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International Journal of Rock Mechanics & Mining Sciences 45 (2008) 29–46

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Upshot of strata movement during underground mining of a thick coalseam below hilly terrain

Rajendra Singha,�, P.K. Mandala, A.K. Singha, Rakesh Kumara, J. Maitib, A.K. Ghosha

aCentral Institute of Mining and Fuel Research (CIMFR, formerly CMRI), Dhanbad 826001, Jharkhand, IndiabIndian Institute of Technology (IIT), Kharagpur 721302, West Bengal, India

Received 27 June 2006; received in revised form 25 February 2007; accepted 28 March 2007

Available online 27 June 2007

Abstract

Underpinning-based simultaneous extraction was successfully used to extract more than 90% of thick and contiguous sections of a

coal seam at Chirimiri mine, lying below a hill cap with a rapid change in depth of cover and is placed above the surrounding ground

level. The last few panels of the mine encountered strata control problems during the depillaring. Underground instrumentation and

observations showed that the last phase depillaring of the property near the free surface of the hill slope experienced shearing of overlying

strata along two incompetent layers. Field investigations revealed that large amount of lateral movement due to the hilly nature of the

overlying strata is the main source of the problem. A simple two-dimensional study on a simulated model also demonstrated the

occurrence of the problem during working below an unconfined rock mass of hilly nature. Different evidences were collected from the site

to identify the main horizons and the direction of lateral movement and, accordingly, two different strata management plans, one for

each section, were successfully executed to overcome the encountered strata control problems. Field investigations in the rest of the

panels of the site showed favourable impact of the adopted strata management plan.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Thick seam; Bord and pillar; Roof stability; Depillaring; Lateral movement; Strata management

1. Introduction

In India, two close coal seams/sections with inter-burdenthickness 9m or less are called contiguous coal seams/sections, and a coal seam with thickness of 4.8m or more iscalled a thick seam. Contiguous sections of around 12.5mthick, Zero seam at Chirimiri colliery were extractedsimultaneously under hill slope with a rapid change indepth of cover. The coal seam below the hill cap was nearlyflat and is located at a horizon above the surroundingground level. The working plan of the Zero seam of themine is shown in Fig. 1. This figure consists of topographiccontours of the overlying steep escarpment of the hill,overlapped over the plan. Dense inaccessible forest andsteep escarpment of the hill restricted the chance of any

e front matter r 2007 Elsevier Ltd. All rights reserved.

mms.2007.03.006

ing author. Tel.: +91326 2211608; fax: +91326 2204362/

ess: [email protected] (R. Singh).

ground movement study from the surface during under-ground mining of the seam.Most of the panels of the mine were depillared without

any strata control problem by underpinning-based simul-taneous extraction method of CMRI [1]. Underpinning isthe application of full column-grouted cable bolt ofsuitable length in the floor of the top section working.However, the last few panels of the mine-experienced stratacontrol problems, mainly in the upper horizon of theworking. Splits and slices experienced failure of overlyingroof strata during extraction of pillars in panels G1 and G2of the mine. Shearing of overlying strata was observed at ahorizon, 2.9m above the roof level of the top section and ata horizon in the parting, 3.6m above the roof level of thebottom section. Prematured failure of the roof strata poseda challenge for safe depillaring of rest part of the propertyof the coal seam. Underground instrumentation andmonitoring of strata behaviour indicated development ofconsiderably large amount of induced horizontal stress,which might have developed due to lateral movement of

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AditsSurface to Zero

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9S

50S48S

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Stress meters installed in vertical and horizontally drilled hole

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S 40˚ 00’ 00”E FULL DIP I IN 182

Fig. 1. Working plan of Zero seam at Chirimiri colliery showing different panel positions with surface contours to depict fast change in cover.

R. Singh et al. / International Journal of Rock Mechanics & Mining Sciences 45 (2008) 29–4630

the overlying strata. Subsequently, strata managementsystems were formulated and successfully adopted tocounter the adverse affect of the lateral movement overthe underground working. This case study of visualisationand management of the active lateral movement duringunderground mining of the thick and contiguous sectionsof the Zero seam under fast change in depth of cover isdetailed in this paper.

2. Ground movement in hilly areas

Once the problem of overlying strata failure wasobserved in panels G1 and G2, the study was focused on

finding out the reason of failure. In the beginning of theproblem, occurrence of hillseams [2] in hilly overburden(Fig. 2) was thought to be the reason of the failure.However, the occurrence of hillseams might have createdproblems for working in all the panels and not only in lastfew panels of the mine. Later, strata control investigationsrevealed that the problem is occurring due to interaction ofthe underground mining structures with the resultedground movement. General ground movement patternand norms for Indian mining conditions are described in[3], while discussing results of surface subsidence investiga-tions of different Indian coalfields. Important differencesbetween the subsidence norms of Indian conditions and

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Exposed hill

slope

Coal

Weathered joint

(Hillseam)

Fig. 2. Occurrence of Hillseams [2] in hill slope with fast change in cover.

R. Singh et al. / International Journal of Rock Mechanics & Mining Sciences 45 (2008) 29–46 31

those given in the Subsidence Engineering Handbook, UK[4] are outlined in the study. Two empirical equations toestimate maximum subsidence (S) for single- and multi-seam cases are given [3] as follows

single seam cases:

S ¼ 0:33f1þ 1:1 tanh½1:4ðx� 1:8Þ�g, (1)

multi seam cases:

S ¼ 0:12f1þ 5:9 tanh½3:8ðx� 1:45Þ�g, (2)

where x is the ratio of width-to-depth ratio (w/h) and non-effective width-to-depth ratio (NEW) [2].

Furthermore, this study proposed a new influencefunction to incorporate subsidence asymmetry and effectof extraction edges of room and pillar mining in theprediction model but all these efforts are valid for a nearlyflat coal seam below continuous cover. The nature ofground movement is observed to be different duringmining below high relief areas [5]. Here, considerableamount of lateral movement of both sides of a valleyinduces large amount of compressive surface strains.However, reported studies on ground movement in highrelief areas are few [6–8] and are aimed at the assessment ofsurface conditions only. The case of Zero seam mining isdifferent from the reported cases of ground movement inhigh relief areas. At Chirimiri colliery, a nearly flat coalseam is located below a hill cap and is placed above thesurrounding ground level, which is approached by a pair ofadits. It was difficult to conduct a surface movement study,mainly due to frequent toppling of boulders along slope-favourable directions after experiencing ground movement.Here, interaction of lateral movement of the unconfined

overlying strata along the weak planes with undergroundmining structures is realised to be rather more important.

