subsidence assessment for longwall mining in the hebden and … · 2017-12-01 · of our assessment...

E

ANNEX

Subsidence Assessment for

Longwall Mining in the

Hebden and Barrett Seams

(SCT Operations Pty Ltd, 2009)

R E P O R T T O : ENVIRONMENTAL RESOURCES MANAGEMENT Part 3A Subsidence Assessment for Mining in Hebden, Barrett and Middle Liddell Seams at Integra Underground Mine GLEN3253

REPORT TO Mike Shelley ERM Australia PO Box 71 THORNTON NSW 2322

SUBJECT Part 3A Subsidence Assessment For Hebden, Barrett and Middle Liddell Mining at Integra Underground REPORT NO GLEN3253 PREPARED BY Ken Mills DATE 11 March 2009

Ken Mills Senior Geotechnical Engineer

Winton J. Gale Managing Director

REPORT: PART 3A SUBSIDENCE ASSESSMENT FOR MINING IN HEBDEN, BARRETT AND MIDDLE LIDDELL

SEAMS AT INTEGRA UNDERGROUND MINE

SCT Operations Pty Ltd - GLEN 3253 - 11 March 2009 Page i

SUMMARY Integra Coal Operations Pty Ltd (ICO) is proposing to mine longwall panels and undertake cut and flit operations in the Hebden and Barrett Seams and cut and flit operations in the Middle Liddell Seam as part of their ongoing operations at Integra Underground Mine. ICO commissioned SCT Operations Pty Ltd to make a subsidence assessment suitable for submission as part of a Part 3A Environmental Assessment being prepared by Environmental Resources Management Australia Pty Ltd. This report presents the results of our assessment of the impacts of mining subsidence on the surface area for the proposed mining geometries. Proposed longwall mining in the Hebden Seam is located below and beyond the area of current longwall panels in the Middle Liddell Seam. There is expected to be interaction between the longwall panels in the two seams in areas of overlap. This interaction will cause subsidence additional to that which has already occurred. Proposed longwall mining in the Barrett Seam east of Glennies Creek is expected to cause single seam subsidence typical of mining geometries that are of supercritical width in subsidence engineering terms. Cut and flit mining operations in the Middle Liddell Seam, Hebden Seam and Barrett Seam are not expected to cause any significant subsidence. The actual subsidence will depend on the panel geometries and overburden depths at each site but subsidence in these areas is expected to be low level and imperceptible for all practical purposes. Subsidence monitoring results from the Middle Liddell Seam longwall panels provide a basis for estimating the subsidence in the Barrett Seam Mining Area and in those parts of the Hebden Seam Mining Area outside of the footprint of the Middle Liddell Seam longwalls. There is currently limited empirical experience on which to base estimates of multi-seam subsidence. A conservative approach to estimating subsidence has been adopted until monitoring information becomes available to refine these estimates. Maximum subsidence in the areas of multi-seam longwall operations has been estimated as 80% of the total extracted thickness of both seams equivalent to 3.5-4m of total subsidence. The actual subsidence is expected to be somewhat less than the 3.5-4m maximum that is predicted at this stage. Horizontal strains and tilts in the Barrett Seam Mining Area are estimated using subsidence experience from the Western Coalfield where the overburden depths and overburden caving characteristics are similar to those in the Barrett Seam Mining Area. Horizontal strains and tilts in the Hebden Seam Mining Area are estimated using subsidence experience from Integra Underground Mine and from the Southern Coalfield where the overburden depths are typically in the 400-500m range. A conservative approach has been adopted for estimating strains and tilts.



The predicted subsidence for the various mining geometries is summarised as follows:

Mining Area

Subsidence from mining

Middle Liddell

Seam (m)

Subs from Proposed Mining

(m)

Total Subs from both

Seams (m)

Max Tensile Strain (mm/m)

Max Comp Strain (mm/m)

Max Tilt

(mm/m)

Barrett Seam N/A 1.5-2.0 N/A 16-27 21-36 50-90

Hebden (No ML Seam workings) 0 1.3-1.6 1.3-1.6 1.7-3.9 2.3-5.2 6.8-15

Hebden (Above Mains (LWs1-6) <0.02 2.1-2.4 2.1-2.4 2.8-5.8 3.7-7.7 11.-23

Hebden Above ML Mains (LWs7-17)

<0.02 1.7-2.0 1.7-2.0 2.2-4.8 3-6.5 9.0-19

Hebden Above ML LWs 1-5 0.7-1.0 2.6-3.4 3.5-4.0 4.6-9.7 6.1-13 18-39

Hebden Above ML LWs 6-17 1-1.5* 2.5-3.4 3.5-4.0 4.6-9.7 6.1-13 18-39

* Subsidence from mining in the Middle Liddell Seam is currently incomplete and will continue to occur over Longwalls 6-17 as a result of ongoing mining.

Mining subsidence impacts in the Barrett Seam Mining Area are mainly limited to the roads, Glennies Creek Community Hall, and three residences located over Longwall B7. Mining subsidence impacts in the Hebden Seam Mining Area include impacts to Bettys Creek and the diversion channel on Bettys Creek across the top of the Eastern Rail Pit, Main Creek, Glennies Creek Road, a bridge over Main Creek, a residence west of Glennies Creek Road, the Mt Owen Rail Line and associated infrastructure, Forest Rd, various agricultural infrastructure including fences, farm dams, farm access roads. Mining impacts on the Main Northern Rail Line, several other residences in the Hebden Seam Mining Area including a disused dwelling (formerly a church) are expected to imperceptible. There is not expected to be any perceptible subsidence impacts in any of the cut and flit mining areas.

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TABLE OF CONTENTS

PAGE NO SUMMARY .............................................................................................. I TABLE OF CONTENTS ................................................................................. III 1. INTRODUCTION ................................................................................... 1 2. SITE DESCRIPTION............................................................................... 1

2.1 Surface Features..................................................................... 32.1.1 General Description ...................................................... 32.1.2 Surface Infrastructure.................................................. 4

2.2 Mining Geometry...................................................................... 4 3. DESCRIPTION OF NATURAL FEATURES AND SURFACE IMPROVEMENTS............... 6

3.1 Natural Features ..................................................................... 63.2 Public Infrastructure .............................................................. 11

3.2.1 Roads........................................................................ 113.2.2 Glennies Creek Community Hall .................................... 133.2.3 Main Northern Rail Line............................................... 133.2.4 Power Lines ............................................................... 143.2.5 Telstra Cables ............................................................ 143.2.6 Survey Trig Station..................................................... 14

3.3 Privately Owned Infrastructure ............................................... 153.3.1 Ravensworth East and Glendell Open Cut Mines ............ 153.3.2 Mt Owen Rail Line....................................................... 163.3.3 Concrete Bridges over Bettys Creek ............................ 163.3.4 Buried Rail Communication Cables ................................ 193.3.5 Rail Maintenance Road ................................................ 193.3.6 Buried Water Supply Pipeline ....................................... 193.3.7 Residential Buildings ................................................... 193.3.8 Disused Building ......................................................... 193.3.9 Fences and Agricultural Infrastructure......................... 233.3.10 Envirogen Electricity Generation Facility ....................... 23

3.4 Integra Owned Infrastructure ................................................. 23 4. PREDICTION METHODOLOGIES AND DISCUSSION OF ASSUMPTIONS ................ 28 5. REVIEW OF THE RESULTS OF PREVIOUS SUBSIDENCE MONITORING ................ 29

5.1 Subsidence Characteristics Measured at Integra Underground Mine ..................................................................................... 295.2 Goaf Edge Subsidence Profiles ................................................ 325.3 Constants for Prediction ........................................................ 33

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6. SUBSIDENCE PREDICTIONS.................................................................. 346.1 Barrett Seam Mining Area...................................................... 34

6.1.1 Vertical Subsidence .................................................... 346.1.2 Strains...................................................................... 346.1.3 Tilts .......................................................................... 35

6.2 Hebden Seam Mining Area ...................................................... 356.2.1 Hebden Seam Mining Only ........................................... 356.2.2 Hebden Longwalls Below Main Headings and Cut and Flit Mining in Middle Liddell Seam ................................. 366.2.3 Hebden Longwalls Below Middle Liddell Seam Longwall Areas ........................................................................ 366.2.4 Hebden Seam Cut and Flit Mining Operations................ 376.2.5 Subsidence Estimates ................................................ 37

6.3 Distribution of Subsidence ...................................................... 386.4 Extent of Subsidence Impacts................................................. 38

7. ASSESSMENT OF SUBSIDENCE IMPACTS ................................................. 41

7.1 Natural Features ................................................................... 417.2 Public Infrastructure .............................................................. 41

7.2.1 Roads........................................................................ 427.2.2 Glennies Creek Community Hall .................................... 437.2.3 Main Northern Rail Line............................................... 437.2.4 Power Lines ............................................................... 447.2.5 Telstra Cables ............................................................ 447.2.6 Survey Trig Station..................................................... 44

7.3 Privately Owned Infrastructure ............................................... 447.3.1 Mt Owen, Ravensworth East and Glendell Open Cut Mines ........................................................................ 447.3.2 Mt Owen Rail Line....................................................... 467.3.3 Concrete Bridges over Bettys Creek ............................ 477.3.4 Buried Rail Communication Cables ................................ 477.3.5 Rail Maintenance Road ................................................ 487.3.6 Buried Water Supply Pipeline ....................................... 487.3.7 Residential Buildings ................................................... 487.3.8 Disused Building ......................................................... 497.3.9 Fences and Agricultural Infrastructure......................... 507.3.10 Envirogen Electricity Generation Facility ....................... 50

7.4 Integra Owned Infrastructure ................................................. 51 8. RECOMMENDATIONS FOR FUTURE MONITORING......................................... 51

8.1 Barrett Seam Mining Area...................................................... 518.2 Hebden Seam Mining Area ...................................................... 51

9. CONCLUSIONS ................................................................................. 53 10. REFERENCES ................................................................................... 54

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SCT Operations Pty Ltd - GLEN 3253 - 11 March 2009 Page 1

1. INTRODUCTION Integra Coal Operations Pty Ltd (ICO) is proposing to mine longwall panels in the Hebden and Barrett Seams, and undertake cut and flit mining operations in the Middle Liddell, Hebden and Barrett Seams as part of their ongoing mining operations at Integra Underground Mine. ICO commissioned SCT Operations Pty Ltd to make a subsidence assessment suitable for submission as part of an Environmental Assessment being prepared by Environmental Resources Management Australia Pty Ltd (ERM). This report presents the results of our assessment of the impacts of mining subsidence on the surface area for the proposed mining geometries. The report is structured to provide the following.

