Longwall facelines: Geology, convergence and powered support rating

Mining Science and Technology, 7 (1988) 243-252 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

243

LONGWALL FACELINES: GEOLOGY, CONVERGENCE AND POWERED SUPPORT RATING I. Porter and N.I. Aziz Department of Civil and Mining Engineering, University of Wollongong, Wollongong, N.S.W. 2500 (Austrafia)

(Received February 1, 1988; accepted May 16, 1988)

ABSTRACT

The primary objective of this paper is to promote an understanding of the relationships between geology, convergence and powered support rating. Investigations were conducted at various collieries in the Illawarra Region of N.S.W. (Austrafia) and the Scottish area of the British Coalfields. Conditions in the two areas are in total contrast: the Scottish area utilises lower load bearing capacity supports due to the "'light" (from a rock mechanics point of view) geological structure surrounding the coal measures and the fact that this structure can accommodate relatively large amounts of convergence with

few adverse effects. On the other hand the "'heavy" geological conditions of the Illawarra region demand a firmer control over convergence and the use of high capacity supports. Results from the investigations conducted are analysed to show how powered support rating and local geological conditions directly affect the amount of convergence which occurs in the central area of a longwall faceline. Depth of cover and height of extraction are encompassed in the overall investigation and an empirical formula is developed to indicate the amount of influence each parameter has on convergence.

INTRODUCTION

strata at the face. To achieve the desired control, an appropriate capacity and type of support must be chosen to complement the local strata conditions. Early longwalls in Australia were a direct result of technology transfer from Europe, and in particular the United Kingdom. This technology transfer m a y have been acceptable in some facets of the operation, but when

The traditional home of longwall mining is Europe, b u t the economic benefits of this method of mining in various geological settings has lead to its rapid acceptance in other mining areas throughout the world. The success of this method of mining relies on, among other factors, adequate control of

244 TABLE 1 Longwall conditions in Australia and the United Kingdom AUSTRALIA

U.K.

Type of rock coal roof

Low strength Sandstone/mudstone

floor

Medium strength mudstone or shale 2.6 m 1.5:1 to 3:1

Predominantly low strength Predominantly seatearth and sandstone Shale, weak seatearth

Average working height Horizontal to vertical stress ratio Average convergence across longwall working span Average subsidence at the surface Average face length Average web width Daily output/face Average support setting load density Average support setting to yield pressure ratio Load bearing capacity per supporting unit

1.6 m 1.7:1 to1:1

5% 45% 150 m 0.9 m 5150 t

17.5% 80% 200m 0.75m 728 t

0.625MPa

0.27 MPa

80%

55%

400 t-900 t

250 t-600 t

related to on-face strata control the available equipment was, to say the least, totally unacceptable. The parameter which had a severe detrimental effect on the early powered supports in Australia was the massive sandstone sequence directly above the coal measures. This massive formation resulted in very " h e a v y " conditions being experienced on the facelines and subsequently the supports developed for the "lighter" British conditions did not have the load bearing capacity to adequately support the roof. A secondary factor was the high racking forces developed b y the immediate roof, due to the high horizontal to vertical stress ratio, which resulted in shear failure of the support system. To combat the problem, modern longwalls in Australia utilise high capacity shield supports which are specifically designed to cope with the high horizontal and vertical loads. Table 1 illustrates the considerable difference in the nature of longwall mining oper-

ations between Australia and the United Kingdom. As mining is a very competitive industry it is not always desirable to choose the biggest and best support b u t rather to choose the optimum capacity and type of support for the given conditions. In order to determine the o p t i m u m capacity of support it is necessary to gain a thorough understanding of s t r a t a / support interaction.