2.1. Influence of lateral movement

Underground mining disturbs the natural state of stressequilibrium due to which vertical and/or horizontal move-ment takes place. The mining of a nearly flat coal seamunder confined and continuous cover causes stimulation ofvertical mining-induced stress (Fig. 3) because most of theoverlying rock mass moves downward in the void under theinfluence of gravity. The value and nature of the inducedvertical stress is influenced by nature of the overlying strataand depth cover [9]. Here, the value of stimulatedhorizontal stress remains very low and, generally, remainsinsignificant for the safety of the associated undergroundstructures. But underground mining of coal under varyingdepth of cover exhibit considerable amount of lateraldisplacement towards mined-out/slope-favourable area [5]due to unconfined condition of the overlying strata.Conventionally, final extraction of pillars (depillaring)starts from one end of the property and terminates at theother end of the mine. Under this scheme of pillarextraction, working in the middle of the property faces anormal situation of strata movement. While, both,commencement and end of the depillaring encounterslope-favourable area for the overburden. However, duringinitial depillaring operation in the vicinity of slope,dimension of the resulted void remains small and issurrounded by intact rock mass. But when the depillaringapproaches towards exhaustion of the property, thecondition of ground movement becomes more complex asmost of the unconfined overlying strata are already in

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Fig. 3. Expected planar movement of confined and un-confined overlying strata under the influnce of mining void.


fractured state due to the implemented depillaring activ-ities. Considerably, large amounts of overlying rock massencounter lateral movement: firstly, due to movement ofthe fractured strata towards the mined-out area andsecondly, because of movement of the overlying stratatowards natural slope-favourable area of the hilly terrain.This may also lead to the development of tensile cracks.

Mining-induced vertical movement fractures the over-burden, which is tackled through proper planning ofnatural and applied supports in the interest of coalproduction. But a possibility of lateral movement maypose serious threat for the safety of the strata in andaround the working horizon and the movement is difficultto be controlled. Occurrence of a lateral movement in thevicinity of working may fracture/disturb the surroundingstrata, and in turn creates problems for undergroundmining of the coal. The mining-induced lateral displace-ment may also stimulate horizontal stress in the overlyingstrata. In fact, the main reason of the lateral movement isthe absence of side confinement due to hilly nature of theoverburden. As the movement originates due to excavatedvoid, its lateral component remains very small in the levelof working horizon. However, the value of lateral move-ment keeps steadily increasing towards overlying stratahaving its largest value at the surface. The development oflateral movement in the overlying strata is not likely toadversely affect the working condition of a seam fornormal height of extraction. But the upper horizon ofunderground working of a thick seam (complete thicknessin single lift) or simultaneous working of contiguoussections/seams may encounter a safety threat by theinduced lateral movement. The nature and value ofmining-induced movement is of strategic importance for

the safety of underground mining structures. Design andperformance of natural and applied supports, includingsystematic support of roof, depend heavily upon the valueand pattern of the mining-induced stress. Considerably,large amount of lateral movement of overlying strataduring underground mining below a hill cap (unconfinedstrata) is likely to produce a difficult condition for themining because of the simple fact that the rock masses tendto be strong in compression but weak in tension and shear[10]. Therefore, in comparison to compressive nature ofloading by the vertical-induced stress, development oflateral movement/stress is likely to create a more failurefavourable condition during underground mining of athick seam below a hill cap (unconfined strata).

3. Details of the site

The Chirimiri colliery of Chirimiri area under SouthEastern Coalfield Limited (SECL) is located in Koreadistrict of a newly formed Chattishgarh state of India. Thecoalfield is a part of the Sonhat–Sohagpur master basinextending over Chattishgarh and Madhya Pradesh.

3.1. Coalification

The Decan trap in the form of igneous intrusion cappedthe coal measure formation from Chirimiri to Sonhat. TheZero seam is the top most seam, underlain by four morecoal seams. The Zero seam appears in single as well asdouble sections in the mine. The thickness of the seam insingle and double sections is 6 and 12.5–13m, respectively.A representative section of the seam where it appears indouble section is shown in Fig. 4. Bartunga hill, 650–690m

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2.8m

3.2m

3.0m

2.5m

Bottom section(6m)

Laminated

parting

Top

0.7-1m

section

Fig. 4. A typical sectional view of contiguous sections of Zero seam.

Table 1

Physico-mechanical properties of the formation of Zero seam (after [11])

Formations RQD

(%)

Strength (MPa) Bulk density

(t/m3)

sc st

Coal (bottom section) 35 51.3 4.7 1.4

Grey shale 13 13.0 1.1 2.3

Carb. shale 24 25.0 1.5 1.6

Coal (top section) 32 43.4 3.9 1.4

Sandstone 75 35.4 5.7 2.3

Carb. shale 35 21.0 1.5 1.6

Mudstone (medium

grained)

15 NO NO 2.2

Sandstone (loose) 26 18.6 2.6 2.2

Sandstone (coarse

grained)

32 16.0 2.8 2.2

NO, not observed (due to unavailability of required shape and size of the

sample).

Table 2

Dimensional details of pillar formation in the seam

Parameter Description

Size of pillar 22� 22m (center to center)

Gallery width 4.5m

Seam thickness 12.5m

Development height 2.5m (top)

Development height 2.8m (bottom)

Thickness of coal band over

bottom section development

3.5m

Depth cover 25–170m

Gradient 1 in 82

Incubation period 6 months


above mean sea level, had steep escarpment and denseinaccessible forest over the coal deposits. Around 1.5 km2

hill cap area in the leasehold contains nearly 10.4 milliontons of coal in Zero seam [11]. Core procurement through afresh drilling from hilltop showed very unstable formationup to the Zero seam coal horizon, and even down below,and provided limited rock samples for laboratory testing.Mudstone layers were found to be of resilient nature underdry condition, but lost competency after coming in contactof moisture/water. The observed physico-mechanical prop-erties of the formations are given in Table 1.