1. A description of the general area including the proposed mining geometry, overburden depth and other parameters of relevance to a subsidence assessment.

2. Specific detail of the features, both natural and man-made that have been identified within the general area as likely to be impacted by mining subsidence.

3. A review of the results of previous subsidence monitoring data at Integra Underground and consideration of multi-seam mining experience.

4. Subsidence estimates based on the previous subsidence monitoring at Integra Underground and elsewhere and an assessment of the likely accuracy of these estimates.

5. Specific assessments of the subsidence impacts on the surface features identified.

6. Recommendations for subsidence monitoring programs and strategies to manage the subsidence impacts identified.

2. SITE DESCRIPTION Figure 1 shows a plan of the proposed longwall and cut and flit mining areas in the Middle Liddell Seam, Hebden Seam and the Barrett Seams. These mining areas are superimposed onto a 1:25,000 topographic series map of the area for general reference purposes. The plan has been updated to show the Mt Owen Rail Line, but details of other changes associated with adjacent mining activity are shown in Figure 3. Figure 2 shows a stratigraphic section that illustrates the relative locations of the Middle Liddell Seam, Barrett Seam and Hebden Seam in the centre of the Hebden Seam Mining Area based on DDH223, the location of which is shown in Figure 1. The Hebden Seam is approximately 60m below the floor of the Middle Liddell Seam at this location. However, the three seams split and coalesce across the lease, so the apparent thickness of the combined seam section can vary significantly from place to place.

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The Hebden Seam Mining Area is located entirely on the north-western side of Glennies Creek. The Barrett Seam Mining Area is located on the south-eastern side of Glennies Creek except for a small area where cut and flit mining is proposed below and west of Glennies Creek. The Hebden and Barrett Seam Mining Areas do not overlap. 2.1 Surface Features Figure 3 shows a plan of the surface area with the major surface infrastructure and mine layout superimposed. Figure 4 shows two panoramas of the general area. The first panorama is looking west across the area above the proposed Hebden Seam Mining Area. The second panorama is looking east across the area above the proposed Barrett Seam Mining Area. The photograph locations are shown in Figure 3. 2.1.1 General Description Most of the surface area above the proposed mining areas comprises open agricultural grazing land with areas of scattered trees and patches of bush located mainly along stream channels. There are open cut mining operations to the northwest, west, south and east of the proposed mining areas but these are mainly outside the proposed mining areas. The surface on the western side of Glennies Creek is predominantly owned by Xstrata through its various subsidiary companies (including Glendell Tenements Pty Ltd, Savage Minerals Ltd & Enex Foydel Ltd, Enex Ravensworth Pty Ltd and Xstrata Mount Owen), but there are also significant areas owned by private landholders, including numerous small landholdings between Glennies Creek Rd and Glennies Creek above the Barrett Seam cut and flit mining area. The surface area on the eastern side of Glennies Creek includes some agricultural grazing land in private ownership west of Middle Falbrook Road and north of Stony Creek Road, with the remaining land owned by ICO and associated companies. In the Hebden Seam Mining Area, the natural surface topography is gently undulating with extensive flat areas adjacent to Glennies Creek, Main Creek and Bettys Creek. The reduced levels range from about RL70m in the southern corner of Longwall H1 to a high point of RL134m over Longwalls H9 and H10. There are numerous ephemeral watercourses draining from this high point and the ridge located north east of the high point. These watercourses flow into Bettys Creek in the west and Main Creek in the east. There are numerous small farm dams located along and adjacent to the ephemeral watercourses in the area. In the Barrett Seam mining area east of Glennies Creek, the surface topography ranges from RL75m adjacent to the Glennies Creek flood plain through to a high point at RL115m above Longwalls B6 and B7. All the ephemeral stream channels flow toward Glennies Creek.





2.1.2 Surface Infrastructure The main surface improvements within the proposed longwall mining areas include Glennies Creek Road, Middle Falbrook Road, Stony Creek Road, Forest Road, five residential buildings and various other buildings including farm buildings, a disused dwelling (formerly a church), and a community hall, electricity lines, buried Telstra cables, a gas-fired electricity generation plant, rail infrastructure, Glendell Mine haul road, Bettys Creek diversion channel, and Integra Coal mine infrastructure. There are also several minor roads, fences and farm dams. Of the five occupied residences located within the general footprint of the longwall panels, three are located over Longwall B7 and the other two are located adjacent to Longwall H1. Numerous other residences are located over the cut and flit mining areas. Rail infrastructure in the area includes the Main Northern Rail Line located adjacent to the southern edge of the Hebden Seam Mining Area and the Mt Owen Rail Line. 2.2 Mining Geometry Three types of mining geometry are used at Integra Underground Mine: mine access roadways, cut and flit mining operations, and longwall mining operations. Mine access roadways comprise main heading developments and gateroad developments also referred to as first workings. First workings comprise headings or roadways (approximately 5.2 m wide) and interconnecting cut-throughs. First workings establish access to the coal resource area, allow movement of personnel and material within the mine, and supply the mine with ventilation air. Cut and flit mining operations are a variation of first workings comprising a similar geometry but with the difference that they are intended primarily to mine about 30% of the coal in plan area by forming a regular grid of roadways with large pillars in between. The intent of this geometry is to form pillars that are long term stable and cause low levels of surface subsidence. A value of 20mm is usually adopted as being a practical cut-off for subsidence movements because movements of this magnitude over large areas are of no practical significance and the magnitude of seasonal variations in ground level that occur naturally is commonly greater than 20mm. Longwall mining involves the total extraction of the target coal seam over panels that are individually some 250m wide and typically several kilometres long. Surface subsidence above longwall panels usually develops incrementally in response to the mining geometry and occurs mainly within the footprint of the longwall mining area. The low levels of subsidence



movement that occur around the periphery of longwall mining areas are generally imperceptible except for very sensitive structures. Current mining activity at Integra Underground is undertaken entirely within the Middle Liddell Seam. The seam section mined is nominally 2.4m. Longwalls 1-8 in the Middle Liddell Seam have already been mined and up to seven more panels in the Middle Liddell Seam are approved and planned to be mined before mining commences in the Hebden Seam. A trial cut and flit mining area has already been approved immediately to the south of the main headings (ERM 2008) but a larger area the cut and flit mining area is planned further south and west of this trial area as shown in Figure 3. In the Hebden Seam Mining Area, fifteen longwalls are proposed with cut and flit mining around the southern edges. The longwall panels are laid out so that gateroads (longwall panel access roadways) in the Hebden Seam are located below the Middle Liddell longwall panels. Hebden longwall panels H1 to H4 are narrower than the subsequent panels in order to fit the gateroads under the first five narrow longwall panels in the Middle Liddell Seam. The subsequent Hebden longwall panels have a total void that is nominally 255m wide. The seam section to be mined in the Hebden Seam Mining Area is likely to vary as seams split or coalesce. For assessment purposes, the seam section has been assumed to be 2.6m. In the Barrett Seam Mining Area, seven longwalls are proposed below areas that are remote from Glennies Creek and its associated alluvium. Cut and flit mining operations are proposed in the remaining areas to the north, south, and west of the longwall panels. The seam section in the Barrett Seam Mining Area ranges from 2.5-3.5m. For assessment purposes an average seam section of 3.3m has been assumed. Table 1 summarises the mining geometry, overburden depth and seam thickness for each of the proposed longwall panels. Table 1: Summary of Mining Geometry, Overburden Depth and Seam Thickness

Longwall Panel

Overburden Depth (m)

Panel Width (m)

Representative W/D Ratio

Seam Thickness (m)

B1 135-180 220 1.3 2.9

B2 130-180 205 1.3 3.2

B3 120-160 255 1.8 3.3

B4 110-180 255 1.7 3.4

B5 130-190 255 1.5 3.4

B6 120-190 255 1.5 3.1

B7 110-190 255 1.6 3.1

H1 310-370 185 0.5 2.6

H2 310-380 140 0.4 2.6

H3 320-390 140 0.4 2.6





Longwall Panel


Panel Width (m)

Representative W/D Ratio

Seam Thickness (m)

H4 320-400 135 0.4 2.6

H5 320-410 245 0.6 2.6

H6 330-430 255 0.6 2.7

H7 330-450 255 0.6 2.8

H8 350-470 255 0.6 2.7

H9 450-510 255 0.5 2.6

H10 460-500 255 0.5 2.6

H11 470-570 255 0.5 2.6

H12 480-560 255 0.5 2.6

H13 480-560 255 0.5 2.6

H14 490-530 255 0.5 2.6

H15 490-510 255 0.5 2.6 Figure 5 shows isopachs of the overburden depth to the proposed mining horizons (Hebden Seam in the Hebden Mining Area and Barrett Seam in the Barrett Seam Mining Area). The overburden varies mainly as a consequence of seam dip to the north-west. The proposed mining area is located within a basin type structure that dips generally to the north-west and rises sharply at either end of the Hebden longwalls to define the effective panel geometries. The seam thickness varies as a result of various minor seams coalescing. 3. DESCRIPTION OF NATURAL FEATURES AND SURFACE IMPROVEMENTS The natural features and surface improvements within the proposed mining areas have been identified on the basis of several site visits to walk over the surface, discussion with colliery personnel and other specialist groups working for the colliery, and from surface feature maps provided by ICO and ERM. In this section, these features and improvements are described to provide a context for the assessment of likely subsidence impacts. 3.1 Natural Features The main natural features in the proposed mining area are Glennies Creek, Bettys Creek, Main Creek and the alluvium associated with Glennies Creek. The locations of these three creeks are shown in Figure 3. Glennies Creek is the most significant of the three. Figure 6 shows photographs of Glennies Creek. Longwall panels have been designed so that the edge of the Glennies Creek alluvium is protected by a barrier of 26.5° angle of draw (or half depth). Longwall mining is planned directly below sections of Bettys Creek and Main Creek.

