With the extraction of adjacent panel (i.e. Panel no.1), the amount of maximum subsidence
was increased to 1.6m. In this panel no arrangements are made for measuring the strain
values and the subsidence pillars were also at a greater distance i.e. 30m distance between
two pillars along centre line.
PANEL NO.1 :
Details of the Panel no.1 are given below.
Panel Length : 500 m
Face Length : 62.5 m
Depth : 48 m(min.), 85 m(max.)
Panel started on : 02-02-2004
Max. Subsidence : 2.07 m (as on 10-4-2004)
Avg. Rate of retreat : 8 m / day
The observed subsidence profile is varying from the predicted profile for this panel
also, may be because of the same reasons specified for Panel no.1A. The Subsidence profiles
are given below.
SUBSIDENCE PROFILE OF PANEL NO.1 ( AS ON 10-04-2004)
Panel no.1 Subsidence Profile
In this panel, strata monitoring study is going on in association with the competent
Scientific Institutions. They designed the subsidence pillars layout on surface over the panel
for measuring both the subsidence and strain (layout is given in PLAN-3). The general strain
observed is 5mm/m in compression and tension. However, in a small zone the compressive
and tensile strains were about 60mm/m. The reason for the same was analyzed and found that
the area at which maximum strain observed was under the influence of underlying goaves
(Strain curves where maximum strain observed are shown in Annexure-III). Extra measures
taken in this panel for knowing the movement of strata are as follows.
1. Increasing the number of subsidence pillars on surface and measurement of distances
to calculate strain.
2. Installation of Multi Point Borehole Extensometer (MPBEx) with 4-anchors grouted
at different depths from surface over the panel.
3. monitoring of load by Load cells with data logger to avoid human mistakes
4. Provision of more number of Tell-tales
5. Convergence indicators for every 10m in Tail and Main gate road ways.
6. Vibrating Wire Stress gauges.
7. Remote convergence indicators for monitoring in goaf.
From the study conducted in this mine regarding subsidence it is clear that, there is a
definite effect of underlying goaves on the final amount of subsidence which is more
predominant in raise side panels lying at shallow depth which is clearly observed in the
profiles of Panel no.1A & 1. With the change in position of goaves below the longwall panels
the location at which maximum subsidence occurred is also changed.
1. For panel no.1A maximum subsidence observed at 240m distance from face
starting, where complete goaf exists below the panel.
2. For panel no.1 maximum subsidence observed at 390m distance from face
starting, where complete goaf exists below the panel.
Moreover, the orientation of Panels in King seam is different from the orientation of
Longwall panels of Top seam.
The empherical equations developed for predicting the subsidence value are site
specific, and includes the geo-mining conditions of the area and need not satisfy the profiles
of other area that are with varying conditions.
The authors are grateful to the Singareni Management in particular Sri CH Krishiah
GM, KGM for encouraging in bringing out this paper. The views expressed by the authors
are of their own and not necessarily of the organization to which they belong.
1. Dhar B.B. (1995), Status of subsidence research in India, Acourse on Subsidence
prediction and management in mining areas by CMRI, 18th-22nd November,
2. Venkata Ramaiah M.S., Sudhakar.Lolla, Venkat Ramana Rao M., “Prediction of
Surface subsidence with interaction of goaf (PRESS WING)”, Presented in
“Geomechanics & Ground Control” seminar conducted by CMRI in 2003.
3. NIRM Report, “Subsidence studies at SCCL”.
4. Orchard R.J., (1964), Surface subsidence resulting from alternate treatment of colliery
goaf, Colliery Engineering, October, pp.428-435.
5. Holla L., (1991), Reliability of Subsidence prediction methods for use in Mining
Decisions in New South Wales.
TOP SEAM WORKING PLAN
OF PVK NO.5 INCLINE,
PLAN SHOWING THE GOAVES OF KING SEAM BELOW THE LONGWALL
PANELS OF TOP SEAM
PLAN SHOWING THE LAYOUT OF SUBSIDENCE PILLARS ON SURFACE OVER
THE LONGWALL PANEL NO.1
- On each side of the centre line pillars were constructed at all the junction points of the
rectangles, column wise distance is 10m and row wise distance is 5m.
- Along centre line the distance between two pillars is 7.5m