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Geomechanical behaviour of laminated, weak coal mine roof strata and the implications for a ground reinforcement strategy
ARTICLE IN PRESS
International Journal of Rock Mechanics & Mining Sciences 41 (2004) 147–157
Geomechanical behaviour of laminated, weak coal mine roof strata and the implications for a ground reinforcement strategy B.K. Hebblewhite*, T. Lu School of Mining Engineering, The University of New South Wales, Sydney, Australia
Abstract This paper describes a component of the results from an ARC-SPIRT funded collaborative research project between UNSW, Powercoal Pty Ltd. and Springvale Coal Pty Ltd. The research project was aimed at identifying strategies for appropriate ground control in particularly weak and soft roof strata in underground coal mining. A field investigation was undertaken at Angus Place Colliery in the Western Coalfield of New South Wales. The investigation incorporated a range of geotechnical instrumentation and was conducted over a period of time from the development face until the site was lost into the goaf of a retreating longwall panel. This paper describes the outcomes from the field investigation, together with a selection of supporting laboratory studies. The paper also presents a number of alternative presentation modes for extensometry data. The results clearly demonstrate the time and faceproximity related influences on roof integrity, and particularly demonstrated the distribution of deformation, bed separation and strata failure into the roof and across the full span of the roadway, together with reflection of this behaviour within the fully encapsulated roof bolt reinforcement system installed. r 2003 Elsevier Ltd. All rights reserved.
1. Introduction
2. Background
The principal objective of this research investigation was to develop an improved understanding of the deformational behaviour of laminated, weak roof strata in underground coal mines, under both first workings and abutment loading conditions, and to then apply that understanding to the development of appropriate reinforcement strategies. The ability to achieve roof reinforcement effectiveness and roadway stability under these particularly extreme geological conditions has continually challenged mining operations at a number of collieries operating in the Lithgow Seam of the Western Coalfield of New South Wales, Australia. This problem is of particular importance in longwall mines where gateroad stability is critical to the efficient operation of the longwall face. Angus Place Colliery, located near Lithgow, NSW, was chosen as an appropriate site for this field investigation.
Angus Place Colliery is located in the southern section of the Western Coalfield of New South Wales, approximately 110 km due west of Sydney, Australia (see Fig. 1). At this colliery, longwall mining has been the primary source of production since 1979. Mining conditions have progressively deteriorated as more severe geological/geotechnical conditions have been encountered. It is claimed by some that Angus Place experiences arguably the most difficult coal mining strata conditions in Australia [1]. The mine roof changes from a combined Lithgow– Lidsdale Seam section (convergence zone) in the north and to the east, through a seam split zone, where the seams diverge. Combinations of three joint sets, namely NW–SE, NE–SW and N–S, can affect the coal roof. If two or more sets occur at higher frequencies, then the roof is effectively composed of large numbers of blocks [2]. Coupled with the structural discontinuities caused by jointing in the roof, the laminated stratigraphic nature of the strata—composed of varying layers of coal, mudstone, sandstone and claystone—further contributes to a roof that exhibits both weak and relatively soft geomechanical characteristics.
*Corresponding author. Tel.: +61-2-9385-5160; fax: +61-2-93137269. E-mail address: [email protected] (B.K. Hebblewhite). 1365-1609/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmms.2003.08.003
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B.K. Hebblewhite, T. Lu / International Journal of Rock Mechanics & Mining Sciences 41 (2004) 147–157
Fig. 1. Western Coalfield of New South Wales, Australia.
3. Mechanical properties of roof rocks In order to understand the mechanism of deformational behaviour in this type of roof material, it was important to have an appreciation of the typical mechanical properties of the various intact rock components of the strata elements and their contribution to the roof rock mass as a whole. Thus, a program of laboratory tests was designed
for this purpose. Fig. 2 shows the proportion of different rock types within the immediate roof strata, and indicates that coal, mudstone and sandstone were the major components of the immediate 5 m of roof. Standard geomechanical testing of this material included: modulus of elasticity (E), uniaxial compressive strength (UCS), indirect tensile strength (TS), determination of cohesion and angle of internal friction, together with shear strength
ARTICLE IN PRESS B.K. Hebblewhite, T. Lu / International Journal of Rock Mechanics & Mining Sciences 41 (2004) 147–157 70 60
Table 1 Mechanical parameters of coal, mudstone and sandstone
50
Rock type
UCS (MPa)
E (GPa)
C
Cr
f
fr
TS (MPa)
Coal Mudstone Sandstone
43 82 49
3 6 6
4 4.5 5
0.2 0.2 0.01
22 38 46
12 26 28
3 5 5.2
61
Average percentage of rock type (%)
149
40 30 18
20
14
10
Mudstone
Sandstone
Coal
Claystone
2
2
Siltstone
Other
Rock types
Fig. 2. Immediate roof rock type distribution.
and triaxial compressive strength under different levels of confinement. These tests were conducted on the dominant three rock types—mudstone, coal and sandstone. The claystone only formed a small proportion of the roof, however, it was also suspected to play a significant part in the deformational behaviour of the roof material underground. In an undisturbed state, the material is strong and stiff, however on exposure to air or water it rapidly weathers to a quite incompetent material. Unfortunately, the combination of this behaviour on recovered samples, plus the thin cross-sections of the material recovered prevented any claystone testing in the laboratory. Tables 1 and 2 summarise the test results obtained for the coal, mudstone and sandstone. Fig. 3 illustrates the differences in shear strength among samples of coal, mudstone and sandstone under increasing normal stresses. This indicates a far more significant strength increase with confinement for the mudstone and sandstone, than for the coal. In addition, tests were conducted to investigate the sensitivity of mudstone to water, so as to gain an indication of the effect of water on the mechanical properties of the major roof component and determine the deterioration mechanism of roof rock masses. The testing results indicated that when water content reached a level of saturation, the maximum uniaxial compressive strength of mudstone reduced to 22 MPa. This reduction rate is up to 74%, compared to the maximum value of 82 MPa for the dry sample (0% moisture). On the basis of these results, the intact roof rocks could be classified as medium to strong, in a dry condition. However, when the water content and discontinuities are taken into consideration, the roof strata must be classified as weak. It should also be noted that the pre-mining virgin stress conditions at Angus Place have been measured on a number of occasions and indicate horizontal to vertical stress ratios of up to 3:1—typical of the Sydney Basin.
Table 2 Water content in natural condition Rock type
Water content (%)
Coal Mudstone Sandstone
3.5 4 3
Shear strength (MPa)
3 0
18 16 14 12 10 8 6 4 2 0
Sandstone Mudstone 4
Coal 7
9
11
12
Normal Stress (MPa) Fig. 3. Shear strength with various levels of normal stresses and rock types.
4. Mine site investigation The major objective of this investigation was to develop an understanding of the detailed roof deformational behaviour and the performance of the installed rock bolting reinforcement system used in the longwall gateroad. The geotechnical monitoring instrumentation consisted of multiple wire and sonic extensometers, plus strain gauge instrumented bolts. Fig. 4 illustrates the monitoring configuration in heading 2, maingate 22, Angus Place Colliery. (The chain pillar is located on the left of this diagram (looking inbye), while the future longwall block is on the right.) This instrumentation pattern was all installed on development, within 2 m of the roadway face, in order to obtain the maximum component of the developmentrelated deformation, as soon as the face advanced away from the site. A key feature of the instrumentation layout—apart from the considerable mine logistical organisation and assistance provided to enable installation of such a large array of instruments at the
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This replicated the normal roof bolt support pattern in the mine, forming a single row in the normal support system.