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Bettys Creek is an ephemeral, meandering watercourse that is incised some 2-3m below the general flood plain. Previous inspections found the channel was generally dry except for occasional short sections of ponded water. Figure 7 shows a section of Bettys Creek on the western edge of the proposed mining area. There are several ponds located on the general flood plain that appear to be remnants of the river channel isolated as the main channel has cut down to its current level. These ponds support reeds and other semi-aquatic vegetation. Figure 8 shows an example of these ponds. Bettys Creek has been diverted across the rehabilitated Eastern Rail Pit at Mt Owen Open Cut. This channel has been engineered to provide a stable gradient. The channel is located within the proposed Hebden Seam Mining Area. A second diversion further upstream diverts flow from Bettys Creek into Main Creek. This second diversion channel is north of the Hebden Seam Mining Area. The northern part of Main Creek located within the proposed mining area is similar to Bettys Creek. Figure 9 shows a photograph from this section of the creek. The section of Main Creek located over the central part of the proposed Hebden Mining Area discharges onto a broad flood plain so that the stream channel is poorly defined. In the south, the channel becomes more deeply incised again and appears to be freshly eroded downstream of the Glennies Creek Road bridge.




3.2 Public Infrastructure Publicly owned or accessible infrastructure identified within the proposed mining area includes three tar-sealed roads, two dirt roads, Glennies Creek Community Hall, the Main Northern Railway line, numerous power lines, buried Telstra lines and a survey trig station. 3.2.1 Roads Glennies Creek Road (Falbrook Road), Middle Falbrook Road and Stony Creek Road traverse the proposed mining area. Glennies Creek Road passes over the Middle Liddell longwall panels but will not be directly undermined by longwall panels in the Hebden Seam. Middle Falbrook Road and Stony Creek Road pass directly over Barrett Seam longwall panels. Figures 10-13 show photographs of these roads. They are all regularly used with speed limits ranging 80-100km/hr. Middle Falbrook Road crosses over an ephemeral waterway below Possum Skin Dam at a concrete culvert (Figure 11). Glennies Creek Road crosses Main Creek at a concrete bridge that is located just beyond the eastern limit of Longwall H1 and above the main headings in the Middle Liddell Seam (Figure 13).





Forest Road and Nobles Lane are not sealed but are accessible to the public. Forest Road crosses the Hebden longwall areas including areas where multi-seam interaction is expected. The condition of the road deteriorates toward the north and access is eventually limited by a locked gate. Comfortable travelling speeds range from 40-60km/hr. Nobles Lane is not located over any of the proposed longwall panels, but is located above the proposed Barrett Seam cut and flit mining area. 3.2.2 Glennies Creek Community Hall Figure 14 shows a photograph of the Glennies Creek Community Hall. This structure is located over Longwall B6. The structure is of lightweight timber and steel construction on a concrete slab with tin cladding. The toilet facilities at the rear are of brick construction. 3.2.3 Main Northern Rail Line The Main Northern Rail Line is located in the south of the proposed Hebden Seam Mining Area adjacent to the main heading developments. There is no longwall mining proposed within the protection barrier as required by the State Rail Authority consistent with the conditions of Coal Lease CL382.




3.2.4 Power Lines Several electricity power lines are located over the proposed mining areas. The locations of these lines are shown in Figure 3. They are single pole structures and they are predominantly located outside the footprint of the proposed longwall panels. Figure 15 shows an example of one of the lines that is typical of the area. The conductors are supported on single wooden poles and are attached through fixed insulators onto wooden cross beams. 3.2.5 Telstra Cables The location of buried Telstra cables is shown in Figure 3. The line following Forest Road and lines alongside Glennies Creek Road and Middle Falbrook Road are located over longwall panels. 3.2.6 Survey Trig Station A survey trig station is located adjacent to Glennies Creek Road in an area that is remote from proposed longwall panels but is located over proposed cut and flit mining operations.



3.3 Privately Owned Infrastructure Privately owned infrastructure within the proposed mining areas includes infrastructure associated with Mt Owen Mine Complex, numerous residential and farm buildings including a disused dwelling (formerly a church), a gas-fired electricity generation facility and associated infrastructure, and numerous agricultural facilities including gates, fences, access roads, and farm dams. 3.3.1 Ravensworth East and Glendell Open Cut Mines The Mt Owen and Ravensworth East Open Cut operations have substantially completed mining operations within the proposed Hebden Seam Mining Area. Future mining west of the railway corridor is likely to be completed and rehabilitated by the time there are any subsidence impacts from proposed




longwall mining in the Hebden Seam. Several tailings pits are currently being filled but are likely to be complete by the time the area is impacted by mining subsidence. The Dam Safety Notification Zone for a prescribed dam (TP1) in Ravensworth East Open Cut extends into the Hebden Seam Mining Area, but by the time that subsidence from the Hebden Seam mining influences the dam, the dam will have been backfilled on both sides and no longer exist. The Glendell Open Cut Mine to the west is currently starting operations and is expected to finish in 2024 (DoP 2008). An overburden storage dump and the main haul road to the Mt Owen rail loading facility are on the periphery of the Hebden Seam Mining Area. The items of infrastructure at the Mt Owen Complex that will be significantly impacted by mining subsidence are the Mt Owen Rail Line and the Bettys Creek diversion across the now rehabilitated Eastern Rail Pit. A diversion of Bettys Creek across the rehabilitated Eastern Rail Pit is located over the northern Hebden Seam longwall panels and, although the flows are reduced by an upstream diversion into Main Creek, this diversion is required to remain operational. 3.3.2 Mt Owen Rail Line The Mt Owen Rail Line is a single track spur line with a rail loop and coal loading facility at its northern end. The rail loop is north of the proposed mining area. The single line crosses the proposed mining area. Xstrata uses the Mt Owen Rail Line as the main coal haulage corridor from the Mt Owen Complex with up to 12 trains per day currently using this line. The rail line is required to be operational until all the mines in the area close, which is currently expected to before 2024. Figure 16 shows two photographs of the rail line where it crosses a shallow cutting 2-3m deep near the southern end of Longwall H6 and continues along a low embankment near the southern end of Longwall H13. The maintenance road that runs alongside the track can also be seen. 3.3.3 Concrete Bridges over Bettys Creek Figures 17 and 18 show the nature of the construction of the two concrete bridges that cross Bettys Creek, one carrying the rail line and the other carrying the maintenance road. Detailed construction drawings have not been sighted, but the bridge construction would appear to comprise a number of concrete piles, concrete pile caps, and pre-stressed concrete beams as the bridge deck. The road bridge is similarly constructed but with lighter concrete beams for the deck.




3.3.4 Buried Rail Communication Cables Communication cables for signalling and other purposes are laid alongside the Mt Owen Rail Line. It is understood from the Australian Rail Track Corporation that the cables are most likely to be direct buried, possibly in sand, covered by at least marking tape and possibly a physical barrier. Depth of burial could range from approximately 0.5m to greater than 1m. All of the cables are of the PVC insulated copper type. 3.3.5 Rail Maintenance Road The rail maintenance road located alongside the rail line is shown in Figures 16 and 17. The road is single lane, unsealed and not accessible to the public. 3.3.6 Buried Water Supply Pipeline The route of a buried water supply pipe that services the Mt Owen Mine Complex is shown in Figure 3. It is understood that the pipeline is used only infrequently but is nevertheless required to remain serviceable. The pipe is understood to be welded polyethylene pipe with an outside diameter of 355mm buried to a depth of approximately 0.5–1m. It is located adjacent to the rail line. At the suction end, the grade is PN12, which reduces to PN10 in the middle section and PN8 at the outlet end. 3.3.7 Residential Buildings There are numerous residential buildings within the proposed mining areas, but only five are located over or close to longwall panels. Three of these residences are located over Longwall B7; the other two are located next to Longwall H1 over the previously mined Longwalls 1 and 2 in the Middle Liddell Seam. Of the three located over Longwall B7, two are under purchase agreements with Integra. Figure 18-22 shows photographs of the five residences located within the proposed mining area that are located over or close to longwall panels. The remaining residences are located over cut and flit mining areas where subsidence is expected to remain at low levels and be imperceptible for all practical purposes. 3.3.8 Disused Building Figure 23 shows a disused dwelling, formerly a church, located next to Glennies Creek Road. The building was constructed in 1925 and used as a Catholic church until about 1980 when it was closed. The building was subsequently sold and used as a residential dwelling. It is now owned by Xstrata and has been not been occupied for some time. A disused dwelling previously located over Longwall H9 has been removed by Xstrata. The power line servicing this structure has been removed and the buried Telstra cable from Forest Road is no longer used and has been left in situ.


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3.3.9 Fences and Agricultural Infrastructure The fences within the proposed mining areas range in condition from new to barely serviceable. Figure 24 shows two examples of fences that cover the range generally observed in the area. Figure 25 shows a steel frame farm building that is located over Longwall H9. The structural framing for this building is founded on individual concrete footings. It has an earth floor and at the time of inspection was empty. There are numerous farm dams located along tributaries of the various ephemeral watercourses in the area. Some examples of these are shown in Figure 26. The dam walls are generally less than 2-3m high and appear to have been constructed by scraping the near surface soil into an embankment. There are several examples where the capacity of the dams has been significantly reduced as a result of piping failures through the dam wall. Figure 27 shows two examples of these types of failure. These types of failure would be expected when dam walls are constructed using dispersive soils. Several of the dams support reed and other semi-aquatic plant populations. There are numerous farm tracks and farm access roads within the proposed mining areas. These range from barely formed four wheel drive tracks to well maintained access roads. 3.3.10 Envirogen Electricity Generation Facility Envirogen operates a seam gas powered, electricity generating plant adjacent to the intersection of Middle Falbrook Road and Nobles Lane. This plant is located in an area above the main headings in the Barrett Seam. The plant is supplied with gas from the Integra underground operations via a network of gas pipelines that collect gas from wells located across the surface area above the longwall panels. The pipelines operate on suction and have been designed to accommodate anticipated subsidence. Most sections of the pipeline are constructed from polypipe and buried in shallow trenches. The above ground sections are constructed from steel. A bridge structure carries the pipeline across Glennies Creek. 3.4 Integra Owned Infrastructure Possum Skin Dam is located over the Barrett Seam Mining Area. This dam will be decommissioned prior to any subsidence occurring in the vicinity. Underground mining in the Barrett Seam Mining Area will be accessed through the void of the North Open Cut (formerly known as Glennies Creek Open Cut). Open cut mining is expected to be finished by about 2015; before the Barrett Seam Mining Area is developed.