STRATA/SUPPORT INTERACTION The understanding of strata / support interaction has been the subject of considerable research [1-6]. Various parameters have been identified as being pertinent to the understanding of s t r a t a / s u p p o r t interaction. The following were subject to a comprehensive programme of study conducted in various collieries in N.S.W. and Scotland:

245

(i) roof to floor convergence (ii) compaction of fines above and below the support, (iii) support leg closures (iv) support setting pressures and pressure build up due to convergence (v) separation and changes of strain across the bedding planes of the immediate strata surrounding the support and its influence on the above parameters (vi) the characteristics of the caved waste In order to analyse the interaction of the above parameters it was decided to derive an empirical relationship which would numerically link convergence, geology and powered support rating.

CONVERGENCE The amount of convergence which is experienced in the central area of a longwall faceline is dependent upon many parameters. A number of researchers have produced empirical relationships to predict the amount of convergence which occurs and subsequently promote a better understanding of the mechanism of convergence and the parameters which have direct influence. Wilson [1] analysed numerous results from investigations conducted in Britain, France and Germany and concluded that the minimum rate for convergence may be calculated from: C = ( 1 0 W = 30) where: C = convergence in millimetres/metre advance, and W = the height of seam extraction in metres. It was later proposed that depth of cover would also effect the rate of convergence but to a lesser magnitude than the two major factors. Supported by additional results from the French coalfields Wilson suggested that convergence varies with depth of cover raised to the power minus one quarter, i.e. Co~H-1/4,

100-

s~ 80 "4.__

~ea " ~r J

60-

X

L

~

40

ca , , -

='~

/Cony~ ~0"25

20

o ka

o

Ido

2~o

360

~6o

s~o

660

7~o

800

Depfh below surface H(m}

Fig. 1. Effect of depth on convergencerate [2].

and therefore depth affected convergence will level off over the 650 metre mark (Fig. 1). This must be requalified by stating that this levelling off will not continue indefinitely, as increased floor heave will adversely affect convergence especially in "softer" floors, as greater depths of cover produce higher abutment loads. It is proposed that the above formula be amended to include the "depth of cover" parameter. This produces the following: C = (10 W + 30) H- °25/H~ o.25 where; H = depth of cover, HD---- depth of cover datum (taken as 650 m). If depths of cover of 650 m and 100 m are taken with an extraction height of 1 metre, values of 40 mm and 64 mm convergence per metre face advance are calculated from the modified formula. These values agree with the values indicated in Fig. 1. It was stated that the original formula, corrected for depth of cover, may be used to estimate roof to floor convergence near the centre of a longwall face to an accuracy of 20-30%. Research results from France helped in the derivation of the above equations, and French investigators themselves have produced equations to estimate convergence. Josien and Gouilloux (1978) produced the following equation which takes into account the load-bearing capacity of the supports and a

246 subsidence factor which depends on the type of waste support: C = ( q . W ) ° V S . H - ° z s . [ 6 8 0 0 / P M + 66], where; C = average convergence in m m / m face advance; W = working thickness of seam in metres: (0.8 < W_< 3); q = a subsidence factor, caving = 1 pneumatic stowing = 0.6 hydraulic stowing = 0.15; H = d e p t h o f cover, (100 < H < 1000 metres); P M = load bearing capacity of the support, expressed in tonnes/linear metre of face (20 < P M < 260). Although this formula takes into account the load-bearing (yield load) capacity of the support it does not allow for the variation between the setting pressure and its final yield pressure, i.e. the mean load density: It may therefore be concluded that the above formula indicates that convergence occurring afte? yield is the limiting factor for estimating total closure between the roof and floor. Since yield occurs in less than 30% of occasions for the average cycle in both Scotland and the Southern Coalfields, it is proposed that the load-bearing capacity of the support be replaced by some other value which indicates the resistance offered by the powered support. The mean load density may seem to be the obvious choice, but as this value is out of the control of the Strata Control Engineer, and is influenced largely by the convergence which occurs, it would be more beneficial to use the setting load density for this particular parameter. The other parameter which should come under closer scrutiny is the subsidence factor. Although it takes into account the method of waste support, it does not allow for any variation in the state of caving which in certain circumstances may cause an increase or decrease in convergence. As caving is ultimately