WE
SE1
SE2
SE3
SE4
SE5
4.1. Roof deformation at different horizons SG1 300
600
750
SG4
900
600
SG5 600
1250
2500
1150
SG7
SG3
SG2
SG6
700
2000 800
850
800
1300 550
400 300
SE6
SG8 SE7
(Looking inbye) 1500
1550
4500
Fig. 4. Detailed instrument layout in heading 2, maingate 22 (vertical section view, all dimensions in metres) (WE–wire extensometer, SE–sonic extensometer, SG–strain gauged bolt).
Fig. 5 shows the roof deformation profile recorded at different time intervals after installation, along the 15 m length of the combined sonic/wire extensometer in the centre of the heading. It indicates the height to which rock deformation (roof softening) occurred during different stages of mining activity. (Note: In the diagram legend, the results are presented as either before or after extraction. This refers to the development stage of the inbye mining activity, and the commencement of the retreating longwall extraction, respectively). Key features of these results can be summarised as follows: *
*
The overall height of measurable, mining-induced deformation extends to at least 12 m, or over 4 times mining height. The small amount of movement in the upper strata (above 7 m) occurred immediately after the development face advanced away from the site, and did not increase thereafter. The major deformation zone, or height of softening, extends to approximately 6 m into the roof, roughly twice the mining height. This upper limit to major movement was evident from the time of initial development away from the face, and remained relatively constant throughout both development and extraction stages. 16
Day 6 (bef. extraction) Day 63 (bef. extraction) Day 78 (bef. extraction)
14
Day 174(bef. extraction)
Distance from roof surface (m)
face—was the large number of monitoring points across the full roadway width (or roof span). Unlike routine monitoring conducted at most mines, where instrumentation is usually restricted to the roadway centreline, it was considered important in this instance to gain a complete profile of the roof deformation across the full span of the roof. In this sense, the comprehensive nature of the dataset obtained from this investigation was quite unusual, if not unique, for the Australian coal industry. In order to gain an absolute, stable reference point in the roof, above the height of roof dilation and sag, a wire extensometer was installed in the centre of the heading roof. Previous experience at Angus Place had indicated that the 7.5 m upper limit of the standard sonic extensometers was at times only marginal for acting as a non-displaced reference point. The reference anchor for the wire extensometer was located 15 m above the roof, horizon with three other measurement anchors located at 12, 9.5 and 7.5 m above the roof, respectively. (The weak and deformable nature of the floor strata, combined with mine traffic obstruction and potential damage, prevented the use of any floor monitoring stations.) Based on the monitoring arrangement adopted, the relative deformation within the 7.5–15 m region of roof could be determined. There were also five sonic extensometers installed, each with 20 anchors at up to 500 mm spacing over a 7.5 m length. These were positioned in a single row across the roadway width, with a centrally located station immediately adjacent to the wire extensometer to enable the two datasets to be combined to create an effective 23 anchor, 15 m long extensometer. Apart from extensometers, six strain gauge instrumented bolts, 2.1 m in length, (9 gauges per side at 200 mm spacing), were also installed across the roadway in the same plane as the extensometer instrumentation.
12
Day 225 (aft. extraction) Day 271(aft. extraction)
10
Coal Claystone Mudstone Sandstone
8
6
4
2
0 02
0
40
60
80
Displacement (mm)
Fig. 5. Roof deformation recorded by wire and sonic extensometers at centre of heading.
ARTICLE IN PRESS B.K. Hebblewhite, T. Lu / International Journal of Rock Mechanics & Mining Sciences 41 (2004) 147–157 *
*
Relative deformation, or dilation of the roof above this 6 m horizon did not exceed 10 mm. The majority of differential movement (+65 mm), or dilation, has occurred within the immediate coal/ claystone roof (0–2 m), and in the mudstone strata from 4 to 5.5 m.
4.2. Roof deformation associated with different mining activities The instrumentation was monitored regularly over a 278 day period, from the time of initial roadway development away from the face, to the time when the retreating longwall face passed the site, which was then lost in the goaf. Fig. 6 is a typical graph showing the cumulative displacement of roof strata measured at each anchor horizon within the sonic extensometers, relative to time. This plot is from SE3, located in the centre of the roadway. Each line represents the cumulative deformation of the roof surface, relative to the respective anchor horizons. The results clearly illustrate five distinct phases of deformation that can be related to both a mining-induced loading environment and a time effect. These five phases, and their characteristics for this particular extensometer dataset, are: 1. Static conditions with minimal deformation, prior to initial development (0–15 days)—less than 5 mm total dilation over full extensometer length while face remained within 2 m of instrumentation site. 2. A primary region of mining-induced effect of development drivage away from the site between 15 and 20 days. This corresponded to a change in face proximity from 2 to 37 m. Total roof dilation reached approximately 15 mm. 3. A secondary development proximity effect between 20 and 60 days, (37–147 m face proximity), during
which maximum roof dilation increased to in excess of 50 mm. Beyond this point all results suggest that mining-induced effects from the development drivage have ceased. 4. A period of ongoing, time-dependent deformation, independent of mining effects, from 60 to 250+days. During this period, there is a reasonably ‘steady-state’ deformation trend, with the total reaching approximately 65 mm. This represents a deformation rate over this period, of approximately 0.08 mm/day. 5. A further mining-induced effect due to the approach of the longwall face, from 250 days until the endpoint of 278 days, during which the total deformation increased from 65 mm to 91 mm. The longwall face was 35 m away at the 250 day stage, indicating the maximum distance at which any significant ‘front abutment effect’ was detected. 4.3. Roof deformation distribution across roadway The cumulative deformation profile from all five sonic extensometers across the roadway is shown in Fig. 7. For each extensometer, the cumulative deformations for each individual anchor, relative to the reference point at the back of the hole are plotted, at six different time intervals (during the development, static and longwall retreating periods). Significant observations from these results are as follows: *
*
In all cases the major deformation is restricted to below the 6 m horizon (base of the sandstone, top of mudstone), with only very minor levels above. In the case of SE5 (closest to the longwall block) the major deformation appears confined to below the 4 m roof horizon (base of the mudstone). At the top of the bolted horizon (2.1 m) and just above, there appears to be a significant deformation
100 Anchor position:0.5m 90 Anchor position:0.8m 80 Anchor position:1.7m
Displacement (mm)
70
Anchor position:2.3m
60
Anchor position:2.9m
50
Anchor position:3.2m
40
Anchor position:4.0m
30
Anchor position:4.4m Anchor position:4.9m
20
Anchor position:5.4m
10
Anchor position:5.9m 0 0
50
100
150
151
200
250
300
Anchor position:7.3m
-10
Time (day) Fig. 6. Cumulative displacement with time recorded by extensometer SE3 (centre) (arrows indicate five loading stages).
