goaves effect on subsidence


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goaves effect on subsidence

  1. 1. 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. ABSTRACT 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 land. 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 surface subsidence. 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.
  2. 2. 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. INTRODUCTION: 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 TOP SEAM leaving coal & shale in roof 2.14 to 10.2 (QUEEN SEAM) and floor 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
  3. 3. 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) Where, 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) Where, Gf = Goaf Factor. P = Percentage of Goaf area in below seam present beneath the longwall panel of top seam at a particular point. Thus, 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.
  4. 4. 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) W/H 0 6 5 4 3 2 1 0 -0.5 Actual Subsidence Profile -1 SUBSIDENCE -1.5 Profile without goaf correction -2 Predicted Profile after Goaf correction -2.5 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.)
  5. 5. 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 0 12A 13 13A 14 14A 15 15A 16 16A 17 17A 18 18A 19 19A 20 20A 21 21A 22 22A 23 23A 24a 24B 25a 25B -0.5 Subsidence (m) -1 -1.5 -2 Actual_Subsidence pred_Subsidence -2.5 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.
  6. 6. CONCLUSION: 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. ACKNOWLEDGEMENT: 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. REFERENCES: 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, 1995.,pp.1-11. 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.
  8. 8. PLAN –2 PLAN SHOWING THE GOAVES OF KING SEAM BELOW THE LONGWALL PANELS OF TOP SEAM PANEL NO.1 PANEL NO.1A PLAN –3 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
  9. 9. ANNEXURE –I OBSERVED SUBSIDENCE PROFILES OF DIFFERENT LONGWALL PANELS Panel no.2 0 6 5 4 W/ H 3 2 1 0 -0.5 -1 -1.5 -2 -2.5 -3 Panel no. 3 0 6 5 4 3 W/ H 2 1 0 -0.5 -1 -1.5 -2 -2.5 Panel no.4 0 5 4 3 W/H 2 1 0 -0.2 -0.4 Subsidence (m) -0.6 -0.8 -1 -1.2 -1.4 -1.6 -1.8 -2 Panel no.5 0 6 5 4 3 2 1 0 W/H -0.5 Subsidence (m) -1 -1.5 -2 -2.5
  10. 10. ANNEXURE –II OBSERVED SUBSIDENCE Vs PREDICTED SUBSIDENCE AFTER APPLYING GOAF FACTOR Panel no.2 0 6 5 4 W/H 3 2 1 0 -0.5 Subsidence (m) -1 -1.5 -2 -2.5 act-sub pred-sub -3 Panel no. 3 0 6 5 4 3 2 1 0 W/H -0.5 Subsidence (m) -1 -1.5 -2 -2.5 p3 pred -3 Panel no.4 0 5 4 3 2 1 0 W/H -0.5 Subsidence (m) -1 -1.5 -2 p4 pred -2.5 Panel no.5 0 6 5 4 3 W/H 2 1 0 -0.5 Subsidence (m) -1 -1.5 p5 -2 pred -2.5
  11. 11. ANNEXURE –III STRAIN CURVE ALONG ‘I’ LINE 15 10 (+) 5 0 -5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 -10 -15 -20 -25 -30 -35 -40 (-) -45 -50 -55 -60 -65 -70 -75 Maximum Compressive strain observed is = 70 mm/m (-) Maximum Tensile strain observed is = 10 mm/m (+) STRAIN CURVE ALONG ‘Q’ LINE 65 60 55 50 45 40 35 (+) 30 25 20 15 10 5 0 -5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 -10 (-) Maximum Compressive strain observed is = 6 mm/m (-) Maximum Tensile strain observed is = 60 mm/m (+)