dependent on the local geology it would seem wise to include a parameter which takes into account the overall geological pattern of the surrounding strata. To this end the authors put forward the following equation: C = 14(G/S) W 0"75.H -0"25 C = convergence ( m m / m e t r e advance) W = height of extraction (m) H = depth of cover (m) S = setting load density (MPa) G = geological factor. The geological factor for any particular face is deduced by analysing the nature of the surrounding strata and its ability to cave in the waste, the in situ rock strength of the strata, the depth of cover if over 650 m (as this will cause heave in soft floors) and the experience of results from various situations. Examples of this factor are as follows: 0.7 = competent roof and floor, good caving 1.0 = weak roof and floor, good caving (assumed datum), 1.4 = competent roof and floor, good caving, heavy conditions. N.B. The datum may be changed by taking a different constant (14 in this case).

GEOLOGY AND ROCK CHARACTERISTICS Scottish coalfield The geology of the Scottish coalfield belongs to the carboniferous system of the upper Palaeozoic group. The coal seams of economic importance occur in two groups; the limestone group and the productive coal measures. The Ayr Hard and Main seams mined at Killoch Colliery belong to the productive coal measures, the Upper Hirst seam of Solsgirth Colliery occurs in the Upper Limestone group and the Chemiss seam mined at Seafield Colliery belongs to the productive coal measures occurring under the Firth of Forth. Details of

247 I.R.S = I N S I T U -R-. S

ROCK S T R E N f T H

>,~.e~4

i.R.__SS

, ~.~.~

.~,.,¢,.~.

I.R.S

i.~4;.] C[oysfone

130 .......... Sandstone 83

:::, :%

Sandstone ,/

,'" ,. -3

-2A~.:L:: 129 ~ S a n d s t o n e

23 ....

..,.,

39 :_----i~-Sh/sO~m5m .

55 =__c.=_ -- Shale with = : : : Sandstone . . . . Lens25m

~,.:':,l{:1 1B " 14 2o ~--"L:%lShate 2m ~^ 25 ~ L o m i n i t e 3m ,'u ~ uul[i 3.Ore 5 5 ~ m l l B u U i Cool 3m Mudsfone 3m 18 18 . . . . . I Shale 3m 77 ii~'~-~ilSondsfone 69 CORRIMAL

APPIN

28 -_-=~_~iShote 3m

~< , :

. "-,:

3

103 ' " ...... Sandstone .::,=~,.~ Shale 0.5m I...aminife 2m, Sandstone kon~(omera~e'B4,. ~uUl 3.urn ~m ~ . . . . BuUi Seam2.5m Sandy Shale 2m ---_-2 Shale 3m Sandstone 65 ii'!i!!:'!i! Sondsfone

SOUTH BULLI

WI~ST CLIFF

Sandstone

Sitfstone

f. ¢, :- L'-:I

12

6 5 12

Shel!,y !~!'C.~ Sandstone Mudstone14m 8 H dut s one 2 m 7 Cool 1.7m 4 ~Coo[2.5m Seafearfh Im 4 Dirty CoalO-3m ~r 5 ~SeafearthO4m 19 Shel[y 40 Mudsfone ~'-,"~!l Sandstone

Si,tston Om

|

T !,i!,(~:iSandstone ~;:: 2m | Silfstone 2m

~

~udstone$3m

Mudstone1]m

[

L.oall.6m

=_-~ Seateorth Ira 3 / C o a t 3.6m . . . . . "Silfsfone 1.2m~n6 ~ SeofeorfhO.Im "~!:!/;!::~ Siltsf on e 09m ]:;:;i '.::i:~!iJSandsfone

,,;=v Sandstone

KILLOCH-AYR HARD KILLOCH-MAINCOAL SEAFIELD-CHEMISS SOLSfilRTH-UPPERHIRST a

Fig. 2. Comparative geological sections of facelines investigated.