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Day43 (before longwalling) Day 99 (before longwalling) Day 224 (before longwalling) Day 248 (after longwalling) Day 262 (after longwalling) Day 277.7 (after longwalling)
Distance from roof surface (m)
7
6
5
4
3
2
7
0
4
3
2
0 20
40
60
80
Day 44 (before longwalling) Day 100 (before longwalling) Day 262 (before longwalling Day 249 (after longwalling) Day 263 (after longwalling) Day 278 (after longwalling)
7 6
40
20
(b)
Displacement (mm) 8
0
100
5 4 3 2 1
80
60
100
Displacement (mm)
8
Day 43 (before longwalling) Day 99 (before longwalling) Day 244 (before longwalling) Day 248 (after longwalling) Day 262 (after longwalling) Day 277 (after longwalling)
7
Distance from roof surface (m)
0
(a)
Distance from roof surface (m)
5
1
1
6 5 4 3 2 1
0
0 0
(c)
Day 43 (before longwalling) Day 99 (before longwalling) Day 224 (before longwalling) Day 248 (after longwalling) Day 262 (after longwalling) Day 277 (after longwalling)
6
Distance from roof surface (m)
152
20
40
60
80
100
8
20
40
60
80
Displacement (mm)
Day 43 (before longwalling) Day 99 (before longwalling) Day 224 (before longwalling) Day 248 (after longwalling) Day 262 (after longwalling) Day 277 (after longwalling)
7
Distancefromroof surface (m)
0
(d)
Displacement (mm)
6 5 4 3 2 1 0 0
(e)
20
10
40
30
50
Displacement (mm)
Fig. 7. (a–e) Roof deformation at different horizons (refer to Fig. 4 for instrument numbers).
*
increase, suggesting that the bolted roof beam is pulling away from the overlying strata, creating major bed separation immediately above the bolts and also within the upper sections of the bolted horizon. This is most evident in SE1–SE3, and to a lesser extent in SE5, but not SE4. This behaviour is observed to commence from the initial development stage, and continue throughout the subsequent static and longwall abutment periods. This could indicate a less than optimum bolt length. Within the bolted horizon (especially evident in SE2, and to a lesser extent SE1 and SE3), the bolts appear to be holding the strata together, as a unit, with very
little differential movement between the roof surface and the 2.1 m horizon. In SE2 the relative deformation over the bolted horizon is excellent, at less than 10 mm, particularly prior to the final dataset. SE3 is of a similar magnitude, although the final longwall approach appears to have caused some significant relative deformation or strain within the lower 0.5 m of roof in the final dataset, suggesting loss of immediate roof control. SE1 has shown excellent bolt reinforcement for the lower 1.5 m of bolt length, although the upper section of the horizon is exhibiting high differentials, or strain levels. This may indicate that the bolts are not constraining the rock
ARTICLE IN PRESS B.K. Hebblewhite, T. Lu / International Journal of Rock Mechanics & Mining Sciences 41 (2004) 147–157
*
*
*
mass significantly, and may actually be pulling out of their anchorage. SE4 and SE5 do not show the same constrained characteristic of bolt reinforcement of the lower roof. They both exhibit a trend of constantly increasing deformation differential throughout the bolted horizon, and above, in the case of SE4. This trend is evident from the time of initial development and results in at least 20 mm differential deformation across the bolted horizon by the time the longwall retreats past the site. This is indicative of a reinforcement system which is at the limit for controlling or constraining the immediate roof beam—possibly in terms of both bolt length and density. SE1–4 all display a further major deformation differential at approximately 4–5 m horizon (within the mudstone). This is most pronounced in SE2–4. Therefore, even if the bolts are constraining the immediate roof sufficiently (more so on the left-hand side), the reinforced roof beam is clearly not stiff enough to constrain the upper strata horizons from softening and incurring bed separation. Again, this trend is evident from the immediate development stage. In other words, the upper roof integrity is being eroded from the time of initial development, even if this may not be evident in the roadway itself. This can only result in reduced stability as subsequent abutment loads are encountered.
Fig. 8 is an alternative presentation of the overall extensometry data, depicting the deformation at a selection of roof horizons—0, 1.86, 3.72, 4.65 and 6.65 m—at the time when the longwall face passed the instrumentation site. This clearly demonstrates the shape and extent of the height of roof softening, characterised by a near-vertical sided arch profile, diminishing only above the 4.65 m horizon, although exhibiting marginal deformation in the centre portion of the roof span at the 6.65 m horizon. This profile also illustrates the non-symmetrical nature of the deformation, with the more dominant deformation on the left (pillar side) of the heading, while on the right, the outer extensometer is not reporting significant deformation above the 3.72 m horizon, compared to 4.65 m on the left. The classic bending nature of the roof beam is also clearly illustrated in this data presentation. 4.4. Analysis of strain levels in roof strata The extensometer data was converted to incremental strain between individual anchor horizons, using original anchor separation distances. The strain results therefore correspond approximately to the slope of the extensometer deformation plots (to the extent that anchor separations were roughly the same throughout). Fig. 9 is
153
an example of the strain results, in this case for SE1, plotted as a vertical stacked set of histograms, grouped for five different time periods (day 277 is the top of each stack of five bars). This illustrates both the strain trend over time at each horizon, as well as the relative magnitudes of strain developed within the strata at the different heights into the roof (and with respect to roof rock type). This set of results provides further evidence that, on the left, or pillar side of the roadway at least, the immediate roof horizon, for at least the first 1 m, is well constrained with negligible strain evident throughout the investigation. However, above that horizon, strain magnitudes and frequency of high strain levels increases markedly, especially within the coal horizons up to 2.4 m and beyond, possibly adjacent to the claystone bands also, and then to a lesser extent into the higher mudstone. Fig. 10 presents the same dataset as a separate series of histograms. This enables easier comparison of the strain distribution relative to time. Apart from highlighting the major region of strain magnitude in the 1–2.5 m region, it also demonstrates the increasing strain activity higher into the roof relative to the initial dataset. The levels of strain developed within the immediate roof strata, as illustrated above, are certainly well in excess of those required to cause tensile failure of intact rock material, or to generate separation along existing bedding planes. The actual condition within the roof is no doubt a combination of both of these effects. Fig. 11 is an alternative analysis of the deformation data across the full roadway width at the final time period (277 days). Rather than processing the deformations as incremental strains distributed across each interval between adjacent anchors, it is a schematic depiction of the deformation as individual crack formations, or bed separations between successive anchors. The correlation of crack continuity between the different extensometers is remarkably consistent. It again highlights the significant development of horizontal fracturing/separation within the main coal roof horizon (1–2.5 m) and the higher mudstone horizon (3.9–6.0 m).
4.5. Instrumented bolt results The results from the strain-gauged bolts installed across the instrumentation site were not as conclusive as the extensometry. Fig. 12 is a composite plot of the maximum axial load recorded on these six bolts, immediately after the development stage of mining, and then again after the longwall retreated back to the site. (Note that the majority of the bolts reported significant unloading within the last 20 m of longwall approach, associated with either bolt/resin/rock bond failure, bolt failure or gauge failure. The first of these possibilities is considered the most likely.)
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Ext1 900
Ext3
Ext2 600
800
Ext4 850
Ext5 800
550
10 mm 20 mm
10 mm 20 mm 30 mm 7.29m 10 mm 20 mm 30 mm 40 mm
6.65m 4.65m
50 mm 3.72m
1.86m
10 mm 20 mm 30 mm 40 mm 50 mm 60 mm 70 mm 10 mm 20 mm 30 mm 40 mm 50 mm 60 mm 70 mm 80 mm 90 mm
Fig. 8. Roof deformation at day 277 (after mining extraction).