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4. PREDICTION METHODOLOGIES AND DISCUSSION OF ASSUMPTIONS The subsidence predictions for the Barrett Mining Area and those parts of the Hebden Mining Area outside of the Middle Liddell mining footprint can be predicted with confidence based on the broad experience of subsidence monitoring above single seam longwall panels at Integra Underground Mine in the Middle Liddell Seam and more generally at other sites in New South Wales. Subsidence prediction for these single seam mining areas is based on an understanding of the mechanics of subsidence behaviour and recognition that longwall subsidence comprises two essentially independent components: sag subsidence and elastic chain pillar compression (Mills 1988). These two components can be independently calculated based on panel geometry, pillar size and overburden depth taking into account the pillar loadings involved and the spans across individual panels. Total subsidence is estimated by adding the two components together. This approach has been found to be generally conservative compared to measured subsidence but provides a broad based estimate of subsidence that might be expected in situations that are outside of the normal range of subsidence experience. The database of subsidence experience for multiple seam operations is much more limited. A conservative approach to prediction has been adopted until results become available from currently planned coal mining operations elsewhere in the Hunter Valley. Estimates of maximum subsidence are based on the expectation that subsidence has the potential to reach up to 80% of the total combined seam section extracted (Li et al 2008). Actual subsidence is likely to be somewhat less than this maximum, but further monitoring is required to determine the factors that influence subsidence behaviour in multi-seam longwall environments. This approach recognises that the subsidence caused by mining in the Hebden Seam may be more than the extracted thickness of the Hebden Seam. Pillars in the Middle Liddell Seam that were previously limiting surface subsidence are expected to be destabilised by mining in the Hebden Seam causing additional subsidence at the Middle Liddell Seam level. The maximum subsidence resulting from longwall mining in the Hebden Seam is likely to increase slightly in areas where there are first workings in the Middle Liddell Seam compared to areas where there is no mining in the Middle Liddell Seam. Maximum strains, tilts and curvatures are predicted on the basis of empirical relationships developed from observations of maximum subsidence at Integra Underground Mine and in other coalfields where the overburden depths are comparable.




5. REVIEW OF THE RESULTS OF PREVIOUS SUBSIDENCE MONITORING Subsidence monitoring has been conducted at Integra Underground Mine since soon after the commencement of mining in the Middle Liddell Seam. The results of the monitoring conducted to the end of Longwall 7 are presented in detail in SCT Report GLEN3311. The key elements that are relevant to subsidence prediction are summarised in this section. 5.1 Subsidence Characteristics Measured at Integra Underground Mine Subsidence characteristics are routinely measured using a line of survey markers placed at regular intervals across the surface typically either perpendicular or parallel to the longwall panels. B Line is the name of one such line that crosses all the panels mined so far in the Middle Liddell Seam. The location of B Line is shown in Figure 28. The overburden depth increases from 280m over Longwall 1 to 400m over Longwall 8, mainly as a result of seam dip. The surface topography is essentially flat rising gently to the west. Figure 29 shows the subsidence measured on B Line at the completion of Longwall 8. The locations of individual longwall panels are also plotted on the same graph at natural scale relative to the surface. The development of surface subsidence in the monitoring results to date shows the effect of interactions between adjacent panels. Table 2 summarises the magnitude of maximum subsidence at the completion of each longwall panel. Table 2: Summary of Subsidence Measured on B Line

Longwall Panel Completed

Panel Width (m)


Maximum Subsidence

(mm)

Location of Maximum

Subsidence

1 100 270-290 <50mm LW1

2 141 280-230 451 LW1/2

3 167 290-320 763 LW2

4 151 300-330 857 LW3

5 242 300-350 950 LW3

6 256 300-360 965 LW3

7 256 310-380 1267 LW6

8 256 320-400 1299 LW7 The location of the chain pillars is apparent in the subsidence profile, indicating that some sag subsidence is occurring over individual panels. Over the first five panels, the relatively small magnitude of this sag subsidence compared to the maximum subsidence measured is consistent with strata compression above and below the chain pillars being the main contributor to and control of, surface subsidence in the panels mined so far. The subsidence behaviour is controlled by a super-panel effect of multiple panels with the maximum subsidence controlled mainly by strata compression.




The subsidence profile from Longwall 6 shows the development of greater levels of sag subsidence (up to about 500mm) with pillar compression still remaining a significant control on surface subsidence. Maximum subsidence at the completion of Longwall 8 was approximately 1.31m, but, without the influence of chain pillars, maximum subsidence is likely to be in the range 55-65% of the effective seam thickness mined or 1.3-1.6m for nominal mining sections of 2.4m. Table 3 summarises the maximum subsidence movements that have been measured up to the completion of Longwall 8 on each of the subsidence lines. These values are not necessarily the final maximum values because subsidence is not complete on most of the lines and further increases are expected. Table 3: Summary of Maximum Subsidence Movements Measured to Completion of Longwall 8

Subsidence Line

Maximum Subsidence

(m)


Maximum Tensile Strain

(mm/m)

Maximum Compressive

Strain (mm/m)

Maximum Tilt

(mm/m)

A 0.91 290 0.8 2.8 5.8

B 1.31 330 2.7 2.7 8.2

D 0.87 300 1.5 1.6 4.4

E 1.28 330 2.1 1.8 8.1

F 1.20 355 1.9 1.9 7.4

H 0.67 360 1.5 2.0 5.0

L 1.21 360 1.3 2.6 7.0

M 0.90 360 1.3 2.0 7.2




Maximum strains of 2.7mm/m in tension and 2.8mm/m in compression were measured at a representative overburden depth of 330m for maximum subsidence of 1.31m. A survey on 13 January 2004, which shows peak tensile strains up to 3.3mm/m has been disregarded because the results were not replicated in subsequent surveys and it is considered probable that some difference in survey conditions or analysis technique has contributed to the higher values rather than any real change. 5.2 Goaf Edge Subsidence Profiles Figure 30 shows goaf edge profiles for A Line, D Line, B Line and L Line overlain on the same plot and normalised with respect to overburden depth. The goaf edge subsidence profile over a single seam can be approximated using an envelope formed by two line segments, one that extends from 20mm at the angle of draw of 26.5° (0.5 times overburden depth) to 250mm at the goaf edge, and a second that extends from 250mm at the goaf edge to maximum subsidence of 1.5m at a distance of approximately 0.6 times overburden depth from the solid goaf edge. While locally higher tilt values are expected at a distance from the goaf edge of approximately 0.3 times overburden depth consistent with predictions of maximum tilt, the subsidence profile can be reasonably estimated as a straight line, particularly given the large variations that are evident in the various goaf edge subsidence profiles measured to date. This profile is used to estimate the goaf edge subsidence for the proposed mining areas.




5.3 Constants for Prediction Strains and tilts across numerous sites have been found to be approximately proportional to the maximum subsidence and inversely proportional to overburden depth. The maximum values can be estimated using the formulae:

Emax (tensile) = K1 Smax / D Emax (compression) = K2 Smax / D Gmax = K3 Smax / D

where Emax (tensile) is the maximum tensile strain, Emax (compression) is the maximum compression strain, Gmax is maximum tilt, Smax is maximum subsidence, D is overburden depth, and K1, K2 and K3 are constants that are particular to each site or general area. The results of monitoring to the end of Longwall 8 in the Middle Liddell Seam provide a basis to estimate K values specific to Integra Underground Mine. These values are then compared with the range of experience at other sites. Subsidence monitoring above previous longwall panels at Integra Underground Mine indicates maximum strains of 2.7mm/m in tension, 2.8mm/m in compression, and 8.2mm/m tilt. Representative K values based on an assumed representative overburden depth of 350m and maximum subsidence of 1.31m are shown in Table 4 together with equivalent values for the Southern, Western and Newcastle Coalfields. Table 4: Summary of Subsidence Constants

Site/Coalfield K1 K2 K3

Integra Underground Mine 720 750 2200

Southern 400 900 3000

Western 1500 2000 5000

Newcastle 400 600 1800

Used for Prediction Barrett Seam Mining Area 1500 2000 5000

Used for Prediction Hebden Seam Mining Area 1000 1000 3000 For subsidence predictions in the Barrett Seam Mining Area, K values consistent with experience in the Western Coalfield are used because the overburden depth in both areas is generally less than 200m. In the Hebden Seam Mining Area, the overburden depth ranges between 310m and 570m and K values from the Southern Coalfield and previous mining in the Middle Liddell Seam at Integra Coal Mine are considered more appropriate to use.