3.2. Development

The seam of nearly 12m thickness was developed in twosections, leaving a nearly 3m thick parting of alternatelayers of calcareous shale, coal and mudstone between the

two contiguous sections. The thicknesses of clean coal inthe top and bottom sections are 3–3.5 and 6–6.5m,respectively. The immediate roof of the top section is shaleand mudstone, which is not competent and had a tendencyto part from roof when exposed. Therefore, a coal band ofat least 0.7–1.0m thickness was left along roof of the topsection development. The roof of the bottom section,excluding coal, is also extremely week and laminated.However, the development height of the bottom sectiongalleries was 2.8m only, which provided a nearly 3m thickband of coal as immediate roof of the bottom section.A pair of adits (Fig. 1) has entered the seam. Both sectionsof the seam were standing on pillars and the developmentin both the sections was done along respective floorsmaintaining coincident verticality of pillars and galleries.The width and height of the gallery are 4.5 and 2.8m,respectively. Table 2 provides dimensional details of pillarformation in both sections of the seam.

3.3. Mining method

Geophysical properties of inter-burden/parting betweentwo close seams/sections play important roles in theselection of a suitable mining method. Competent inter-burden strata can control adverse effect of pillar loadtransfer phenomenon during working of the two closeseams/sections. It is always very difficult to extract pillarsof two developed coal seams/sections with weak andlaminated inter-burden parting of around 3m thickness.To achieve an ideal pillar load transfer condition, the pillarformation in the two contiguous sections is kept super-imposed. Conventional approach to depillar the developedtop section (with caving) first and then depillaring of thebottom section encounters a number of safety problemsand found to be uneconomical. A study on simulatedmodels was undertaken by CMRI [12] to visualise safetyduring contiguous seams/sections working. It was con-cluded during this study that the simultaneous depillaringof the superimposed pillars of contiguous seams/sectionsprovides better safety. Further, on the basis of the study onsimulated models, it was observed that the stability of thelaminated parting of around 3m thickness could be

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improved by rock mass reinforcing technique [13] duringthe simultaneous extraction.

Thick bottom section of the Zero seam experienced pillarformation in 1990 by conventional board and pillar (B&P)method. Since then, mine management was looking for asuitable/feasible mining method for the final safe extractionof the seam. A number of agencies were invited to offer anappropriate mining method. The Blasting Gallery (BG)method of mining of Cdf International, France, was firstconsidered for depillaring of the seam. This idea wasabandoned due to the fragile nature of the top section roofand similar nature of parting between the two sections.Joy-Kier joint venture attempted to introduce continuousminer-based depillaring using floor bolting. However,extraction of the thick bottom section in presence of theroof bolt support in overlying coal bed was difficult tohandle by the continuous miner. The idea of underpinning-based simultaneous extraction [1] of both the sections byCMRI was finally selected by the industry for depillaring ofthe Zero seam. In addition to consolidating the laminatedparting, underpinning worked as cable bolt support for thethick bottom section of the seam. The idea of under-pinning, therefore, had sound backup of an excellentexperience of underground extraction of a thick seam insingle lift [14] using cable bolt at NCPH mine of theChirimiri area. Use of grouted steel ropes under tension forsupporting a high roof as well as an overlying coal band, andto improve safe span of overhanging strata near the goafedge was successfully applied for depillaring of completethickness of a thick coal seam in single lift. This techniqueimposed some improvements over existing BG method toextract full thickness of a thick coal seam in one lift.

3.4. Extraction sequence

The laminated 3.0m thick parting of alternate layers ofshale, carbonaceous shale, coal and mudstone receivedadditional thickness of 3.2m coal band due to formation ofsuperimposed pillars along floors of both sections of theseam. At least 0.7m thick coal band in the roof of topsection galleries was left to protect the working fromadverse effect of shale roof swelling. At the time of pillarformation, non-retractable roof bolts supported super-imposed galleries of both the sections. Before commence-ment of depillaring, the composite parting of 6.2m betweenthe top and bottom section galleries was reinforced byunderpinning to sustain the burden load of the top sectioncaving and to facilitate the extraction of the bottomsection.

Sequence of extraction was kept completely identical tothat of a conventional depillaring by B&P method ofworking except simultaneous extraction of both thesections. The extraction of top and bottom sections wasdone simultaneously by caving method. Splitting andslicing process was adopted for pillar extraction from boththe sections of the seam. During the simultaneousextraction, generally superimposition of working in both

the sections was maintained with a special attention that, inany case, the working face of bottom section does not crossthat of the top section. The underpinning worked as cablebolt support [14] for the bottom section working and theroof coal band was taken during retreat. Bottom sectionworking was restricted up to 5m height in a go; leaving 1mstable coal band along the roof of the bottom section.

3.5. Support

Chirimiri colliery was apparently the first undergroundmine in India to adopt only roof bolt support duringdevelopment of both the sections on pillars. Installation offull column-grouted 6–6.5m long rope dowels (Fig. 5) at agrid pattern of 1.2� 1.2m in the floor of the top section inthe development gallery, split and slices consolidated thecomposite parting of nearly 6.2m thickness. Duringheightening of the bottom section, the immediate roofexperienced effective support in advance by the fullcolumn-grouted cable bolts in form of underpinning fromthe top section. Full column-grouted roof bolt of 1.5mlength were placed at a grid pattern of 1.2� 1.2m indevelopment gallery, split and slices of top and bottomsections. Conventional supports like pit props and steelchocks were used in addition to roof bolts during thedepillaring.

4. Field observations

Systematic underground instrumentation and monitor-ing was undertaken in each depillaring panel of the minebecause the mining of contiguous sections of the Zero seamwas experiencing special mining method under fast changein depth of cover. Four types of instruments, i.e.,convergence indicators, load cells, stress meters andmulti-point borehole extensometers, were installed atdifferent selected stations (Fig. 6) of each panel for thestudy of strata behaviour. Monitoring of readings of theseinstruments with respect to advance of the diagonal line ofextraction was done to facilitate strata behaviour study ineach panel.Telescopic convergence points were installed in the

development galleries, splits and slices and observationsat these points were taken till the site remained accessibleduring the pillar extraction. Remote instruments, i.e., stressmeters and convergence indicators were installed atselected stations and the connecting cables of theseinstruments were taken out of the working to a safeposition to get information even after the instrumentedstation became inaccessible. The rib positions in the pillarwere fixed in advance for the instrumentation purpose andgrooves (like manholes) up to development height weremade in the proposed ribs. After the installation of theinstruments within the grooves; the place was protected bychocks; leaving the instruments within the groove. Mag-netic-type borehole extensometers were installed in theparting between top and bottom sections. Observations of

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Fig. 5. Schematic diagram showing simultaneous depillaring of top and bottom sections with underpinning and roof bolt support.