From the monitoring data, it was noted that the maximum axial load measured by the six instrumented bolts installed on the roof strata varied from 91 to 227 kN after development and the static loading phase, and from 210 to 256 kN as the longwall approached. The change from a non-symmetrical loading profile after development, to a very consistent, almost flat load profile across the heading, just after the onset of the longwall abutment, is interesting to note. This nonsymmetrical loading development is further illustrated in Figs. 13(a) and (b). In Fig. 13(a) it is clear that the six bolts have loaded up quite differently, even before any mining had taken
place, and the bolts were still within 2 m of the face. In particular, bolts 1 and 5 developed quite significant loads (approx. 140 and 200 kN), whilst the other bolts remained below 80 kN. After development and the timedependent ‘static’ loading stage, Fig. 13(b) indicates that bolts 1, 5 and 6 were all loaded to approximately 200 kN, whilst the other three bolts had only increased to approximately 80–100 kN. It is difficult to interpret these differing behaviours, other than to compare them with the extensometer data across the heading. As was noted from Fig. 6, extensometer SE1 displayed significant differential deformation in the upper section of the bolted horizon, whilst SE4 and 5 (closest to bolts
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Day 277 Day 271 Day 224 Day 141 Day 36
7 .1 6 .6 6 .1 5 .6 5 .1 4 .6 4 .1
Distance from roof surface
3 .7 3 .3 3 .0 2 .7 2 .4 2 .1 1 .8 1 .5 1 .3 0 .9 0 .6 0 .3
7.1
7.1
6.6
6.6
6.6
6.1
6.1
6.1
6.1
6.1
6.1
5.6
5.6
5.6
5.6
5.6
5.6
5.1
5.1
5.1
5.1
5.1
5.1
4.6 4.1 3.7 3.3 3.0 2.7 2.4 2.1 1.8 1.5
4.6 4.1 3.7 3.3 3.0 2.7 2.4 2.1 1.8 1.5
1.3
1.3
0.9
0.9
0.6
0.6
0.3
0.3 0
1
2
0
10
20
Strain (mm/mm)
Strain (mm/mm)
Day 5
Day 36
4.6 4.1 3.7 3.3 3.0 2.7 2.4 2.1 1.8 1.5
4.6 4.1 3.7 3.3 3.0 2.7 2.4 2.1 1.8 1.5
Distance from roof surface (m)
7.1
6.6
Distance from roof surface (m)
7.1
6.6
Distance from roof surface (m)
7.1
6.6
Distance from roof surface (m)
7.1
Distance from roof surface (m)
Distance from roof surface (m)
Fig. 9. Strain between adjacent anchors along extensometer (SE1).
4.6 4.1 3.7 3.3 3.0 2.7 2.4 2.1 1.8 1.5
4.6 4.1 3.7 3.3 3.0 2.7 2.4 2.1 1.8 1.5
1.3
1.3
1.3
1.3
0.9
0.9
0.9
0.9
0.6
0.6
0.6
0.6
0.3
0.3
0.3
0
5 10 15 20 25
0
10
20
0.3 0
10
20
30
0
20
40
Strain (mm/mm)
Strain (mm/mm)
Strain (mm/mm)
Strain (mm/mm)
Day 62
Day 77
Day248
Day 277.7
Fig. 10. Strain distribution within roof strata relative to time (SE1).
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Ext2
Ext1 900
Ext4
Ext3
600
800
850
800
Ext5 550
8(m) 20 7
6
20
20
20
19
19
19
19
19
18
18
18
18
18
17
17
17
17
17
16
16
16
10.6mm
6.7mm
6.9mm
16
15
15
5
14
3.4mm
15
8.9mm
15.8mm
15
14
14
14
16.5mm
13.7mm
13
13
4
15
13.6mm
9.4mm
16
14 13
13
13
17mm
3
12
11 10 9
11 10
11
10 9 8
8
11
11
10 9
8.1mm
9mm
6.4mm
4.1mm
6 10.2mm 5
7 6 5
9.1mm
1.5mm
3
2
1mm
2
1
3.4mm
7
7
6
6
6
5
5
5
4 2mm 04mm
1
7.5mm
8
11.8mm
4
4 3
10 9
9 8 7
15.2mm 9.8mm
1
12
12
8 7
2
12
12
3
2.4mm
4 3
2
6.7mm
2
4 3.8mm 2.5mm
1
3
3.7mm
2
2mm
1
Roof surface Fig. 11. Bed separation/horizontal fracturing within roof strata (day 277).
3 00
5. Conclusions
after development
Axial load (kN)
2 50
after longwall extaction commenced
2 00
Longwall Face
1 50
1 00
50
0 Bolt 1
Bolt 2
Bolt 3
Bolt 4
Bolt 5
Bolt 6
Roof surface of heading Fig. 12. Axial load carried by bolts across the heading (after Ref. [3]).
This field investigation provided an invaluable, quantitative dataset of information to assist in the research investigations into reinforcement of weak, soft coal mine roof strata. The comprehensive nature of the geomechanical monitoring program undertaken at the site at Angus Place Colliery was unusual, if not unique in providing the breadth of information about strata deformation and reinforcement response under both development and longwall abutment loading conditions. In terms of specific conclusions of relevance to the Angus Place site, the following is a summary of the findings: *
5 and 6) displayed a lack of confinement and a significant degree of differential deformation from the start of development mining, consistent with higher bolt loading.
The immediate roof geology consisted of moderate to strong mudstones, sandstones and claystone bands, together with various relatively weak and structured coal plies—under dry conditions. Both the mudstones and the claystones suffered severe loss of strength when exposed to moisture, resulting in an immediate
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*
180
Axial load (kN)
160 140 120 100 80
Bolt1
60
Bolt2 Bolt3
40
Bolt4
20
Bolt5 Bolt6
0 0
1
3
2
4
5
6
Time (day)
(a) 250
*
157
In circumstances where the upper strata deformation required reduction, the data suggests that reinforcement of the upper horizons by longer tendon support (6 m or greater) must be installed at the time of initial development, not as a later secondary support, if it is to be most effective. It may be that a thicker immediate roof beam created by longer primary bolts might be sufficient to prevent the upper strata deformation, without these longer tendons. There is a suggestion in the data that there might be a case for a non-symmetrical reinforcement pattern in the primary bolting density, although this would need further validation before adoption.
Axial load (kN)
200 150
Acknowledgements Bolt1
100
Bolt2 Bolt3
50
Bolt4 Bolt5 Bolt6
0 0
(b)
50
100
150
200
250
Time (day)
Fig. 13. (a,b) Time-dependent development of maximum axial bolt loads.
*
*
*
roof strata of deteriorating strength, and a high density of structure. The roof strata under development loading exhibited immediate deformation across the heading width, and almost immediate significant deformation of the first 6 m of roof, with lesser movement up to 12 m. This indicated that the bolted immediate roof beam was not sufficiently stiff to resist upper strata delamination. The coal/claystones in the 1–2.5 m region of roof, and the mudstone between 4 and 5.5 m were shown to be the major sources of deformation and dilation. The results indicated that some benefit could be derived from longer primary bolting, beyond the current 2.1 m horizon, possibly to the 2.5 m horizon.
The authors wish to acknowledge the following organisations for support of this research project during the period 1996–2000: Australian Research Council— SPIRT Program; Powercoal Pty Ltd. and Springvale Colliery Pty Ltd. In particular, the assistance provided by personnel at Angus Place Colliery, especially Messrs Peter Doyle and Lyndon Bryant, is gratefully acknowledged. The views expressed in this paper are those of the authors, and not necessarily those of any of the above supporting organisations, or of The University of New South Wales.