6. SUBSIDENCE PREDICTIONS In this section, the subsidence is estimated for each of the proposed mining areas. Surface subsidence impacts are expected to occur mainly within the footprint of the longwall panels and the areas in close proximity to the longwall panels. The proposed cut and flit mining areas adjacent to the longwall panels are planned to be designed to cause low levels of surface subsidence and consequently surface subsidence impacts are expected to be imperceptible in these areas. The Hebden Seam Mining Area substantially overlaps the existing and approved future Middle Liddell Seam longwall panels. Interaction between the longwall panels in the two seams is expected to cause subsidence in the Middle Liddell Seam additional to that which has already occurred, as well as subsidence from longwall mining in the Hebden Seam itself. Longwall mining in the Hebden Seam will also interact with the cut and flit area in the Middle Liddell Seam and with the main heading pillars. While some additional subsidence is expected as a result of the pillars in the Middle Liddell Seam, the magnitude of this additional subsidence is likely to be small compared to the subsidence caused by mining in the Hebden Seam. 6.1 Barrett Seam Mining Area The Barrett Seam Mining Area is located in an area where the overburden depth ranges from 110m to 190m. Individual longwall panels are 205-230m wide, so each panel is at or close to supercritical width in subsidence engineering terms. Supercritical width means that maximum subsidence in the centre of each panel is expected to be in the range of 50-65% of the seam thickness mined depending on the reconsolidation characteristics of the overburden strata. A value of 60% is chosen for subsidence prediction purposes as being most likely to be an upper limit. 6.1.1 Vertical Subsidence For the proposed mining section ranging from 2.9m to 3.4m, and assuming maximum likely subsidence of 60% of seam thickness, maximum subsidence is expected to be in the range 1.5-2.0m. Subsidence over the chain pillars is likely to be in the range 0.1-0.3m with the magnitude increasing with overburden depth. The full range of subsidence movements is expected over each individual longwall panel with transient tilts and strains above the retreating longwall face and permanent tilts and strains along each of the panel edges. 6.1.2 Strains Maximum strains are estimated to range 16-27mm/m in tension (using K1=1500) and 21-36mm/m in compression (K2=2000) with the larger values




at shallower overburden depths, but strains are considered likely to peak at 5-10mm/m in most areas. Maximum tensile strains are expected to occur around the edges of each longwall panel and at topographic high points. Maximum compressive strains are likely to occur mainly in the central part of each panel and at topographic low points. 6.1.3 Tilts Maximum tilts are predicted to range up to 50-90mm/m (using K3=5000) depending on overburden depth. Maximum tilt is expected to occur at a distance from the goaf edge of 30-60m (approximately 0.3 times overburden depth). However, based on previous monitoring at other sites in the Hunter Valley at similar depths tilts, are considered most likely to peak in the range 15-25mm/m. 6.2 Hebden Seam Mining Area The Hebden Seam Mining Area is located north-west of Glennies Creek. This area can be divided into four parts based on the potential for subsidence.

a) Hebden Seam longwalls with no overlying Middle Liddell Seam workings.

b) Hebden Seam longwalls below main heading development pillars or cut and flit mining areas in overlying Middle Liddell Seam.

c) Hebden Seam longwalls below extracted longwall panels in the Middle Liddell Seam.

d) Hebden Seam cut and flit mining areas.

The characteristics of the subsidence expected in these areas are discussed in the following sections. 6.2.1 Hebden Seam Mining Only In areas where the Hebden Seam is outside of the Middle Liddell Mine workings, subsidence is likely to be similar to that which has occurred so far in the Middle Liddell Seam. Subsidence up to 1.7m is considered possible on a 2.8m mining section. However, as the mining section is 2.6m in most places, maximum subsidence in the range 1.3-1.6m is expected. The maximum subsidence is likely to be controlled by elastic compression of the Hebden Seam chain pillars with up to 0.2-0.3m of sag subsidence over the centre of each panel. Maximum tensile strains in the range 2-4mm/m, maximum compressive strains in the range 3-6mm/m, and maximum tilts in the range 9-16mm/m are expected in areas where only the Hebden Seam is longwall mined. Each parameter is expected to be inversely proportional to overburden depth.




6.2.2 Hebden Longwalls Below Main Headings and Cut and Flit Mining in Middle Liddell Seam In areas where the longwall panels in the Hebden Seam are located below the Middle Liddell Seam main headings or below areas that have been developed as standing pillars using cut and flit, there is potential for longwall mining in the Hebden Seam to destabilise the overlying pillars causing additional subsidence. At the ends of Longwalls 1-6, the Middle Liddell Seam pillars are approximately 32m by 22m and the Middle Liddell Seam is nominally 2.4m thick. Assuming that the main heading pillars are fully destabilised and the volume of the coal in the pillars is able to fully occupy the roadway volume, there would be potential for additional subsidence of up to 0.8m. While dilation of the coal would reduce this value, it represents an absolute upper limit. The total subsidence resulting from mining the Hebden Seam in this area would therefore be 2.1-2.4m (1.3-1.6m from Hebden Seam plus 0.8m from the destabilised Middle Liddell Seam pillars). At the ends of Longwalls 7-9, the Middle Liddell Seam pillars are larger and average 40m by 55m, reducing the volume available to be filled by coal in the event of the pillars becoming destabilised, and reducing maximum additional subsidence to 0.4m. The total subsidence in this area from mining the Hebden Seam would therefore be up to 1.7-2.0m (1.3-1.6m from Hebden Seam plus 0.4m from the destabilised Middle Liddell Seam pillars). For the cut and flit operations proposed in the trial area, the extraction ratio is typically of the order of 32%. The maximum contribution to subsidence if pillars in a 2.3m high seam became destabilised by longwall mining in the Hebden Seam would be approximately 0.7m. In practice, the main heading pillars and the cut and flit pillars are large enough to withstand the goaf edge loading during longwall mining in the lower seam and are expected to remain substantially intact throughout the subsidence cycle. Furthermore, the total width of the main headings and proposed cut and flit operations is expected to be much less than about 0.7 times overburden depth, so the overall panel width is not sufficient for full subsidence to develop at the surface. On balance, the contribution to total subsidence of the Middle Liddell Seam cut and flit pillar areas and main heading pillars is considered likely to be generally less than about 0.3m, but the upper limit values of 0.4-0.8m have been used until monitoring data becomes available to confirm the magnitude of the multi-seam interaction in these areas. Strains and tilts are expected to be less than half the maximum values currently observed in the Middle Liddell Mining Area shown in Table 3. 6.2.3 Hebden Longwalls Below Middle Liddell Seam Longwall Areas Subsidence in the areas where there is longwall mining in both seams is estimated as 80% of the total combined extraction in both seams. This




approach is intended to be conservative and will be updated as field data becomes available from other sites where multi-seam extraction has been undertaken. In areas where there are longwall panels in both the Middle Liddell and Hebden Seams, maximum subsidence is estimated to be 80% of the combined 5m coal section or 4m. The maximum total subsidence is estimated to be in the range 3.5-4.0m. Some 0.8-1.0m of this total subsidence from mining in the Middle Liddell Seam has already occurred over the area of Longwalls 1-5 and some 1-1.3m has so far occurred over Longwalls 6, 7 and 8, with up to 1-1.5m expected as subsequent panels are mined. Mining in the Hebden Seam is expected to cause additional subsidence of up to 2.6-3.4m above the longwall panels in the Middle Liddell Seam. The additional subsidence associated with mining the Hebden Seam represents 100-130% of a 2.6m thick mining section. These values give an estimate of the range of maximum subsidence that can reasonably be expected based on the information that is currently available. A conservative approach has been adopted and as further information becomes available from other field sites, the estimates can be updated. 6.2.4 Hebden Seam Cut and Flit Mining Operations Subsidence associated with cut and flit mining operations in the Hebden Seam is difficult to estimate without detail of the pillar geometry in this area. The intention is to limit surface subsidence to low levels that are imperceptible on the surface for all practical purposes and this is achievable with mining geometries similar to those approved for the trial cut and flit mining area. 6.2.5 Subsidence Estimates Table 5 summarises the maximum subsidence that is expected in areas where there are no Middle Liddell workings, above Middle Liddell pillar areas, and above the Middle Liddell longwall panels. Table 5: Summary of Subsidence Associated with Mining in Hebden Seam

Area above Middle Liddell (ML) workings where mining in the Hebden Seam is expected to cause subsidence

Subsidence from Mining

Middle Liddell Seam (m)

Subsidence from Mining Hebden Seam

(m)

Total Subsidence From Both Seams (m)

No ML Seam workings 0 1.3-1.6 1.3-1.6

Above ML Mains (LWs1-6) <0.02 2.1-2.4 2.1-2.4

Above ML Mains (LWs7-17) <0.02 1.7-2.0 1.7-2.0

Above ML Longwalls 1-5 0.7-1.0 2.6-3.4 3.5-4.0

Above ML Longwalls 6-17 1-1.5* 2.5-3.4 3.5-4.0 * Subsidence from mining in the Middle Liddell Seam is currently incomplete and will continue to occur over Longwalls 6-17 as a result of ongoing mining.




Maximum strains and tilts are estimated using the empirical relationships for single seam workings. It is recognised that there is an element of extrapolation associated with assuming that empirical relationships developed for single seam environments where the extraction height is 2-3m can be applied to multi-seam environments where the total subsidence is much larger, but in the absence of other information, this is considered to be a reasonable approach until such times as field data becomes available. Table 6: Subsidence Parameters for Hebden Seam Mining Areas

Hebden Seam Mining Area

Total subsidence from both seams (m)

Max Tensile Strain (mm/m)

Max Comp Strain (mm/m)

Max Tilt

(mm/m)

No ML Seam workings 1.3-1.6 1.7-3.9 2.3-5.2 6.8-15

Above ML Cut and Flit & LW1-6 Mains 2.1-2.4 2.8-5.8 3.7-7.7 11.-23

Above ML Mains (LWs7-17) 1.7-2.0 2.2-4.8 3-6.5 9.0-19

Above ML Longwalls 3.5-4.0 4.6-9.7 6.1-13 18-39 6.3 Distribution of Subsidence Figure 31 shows contour plots of the full subsidence expected at the completion of mining in the Middle Liddell, Hebden and Barrett Seams at the Integra Underground Mine overlain on the mine geometry, surface infrastructure and topographic contours. The subsidence contours include the subsidence that has already occurred from mining in the Middle Liddell Seam. Figure 32 shows the same subsidence contours plotted on a 1:25,000 topographic map. While this map is dated 1977 and does not include several significant changes to the surface infrastructure and surface topography since then, it nevertheless provides a context for showing the subsidence relative to surface features. Figure 33 shows the final subsidence profile that is expected in the Hebden Seam Mining Area along an alignment that is close to a composite of B and G Lines (perpendicular to the panels). The subsidence measured to the completion of Longwall 8 in the Middle Liddell Seam is shown. Figure 34 shows the subsidence profile that is expected in the Barrett Seam Mining Area along a cross-section perpendicular to the panels. 6.4 Extent of Subsidence Impacts The extent of subsidence associated with mining in the Hebden and Barrett Seams is expected to occur predominantly within an angle of draw of 26.5° (half depth) outside of the total extraction area. Some low levels of subsidence may occur just outside this area as the overburden depth increases to the northwest because angle of draw typically increases with overburden depth.