30 L

31 L

33 L

3 R

32 L

4 R 5R

34 L

9R 10R 11R 12R 6R 7R 8R

*

* *

* *

* *

*

*

+

+

Not to scale

Top Section Bottom Section

Remote Monitoring station having: Remote Monitoring station having:Remote Convergence Indicator Remote Convergence Indicator Stress Capsule Stress CapsuleMultipoint Borehole Extensometer Multipoint Borehole Extensometer

Remote Indicating Load Cell Remote Indicating Load Cell

Telescopic Convergence Indicator Telescopic Convergence Indicator * Graphic Convergence Indicator+ Magnetic Type Borehole Extensometer

Φ

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Fig. 6. An instrumentation plan of a panel (D) showing a typical instrumentation pattern for field observations in different panels.


borehole extensometers and load cells were discontinuedafter one and two panels of the study, respectively, as theyfailed to provide any significant information. A simplestatistical analysis of the field observations in differentpanels of the mine was done to fix a limiting value of stratacontrol parameters (called threshold limit value) atdifferent positions of the working for safety of the manand machines. A sample of the analysis, done for threedifferent panels of the mine, namely C1, C2 and C3 is givenin Table 3. Further analysis of data of other panels caused

development of a novel process called combined-instru-ment-approach (CIA) for better assessment of the stratabehaviour. A patent application is under progress for CIA,and falls out side the scope of this paper.

4.1. Problems encountered

More than 30 pair of panels (top and bottom) of themine were successfully simultaneously extracted withunderpinning of the parting as a major support system.

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Table 3

Maximum rate of observed convergence and mining induced stress at different stages of depillaring in different panels

Panel At the goaf edge (before fall) At the goaf edge (at the time of fall) Inside goaf edge (before fall)

Convergence

(mm/shift)

Stress (kPa/shift) Convergence

(mm/shift)

Stress (kPa/shift) Convergence

(mm/shift)

Stress (kPa/shift)

Top Bottom Top Bottom Top Bottom Top Bottom Top Bottom Top Bottom

Panel C1 2.8 0.8 2.0 3.0 0.8 NO 1.0 NO 5.6 5.6 12.0 31.0

Panel C2 1.6 1.6 4.0 2.0 1.6 2.4 4.0 6.0 2.8 2.4 150 3.0

Panel C3 4.0 2.6 89.0 15.0 3.1 NO 24.0 15.0 4.4 5.2 204.0 103.0

NO, not observed (due to inaccessibility/disturbance of the observation station).

Fig. 7. Nature of movement of hill cap towards goaved out area with respect to approximate position of panel G1 (a sectional view).


Field monitoring showed that underpinning offeredadequate anchorage to consolidate the parting stability.Strata behaviour observations conducted in differentpanels also showed improvement in competency of theunderpinned parting for safe and efficient working of thecontiguous sections.

However, after exhaustion of nearly 90% of the propertyof the seam and when the working came under theinfluence of other side of the hill slope (Fig. 7), thedepillaring of last few panels of the mine started experien-cing strata control problems. Appearance of tensile cracksin roof strata followed by collapse in galleries and galleryjunctions made it difficult to continue the depillaring.Although some local tensile failure of roof strata wasobserved during depillaring of few earlier panels also, themajor problem was encountered during depillaring of G1and G2 panels of the mine. At this juncture, only fivepanels G1, G2, G3, J1 and J2 (Fig. 8) were left fordepillaring at the mine. Roof falls and pillar instabilitywere observed in G1 and G2 panels just after extraction ofa few initial pillars, which hindered the depillaring process.Encountered strata control problems in these two panelsare detailed below.

4.2. Panel G1

Panel G1 of the mine was located close to the boundaryof the leasehold area of the Zero seam at Chirimiri colliery.Only bottom section coal of nearly 6m thickness was to beextracted here and the top section of the seam was notpresent in this panel. The coal seam in the panel wasdeveloped along the floor of the seam with a height of2.8m. Coal band of the roof was supported with cablebolts (4.5m long) and was supposed to be extracted byheightening during retreat of the depillaring operation. Anold air shaft of the seam was also in this panel at thelocation 18LW/16R. Brick shaft lining of the air shaft wasquite intact before commencement of extraction of thepanel. Depillaring of the panel G1 started after completionof successful depillaring of most of the property of theseam. After complete extraction of one pillar (first pillar)and while operation was going on in the second diagonalrow of pillars, a longitudinal crack of about 10m lengthappeared along roof of the 20LW gallery between 15R and16R. The development of crack did not cause any adverseaffect over the working. This crack was developed inthe northern side of the level gallery, which was the

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ABCDEF

G

H

I

Panel G1

Panel G2

Panel J1

Panel G3

Panel J2

DriftSurface to Zero seam

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S3000

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S2250S3250

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50S48S

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N

FULL DIP I IN 182 S 40 00’ 00” E

Stress meters installed in vertically and horizontally drilled holes

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O

Last patch of the property to be extracted encountered problems

Fig. 8. Location of remaining panels and positional correlation to plot surface profile along the five panels to be extracted.


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0

5

10

15

20

25

30

-10 -5 0 5 10 15 20 25 30 35 40 45 55

Goaf edge distance (m)

Ind

uce

d s

tres

s (k

g/c

m2) Induced vertical stress

Induced horizontal stress

50

Fig. 9. Development of induced stresses (vertical and horizontal) in and

around working horizons at location 20LW/14-15R of panel G1.


slope-favourable direction of the hill. At the same time, alateral shearing of brick lining in the old air shaft at ahorizon of 3.6m above the roof level of the panel was alsonoticed. The noticed amount of lateral shift of the bricklining in that particular horizon due to the shearing variedbetween 5 and 10 cm. Direction of the lateral shift wasobserved to be in the slope-favourable direction of the hill.