References [1] Doyle P. Angus Place Colliery Case Study—Longwall production versus roadway support (unpublished), 1998. [2] Shepherd J, Temby P. Evaluation of the tectonic and stratigraphic controls on mining conditions in the Lithgow Seam at Angus Place and Springvale Collieries, Western Coalfield, Sydney Basin, UNSW Mining Research Centre Research Report RR1/97, 1997. [3] Lu T, Hebblewhite B. Field monitoring of rock bolting performance in weak roof strata, 17th International Conference on Ground Control in Mining, WVU, USA, 1998, 243–8.
International Journal of Rock Mechanics & Mining Sciences 41 (2004) 147–157
Geomechanical behaviour of laminated, weak coal mine roof strata and the implications for a ground reinforcement strategy B.K. Hebblewhite*, T. Lu School of Mining Engineering, The University of New South Wales, Sydney, Australia
Abstract This paper describes a component of the results from an ARC-SPIRT funded collaborative research project between UNSW, Powercoal Pty Ltd. and Springvale Coal Pty Ltd. The research project was aimed at identifying strategies for appropriate ground control in particularly weak and soft roof strata in underground coal mining. A field investigation was undertaken at Angus Place Colliery in the Western Coalfield of New South Wales. The investigation incorporated a range of geotechnical instrumentation and was conducted over a period of time from the development face until the site was lost into the goaf of a retreating longwall panel. This paper describes the outcomes from the field investigation, together with a selection of supporting laboratory studies. The paper also presents a number of alternative presentation modes for extensometry data. The results clearly demonstrate the time and faceproximity related influences on roof integrity, and particularly demonstrated the distribution of deformation, bed separation and strata failure into the roof and across the full span of the roadway, together with reflection of this behaviour within the fully encapsulated roof bolt reinforcement system installed. r 2003 Elsevier Ltd. All rights reserved.
1. Introduction
2. Background
The principal objective of this research investigation was to develop an improved understanding of the deformational behaviour of laminated, weak roof strata in underground coal mines, under both first workings and abutment loading conditions, and to then apply that understanding to the development of appropriate reinforcement strategies. The ability to achieve roof reinforcement effectiveness and roadway stability under these particularly extreme geological conditions has continually challenged mining operations at a number of collieries operating in the Lithgow Seam of the Western Coalfield of New South Wales, Australia. This problem is of particular importance in longwall mines where gateroad stability is critical to the efficient operation of the longwall face. Angus Place Colliery, located near Lithgow, NSW, was chosen as an appropriate site for this field investigation.
Angus Place Colliery is located in the southern section of the Western Coalfield of New South Wales, approximately 110 km due west of Sydney, Australia (see Fig. 1). At this colliery, longwall mining has been the primary source of production since 1979. Mining conditions have progressively deteriorated as more severe geological/geotechnical conditions have been encountered. It is claimed by some that Angus Place experiences arguably the most difficult coal mining strata conditions in Australia [1]. The mine roof changes from a combined Lithgow– Lidsdale Seam section (convergence zone) in the north and to the east, through a seam split zone, where the seams diverge. Combinations of three joint sets, namely NW–SE, NE–SW and N–S, can affect the coal roof. If two or more sets occur at higher frequencies, then the roof is effectively composed of large numbers of blocks [2]. Coupled with the structural discontinuities caused by jointing in the roof, the laminated stratigraphic nature of the strata—composed of varying layers of coal, mudstone, sandstone and claystone—further contributes to a roof that exhibits both weak and relatively soft geomechanical characteristics.
*Corresponding author. Tel.: +61-2-9385-5160; fax: +61-2-93137269. E-mail address: [email protected] (B.K. Hebblewhite). 1365-1609/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmms.2003.08.003
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Fig. 1. Western Coalfield of New South Wales, Australia.
3. Mechanical properties of roof rocks In order to understand the mechanism of deformational behaviour in this type of roof material, it was important to have an appreciation of the typical mechanical properties of the various intact rock components of the strata elements and their contribution to the roof rock mass as a whole. Thus, a program of laboratory tests was designed
for this purpose. Fig. 2 shows the proportion of different rock types within the immediate roof strata, and indicates that coal, mudstone and sandstone were the major components of the immediate 5 m of roof. Standard geomechanical testing of this material included: modulus of elasticity (E), uniaxial compressive strength (UCS), indirect tensile strength (TS), determination of cohesion and angle of internal friction, together with shear strength
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Table 1 Mechanical parameters of coal, mudstone and sandstone
50
Rock type
UCS (MPa)
E (GPa)
C
Cr
f
fr
TS (MPa)
Coal Mudstone Sandstone
43 82 49
3 6 6
4 4.5 5
0.2 0.2 0.01
22 38 46
12 26 28
3 5 5.2
61
Average percentage of rock type (%)
149
40 30 18
20
14
10
Mudstone
Sandstone
Coal
Claystone
2
2
Siltstone
Other
Rock types
Fig. 2. Immediate roof rock type distribution.
and triaxial compressive strength under different levels of confinement. These tests were conducted on the dominant three rock types—mudstone, coal and sandstone. The claystone only formed a small proportion of the roof, however, it was also suspected to play a significant part in the deformational behaviour of the roof material underground. In an undisturbed state, the material is strong and stiff, however on exposure to air or water it rapidly weathers to a quite incompetent material. Unfortunately, the combination of this behaviour on recovered samples, plus the thin cross-sections of the material recovered prevented any claystone testing in the laboratory. Tables 1 and 2 summarise the test results obtained for the coal, mudstone and sandstone. Fig. 3 illustrates the differences in shear strength among samples of coal, mudstone and sandstone under increasing normal stresses. This indicates a far more significant strength increase with confinement for the mudstone and sandstone, than for the coal. In addition, tests were conducted to investigate the sensitivity of mudstone to water, so as to gain an indication of the effect of water on the mechanical properties of the major roof component and determine the deterioration mechanism of roof rock masses. The testing results indicated that when water content reached a level of saturation, the maximum uniaxial compressive strength of mudstone reduced to 22 MPa. This reduction rate is up to 74%, compared to the maximum value of 82 MPa for the dry sample (0% moisture). On the basis of these results, the intact roof rocks could be classified as medium to strong, in a dry condition. However, when the water content and discontinuities are taken into consideration, the roof strata must be classified as weak. It should also be noted that the pre-mining virgin stress conditions at Angus Place have been measured on a number of occasions and indicate horizontal to vertical stress ratios of up to 3:1—typical of the Sydney Basin.
Table 2 Water content in natural condition Rock type
Water content (%)
Coal Mudstone Sandstone
3.5 4 3
Shear strength (MPa)
3 0
18 16 14 12 10 8 6 4 2 0
Sandstone Mudstone 4
Coal 7
9
11
12
Normal Stress (MPa) Fig. 3. Shear strength with various levels of normal stresses and rock types.
4. Mine site investigation The major objective of this investigation was to develop an understanding of the detailed roof deformational behaviour and the performance of the installed rock bolting reinforcement system used in the longwall gateroad. The geotechnical monitoring instrumentation consisted of multiple wire and sonic extensometers, plus strain gauge instrumented bolts. Fig. 4 illustrates the monitoring configuration in heading 2, maingate 22, Angus Place Colliery. (The chain pillar is located on the left of this diagram (looking inbye), while the future longwall block is on the right.) This instrumentation pattern was all installed on development, within 2 m of the roadway face, in order to obtain the maximum component of the developmentrelated deformation, as soon as the face advanced away from the site. A key feature of the instrumentation layout—apart from the considerable mine logistical organisation and assistance provided to enable installation of such a large array of instruments at the
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This replicated the normal roof bolt support pattern in the mine, forming a single row in the normal support system.