the representative geological sections around all four seams are shown in Fig. 2. Basically the strata of the immediate roof consists largely of shale, mudstone and variegated sandstone. These rocks are relatively low strength which tend to cave readily behind the powered supports. The caved waste offers immediate support to the overlying competent beds, transferring a significant portion of the upper roof load away from the powered supports and hence the use of lower capacity supports ~is possible. Under these conditions relatively high amounts of convergence would have little detrimental effect on the operation of the longwall face and in fact convergence of the immediate roof has in the past been utilised to assist in the mining of hard thin seams. The immediate floor of all four faces investigated consists of layers of seatearth, siltstone and sandstone. Seatearth has a low load bearing capacity which tends to fail beneath

the powered support base, resulting in excessive floor debris. Southern coalfield of N.S.W.

In contrast the seams of the Southern coalfield of N.S.W. belong to the Illawarra Coal Measures of the Permian series. The Bulli seam is the uppermost formation of the Illawarra Coal Measures and all the longwall faces in the district are currently located in this seam. The Horizon directly above the Bulli seam consists mainly of sandstone, with the lower section being of variable composition comprising shale, mudstone and s a n d s t o n e / s h a l e laminations (Fig. 2). The sandstone varies in strength from a weak conglomerate sandstone deficient in cementitious matrix to a relatively high strength sandstone which exhibits bridging capabilities with the immediate caving range.

1.6 520 237

G-factor Web width (m)

Support capacity (t) Leg bore (m) Support spacing (m) Face to waste distance (m) Nominal setting press (MPa) Average setting press (MPa) Average setting load density (MPa) Caving

2.4 759 166

Ayr Hard A23 Retreat

Seam Face Longwall system Height of extraction (m) Depth (m) Face length (m) Rate of advance (m/wk) Cycle time (rains.) Support type 1.5 242 180

Chemiss C24 Retreat

Seafield

2.25 365 171

Upper Hirst 62 Advance

Solsgirth

2.8 500 175

Bulli LW9 Retreat

Appin

2.4 430 104

Bulli LW5 Retreat

Corrimal

2.8 450 183

Bulli LW205 Retreat

South Bulli

N.S.W. (Aust) Southern Coalfield mines

2.5 480 130

Bulli LW2 Retreat

West Cliff

3.3 13.8 10.1

3.3

12.1

10.2

1.0 0.6 0.8 0.6

0.14 Immediate

1.2

1.1

0.12 Immediate

240 0.108

180 0.095

0.7 0.6

0.11 Immediate

11.0

13.8

3.3

1.1

180 0.095

0.9 0.6

0.15 Immediate

10.5

12.4

3.3

1.2

240 0.108

0.43 0.5 m Overhang 1.4 0.7

27.6

32

5.4

1.5

628 0.20

0.41 1.5 m Overhang 1.6 0.8

26.5

32.5

6.6

1.5

630 0.22

1.5 0.7

0.56 Immediate

0.0

32.5

5.1

1.5

630 0.22

0.73 0.5 m Overhang 1.4 0.8

27.8

36

5.0

1.5

900 0.25

18 5 15 15 65 50 40 70 90 180 100 100 80 55 95 45 Gullick Dobson Gullick Dobson Gullick Dobson Gullick Dobson Westfalia Dowty Dowty Gullick Dobson 6 leg chock 6 leg chock 5 leg chock 6 leg chock 4 leg shield 4 leg shield 4 leg shield 4 leg-chock shield

Main Coal N56 Advance

Killoch

Details

Killoch

Scottish mines

Site

Investigation sites details for Scottish and N.S.W. Southern Coalfield longwall faces

TABLE 2

b,.) OC

249

In this situation the existence of distinct parting planes in the immediate roof governs the state of caving and may result in the transfer of excessive load on to the face supports and the coal face causing face flushing and hence poor roof control. Therefore the need to minimise the amount of roof to floor convergence through increased powered support capacity is a pre-requisite for efficient and safe working under massive roof formations such as those encountered when working the Bulli seam. The floor of the Bulli seam consists mainly of strong shales and mudstone. Some variation in strength is observed from one mine to another and even individual faces in the same mine. In South Bulli Colliery floor intrusions occur in the coal seam and are in the form of stone rolls which can be up to 1.5 m high.