With further progress of the depillaring and judiciousheightening of the roof coal during retreat under cable boltsupport, a fall of cable bolts was observed near location20LW/15-16R of the panel. In fact, this fall of cable boltswas an unexpected happening. The cable bolts weresupposed to remain intact and keep hanging in roof afterwinning of the overlying coal band. Inspection of the fallencables of the cable bolts revealed that the lengths of thefallen cables at 20LW/15-16R gallery were 3.6m only. Itwas observed that the fallen wire ropes were cut byshearing along a horizon of nearly 3.6m above the originalgallery roof. As an immediate action to manage thelongitudinal crack in the original roof, the applied supportwas strengthened by chock supports and additionalreinforcement. But all these three events: (i) appearanceof crack in the original roof, (ii) horizontal shifting of bricklining of air shaft and (iii) shear cut of 4.5m long installedcable bolt rope at 3.6m depth inside roof coal band;indicated considerable lateral movement in the overlyingstrata. All these correlations indicated that the inducedlateral movement was active in roof strata along a horizon,nearly 3.6m above roof level of the original gallery of thepanel. Interaction of the resulted lateral movement with theunderground mining structures created an adverse stratacondition for depillaring of the panel G1.

4.2.1. Instrumentation and observations

Under the normal strata movement study programme ofthe mine, a number of stress meters were installed inhorizontally drilled holes in pillar to measure vertical-induced stress. However, to visualise the development ofinduced lateral stress due the observed lateral movement,an additional stress meter was installed at location 20LW/14-15R of the panel in a vertical drill hole in the overlyingroof strata. This stress meter was placed in a vertical hole,4m above the roof level, to intercept the induced lateralstress. At this location (20LW/14-15R), one stress meterwas already existed in the pillar to measure vertical-induced stress. Regular monitoring of variation of readingsof both (vertical and horizontal) the stress meters showed,relatively, large variation in induced horizontal stress at thestation 20LW/14-15R. Here, maximum observed-inducedvertical and horizontal stress were 2.8 and 0.87MPa,respectively. A plot of variation of induced stresses(vertical and horizontal) at the location 20LW/14-15Rwith respect to goaf edge distance during depillaring of thepanel G1 is shown in Fig. 9. Earlier attempt [15] to measurevertical- and horizontal-induced stresses during depillaringunder confined strata showed predominance of vertical-induced stress only. Even at the time of stook/pillar

crushing, the maximum observed value of ratio ofhorizontal to vertical-induced stresses was 0.08, while thesame was observed to be 0.3 (quite high) in the presentcase. Further, the additionally installed instrument mighthave missed considerable amount-induced horizontal stressas it was installed in the middle of the process of thedepillaring and induced stress development.

4.2.2. Positional correlation

All five remaining panels to be depillared were under theinfluence of strata movement due to the goaves of differentexhausted panels of the mine from one side, while the freesurface of hill slope was on the other side. All instrumentedstations of these panels were corelated with surface andwere plotted on the mine plan. Overlapping of the surfacecontours on the working plan was used for positionalcorrelation of the panel with respect to the hill slope.Assuming the peak of hilltop as centre (maximum depth ofcover), different surface profiles (sectional) were drawnalong different lines passing through the remaining fivepanels to be depillared. A surface profile on a vertical planealong a line OA (Fig. 8), starting from the peak to thehill slope and passing through the panel G1, is shown inFig. 10. From this profile, it is obvious that the affectedpanel lies beneath high escarpment zone of the hill and isunder the influence of the free face of the hill slope.

4.3. Panel G2

G2 was another panel of the mine, which encounteredstrata control problem during the depillaring. This panelwas next adjacent panel in succession to G1, to beextracted. In comparison to the location of panel G1, thispanel was relatively inside from the free face of the hillslope and consisted, both, top and bottom sections of theseam. Simultaneous depillaring in both sections of thepanel started in the month of May 2003. During slicing offirst pillar in both the sections of the panel, longitudinalcracks were observed in the roof of, both, top and bottom

ARTICLE IN PRESS

175165

155145

130120

110100

9085

7060

5040

30

0

25

50

75

100

125

150

175

0 40 80 120 200 280 320360 400

Horizontal distance from maximum depth cover (m)

Dep

th c

ov

er (

m)

440240160

Fig. 10. Surface profile along a vertical cross-section above panel G1 of

the mine.

8.5 m

2.5 m0.70 m0.58 m0.77 m

3.09 m

0.39 m0.27 m3.0 m1.24 m0.96 m2.5 m

1.65 m0.95 m 0.45 m

2.45 m

2.8 m

Top section3.46 m

5.25 m

Gallery

Coarse grainedsandstone

Medium grainedsandstone

Coal

Loose sandstone

Shale

Mudstone

Bottom section

32.8m

LEGEND

Fig. 11. Geo-mechanical properties of Zero seam and immediate over-

lying strata intercepted in an upward drilled bore hole at location 21S/29-

30LW.


section galleries, at 20LW/11–12R. Cracks of both thesection were almost vertically superimposed to each other.As an immediate measure, roof near these cracks wereadditionally supported with heavy duty steel chocksupports in bottom section and full column-grouted cablebolts of 4.5m length in the top section to continue theextraction in the panel. Roof coal of bottom section of thein-bye development headings was blasted down and with-drawn before the stooks were attacked. During heighteningof the bottom section, a local fall at 20LW/11-12Rgallery was observed. This fall occurred along with ropesof the applied underpinning (cable bolt) and the observedlength of the fallen ropes was 3.6m only. Here also, thefallen ropes of cable bolts were cut by cross-sectionalong a horizon, nearly 3.6m above the original galleryroof. Local fall in the top section also took place alongwith the ropes of the cable bolts of 2.9m length only.Inspection of these fallen ropes indicated that theyexperienced a shear cut by cross-section along a horizonof nearly 2.9m above the original gallery roof of the topsection. As per the experience of high-induced horizontalstress in the previous panel G1, a number of stress meterswere installed at different locations of the panel to studyvertical and horizontal-induced stresses in both thesections. Observations of these stress meters indicated thatthe value of induced horizontal stress was considerablyhigh during the depillaring. Positional correlation exercisealso indicated that the panel G2 lies beneath highescarpment zone of the hill and is under influence of thefree face of the hill slope.