WE
SE1
SE2
SE3
SE4
SE5
4.1. Roof deformation at different horizons SG1 300
600
750
SG4
900
600
SG5 600
1250
2500
1150
SG7
SG3
SG2
SG6
700
2000 800
850
800
1300 550
400 300
SE6
SG8 SE7
(Looking inbye) 1500
1550
4500
Fig. 4. Detailed instrument layout in heading 2, maingate 22 (vertical section view, all dimensions in metres) (WE–wire extensometer, SE–sonic extensometer, SG–strain gauged bolt).
Fig. 5 shows the roof deformation profile recorded at different time intervals after installation, along the 15 m length of the combined sonic/wire extensometer in the centre of the heading. It indicates the height to which rock deformation (roof softening) occurred during different stages of mining activity. (Note: In the diagram legend, the results are presented as either before or after extraction. This refers to the development stage of the inbye mining activity, and the commencement of the retreating longwall extraction, respectively). Key features of these results can be summarised as follows: *
*
The overall height of measurable, mining-induced deformation extends to at least 12 m, or over 4 times mining height. The small amount of movement in the upper strata (above 7 m) occurred immediately after the development face advanced away from the site, and did not increase thereafter. The major deformation zone, or height of softening, extends to approximately 6 m into the roof, roughly twice the mining height. This upper limit to major movement was evident from the time of initial development away from the face, and remained relatively constant throughout both development and extraction stages. 16
Day 6 (bef. extraction) Day 63 (bef. extraction) Day 78 (bef. extraction)
14
Day 174(bef. extraction)
Distance from roof surface (m)
face—was the large number of monitoring points across the full roadway width (or roof span). Unlike routine monitoring conducted at most mines, where instrumentation is usually restricted to the roadway centreline, it was considered important in this instance to gain a complete profile of the roof deformation across the full span of the roof. In this sense, the comprehensive nature of the dataset obtained from this investigation was quite unusual, if not unique, for the Australian coal industry. In order to gain an absolute, stable reference point in the roof, above the height of roof dilation and sag, a wire extensometer was installed in the centre of the heading roof. Previous experience at Angus Place had indicated that the 7.5 m upper limit of the standard sonic extensometers was at times only marginal for acting as a non-displaced reference point. The reference anchor for the wire extensometer was located 15 m above the roof, horizon with three other measurement anchors located at 12, 9.5 and 7.5 m above the roof, respectively. (The weak and deformable nature of the floor strata, combined with mine traffic obstruction and potential damage, prevented the use of any floor monitoring stations.) Based on the monitoring arrangement adopted, the relative deformation within the 7.5–15 m region of roof could be determined. There were also five sonic extensometers installed, each with 20 anchors at up to 500 mm spacing over a 7.5 m length. These were positioned in a single row across the roadway width, with a centrally located station immediately adjacent to the wire extensometer to enable the two datasets to be combined to create an effective 23 anchor, 15 m long extensometer. Apart from extensometers, six strain gauge instrumented bolts, 2.1 m in length, (9 gauges per side at 200 mm spacing), were also installed across the roadway in the same plane as the extensometer instrumentation.
12
Day 225 (aft. extraction) Day 271(aft. extraction)
10
Coal Claystone Mudstone Sandstone
8
6
4
2
0 02
0
40
60
80
Displacement (mm)
Fig. 5. Roof deformation recorded by wire and sonic extensometers at centre of heading.
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*
Relative deformation, or dilation of the roof above this 6 m horizon did not exceed 10 mm. The majority of differential movement (+65 mm), or dilation, has occurred within the immediate coal/ claystone roof (0–2 m), and in the mudstone strata from 4 to 5.5 m.
4.2. Roof deformation associated with different mining activities The instrumentation was monitored regularly over a 278 day period, from the time of initial roadway development away from the face, to the time when the retreating longwall face passed the site, which was then lost in the goaf. Fig. 6 is a typical graph showing the cumulative displacement of roof strata measured at each anchor horizon within the sonic extensometers, relative to time. This plot is from SE3, located in the centre of the roadway. Each line represents the cumulative deformation of the roof surface, relative to the respective anchor horizons. The results clearly illustrate five distinct phases of deformation that can be related to both a mining-induced loading environment and a time effect. These five phases, and their characteristics for this particular extensometer dataset, are: 1. Static conditions with minimal deformation, prior to initial development (0–15 days)—less than 5 mm total dilation over full extensometer length while face remained within 2 m of instrumentation site. 2. A primary region of mining-induced effect of development drivage away from the site between 15 and 20 days. This corresponded to a change in face proximity from 2 to 37 m. Total roof dilation reached approximately 15 mm. 3. A secondary development proximity effect between 20 and 60 days, (37–147 m face proximity), during
which maximum roof dilation increased to in excess of 50 mm. Beyond this point all results suggest that mining-induced effects from the development drivage have ceased. 4. A period of ongoing, time-dependent deformation, independent of mining effects, from 60 to 250+days. During this period, there is a reasonably ‘steady-state’ deformation trend, with the total reaching approximately 65 mm. This represents a deformation rate over this period, of approximately 0.08 mm/day. 5. A further mining-induced effect due to the approach of the longwall face, from 250 days until the endpoint of 278 days, during which the total deformation increased from 65 mm to 91 mm. The longwall face was 35 m away at the 250 day stage, indicating the maximum distance at which any significant ‘front abutment effect’ was detected. 4.3. Roof deformation distribution across roadway The cumulative deformation profile from all five sonic extensometers across the roadway is shown in Fig. 7. For each extensometer, the cumulative deformations for each individual anchor, relative to the reference point at the back of the hole are plotted, at six different time intervals (during the development, static and longwall retreating periods). Significant observations from these results are as follows: *
*
In all cases the major deformation is restricted to below the 6 m horizon (base of the sandstone, top of mudstone), with only very minor levels above. In the case of SE5 (closest to the longwall block) the major deformation appears confined to below the 4 m roof horizon (base of the mudstone). At the top of the bolted horizon (2.1 m) and just above, there appears to be a significant deformation
100 Anchor position:0.5m 90 Anchor position:0.8m 80 Anchor position:1.7m
Displacement (mm)
70
Anchor position:2.3m
60
Anchor position:2.9m
50
Anchor position:3.2m
40
Anchor position:4.0m
30
Anchor position:4.4m Anchor position:4.9m
20
Anchor position:5.4m
10
Anchor position:5.9m 0 0
50
100
150
151
200
250
300
Anchor position:7.3m
-10
Time (day) Fig. 6. Cumulative displacement with time recorded by extensometer SE3 (centre) (arrows indicate five loading stages).
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Day43 (before longwalling) Day 99 (before longwalling) Day 224 (before longwalling) Day 248 (after longwalling) Day 262 (after longwalling) Day 277.7 (after longwalling)
Distance from roof surface (m)
7
6
5
4
3
2
7
0
4
3
2
0 20
40
60
80
Day 44 (before longwalling) Day 100 (before longwalling) Day 262 (before longwalling Day 249 (after longwalling) Day 263 (after longwalling) Day 278 (after longwalling)
7 6
40
20
(b)
Displacement (mm) 8
0
100
5 4 3 2 1
80
60
100
Displacement (mm)
8
Day 43 (before longwalling) Day 99 (before longwalling) Day 244 (before longwalling) Day 248 (after longwalling) Day 262 (after longwalling) Day 277 (after longwalling)
7
Distance from roof surface (m)
0
(a)
Distance from roof surface (m)
5
1
1
6 5 4 3 2 1
0
0 0
(c)
Day 43 (before longwalling) Day 99 (before longwalling) Day 224 (before longwalling) Day 248 (after longwalling) Day 262 (after longwalling) Day 277 (after longwalling)
6
Distance from roof surface (m)
152
20
40
60
80
100
8
20
40
60
80
Displacement (mm)
Day 43 (before longwalling) Day 99 (before longwalling) Day 224 (before longwalling) Day 248 (after longwalling) Day 262 (after longwalling) Day 277 (after longwalling)
7
Distancefromroof surface (m)
0
(d)
Displacement (mm)
6 5 4 3 2 1 0 0
(e)
20
10
40
30
50
Displacement (mm)
Fig. 7. (a–e) Roof deformation at different horizons (refer to Fig. 4 for instrument numbers).