CASE STUDIES In order to formulate any empirical relationship a number of case studies must be completed. As stated previously these investigations were conducted in the vastly differing conditions and working practices of N.S.W. and Scotland. Table 2 contains the details of the sites investigated.

ANALYSIS OF RESULTS The data gathered from the site investigations was analysed and the value obtained from convergence measurements was compared with that predicted by the three empirical equations. The following table presents the results for direct comparison. From the above table it can be seen that the formula which includes the setting load density (S.L.D.) and a geological factor predicts values for convergence which are consistently closer to the actual value than those predicted using the British and French formulae. It should be noted that the British formula was first developed in the 1960's when it may have seemed reasonable to ignore the resistance offered by the face support system. However, it can be seen from Table 3 that a modern powered support system offers considerable resistance to convergence. Assuming that the values predicted using the British equation are accurate for older support systems, a modern powered support will help reduce convergence by more than 35%, even in massive geological conditions. From the above the first conclusion can be drawn, i.e. that it is essential to include some value for support resistance in any formula which is used for predicting convergence.

TABLE 3 Predicted a n d actual convergence values Area

Face

Predicted convergence ( m m ) British

French

Authors

Actual

Scotland

A23 Killoch N56 Killoch C24 Seafield 62 Solsgirth LW4 A p p i n LW5 Corrimal LW205 South Bulli LW2 West Cliff * V105 Deep Navigation

49 52 58 61 62 60 64 59 57

32 37 37 42 38 35 39 33 37

35 29 31 35 21 23 18 11 19

30 27 29 33 20 9 17 7 22

Illawarra

250 Which value should be used for support resistance? The French equation indicates that the support systems load bearing capacity per metre of face length should be used. This appears to hold for European conditions, where the predicted convergence values are within 25% of those recorded in situ. When it comes to Australian conditions the figures predicted are all over 45% too high. It should be noted that the French equation is only applicable for support capacities between 20 and 260 t o n n e s / l i n e a r metre of face, whereas the supports used in Australia have capacities greater than 400 t o n n e s / m e t r e of face. It may also be pointed out that the French equation does not include any significant geological parameter, but as the geological factors proposed for Australian conditions increase the predicted convergence values by more than 30%, this would make the French equation even more inaccurate. From this it would appear that load bearing capacity is not the figure which should be used for powered support resistance in the prediction of convergence on a longwall face. The third equation, using S.L.D. as resistance, predicts values which, in all cases except A23 Killoch, are closer to the value measured in situ. Even if a geological factor is not included, the third equation produces comparable values in all but two cases. It was accepted that the parameters used in the third equation were relevant to the accurate prediction of convergence, and subsequently the effect of proximate geology was considered. When geology was taken into account, two faces; namely LW5 Corrimal and LW2 West Cliff (both Australia), had 180% and 60% less convergence than that predicted. At first it was considered that the geological factor used was too high, as a figure of 1.0 would produce reasonable values, but after further consideration of the geology and known conditions which existed at that time, it was concluded that the values of 1.6 for LW5 and 1.4 for