4.4. Properties of overlying strata

On the basis of different observations, it was realisedthat strata control problems encountered in panels G1 andG2 of the mine were mainly due to lateral movement of theoverlying strata under influence of the hill slope andpresence of large mined-out area. The horizon of lateralmovement could easily be detected through the length ofthe fallen wire ropes of the cable bolts in both the sections.

Identified two horizons of considerable lateral movementswere located at 3.6 and 2.9m above roof level of theoriginal galleries of bottom and top sections, respectively.About 72% of the total overlying formation was found tobe sandstone, while the remaining portion was clay toclayey shale and mudstone, mainly in the upper horizon.The formation on the whole was laminated and weak,especially due to presence of intrusions and weatheringalong the slope of the hill escarpment. The weatheredformation had low strength, poor core recovery, normallyreduced to powder or sludge. An upward borehole ofnearly 40m length was drilled from the bottom section toprocure cores for the assessment of rock quality at differenthorizons. The stratigrahic column over the seam obtainedthrough this freshly drilled borehole is shown in Fig. 11. Itis evident from this figure that the identified horizons (asper the length of the fallen rope of cable bolts) of lateralmovements above top and bottom sections consistedincompetent strata of mudstone, which allowed theshearing to happen under the influence of the inducedlateral movement due to underground mining near the hillslope.

5. Numerical modelling

Ground movement study by simulation is difficult andeven elasto-plastic analysis by a continuum method ofnumerical modelling usually fails [3] to provide a realisticvalue of ground movement during an underground mining.Depillaring is a three-dimensional phenomenon and,therefore, a three-dimensional simulation of the problem

ARTICLE IN PRESS

Table 4

Material properties used in numerical modelling

Material Young’s

modulus

(GPa)

Bulk

modulus

(GPa)

Shear

modulus

(GPa)

Density

(g/cm3)

Sandstone

(floor)

7.00 4.67 2.80 2.50

Coal 2.19 1.46 0.88 1.48

Shale 2.16 1.44 0.86 1.52

Mudstone 2.30 1.53 0.92 2.10

Sandstone

(medium)

3.15 2.10 1.26 2.02

Sandstone

(loose)

0.50 0.33 0.20 1.88

Sandstone

(coarse)

1.47 0.98 0.59 1.95

Poisson’s ratio for all formations is taken as 0.25.


would provide better results. However, even in a three-dimensional model, it is very difficult to simulate thebroken and jig-jag nature of a depillaring face and is beingfurther constrained by the required computational time.However, numerical modelling in idealised conditions oflaboratory provides scope of parametric investigation.Further, up to some extent, the simulation study reducesthe discreteness of field measurements to assess strata/ground movement during underground mining. Consider-ing these facts, a simple two-dimensional simulation studywas taken up to have bit more understanding about thenature of ground movement at different stages of depillar-ing below the hilly nature of the overburden.

The condition of the site was simulated to a two-dimensional problem using plain strain modelling ofFLAC-3D [16], which is based on a finite difference code.A simplified geometry of the pillar extraction below hillslope was simulated in an elastic model of the package.Height and slope of the hill in the simulated model waskept similar to the field conditions. A half symmetry modelof 11 pillars, consisting of 30,986 grid points (Fig. 12), wasanalysed for the movement study. All dimensional valuesand sequence of working were kept similar to those of thesite in the model except gallery width and total number ofpillars across the hill. The gallery width was kept at 4minstead of 4.5m for the convenience of grid generation. Tokeep the size of the model small, only 11 pillars (includingone barrier pillar) were formed in the model for theanalysis. Thickness of the strata was taken as per theborehole information given in Fig. 11 and their propertiesare given in Table 4. Freshly procured core samples of

Fig. 12. Finite difference mesh used for modelling of the half of the hilly ter

different stratum were subjected to laboratory tests fortheir properties. All the tests for physico-mechanicalproperties were conducted at Central Institute of Miningand Fuel Research (CIMFR), Dhanbad. For most of thetesting, the adopted testing procedures and samplepreparation were in accordance with the InternationalSociety of Rock Mechanics (ISRM) norms [17]. Values ofmaterial properties obtained by laboratory tests of rockspecimens were converted to their respective rock massvalues using Sheorey’s [18] approach, which were later usedfor the simulation.The available numbers of in situ stress measurement data

for Indian coalfields are only a few [19]. Among these, the

rain (geometrical symmetry) above contiguous sections of the coal seam.

ARTICLE IN PRESSR. Singh et al. / International Journal of Rock Mechanics & Mining Sciences 45 (2008) 29–46 41

recent three measurements [20] are of considerableimportance and observed that the horizontal stress field isnot highly anisotropic. On the basis of this measurement,the equations for mean vertical and horizontal stresses are

Sv ¼ 0:025H MPa and SH ¼ Sh ¼ 2:4þ 0:01H MPa;

(3)

where H is the depth cover (m), Sv the vertical in situ stress(MPa), SH the major horizontal in situ stress (MPa) and Sh

is the minor horizontal in situ stress (MPa).To simulate the weak plane of mudstone in the

formation, ‘‘interface’’ facility of FLAC-3D was used. Aninterface contact in the package is defined by the normaland shear stiffness that exist at a point in space that is incontact with a finite plane. FLAC-3D represents interfacesas collections of triangular planes (interface elements) andpoints in space (interface nodes). The interfaces arecharacterised by Coulomb sliding and/or tensile separa-tion. The Coulomb shear-strength criterion limits the shearforce (Fsmax) by the following relation:

F s max ¼ cAþ Fn tan f, (4)

where c is the cohesion along the interface, A therepresentative area associated with the interface node, Fn

the normal force and f is the friction angle (degrees) of theinterface surface.