Subsidence impacts are expected to be limited to within the combined footprint of the longwall panels in both seams, except in the vicinity of the Longwalls 1 and 2 in the Middle Liddell Seam which extend outside the footprint of the Hebden Seam Mining Area. Additional subsidence may occur over Longwalls 1 and 2 beyond the 26.5° angle of draw from the Hebden goaf edge, but any additional subsidence would be expected to remain within 26.5° angle of draw of the Middle Liddell longwall panels. 7. ASSESSMENT OF SUBSIDENCE IMPACTS In this section, the impacts of subsidence on the natural features and surface improvements are assessed and described. 7.1 Natural Features Natural features in the proposed mining areas include Glennies Creek, Bettys Creek, Main Creek and the alluvium associated with Glennies Creek. The longwall panels have been laid out so that the edge of the Glennies Creek alluvium is protected by a barrier of 26.5° angle of draw, which is equivalent to half overburden depth. This barrier is expected to be sufficient to provide a high level of protection to the alluvium associated with Glennies Creek and an even higher level of protection to Glennies Creek itself, particularly given that the measured angle of draw has so far been in the range 12-22° for longwall panels in the Middle Liddell Seam. No mining induced subsidence movements are expected to occur in either Glennies Creek or its associated alluvium as a result of the proposed longwall mining activity. Cut and flit operations are planned to go directly below both Glennies Creek and the alluvium. The detail of the geometry of these operations has yet to be determined, but they are planned to be long term stable and to cause low levels of surface subsidence that is imperceptible for all practical purposes. Longwall mining is planned directly below sections of Bettys Creek and Main Creek in the Hebden Seam Mining Area. Subsidence of up to 3.5-4m is expected under sections of these creeks where there are longwall panels in both the Hebden and Middle Liddell Seams. There is considered to be potential for significant changes in creek alignment and increased surface ponding in periods of surface flooding. This potential has been addressed by WRM (2008). Cut and flit operations are not expected to have any perceptible impact on natural surface features. 7.2 Public Infrastructure Publicly owned or accessible infrastructure identified within the proposed mining area includes three tar-sealed roads, two unsealed roads, Glennies Creek Community Hall, the Main Northern Railway line, numerous power lines, buried Telstra lines, and a survey trig station.




7.2.1 Roads The proposed mining areas are crossed by Glennies Creek Road (Falbrook Road), Middle Falbrook Road, Stony Creek Road, Forest Road and Nobles Lane. Glennies Creek Road passes over the first of the Middle Liddell longwall panels but is not directly undermined by longwall panels in the Hebden Seam. Subsidence in the vicinity of Glennies Creek Road is expected to occur mainly as a result of multi-seam interaction with the first of the Middle Liddell Seam longwall panels. Subsidence is estimated to be up to 0.5m, but the surface impacts are likely to be generally small. Surface cracking of the pavement is considered possible, especially where the road crosses the southern end of Longwall 2 (in the Middle Liddell Seam). The bridge over Main Creek will need a specific structural assessment to determine its tolerance to expected vertical subsidence movements of 30mm, tilt of 0.5mm/m across the bridge (rather than along the bridge in the direction of travel), and horizontal strains of 0.5mm/m in tension, also across the bridge. Some monitoring and minor mitigatory works may be required, but the bridge is expected to remain serviceable during and after mining. Middle Falbrook and Stony Creek Roads pass directly over the Barrett Seam longwall panels. These are both tar-sealed roads. Mining subsidence of 1.5-2m, maximum strains in the 20-30mm/m range and tilts of 50-90mm/m are expected. These subsidence movements are expected to cause significant disturbance to the road surface and vertical alignment. Road closure is not expected to be necessary but a suitable management plan will need to be developed to manage traffic under conditions where ground clearance is likely to be reduced by sharp changes in grade and compression bumps. Surface cracks of up to 100-150mm wide but generally much less are expected to develop gradually over a period of one to two weeks when the longwall face passes under the road, with exact timing depending on the progress of mining and the geometry of the road relative to the longwall panel. Changes are likely to be transitory in the direction of longwall retreat and permanent where the road surface crosses the panel edges. Each longwall panel is expected to affect a different section of road with some overlap at the chain pillars. Regrading and resurfacing of the roads is expected to be required following the passage of each longwall panel. Reconstruction of the culvert downstream of Possum Skin Dam is likely to be required. However, these impacts are expected to be manageable. Forest Road crosses the Hebden Seam Mining Area including areas where multi-seam interaction is expected. Surface subsidence of up to 3.5-4.0m, maximum horizontal strains of 5-13mm/m and maximum tilts of 20-40mm/m are expected to occur. Surface cracking of up to 50mm is expected over




solid goaf edges. A reduction in ground clearance is expected at changes of grade and where compression bumps develop. Subsidence movements are expected to develop gradually over a period of several weeks. Changes in surface level may cause ponding after heavy rain to impact on the road surface. Intermittent regrading and road maintenance is likely to be required during the period of mining directly below the road if the road is required to remain fully serviceable. Nobles Lane is not located over any of the proposed longwall panels, but is located above the proposed cut and flit operations. No perceptible changes are expected to occur at Nobles Lane as a result of proposed mining activities. 7.2.2 Glennies Creek Community Hall The location of Glennies Creek Community Hall is shown in Figures 1 and 3. The hall is located some 10-30m from the goaf edge of Longwall B6 where the overburden depth is approximately 170m. Mining subsidence at the location of the hall is expected to be in the range 100-500mm. The horizontal strains are expected to be mainly tensile or stretching in nature with a magnitude of up to 15-30mm/m, but most likely 5-10mm/m, and permanent tilts of up to 50-90mm/m, but most likely in the range 15-25mm/m. These levels of subsidence movement are expected to be sufficient to cause perceptible damage to the building requiring considerable structural work, re-levelling and remedial work to surface water drainage and sewerage. 7.2.3 Main Northern Rail Line The Main Northern Rail Line is located in the south of the proposed Hebden Mining Area south of and adjacent to the proposed main heading developments. There are not expected to be any perceptible subsidence movements as a result of the proposed mining. However “mining of any kind within the protection zone can take place only with the full consultation of the [State Rail] Authority and after aspects of safety and integrity of the rail network and liability for death, injury, costs of investigation and remedial works have been fully covered” (NSW Department of Mineral Resources 1994). The protection zone referred to is defined by 35° angle of draw (0.7 times overburden depth) plus 30m from the edge of the rail property. The overburden depth is approximately 340m in this area, so the protection barrier is approximately 270m wide. There is no longwall mining proposed within this barrier so subsidence movements are expected to be imperceptible at the location of the rail line. Consultation with State Rail will nevertheless be necessary and an early start to the dialogue is recommended.




7.2.4 Power Lines Several electricity power lines are located over the proposed mining areas. These are all single pole structures and they are predominantly located outside the longwall panels where subsidence movements are low. Subsidence movements are likely to reduce ground clearances to conductors and cause tilting of individual poles in some areas, but these effects are manageable on a panel by panel basis consistent with the experience of mining under numerous similar structures at other sites. Installation of sheaves is likely to be required to prevent overloading of the cross beams during the period of mining and some resetting of poles may be necessary. 7.2.5 Telstra Cables Buried Telstra cables are located within the proposed mining areas. Maximum strains in the Barrett Seam Mining Area have the potential to exceed the 15-20mm/m tolerance typically expected of buried Telstra cables so some provision for alternative telecommunications may be necessary in case the cables do become overloaded. Maximum strains in the Hebden Seam Mining Area are expected to be less than 13mm/m and therefore less than the 15-20mm/m tolerance typically expected of Telstra cables. While it is still possible that mining subsidence movements could impact the buried cables, an interruption to service is considered unlikely based on previous experience. 7.2.6 Survey Trig Station A survey trig station is located adjacent to Glennies Creek Road in an area that is remote from proposed longwall panels but would be located over proposed cut and flit mining operations. No significant movements are expected at the trig, but a post-mining resurvey is recommended to confirm that no movements have occurred. 7.3 Privately Owned Infrastructure Privately owned infrastructure within the proposed mining areas includes infrastructure associated with Mt Owen Mine Complex, numerous residential and farm buildings including a disused dwelling (formerly a church), a gas-fired electricity generation facility and associated infrastructure, and numerous agricultural facilities including gates, fences, access roads, and farm dams. 7.3.1 Mt Owen, Ravensworth East and Glendell Open Cut Mines The Mt Owen and Ravensworth East Open Cut have substantially completed mining operations within the proposed Hebden Seam Mining Area and by the




time that mining subsidence starts to impact on this area, the open cut mines are likely to have ceased operation. The subsidence impacts on the Glendell and Ravensworth East Open Cuts operations are likely to be insignificant and imperceptible for most practical purposes because of the location of most of these operations outside the longwall panels. The items of infrastructure at the Mt Owen Complex that will be significantly impacted by mining subsidence are the Mt Owen Rail Line and the Bettys Creek diversion across the now rehabilitated Eastern Rail Pit. These will experience the full range of subsidence with potential for approximately 1.5m of subsidence at the completion of the Middle Liddell longwall panels and additional subsidence of 1.5-2.5m a decade or so later when the Hebden Seam is mined. During longwall mining in the Hebden Seam, strains of up to13mm/m and tilts up to 20-40mm/m are expected. The Bettys Creek diversion will require earthworks to regrade the stream channel and re-establish a stable stream bed once subsidence has occurred. Subsidence will occur incrementally during mining of Longwalls H11 to H13, so several stages of temporary regrading are likely to be necessary, particularly at the upstream edge of the subsidence zone. Control measures to prevent nick point erosion will be necessary. This work will need to be conducted on land owned by Xstrata. Surface mining activity in the area is expected to be complete. It is anticipated that specific management plans will be developed as part of the SMP Approval process. There may be potential to impact on stockpile infrastructure such as lights and drainage channels in close vicinity to the footprint of the longwall panels if the stockpile area is still in operation at the time of mining the Hebden Seam longwall panels. Given the likely changes in infrastructure and the timeframe of subsidence, a more detailed assessment of specific impacts and management strategies is recommended closer to the time of mining in consultation with Xstrata. Tailings Pit 2 is located directly over Longwalls H14 and H15, but Xstrata has advised that this pit will be backfilled with tailings and capped well before the Hebden Seam longwall panels cause subsidence in the area. The dam at the downstream end of Tailings Pit 1 is a prescribed dam administered by the Dam Safety Committee, but by the time that the Hebden Seam longwall panels cause subsidence in the area this dam will be backfilled on both sides and will no longer exist. The nearest point on the West Pit currently being mined is approximately 150m from the corner of Longwall 15 and is unlikely to experience perceptible subsidence impacts. This pit is likely to have been rehabilitated by the time that Hebden Seam longwall panels cause any subsidence in the area An overburden storage dump and the main haul road to the Mt Owen rail loading facility are on the periphery of the Hebden Seam Mining Area but are outside the area where significant subsidence movements are expected. Any overburden stockpiles that are mined under may need some remediation