LW2 were not unreasonable. This conclusion is Supported by the basic fact that all four Australian faces work the Bulfi Seam, and although geology varies from colliery to colliery, the same massive formation still dominates the geological sequence. Closer inspection of the site details produced one particular commonality between both faces, i.e., both faces were considerably shorter than all the other faces. Another point that stood out was that LW2 West Cliff, which was the longer of the two at 130 m, was working in almost virgin strata, only LW1 had been extracted in that area and a 30 m barrier pillar was left between that panel and LW2. In both situations the short face length may have allowed the upper bridging beds to remain intact, spanning between the barrier pillars and virgin strata and subsequently throwing less load onto the support via convergence of the immediate roof. An affect similar to this was predicted by Wold and Pala [7] using voussoir plate analysis to explore the negligible subsidence (about 80 mm) which occurred after the completion of mining longwall block No. I at Ellalong Colliery N.S.W. Another anomaly occurred at C24 face Seafield shortly after investigations there had ceased. As the face advanced towards a kno';vn fault a considerable increase in convergence was experienced, to such an extent that many powered supports became iron bound. The probable reason for this increased convergence was that the change in local conditions, related to the presence of the fault, p r o d u c e d an increased stress concentration and subsequently increased convergence in that region. The asterisked area in Table 3 shows results obtained from an independent investigation conducted by Isaac and Smith [8] at Deep Navigation Colliery in the South Wales are of the United Kingdom. The face was 165 m long extracting a 2.67 m section at a depth of 650 m. The face was equipped with Fletcher Sutcliffe Wild, 4/450

251 tonne Hydrostore chock shield supports at 1.5 m centres producing a setting load density of 0.269 MPa. The average convergence per cycle was 17.7 mm, which is equivalent to 22.1 m m / m face advance. Both the British and French equations predict values over 50% in excess, but the authors' equation, (using a geology factor of 0.9-mudstone floor with weak immediate roof at 650 m depth) predicts convergence within 15% of the actual value. In this situation geology has limited influence, the dominant effect is the application of the average setting load density of 0.269 MPa which is over twice the average of the four Scottish faces while working similar conditions.

CONCLUSIONS In the past it was acceptable to assume that convergence on a longwall face was due primarily to face advance height of extraction and was influenced to a limited extent by depth of cover. However, results from recent investigations indicate that increased setting pressures will in fact reduce convergence, probably by compacting debris at the time of advance and therefore limiting relatively uncontrolled movement. From this it may be concluded that the nature and strength of the immediate roof and floor will also have a direct influence on convergence, as these factors may limit the effectiveness of increased setting pressures. Convergence is also affected by local geology. The lighter conditions which exist in the Scottish Coalfields are due to the immediate roofs ability to cave in the waste, bulk and subsequently offer immediate resistance to the bridging beds of the upper roof. This aids the powered support in performing its task of counteracting roof to floor convergence and subsequently allows a lighter support rating.

Conversely the massive strata conditions of the Illawarra do not provide this supporting medium in the waste. Consequently a higher support rating is required to limit convergence. This conclusion is supported by analysis of the three equations used to predict convergence. Using the equation; C = a W + b, which includes parameters for amount of advance, height of extraction and, with the use of a conversion factor, depth of cover, rates of convergence over 50% in excess of the measured value were predicted in some cases. The equation, C = (qW) 0"75.H-0'25.[6800/ P + 66], which includes parameters for: amount of advance, height of extraction, depth of cover, support load bearing capacity and the method of waste support, predicts values considerably closer to the measured convergence, when compared with the former equation, but it is only relevant to the strata conditions which exist in European mines. When it is applied to Australian mines there is no comparison between the measured and predicted values. The equation; C = 14.G/S.W°VS.H -°'25 includes the above parameters, but substitutes setting load density for load bearing capacity, and includes the caving parameter in an overall geological parameter. The above equation predicts results which are within 15% of the measured value in the majority of circumstances. Exceptions to the above equations could be related to local anomalies. An overall decrease from the "expected" amount of convergence will result from a face which is considerably shorter than average. This "decrease" in convergence may also result from working in virgin strata. One factor which may cause an increase in convergence is the approach of a geological fault. This is probably due to increased stress concentrations in the area surrounding the fault.