If the criterion is satisfied (i.e., if |F s|XFs max), thenFs ¼ Fs max, with the direction of shear preserved. Ifthe shear force exceeds the shear-force limit, then anyfurther shear displacement causes an increase inthe effective normal stress (sn) on the joint according to

Job Title: Working under Bartunga Hill

Lateral movement along

weak plane Hill

slope

Fig. 13. Deformation of grid due to lateral movement along weak planes afte

slope.

the relation

sn ¼ sn þjF sj � F s max

Akstan ckn; (5)

where sn is the normal stress, Fs the shear force, kn thenormal stiffness, ks the shear stiffness, c the dilation angle(degrees) of the interface surface and |Fs| is the magnitudebefore the above correction is made.If tension exists across the interface and exceeds the

tensile strength of the interface, then the interface breaksand the shear and normal forces are set to zero. Estimationof properties for the interface simulation is described in [1].In model study, it is observed that the depillaring in the

central part of the seam experienced normal nature ofstrata movement while the extraction of a pillar near theperiphery of the property (under the influence of hill slope)encountered lateral movement towards slope-favourabledirection. Extraction of the first pillar (leaving one barrierpillar) towards the hill slope initiated shearing along theweak planes. The observed grid deformation in the modelfor this working is shown in Fig. 13. The lateral movementof the strata towards free face of the hill influenced thestability of galleries ahead of the resulted void. Fig. 14shows a close view of lateral movement along the weakplane of the parting between the superimposed galleries ofthe two sections ahead of the void. Further, when the firstpillar (leaving one barrier pillar) towards the hill slope wasextracted, the velocity vectors of different grid points wereobserved to be in slope-favourable direction of the hill.Here, the observed nature of velocity vectors of differentgrid points is shown in Fig. 15. Velocity vectors of grid

Void

du

e to

on

e p

illa

rextr

act

ion

11.5

m

r extraction of the first pillar (leaving one barrier pillar) towards the hill

ARTICLE IN PRESS


Void

due

to o

ne

pil

lar

extr

acti

on

11.5

m

Top

Bottom

4m3m

2.5

m

Horizon of lateral movement along weak plane

Fig. 14. Lateral movement of strata (ahead of working) along weak plane of the parting after extraction of the first pillar (leaving one barrier pillar)

towards the hill slope.

Void due to depillaring at the edge of the property

Hill

slop

e


Fig. 15. Nature of velocity vectors of different grid points around an excavation in the vicinity of the hill slope.


points close the excavation have orientation towards thevoid, while that of the overlying points were observed to bein the slope-favourable direction, which demonstrates the

phenomenon of shearing. When a pillar nearest to thecentre of the hill was extracted, the grid points experiencednormal movement towards the void. The observed trend of

ARTICLE IN PRESSR. Singh et al. / International Journal of Rock Mechanics & Mining Sciences 45 (2008) 29–46 43

the velocity vectors by the model for this position of pillarextraction is shown in Fig. 16. Nature of velocity vectors ofthe overlying strata was observed to be different fordifferent positions of the pillar extraction. The comparedvelocity vectors shown in Figs. 15 and 16 are taken at thetime of convergence of the respective models.

Models were also run to investigate the nature of shearstress development for different positions of pillar extrac-tion. Here, properties of the strata were kept same butinterfaces were not added in the model to simulate weakplanes of mudstone strata. In fact, simulation of interfacesfor the weak planes hindered development of shear stress inthe model. Zone contour of shear stress development inand around the parting between the two sections wereobtained for different positions of pillar extraction in themodel (Figs. 17 and 18). When first pillar (leaving onebarrier pillar) towards the hill slope was extracted, theshear stress experienced by the parting between the twosections (just one pillar ahead) was observed to besignificantly higher than that experienced by the corre-sponding parting for the fifth (from the barrier) pillarextraction. Value of the developed shear stress becameinsignificant during pillar extraction in the central part ofthe model. Although the nature of the shear stressredistribution was more or less similar for the two positions(Figs. 17 and 18) of the pillar extraction, the observed


Void due to dep

Fig. 16. Normal nature of the velocity vectors of different grid points aro

substantial change in the magnitude of the developed shearstress was, probably, the reason of the experiential cablecut and roof fall in the field. This model study revealed thatthe horizon of strata in the parting, from where instabilityoccurred in the field, experienced high concentration of thedeveloped shear stress.

6. Strata management

Both, field and laboratory studies showed that theexperienced strata control problems in G1 and G2 panelsare due to the lateral movement of strata along a weakbedding plane in the overlying rock mass. To manage themining-induced lateral movement, it is required either toincrease the competence of the weak bedding plane by anyphysical means or to create an effective collapse/fracture inthe strata, which are pulling the mass towards the slope/void favourable side. The first solution was not foundsuitable in the top section due to operational and techno-economical constraints. It was difficult to provide aneffective reinforcement to the laterally moving overlyingstrata because the applied additional cable bolts in thetop section got cut by the shearing. After a detailedconsideration of the situation of the site, it was realised thatunless the laterally moving overlying strata of the topsection is stabilised by induced fracturing, the efficacy of

illaring in the central part of the property

Bed separation

und an excavation in the central part of the property below the hill.

ARTICLE IN PRESS

FLAC3D 2.10

CMRI, DHANBAD, INDIA

Step 149756 Model Perspective

Center:

X: 1.304e+002

Y: 1.000e-001

Z: 5.604e+001

Rotation:

X: 0.000

Y: 0.000

Z: 0.000

Dist: 7.884e+002 Mag.: 35.5

Ang.:22.500

Contour of SXZ

Magfac =0.000e+000

Gradient Calculation

-2.0919e+006 to -2.0000e+006

-2.0000e+006 to -1.9000e+006

-1.9000e+006 to -1.8000e+006

-1.8000e+006 to -1.7000e+006

-1.7000e+006 to -1.6000e+006

-1.6000e+006 to -1.5000e+006

-1.5000e+006 to -1.4000e+006

-1.4000e+006 to -1.3000e+006

-1.3000e+006 to -1.2000e+006

-1.2000e+006 to -1.1000e+006

-1.1000e+006 to -1.0000e+006

-1.0000e+006 to -9.0000e+005

-9.0000e+005 to -8.0000e+005

-8.0000e+005 to -7.0000e+005

-7.0000e+005 to -6.0948e+005

Interval =1.0e+005

Top section

Bottom section


13:01:18 Fri Jun 02 2006

Fig. 17. Zone contour of shear stress in and around the parting ahead of the first pillar extraction from the barrier side (in the vicinity of the slope).


reinforcement is difficult to be achieved even in the bottomsection for the strata management. Accordingly, blasting-induced fracturing/caving of overlying strata was at-tempted successfully for the top section. However, applica-tion of the blasting-induced fracturing was also not verystraight forward and, therefore, the experience of inducedcaving [21] through long hole drilling and blasting wasutilised for the purpose. Drilling and blasting of roof stratawas done after identification of the appropriate strata anddirection of the lateral movement. Induced blasting waspracticed in roof strata of the top section near goaf edge upto a height of 5–7m from the original roof level at the timeof withdrawal of supports.