of surface cracks to avoid surface infiltration of rainwater that could contribute to slope instability and potential spontaneous combustion of any coal residue within the stockpile. Surface cracks are likely to be concentrated along the brow of the slope and are typically able to be filled without undue difficulty. Impacts on the Mt Owen Rail Line and associated infrastructure are discussed in more detail in the following sections. 7.3.2 Mt Owen Rail Line The Mt Owen Rail Line is a single track spur line that crosses the proposed Hebden Seam Mining Area including areas that have previously been subsided by mining in the Middle Liddell Seam. Waddington and Barbato (2004) report that, “from an engineering standpoint, there are no insurmountable problems in regard to undermining railways and there are no reasons why mining can not be safely carried out beneath railways, so long as adequate plans for the management of the mining impacts and appropriate financial guarantees are put in place before mining”. Experience gained from mining the Middle Liddell Seam below the Mt Owen Rail Line indicates that mine subsidence of up to approximately 1m can be successfully managed with regular monitoring and appropriate response strategies. There does not appear to be any fundamental reason why a similar strategy could not be applied to manage the rail line for the proposed Hebden Seam mining although the subsidence movements are expected to be much greater. Relocating the line to an area not impacted by mining subsidence may be an option worth consideration. The magnitude of the subsidence movements is likely to be significantly greater than the magnitude of movements observed and successfully managed so far. While subsidence movements of up to 3.5-4.0m have been used as a conservative upper limit, it is considered probable that final subsidence will be in the range 2.5-3.0m and this value may be more appropriate to use for assessing the merits or otherwise of relocating the rail line. For subsidence of 3.5-4.0m, peak strains of 5-10mm/m in tension and 6-13mm/m in tension and maximum tilts of 20-40mm/m are estimated. However, there is potential for significant variation in these values. At locations where gently dipping bedding planes outcrop, there is potential for shear movements to concentrate and cause local elevations in strain and tilt values. At this stage, there is limited experience available to base an accurate prediction for multi-seam subsidence impacts, but by the time that mining is due to commence (indicatively in 2018), subsidence results are likely to be available from several other sites. In general, it is likely that subsidence movements in multi-seam mining operations will be less regular and less predictable than for single seam operations because of the greater levels of disturbance to the overburden strata and greater potential for small differences in interaction between goafs to impact on the final surface subsidence profile.




The rail line passes over several embankments. Most of these are only a few metres high and although a much higher embankment is located on the northern edge of the proposed mining area, it is substantially outside of the area that will be affected by subsidence movements. Mining subsidence is expected to cause the embankments to spread laterally and to settle vertically a small amount as a result, with the magnitude of spreading likely to be a function of embankment height and compaction of the fill material. Previous surface mining of the Eastern Rail Pit on one side of the Rail Line and the presence of the Tailings Pit 2 on the other side has the effect of forming an embankment in natural rock between the two excavations. Mining subsidence would be expected to cause lateral movement of this rock embankment with potential to cause additional vertical movements at the level of the rail line. Given that the voids on both sides of this rock embankment will have been backfilled by the time mining subsidence from the Hebden Seam longwall panels impact on the site, the potential for lateral spreading is likely to be reduced. Any subsidence impacts associated with the embankment are expected to be manageable using the approach that has been adopted in the existing rail management plan. 7.3.3 Concrete Bridges over Bettys Creek The two concrete bridges over the Bettys Creek one carrying the rail line and the other carrying the maintenance road, are likely to require specific mitigation works if they are to remain serviceable through mining in the Hebden Seam. These structures are expected to experience the full range of subsidence movements associated with longwall mining in both seams. Vertical subsidence is expected to reduce the effective clearance of the rail line above flood levels in Bettys Creek because these levels are likely to be controlled by the original level of Bettys Creek downstream where there has been no subsidence. Across the 10m footprint of the bridge structure, maximum strains of 13mm/m and maximum tilts of 40mm/m have the potential to cause peak differential horizontal movements of up to 130mm and tilting of 400mm. The articulated structure of the bridge deck beams supported on pile caps is expected to allow peak horizontal and tilt movements to be accommodated with some mitigatory work. However, a specific structural assessment of the bridge foundations is required to confirm the level of relative movement that can be accommodated. 7.3.4 Buried Rail Communication Cables Communication cables for signalling and other purposes are laid alongside the rail line. These cables are expected to remain serviceable during subsidence, but connection and junction box fixtures may fail depending on particular detail. The monitoring procedures that are currently being used to manage subsidence associated with mining in the Middle Liddell Seam are expected to be suitable to manage any failure of the buried communications cables caused by subsidence movements.




7.3.5 Rail Maintenance Road The rail maintenance road located alongside the rail line is single lane, unsealed, and not accessible to the public. This road is expected to experience the full range of subsidence movements but with regrading and regular maintenance during the period of active mining, the road is expected to remain serviceable. 7.3.6 Buried Water Supply Pipeline The buried water supply pipeline for Mt Owen Mine that runs alongside the rail line is likely to be subjected to the full range of subsidence movements with horizontal strains in the range 2-8mm/m. The safe working strain for polyethylene pipe is 6-10mm/m and so failure of the pipe is considered unlikely. A failsafe strategy would involve uncovering the pipe prior to mining, but this would appear to be unnecessary. Experience over the earlier longwall panels would help determine the requirement for the pipe to be uncovered over subsequent panels. 7.3.7 Residential Buildings There are three residences located directly over Longwall B7 and two located adjacent to Longwall H1 in an area that is likely to experience low levels of additional subsidence as a result of interaction with the first two Middle Liddell Seam longwalls. All mining induced subsidence damage is expected to be covered by the Mine Subsidence Board. Development of a subsidence management plan for each structure in consultation with the owners and the MSB is recommended. The two residences located over Longwall B7 on the eastern side of Middle Falbrook Road are expected to experience the full range of subsidence movements with final subsidence of approximately 2m expected. Transient horizontal strains of up to 15-30mm/m, but most likely in the range 5-10mm/m and transient tilts of up to 50-90mm/m, but most likely 15-20mm/m, are expected. These levels of strain and tilt are expected to cause significant structural damage and cracking of brickwork and any on-ground floor slabs. Buried services are likely to be disrupted and require repair. The residence on the western side of Middle Falbrook Road is located approximately 10-40m from the edge of the longwall panel at an overburden depth of approximately 160m. Vertical subsidence of 100-500mm is expected at this location. Permanent horizontal strains are expected to be mainly tensile or stretching in nature with a magnitude of up to 15-30mm/m, but most likely 5-10mm/m, and permanent tilts of up to 50-90mm/m, but most likely in the range 15-25mm/m. The levels of subsidence movement anticipated are expected to cause significant structural damage and permanent tilt. The two residences located west of Glennies Creek Road adjacent to Longwall H1 are both located in areas that have previously experienced subsidence movements from mining in the Middle Liddell Seam. The southern




residence has previously experienced 600-800mm of subsidence, permanent tilts of 3-4mm/m and horizontal strains of approximately 2mm/m. The strains and tilts are unlikely to have been significant, but the residence is located in a flat area so that vertical subsidence and associated surface ponding after heavy rain is a significant issue. The northern residence is located in a gently sloping area and previous subsidence is likely to have been less than 60mm with imperceptibly low levels of tilt and strain. When Longwall H1 is mined, vertical subsidence at the location of the southern residence is likely to increase to about 1m with permanent tilt of up to 20-40mm/m but most likely in the range 10-15mm/m and horizontal tensile strains of up to 5-10mm/m, but most likely in the range 3-4mm/m. While the increased vertical subsidence is likely to lower this residence relative to the general flood plain, the ground tilt is expected to reduce potential for ponding. The permanent strains and tilts are likely to be perceptible but repairable with minor structural work. The northern residence is located over the solid edge of Longwall 2 in the Middle Liddell Seam in an area beyond the end of Longwall 1. Subsidence movements associated with mining Longwall H1 are expected to be imperceptible at this location. Vertical subsidence is expected to be less than 50mm and tilts and strains less than 1mm/m. Residences and farm buildings located between Glennies Creek and Glennies Creek Road are located outside the area where subsidence from longwall mining is expected to be perceptible. Vertical subsidence associated with longwall mining is expected to be less than 50mm and tilts and strains less than 1mm/m for those buildings immediately east of Glennies Creek Road. Residences located over cut and flit mining areas are not expected to experience subsidence of more than 20mm as a result of the proposed mining. This level of subsidence is imperceptible for all practical purposes. 7.3.8 Disused Building The disused dwelling (formerly a church) next to Glennies Creek Road is located above the edge of Longwall H1 in an area that is not overworked in the Middle Liddell Seam. The overburden depth is approximately 350m. Subsidence is expected to range from 50mm to 150mm over the footprint of the structure, which is estimated to be approximately 9m wide and 15m long. Previous subsidence monitoring at ICO indicates that at the solid goaf edge above a single seam longwall panel, the radius of curvature is likely to be in the range 10-30km. Permanent tilt is likely to range from 2-3mm/m along the length of the structure and horizontal strains are likely to be generally less than 1mm/m. MSEC (2007) present a summary of the allowable subsidence movements for various types of building structures that indicates solid masonry buildings can typically tolerate radius of curvature down to about 5km for a 15m long