252 T h e a b o v e c o n c l u s i o n s emphasise the n e e d for a s u p p o r t resistance p a r a m e t e r in the e q u a t i o n for p r e d i c t i n g c o n v e r g e n c e and, in m o s t circumstances, the need for a geological factor. In general, the average rate of a d v a n c e ; m e t r e s / w e e k , does n o t dictate the rate of convergence. But on faces with similar seam section and geology, a faster a d v a n c e rate will p r o d u c e a lower rate of convergence. Finally, it is c o n c l u d e d that a m o u n t of advance, height of e x t r a c t i o n a n d s u p p o r t resistance are the d o m i n a n t p a r a m e t e r s w h e n analysing c o n v e r g e n c e . H o w e v e r , w h e n m a k ing c o m p a r i s o n b e t w e e n certain strata conditions, geology m a y h a v e the effect of causing a 90% v a r i a t i o n in the a m o u n t of c o n v e r g e n c e e x p e r i e n c e d in the central faceline zone.

ACKNOWLEDGEMENTS T h e a u t h o r s wish to t h a n k the staff o f the D e p a r t m e n t of Civil a n d M i n i n g Engineering, T h e U n i v e r s i t y o f W o l l o n g o n g and the staff o f the D e p a r t m e n t of M i n i n g and P e t r o l e u m Engineering, T h e U n i v e r s i t y o f Strathclyde. T h a n k s are also given to British Coal, T h e J o i n t Coal B o a r d , Australia, a n d the p e r s o n nel of the various mines in S c o t l a n d a n d N.S.W., w h e r e the research was carried out.

REFERENCES 1 Wilson, A.H., 1964. Conclusions from recent strata control measurements made by the mining research establishment. Mining Engineer, 123 (April) 367380.

2 Wilson, A.H., 1975. Support load requirements on longwall faces. Mining Engineer, 134 (June): 479-491. 3 Bates, J.J., 1978. Analysis of powered support behaviour. The Mining Engineer, 137: 68i-693. 4 Peng, S.S., Chiang, H.S. and Lu, D.F., 1982. Roof behaviour and support requirements for the shieldsupported longwall faces. State of the Art of Ground Control in Longwall Mining and Mining Subsidence, A.I.M.E. Conf. 5 Smart, B.G.D. and Aziz, N.I., 1986. Influence of caving in Hirst and Bulli seams on powered support rating. Proc. Symp. on Ground Movement and Control Related to Coal Mining, Illawarra Branch Aus.I.M.M., August, pp. 182-193. 6 Smart, B.G.D., Isaac, K. and Hinde, C.G., 1980. Investigations into the relationship between powered support performance and the working environment, with particular reference to strata convergence. Proc. 21st U.S. Rock Mechanics Symp., Rolla, Missouri. 7 Wold, M.B. and Pala, A., 1986. The response of powered supports and pillars to initial longwalling under a strong roof main roof. Proc. Symp. on Ground Movement and Control Related to Coal Mining, Illawarra Branch Aus.I.M.M., pp. 194-203. 8 Isaac, A.K. and Smith, P.E., 1986. Support of longwall workings in thick seam extraction. Proc. Symp. on Ground Movement and Control Related to Coal Mining, Illawarra Branch Aus.I.M.M., August, pp. 204-213. 9 Aziz, N.I., 1983. The Performance of the Powered Support System at Longwall Coal Face LW2, West Cliff Colliery. Department of Civil and Mining Engineering Report in the Field of Strata Mechanics, Report No. 83/2. 10 Josien, J.P. and Gouilloux, C., 1978. Present and future roof control and support in longwall faces in French coal mines: Colliery Guardian, October. 11 Porter, I., 1983. A study of certain aspects of strata control and powered support performance on longwall facelines in various structural environments. Ph.D. Thesis, Department of Mining and Petroleum Engineering, University of Strathclyde, U.K.