*
increase, suggesting that the bolted roof beam is pulling away from the overlying strata, creating major bed separation immediately above the bolts and also within the upper sections of the bolted horizon. This is most evident in SE1–SE3, and to a lesser extent in SE5, but not SE4. This behaviour is observed to commence from the initial development stage, and continue throughout the subsequent static and longwall abutment periods. This could indicate a less than optimum bolt length. Within the bolted horizon (especially evident in SE2, and to a lesser extent SE1 and SE3), the bolts appear to be holding the strata together, as a unit, with very
little differential movement between the roof surface and the 2.1 m horizon. In SE2 the relative deformation over the bolted horizon is excellent, at less than 10 mm, particularly prior to the final dataset. SE3 is of a similar magnitude, although the final longwall approach appears to have caused some significant relative deformation or strain within the lower 0.5 m of roof in the final dataset, suggesting loss of immediate roof control. SE1 has shown excellent bolt reinforcement for the lower 1.5 m of bolt length, although the upper section of the horizon is exhibiting high differentials, or strain levels. This may indicate that the bolts are not constraining the rock
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*
*
*
mass significantly, and may actually be pulling out of their anchorage. SE4 and SE5 do not show the same constrained characteristic of bolt reinforcement of the lower roof. They both exhibit a trend of constantly increasing deformation differential throughout the bolted horizon, and above, in the case of SE4. This trend is evident from the time of initial development and results in at least 20 mm differential deformation across the bolted horizon by the time the longwall retreats past the site. This is indicative of a reinforcement system which is at the limit for controlling or constraining the immediate roof beam—possibly in terms of both bolt length and density. SE1–4 all display a further major deformation differential at approximately 4–5 m horizon (within the mudstone). This is most pronounced in SE2–4. Therefore, even if the bolts are constraining the immediate roof sufficiently (more so on the left-hand side), the reinforced roof beam is clearly not stiff enough to constrain the upper strata horizons from softening and incurring bed separation. Again, this trend is evident from the immediate development stage. In other words, the upper roof integrity is being eroded from the time of initial development, even if this may not be evident in the roadway itself. This can only result in reduced stability as subsequent abutment loads are encountered.
Fig. 8 is an alternative presentation of the overall extensometry data, depicting the deformation at a selection of roof horizons—0, 1.86, 3.72, 4.65 and 6.65 m—at the time when the longwall face passed the instrumentation site. This clearly demonstrates the shape and extent of the height of roof softening, characterised by a near-vertical sided arch profile, diminishing only above the 4.65 m horizon, although exhibiting marginal deformation in the centre portion of the roof span at the 6.65 m horizon. This profile also illustrates the non-symmetrical nature of the deformation, with the more dominant deformation on the left (pillar side) of the heading, while on the right, the outer extensometer is not reporting significant deformation above the 3.72 m horizon, compared to 4.65 m on the left. The classic bending nature of the roof beam is also clearly illustrated in this data presentation. 4.4. Analysis of strain levels in roof strata The extensometer data was converted to incremental strain between individual anchor horizons, using original anchor separation distances. The strain results therefore correspond approximately to the slope of the extensometer deformation plots (to the extent that anchor separations were roughly the same throughout). Fig. 9 is
153
an example of the strain results, in this case for SE1, plotted as a vertical stacked set of histograms, grouped for five different time periods (day 277 is the top of each stack of five bars). This illustrates both the strain trend over time at each horizon, as well as the relative magnitudes of strain developed within the strata at the different heights into the roof (and with respect to roof rock type). This set of results provides further evidence that, on the left, or pillar side of the roadway at least, the immediate roof horizon, for at least the first 1 m, is well constrained with negligible strain evident throughout the investigation. However, above that horizon, strain magnitudes and frequency of high strain levels increases markedly, especially within the coal horizons up to 2.4 m and beyond, possibly adjacent to the claystone bands also, and then to a lesser extent into the higher mudstone. Fig. 10 presents the same dataset as a separate series of histograms. This enables easier comparison of the strain distribution relative to time. Apart from highlighting the major region of strain magnitude in the 1–2.5 m region, it also demonstrates the increasing strain activity higher into the roof relative to the initial dataset. The levels of strain developed within the immediate roof strata, as illustrated above, are certainly well in excess of those required to cause tensile failure of intact rock material, or to generate separation along existing bedding planes. The actual condition within the roof is no doubt a combination of both of these effects. Fig. 11 is an alternative analysis of the deformation data across the full roadway width at the final time period (277 days). Rather than processing the deformations as incremental strains distributed across each interval between adjacent anchors, it is a schematic depiction of the deformation as individual crack formations, or bed separations between successive anchors. The correlation of crack continuity between the different extensometers is remarkably consistent. It again highlights the significant development of horizontal fracturing/separation within the main coal roof horizon (1–2.5 m) and the higher mudstone horizon (3.9–6.0 m).
4.5. Instrumented bolt results The results from the strain-gauged bolts installed across the instrumentation site were not as conclusive as the extensometry. Fig. 12 is a composite plot of the maximum axial load recorded on these six bolts, immediately after the development stage of mining, and then again after the longwall retreated back to the site. (Note that the majority of the bolts reported significant unloading within the last 20 m of longwall approach, associated with either bolt/resin/rock bond failure, bolt failure or gauge failure. The first of these possibilities is considered the most likely.)
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Ext1 900
Ext3
Ext2 600
800
Ext4 850
Ext5 800
550
10 mm 20 mm
10 mm 20 mm 30 mm 7.29m 10 mm 20 mm 30 mm 40 mm
6.65m 4.65m
50 mm 3.72m
1.86m
10 mm 20 mm 30 mm 40 mm 50 mm 60 mm 70 mm 10 mm 20 mm 30 mm 40 mm 50 mm 60 mm 70 mm 80 mm 90 mm
Fig. 8. Roof deformation at day 277 (after mining extraction).