For the roof of the bottom section (i.e., parting), it wasdifficult to practice long hole drilling and blasting due tosimultaneous working in the top section. However, up tosome extent, the adoption of induced caving throughblasting in the roof strata of the top section alleviated theshearing problem of the bottom section roof strata also.Further, entire parting including the proved weak beddingplane was strengthened by enhanced reinforcement of the

parting, i.e., increasing the density of underpinning toarrest the chance of lateral movement. Normally, threerows of underpinning, in a grid pattern of 1.2� 1.2m, werepracticed in different panels. But the last five panels,affected by the strata control problems, adopted five rowsof underpinning to consolidate multi-layered formation ofthe parting and prevent the shearing in the parting abovethe bottom section.

6.1. Efficacy of strata management

The results of the adopted preventive actions wereencouraging and no longitudinal cracks appeared in theroof during working in rest of the panels. A practice ofinduced caving of roof strata in top section and increaseddensity of underpinning for the bottom section to counterthe induced lateral movement was observed to be effectiveand the depillaring in these last panels remained almostfree from any strata control problem. To have a cross-check of the efficacy of the adopted system of stratamanagement against the induced lateral movement, few

ARTICLE IN PRESS

FLAC3D 2.10

Step 96584 Model Perspective

13:05:16 Fri Jun 02 2006

Center:

X: 1.304e+002

Y: 1.000e-001

Z: 5.550e+001

Rotation:

X: 0.000

Y: 0.000

Z: 0.000

Dist: 7.884e+002 Mag.: 35.5

Ang.: 22.500


Contour of SXZ

Magfac = 0.000e+000

-1.5000e+006 to -1.4000e+006

-1.4000e+006 to -1.3000e+006

-1.3000e+006 to -1.2000e+006

-1.2000e+006 to -1.1000e+006

-1.1000e+006 to -1.0000e+006

-1.0000e+006 to -9.0000e+005

-9.0000e+005 to -8.0000e+005

-8.0000e+005 to -7.0000e+005

-7.0000e+005 to -6.0000e+005

-6.0000e+005 to -5.0000e+005

-5.0000e+005 to -4.0000e+005

-4.0000e+005 to -3.4879e+005

Interval = 1.0e+005

Top section

Bottom section

Gradient Calculation

-1.5884e+006 to -1.5000e+006

CMRI, DHANBAD, INDIA

Fig. 18. Zone contour of shear stress in and around the parting ahead of the fifth pillar extraction from the barrier side (nearly 100m inside from edge of

the slope).

Table 5

Result of strata monitoring after adoption of the strata management plan

in panels G3, J1 and J2

Panel

name

Maximum observed value of the strata control parameter before

roof fall

Convergence

(mm)

Vertical stress

(MPa)

Horizontal

stress (MPa)

Remarks

G3 55.07 7.95 0.07 Top section

46.20 7.55 0.06 Bottom section

J1 40.02 11.04 0.05 Top section


J2 61.50 7.24 0.06 Top section



more pair of stress meters were installed in both verticaland horizontal holes at different selected stations in thesepanels of the mine. Readings of these instruments were

monitored during pillar extraction from the panels. Themonitoring showed no significant development of inducedlateral stress. A simple analysis of readings of theinstruments installed in three panels; G3, J1 and J2 aregiven in Table 5. This analysis shows least significantchange in values of the induced lateral stress in comparisonto their counterparts of panels G1 and G2, where theproblem was first encountered.

7. Conclusions

Although void formed due to underground mining isthe origin of the strata movement, side confinement of thestrata played important role for the direction of themovement. Generally, underground coal mining startsfrom one side (dip most) and ends at the other side ofthe property. Underground coal mining with caving belowhilly terrain provides poor side confinement for both initialand final workings, as they are located at the periphery ofthe property near hill slope with poor side confinement.

ARTICLE IN PRESSR. Singh et al. / International Journal of Rock Mechanics & Mining Sciences 45 (2008) 29–4646

However, the initial working at this mine did notexperience strata control problem because the voidremained, relatively, small resulting less disturbance inthe intact overlying strata. The working in the middle ofthe property below hilly terrain experienced normal groundmovement due to presence of considerable side confine-ment. But on the verge of exhaustion of the reserve theworking reached other side of the property, where itexperienced difficult ground movement. Here, the workingencountered large mined-out area at one side and freesurface of hill slope at the other side. Under the condition,the resulted strata/ground movement became threat for thestability of underground structures because a thick seam(complete thickness in single lift) with another contiguoussection was extracted simultaneously. Presence of anincompetent/weak bed in the overlying strata provided achance of slope-favourable lateral displacement. It wasobserved to be a difficult task to defend an undergroundmining structure against the induced lateral displacement.Based on the geo-mining conditions of the site, a localstrategy was formulated to counter the menace of theencountered lateral movement. The reported case studysuccessfully adopted-induced caving of roof strata andconsolidation of bedded layers in top and bottom sections,respectively, to overcome the problems related with theinduced lateral movement during simultaneous working ofthick and contiguous sections below a hilly terrain.

Acknowledgements

The authors are obliged to the Director, CMRI, for hispermission to publish this paper. The valuable supportduring the field investigations by Sahendra Ram and AmitKumar Singh, Senior Scientific Assistants, CMRI issincerely acknowledged. Thanks are due to the manage-ment of the Chirimiri colliery for their valuable co-operation during the field observation. A part of studyreported in this paper is based on a project funded by theSouth Eastern Coalfield Limited (SECL) of Coal IndiaLimited (CIL). The views expressed in this paper are thatof the authors and not necessarily of the organisation towhich they belong.

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