building. Cracking may just begin to become apparent at 1mm/m of tensile strain and tilts of less than 2-3mm/m do not cause perceptible damage. Proposed mining in the Hebden Seam is considered unlikely to cause more than minor cracking. 7.3.9 Fences and Agricultural Infrastructure Fences, farm dams, access roads and other agricultural infrastructure are located right across the proposed mining areas and are expected to experience the full range of subsidence movements. Fences within the proposed mining areas vary greatly in condition. Some tightening and slackening of the wires is likely to occur as a result of mining induced subsidence movements. In most areas, these effects are not out of character with the original condition of the fence. In areas where livestock control is critical, temporary electric fencing is recommended during the period of active mining until fences are re-tensioned. Farm dams are typically fairly tolerant to mining subsidence movements, but surface cracking may occur. However, some of the dams located above the proposed mining areas are constructed from dispersive soils. Any subsidence cracks that form in the dam wall may become sites for dispersive erosion and require remediation. If dams are full at the time of undermining, there would be benefit in drawing them down to avoid the potential for leakage through subsidence cracks to cause dispersive erosion to the dam wall. Farm access tracks across the proposed mining areas are likely to experience the full range of subsidence movements. Surface cracking and changes in grade are expected, but the nature of the changes induced by subsidence is typically within the range of conditions normally encountered on these tracks. Some minor remedial work may be required to repair surface cracks, but the effects of mining subsidence on farm access tracks is not expected to significantly impair their utility. A farm shed located over Longwall H9 is expected to experience the full range of subsidence movements for mining in the Middle Liddell Seam and then subsequently for mining in the Hebden Seam. The building is 20-25m long by 8m wide, and 4-5m high. It is likely that relative subsidence movements across the length of the building would cause cladding on the building to be stretched perceptibly, but the building itself is likely to remain serviceable as a hay storage shed. Any repair work on this structure is expected to be covered by the Mine Subsidence Board and development of a management plan in consultation with the MSB and the buildings owners is recommended. 7.3.10 Envirogen Electricity Generation Facility The Envirogen electricity generating plant adjacent to the intersection of Middle Falbrook Road and Nobles Lane is remote from the proposed longwall




panels and no mining subsidence impacts are expected as a result of the proposed mining. A network of buried polyethylene gas pipelines associated with the capture of mine gas is located across the surface area expected to be impacted by mining subsidence. Maximum predicted horizontal compressive strains of 13mm/m are expected to remain less than the maximum strain capacity of the pipeline. The pipeline operates in suction so the integrity of pipeline is monitored on a routine basis. In the unlikely event of damage to the pipe network, there would be limited potential for mine gas to escape. 7.4 Integra Owned Infrastructure Possum Skin Dam is located over the Barrett Seam Mining Area. This dam will be decommissioned prior to any subsidence occurring in the vicinity. Underground mining in the Barrett Seam Mining Area will be accessed through the void of the North Open Cut (formerly known as Glennies Creek Open Cut). Open cut mining is expected to be finished by about 2015, long before the Barrett Seam Mining Area is developed. 8. RECOMMENDATIONS FOR FUTURE MONITORING The following subsidence monitoring programs are recommended for the Barrett and Hebden Seam Mining Areas. 8.1 Barrett Seam Mining Area Conventional three-dimensional subsidence monitoring over the Barrett Seam Mining Area is recommended on cross lines over each of the proposed longwall panels. The pegs should be spaced at a distance equal to 1/20th depth (approximately 7m over most of the area) and surveyed, as a minimum, at the completion of each longwall panel. Ideally the lines should be perpendicular to the panels, but following convenient surface alignments such as Middle Falbrook Road would reduce the potential for disturbance and disruption to other activities in the area. Specific monitoring of the subsidence movements at particular locations of interest is also recommended. The road alignments, residential dwellings, power poles, and other infrastructure should be surveyed prior to, during and after mining as part of management plans for each of these features to confirm the levels of subsidence movements and the corresponding impacts. 8.2 Hebden Seam Mining Area The Hebden Seam Mining Area includes a range of single seam and multi-seam mining geometries that are expected to provide useful information for confirming the level of subsidence that has been predicted and allowing refinement of the predictions in future panels.




A conventional cross-line is recommended in the start area of the Hebden Seam longwall panels where there is no interaction with mining in the Middle Liddell Seam. We recommend three dimensional surveying of pegs located at 1/20th depth or approximately 15m over the first few longwall panels. The cross line should be located at a distance from the end of the panel of greater than 300m to avoid panel end effects and surveyed as a minimum at the completion of each longwall panel. The pegs on this cross-line line should be numbered from east to west. Depending on the final lengths of individual panels, a longitudinal line at the start of a panel extended beyond the end of adjacent panels is recommended. This line would be most useful above the first of the wider panels because only low levels of sag subsidence are expected over the first few narrower panels. This line would need to be resurveyed at frequent intervals during the initial stages of mining the panel to measure the dynamic subsidence characteristics of the overburden strata. A cross line located over the central section of the longwall panels is recommended to monitor the development of subsidence in a multi-seam mining environment. B Line would be suitable to reuse for the first half of the Hebden Seam longwall panels. G Line would be suitable for the second half. A cross line located over the Middle Liddell main headings would be helpful to measure the subsidence relating to longwall mining under pillar areas. Pegs spaced at 20m centres along the centreline of main headings should be surveyed in three dimensions at the completion of each longwall panel. In addition to the generic subsidence monitoring, specific subsidence monitoring of significant infrastructure items would also be recommended to meet the requirements of structure specific management plans. The items of infrastructure that would be recommended to monitor include the following.

� Mt Owen Rail Line and associated infrastructure including bridge over Bettys Creek.

� Bettys Creek diversion channel over the Eastern Rail Pit at Ravensworth East Open Cut Mine.

� Dwellings alongside Glennies Creek Road, two to the west and one to the east of the road, and the disused dwelling (formerly a church).

� Road bridge on Glennies Creek

� Barrier to Northern Rail Line Typically three dimensional survey marks located around the perimeter of structures and distributed around the general area provide a measure of any subsidence movements. The number of surveys required to be conducted depends on the sensitivity of each structure to subsidence movements and




the amount of subsidence predicted and is typically determined as part of the management plan for each structure. 9. CONCLUSIONS Subsidence monitoring results from the Middle Liddell Seam longwall panels provide a basis for estimating the subsidence in the Barrett Seam Mining Area and in those parts of the Hebden Seam Mining Area outside of the footprint of the Middle Liddell Seam longwalls. Horizontal strains and tilts in the Barrett Seam Mining Area have been estimated using subsidence experience from the Western Coalfield where the overburden depths and overburden caving characteristics are similar to those at Integra Underground Mine. There is currently limited information on which to base estimates of subsidence in those parts of the Hebden Seam Mining Area located below longwall panels in the Middle Liddell Seam. Thus a conservative approach to estimating subsidence has been adopted until it can be refined as monitoring information becomes available. Maximum subsidence in the areas of multi-seam longwall operations has been estimated as 80% of the total extracted thickness of both seams equivalent to 3.5-4m of total subsidence. The actual subsidence is expected to be somewhat less than the 3.5-4m maximum that is predicted at this stage. Horizontal strains and tilts in the Hebden Seam Mining Area have been estimated using subsidence experience from Integra Underground Mine and from the Southern Coalfield where the overburden depths are typically in the 400-500m range. Mining subsidence impacts in the Barrett Seam Mining Area are mainly limited to the tarsealed roads, Glennies Creek Community Hall, and three residences located over Longwall B7. Mining subsidence is expected to cause perceptible damage to these structures and to the road surface. The Mine Subsidence Board operates in the coal mining regions of New South Wales and is responsible for managing the provision of compensation and repairs to structures impacted by mining subsidence. The costs associated with compensation and remediation of any damage to buildings and other structures caused by mining subsidence is expected to be covered by the MSB. Mining subsidence impacts in the Hebden Seam Mining Area include impacts to Bettys Creek and the diversion channel on Bettys Creek across the top of the Eastern Rail Pit, Main Creek, Glennies Creek Road, a bridge over Main Creek, a residence west of Glennies Creek Road, the Mt Owen Rail Line and associated infrastructure, Forest Rd, various agricultural infrastructure including fences, farm dams, farm access roads. Mining impacts on the Main Northern Rail Line, several other residences in the Hebden Seam




Mining Area including a disused dwelling (formerly a church) are expected to imperceptible. There is not expected to be any perceptible subsidence impacts in any of the cut and flit mining areas. 10. REFERENCES Co-Resources Pty Ltd, 2005, ‘Mt Owen Complex / Glennies Creek U/G Locality Plan 2005’ Drawing Number 00674 Revision A dated 18 July 2005. Holla, L. 1985, ‘Mining subsidence in New South Wales 1. Surface subsidence prediction in the Southern Coalfield’ Department of Mineral Resources, Sydney. Holla, L. 1987, ‘Mining subsidence in New South Wales 2. Surface subsidence prediction in the Newcastle Coalfield’ Department of Mineral Resources, Sydney. Holla, L. 1988, ‘Effects of underground mining on domestic structures – prediction versus performance’ Proceedings of 5th Australia-New Zealand Conference on Geomechanics, pp 351-355 Institution of Engineers, Canberra. Holla, L. 1991, ‘Mining subsidence in New South Wales 3. Surface subsidence prediction in the Western Coalfield’ Department of Mineral Resources, Sydney. Holla, L. & Barclay, E. 2000, ‘Mine Subsidence in the Southern Coalfield, NSW, Australia’ NSW Dept. of Mineral Resources, ISBN 0 7313 9225 6. Mills, K.W. 1998, ‘Subsidence mechanisms about Longwall Panels’ in proceedings of the 1998 International Conference on Geomechanics/Ground Control in Mining and Underground Construction, Wollongong 14-17th July 1998. Eds Aziz and Indraratna pp.745-756. MSEC, 2007, “Mine Subsidence Damage to Building Structures” prepared by Mine Subsidence Engineering Consultants (www.minesubsidence.com) NSW Department of Mineral Resources 1994 “Guidelines for Mining Coal Under Railways” MINFO 43, 1994. Umwelt 2003, ‘Mt Owen Operations Environmental Impact Statement’ Waddington, A. & Barbato, J. 2004, ‘Mining Under Railways’ in proceedings of the 6th Triennial Conference on Mine Subsidence, ISBN 0-9585779-2-7 pp.173-182. Water Resources Management 2008 ‘Surface Water Assessment for Integra Coal Mine Barrett/Hebden Underground Mine Extension Project Environmental Assessment’ Report to Integra Coal Operations.





Xstrata Mt Owen Pty Ltd, 2006 ‘Mt Owen Complex Land Management Units’ Drawing Number 01539 dated 15 May 2006. Xstrata 1998 ‘Glendell Mine Operating Plan’ on display at DPI offices Maitland as latest MOP for Glendell (expired 2004).

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