The Hume Coal project has been designed to minimise the potential surface impacts from mine subsidence to levels that are negligible and imperceptible.

The primary way Hume Coal has achieved this is by designing the mine using a first-workings mining method which does not result in large movements of the overlying rock above the mine. This method will economically extract about 1/3 of the coal on average, with the remainder being left in place to permanently support the overlying rock.

The maximum predicted surface settlement is less than 20 millimetres, which means that surface features are highly unlikely to be affected. During mining, there will be ongoing monitoring to check that the mine design objectives are achieved.

KEY FINDINGS

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SUBSIDENCE AND GROUND STABILITY

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The Hume Coal Project will use a mine design which involves mining about one third of the coal reserve, leaving behind large blocks of coal to support the rock strata above. About two thirds of the coal will be left in place. This will prevent the rock above the mined areas from collapsing or caving. This is a key point of difference between the Hume Coal project and more traditional Southern Coalfield mines.

This mine design will ensure that the characteristics of the overlying rock will remain virtually unchanged from the pre-mining conditions, intact and undisturbed. In addition to the low extraction ratio and stability of the overlying rock, the mine will be divided up into discrete mining areas, or panels. Once mined, the panels will contain open voids that will be partially backfilled with rock and stone material that has been separated from the coal in the coal preparation plant (CPP) and returned underground.

The subsidence impact assessment carried out as part of the Hume Coal Environmental Impact Statement was based on decades of evidence from coalfields in Australia and around the world on subsidence and pillar behaviour. The assessment concluded that surface features – both natural and man-made – are highly unlikely to be affected as subsidence will be ”negligible to imperceptible”. The predicted maximum surface settlement is less than 20 millimetres.

To place this assessment in perspective, the level of subsidence will be less than the range of expansion and contraction of soils and clays in the region due to seasonal factors such wet and dry periods. By comparison, a typical longwall mine can result in between 600mm and 2500mm of surface subsidence.

INTRODUCTION

The lowering of the land surface above mine workings is called subsidence. Subsidence occurs at underground mines when the earth above the coal (overburden) caves into the void created when the coal is mined. Sometimes the depressions and troughs can be very deep and cause damage to buildings and infrastructure on the surface. On the other hand, they can be so shallow they cannot be seen and do not affect features on the surface.

Traditionally, any mine resulting in less than 20mm of subsidence would have been considered a “zero

subsidence” mine, owing to the fact that 20mm was the minimum vertical precision of the available surveying equipment. Modern surveying techniques can now detect very small ground movements and settlements that were once imperceptible with surveying equipment a couple of decades ago. This is why the project has been required to undertake a subsidence assessment.

Hume Coal engaged an expert geotechnical engineer to undertake a subsidence impact assessment.

SUBSIDENCE

The assessment considered potential effects on items of Aboriginal significance such as rock shelters and grinding grooves, items of historic significance such as houses, gardens and bridges, infrastructure such as roads, bridges, powerlines, dams and pipelines as well as buildings and built features such as houses, out buildings and fences.

Engineering methods were used to predict the maximum worst-case subsidence levels that could be generated by the project, based on conservative assumptions.

The primary factors considered included mining parameters such as the key dimensions (width, height and length) of the mine tunnels and pillars, and geological characteristics such as the depth of the mine and the attributes of the rock and coal.

Mining will be in the Wongawilli Seam, which is generally overlain by Hawkesbury Sandstone. The image on the following page shows the rock types and thickness in descending order from the surface.

SUBSIDENCE IMPACT ASSESSMENT

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The subsidence impact assessment determined the likely surface settlement from the Hume Coal project will be less than 20mm and the associated effects on man-made structures and natural features above the mine, including aquifers or groundwater storage will be negligible to imperceptible.

For more information on the groundwater systems see the Water Assessment Summary.

Mining for the Hume Coal Project will be ‘first workings’ only in the Wongawilli Seam, from around 80 metres underground to 180 metres depth. The underground tunnels will criss-cross the coal seam, leaving approximately two thirds of the coal in place.

This will leave large, strong and stable pillars or blocks of coal between the mined areas, and result in surface settlement that will be imperceptible or negligible. The assessment found that with such low levels of subsidence, man-made structures and natural surface features are very unlikely to be affected as the ground will remain permanently stable.

Surface settlement will be monitored throughout the life of the mine using sophisticated modern surveying equipment. Modifications to the mine layout and mining method will be considered if more settlement occurs than was predicted.

SUBSIDENCE (continued)

Figure 1 Typical stratigraphic section of the Hume Coal Project area

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Sag and compression

Subsidence can be caused directly above a mine by:• Sag - where the rock formations deform (sag)

where all of the coal has been extracted between two solid pillars of coal (or strata).

• Compression - this occurs because the remaining coal pillars have to support the original weight of the rock above them plus a share of the weight from the mined areas adjacent to them, and therefore compress slightly.