From the monitoring data, it was noted that the maximum axial load measured by the six instrumented bolts installed on the roof strata varied from 91 to 227 kN after development and the static loading phase, and from 210 to 256 kN as the longwall approached. The change from a non-symmetrical loading profile after development, to a very consistent, almost flat load profile across the heading, just after the onset of the longwall abutment, is interesting to note. This nonsymmetrical loading development is further illustrated in Figs. 13(a) and (b). In Fig. 13(a) it is clear that the six bolts have loaded up quite differently, even before any mining had taken
place, and the bolts were still within 2 m of the face. In particular, bolts 1 and 5 developed quite significant loads (approx. 140 and 200 kN), whilst the other bolts remained below 80 kN. After development and the timedependent ‘static’ loading stage, Fig. 13(b) indicates that bolts 1, 5 and 6 were all loaded to approximately 200 kN, whilst the other three bolts had only increased to approximately 80–100 kN. It is difficult to interpret these differing behaviours, other than to compare them with the extensometer data across the heading. As was noted from Fig. 6, extensometer SE1 displayed significant differential deformation in the upper section of the bolted horizon, whilst SE4 and 5 (closest to bolts
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Day 277 Day 271 Day 224 Day 141 Day 36
7 .1 6 .6 6 .1 5 .6 5 .1 4 .6 4 .1
Distance from roof surface
3 .7 3 .3 3 .0 2 .7 2 .4 2 .1 1 .8 1 .5 1 .3 0 .9 0 .6 0 .3
7.1
7.1
6.6
6.6
6.6
6.1
6.1
6.1
6.1
6.1
6.1
5.6
5.6
5.6
5.6
5.6
5.6
5.1
5.1
5.1
5.1
5.1
5.1
4.6 4.1 3.7 3.3 3.0 2.7 2.4 2.1 1.8 1.5
4.6 4.1 3.7 3.3 3.0 2.7 2.4 2.1 1.8 1.5
1.3
1.3
0.9
0.9
0.6
0.6
0.3
0.3 0
1
2
0
10
20
Strain (mm/mm)
Strain (mm/mm)
Day 5
Day 36
4.6 4.1 3.7 3.3 3.0 2.7 2.4 2.1 1.8 1.5
4.6 4.1 3.7 3.3 3.0 2.7 2.4 2.1 1.8 1.5
Distance from roof surface (m)
7.1
6.6
Distance from roof surface (m)
7.1
6.6
Distance from roof surface (m)
7.1
6.6
Distance from roof surface (m)
7.1
Distance from roof surface (m)
Distance from roof surface (m)
Fig. 9. Strain between adjacent anchors along extensometer (SE1).
4.6 4.1 3.7 3.3 3.0 2.7 2.4 2.1 1.8 1.5
4.6 4.1 3.7 3.3 3.0 2.7 2.4 2.1 1.8 1.5
1.3
1.3
1.3
1.3
0.9
0.9
0.9
0.9
0.6
0.6
0.6
0.6
0.3
0.3
0.3
0
5 10 15 20 25
0
10
20
0.3 0
10
20
30
0
20
40
Strain (mm/mm)
Strain (mm/mm)
Strain (mm/mm)
Strain (mm/mm)
Day 62
Day 77
Day248
Day 277.7
Fig. 10. Strain distribution within roof strata relative to time (SE1).
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Ext2
Ext1 900
Ext4
Ext3
600
800
850
800
Ext5 550
8(m) 20 7
6
20
20
20
19
19
19
19
19
18
18
18
18
18
17
17
17
17
17
16
16
16
10.6mm
6.7mm
6.9mm
16
15
15
5
14
3.4mm
15
8.9mm
15.8mm
15
14
14
14
16.5mm
13.7mm
13
13
4
15
13.6mm
9.4mm
16
14 13
13
13
17mm
3
12
11 10 9
11 10
11
10 9 8
8
11
11
10 9
8.1mm
9mm
6.4mm
4.1mm
6 10.2mm 5
7 6 5
9.1mm
1.5mm
3
2
1mm
2
1
3.4mm
7
7
6
6
6
5
5
5
4 2mm 04mm
1
7.5mm
8
11.8mm
4
4 3
10 9
9 8 7
15.2mm 9.8mm
1
12
12
8 7
2
12
12
3
2.4mm
4 3
2
6.7mm
2
4 3.8mm 2.5mm
1
3
3.7mm
2
2mm
1
Roof surface Fig. 11. Bed separation/horizontal fracturing within roof strata (day 277).
3 00
5. Conclusions
after development
Axial load (kN)
2 50
after longwall extaction commenced
2 00
Longwall Face
1 50
1 00
50
0 Bolt 1
Bolt 2
Bolt 3
Bolt 4
Bolt 5
Bolt 6
Roof surface of heading Fig. 12. Axial load carried by bolts across the heading (after Ref. [3]).
This field investigation provided an invaluable, quantitative dataset of information to assist in the research investigations into reinforcement of weak, soft coal mine roof strata. The comprehensive nature of the geomechanical monitoring program undertaken at the site at Angus Place Colliery was unusual, if not unique in providing the breadth of information about strata deformation and reinforcement response under both development and longwall abutment loading conditions. In terms of specific conclusions of relevance to the Angus Place site, the following is a summary of the findings: *
5 and 6) displayed a lack of confinement and a significant degree of differential deformation from the start of development mining, consistent with higher bolt loading.
The immediate roof geology consisted of moderate to strong mudstones, sandstones and claystone bands, together with various relatively weak and structured coal plies—under dry conditions. Both the mudstones and the claystones suffered severe loss of strength when exposed to moisture, resulting in an immediate
ARTICLE IN PRESS B.K. Hebblewhite, T. Lu / International Journal of Rock Mechanics & Mining Sciences 41 (2004) 147–157 200
*
180
Axial load (kN)
160 140 120 100 80
Bolt1
60
Bolt2 Bolt3
40
Bolt4
20
Bolt5 Bolt6
0 0
1
3
2
4
5
6
Time (day)
(a) 250
*
157
In circumstances where the upper strata deformation required reduction, the data suggests that reinforcement of the upper horizons by longer tendon support (6 m or greater) must be installed at the time of initial development, not as a later secondary support, if it is to be most effective. It may be that a thicker immediate roof beam created by longer primary bolts might be sufficient to prevent the upper strata deformation, without these longer tendons. There is a suggestion in the data that there might be a case for a non-symmetrical reinforcement pattern in the primary bolting density, although this would need further validation before adoption.
Axial load (kN)
200 150
Acknowledgements Bolt1
100
Bolt2 Bolt3
50
Bolt4 Bolt5 Bolt6
0 0
(b)
50
100
150
200
250
Time (day)
Fig. 13. (a,b) Time-dependent development of maximum axial bolt loads.
*
*
*
roof strata of deteriorating strength, and a high density of structure. The roof strata under development loading exhibited immediate deformation across the heading width, and almost immediate significant deformation of the first 6 m of roof, with lesser movement up to 12 m. This indicated that the bolted immediate roof beam was not sufficiently stiff to resist upper strata delamination. The coal/claystones in the 1–2.5 m region of roof, and the mudstone between 4 and 5.5 m were shown to be the major sources of deformation and dilation. The results indicated that some benefit could be derived from longer primary bolting, beyond the current 2.1 m horizon, possibly to the 2.5 m horizon.
The authors wish to acknowledge the following organisations for support of this research project during the period 1996–2000: Australian Research Council— SPIRT Program; Powercoal Pty Ltd. and Springvale Colliery Pty Ltd. In particular, the assistance provided by personnel at Angus Place Colliery, especially Messrs Peter Doyle and Lyndon Bryant, is gratefully acknowledged. The views expressed in this paper are those of the authors, and not necessarily those of any of the above supporting organisations, or of The University of New South Wales.
References [1] Doyle P. Angus Place Colliery Case Study—Longwall production versus roadway support (unpublished), 1998. [2] Shepherd J, Temby P. Evaluation of the tectonic and stratigraphic controls on mining conditions in the Lithgow Seam at Angus Place and Springvale Collieries, Western Coalfield, Sydney Basin, UNSW Mining Research Centre Research Report RR1/97, 1997. [3] Lu T, Hebblewhite B. Field monitoring of rock bolting performance in weak roof strata, 17th International Conference on Ground Control in Mining, WVU, USA, 1998, 243–8.