In the case of the Hume Coal project, the subsidence occurs because of compression, due to the fact that about 1/3 of the coal seam will be extracted, so the remaining 2/3 will bear some increased load from the weight of the overlying rock.

The predicted maximum surface settlement due to the compression of the pillars is less than 20 millimetres (less than an inch in the imperial scale), which is expected to form as a fairly uniform lowering across the whole mining area. Until the advent of modern sophisticated surveying equipment over the past two decades, 20 millimetres was considered to be “zero” subsidence as it was the limit of

measurement precision for vertical movements. It is also less than the range of expansion and contraction of clay-rich soils in the region due to seasonal factors such as high rainfall periods, drought etc. By comparison, a typical longwall mine can result in between 600mm and 2500mm of surface subsidence.

As the subsidence is predicted to be negligible to imperceptible it is highly unlikely to affect man-made and natural surface features. This is demonstrated in Table 1, which compares the project’s subsidence parameters to some of those that could lead to damage of surface features.

Mining will result in some decrease in groundwater levels in the overlying strata. This is because some water from the overlying rock will flow into the mine. However, the mine method is designed to reduce the potential for decrease in groundwater levels by preventing fracturing of sandstone above the mine workings. The subsidence movements in the overlying rock will be so low that there will be no additional or enlarged fractures and fissures created for water to pass through more readily.

The table below shows potential subsidence affects on surface features.

VERTICAL SUBSIDENCE

Surface feature Description Impact

Man-made

Buildings The maximum predicted tilt for the project is 0.26 mm/m, which is significantly less than the tilt (5 mm/m) above which remedial work may be required on buildings.

Negligible

Roads The extent of mine workings under the Hume Highway is limited to some road crossings.

There is local evidence of roads being successfully undermined with no significant impact at much higher subsidence, tilt and horizontal strain values than those determined for the project.

Negligible

Transmission towers Power pole instability and cable issues commence at tilt levels around 20 mm/m. The maximum predicted tilt for the project is 0.26 mm/m.

No effects

Gas pipelines Gas pipelines have been successfully undermined with subsidence of 760 mm to 1000 mm. The predicted maximum subsidence from the project is less than 20 mm.

No effects

Water pipelines, telecommunication cables and optical fibre cables

Evidence shows that the predicted maximum values of subsidence, tilt and horizontal strain for the project will not have potential to damage any of this infrastructure.

No effects

Wire fences Fences can withstand tilts up to 10 mm/m and strains to 5 mm/m. The maximum predicted tilt for the project is 0.26 mm/m and strain is 0.36mm/m.

No effects

Vineyards There are many examples of vineyards occurring above long wall mining operations in Australia, which have far greater subsidence affects than those predicted for the project.

Negligible

Aboriginal items Aboriginal items above the mining areas are unlikely to be affected by subsidence given the predicted negligible to imperceptible levels of subsidence.

No effects

Historic items All known historic features are outside the mining area. No effects

Natural

Cliffs The types of cliffs and steep rock exposures in the project area do not conform to cliff lines requiring protection from first workings (i.e. greater than 50 m high, overhanging and may have Aboriginal significance, or contain hanging swamps).

Negligible

Flora and fauna Subsidence impacts on vegetation such as shearing of roots and local ponding will not occur.

No effects

Water resources Subsidence impacts on surface water features such as realignment of drainage lines, bed scouring and cracking of stream beds will not occur.

No effects

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CONCLUSIONThe Hume Coal project has been designed to minimise the potential surface impacts from mine subsidence to levels that are negligible and imperceptible.

The primary way Hume Coal has achieved this is by designing the mine using a first-workings mining method which does not result in large movements of the overlying rock above the mine. This method will economically extract about 1/3 of the coal on average, with the remainder being left in place to permanently support the overlying rock.

The maximum predicted surface settlement is less than 20 millimetres, which means that surface features are highly unlikely to be affected. During mining, there will be ongoing monitoring to check that the mine design objectives are achieved.

W www.humecoal.com.auE info@humecoal.com.au

Hume Coal Main OfficeUnit 7-8, Clarence House 9 Clarence StreetMoss Vale, NSW 2577Phone 02 4869 8200

Berrima Community ShopPost Office CornerShop 3/30 Old Hume HighwayBerrima NSW 2577Phone 02 4877 2481

Hume Coal Pty. Ltd.ABN 90 070 017 784

Engagement HQ www.yoursay.humecoal.com.au

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SUMMARY

SUBSIDENCE AND GROUND STABILITY

The coal pillars between the mine’s network of tunnels will be monitored and surveyed on an ongoing basis to ensure that they are the correct dimensions to achieve the required engineering outcomes.

Natural surface features which are inherently unstable and large man-made structures such as bridges may require specific monitoring to provide evidence that surface settlements are lower than the predicted maximum and any changes to their condition are not as a result of mining.

SUBSIDENCE MONITORING