Tahmoor Colliery Longwalls 25 to 26

Transcription

1 Tahmoor Colliery Longwalls 25 to 26 SYDNEY WATER SEWER INFRASTRUCTURE SURFACE SAFETY AND SERVICEABILITY MANAGEMENT PLAN REVISION F Mine Subsidence Engineering Consultants Level Victoria Avenue Chatswood NSW 2067 PO Box 3047 Willoughby North NSW 2068 Tel. (02) Fax. (02) enquiries@minesubsidence.com September 2008

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3 TABLE OF CONTENTS TABLE OF CONTENTS ii CHAPTER 1. INTRODUCTION Background Predicted Subsidence Movements Limitations Objectives Scope Proposed Mining Schedule Definition of Active Subsidence Zone 4 CHAPTER 2. RISK MANAGEMENT METHOD General Consequence Likelihood Hazard Risk 5 CHAPTER 3. RISK ASSESSMENT Sewage Disposal Infrastructure Hazard Identification Hazard 1 Damaged Joints and Pipes Hazard 2 Sewer Main Grades Reduced Below the Minimum for Self-Cleansing Hazard 3 Rising Mains Hazard 4 - Blockages or Overflows Hazard 5 - Sewage Pumping Station Hazard 6 - Power Line to the Sewage Pumping Station Hazard 7 - Concrete Encasements and Horizontal Bores Typical Concrete Encasements and Horizontal Bores mm Diameter Sewer Main between Huen Place and Castlereagh Street Hazard 8 Creek Crossings Investigations of Myrtle Creek Crossings after mining of Longwall 24B Summary of Risk Analysis for Sewer System Experience of mining Longwall 24A Potential Increased Subsidence during the mining of Longwall CHAPTER 4. RISK CONTROL PROCEDURES Sewer Management Group (SMG) Avoidance and Mitigation Measures Monitoring Plan 29 Mine Subsidence Engineering Consultants ii Tahmoor Colliery

4 Ground Monitoring Lines Visual Inspections Automated Continuous Tilt Monitoring Water Quality Monitoring Triggers and Responses 32 CHAPTER 5. SMG MEETINGS 38 CHAPTER 6. AUDIT AND REVIEW 38 CHAPTER 7. RECORD KEEPING 38 CHAPTER 8. CONTACT LIST 39 Appendix A - Glossary of Terms and Definitions 40 Appendix B Drawings and Illustrations 42 Appendix C Reports 43 Drawings Drawings referred to in this report are included in Appendix B at the end of the report. Drawing No. Description Rev MSEC Sewerage Infrastructure Pipe Size C MSEC Sewerage Infrastructure Pipe Type C MSEC Sewerage Infrastructure Pipes Less than Self-Cleansing Grade C MSEC Sewerage Infrastructure Pipes with Grades Less than MSB Requirements C MSEC Predicted Change in Sewer Grades after LW26 C MSEC Sewer Pipes with Grades Less than Self-Cleansing Grade after LW26 C MSEC Longwall 24A Sydney Water Monitoring Plan C Mine Subsidence Engineering Consultants iii Tahmoor Colliery

5 CHAPTER 1. INTRODUCTION 1.1. Background Tahmoor Colliery is located approximately 80 kilometres south west of Sydney in the township of Tahmoor NSW. It is managed and operated by Xstrata Coal. Tahmoor Colliery has previously mined 24 longwalls to the north and west of the mine s current location. Longwalls 25 to 26 are a continuation of a series of longwalls that extend into the Tahmoor North Lease area, which began with Longwall 22. The longwall panels are located between the Bargo River in the south-east, the township of Thirlmere in the west and Picton in the north. A portion of each longwall is located beneath the urban area of Tahmoor. Infrastructure owned by Sydney Water is located within these areas. It is noted that while changes in sewer grade have occurred as a result of mine subsidence, Longwalls 22 to 24A have directly mined beneath approximately 8.1 kilometres of sewer pipes and no noticeable impacts have been recorded so far. The observed frequency of incidences is similar to those in areas not affected by subsidence. This Management Plan provides detailed information about how the risks associated with the mining beneath sewer infrastructure will be managed by Tahmoor Colliery and Sydney Water. The Management Plan is a live document that can be amended at any stage of mining, to meet the changing needs of Tahmoor Colliery and Sydney Water Predicted Subsidence Movements A summary of the predicted maximum incremental parameters over the whole subsided area, due to the extraction of each longwall, is shown in Table 1.1. Table 1.1 Maximum Predicted Incremental Subsidence Parameters Subsidence Parameter LW 22 LW 23 LW 24 LW 25 LW 26 Vertical Subsidence (mm) Transverse Tilt (mm/m) Longitudinal Tilt (mm/m) Transverse Tensile Strain (mm/m) Longitudinal Tensile Strain (mm/m) Transverse Compressive Strain (mm/m) Longitudinal Compressive Strain (mm/m) Transverse Hogging Curvature (km -1 ) Longitudinal Hogging Curvature (km -1 ) Transverse Sagging Curvature (km -1 ) Longitudinal Sagging Curvature (km -1 ) The maximum predicted cumulative subsidence parameters, after the extraction of each longwall, are shown in Table 1.2. Mine Subsidence Engineering Consultants 1 Tahmoor Colliery

6 Table 1.2 Maximum Predicted Cumulative Subsidence Parameters Subsidence Parameter LW 22 LW 23 LW 24 LW 25 LW 26 Vertical Subsidence (mm) Transverse Tilt (mm/m) Longitudinal Tilt (mm/m) Transverse Tensile Strain (mm/m) Longitudinal Tensile Strain (mm/m) Transverse Compressive Strain (mm/m) Longitudinal Compressive Strain (mm/m) Transverse Hogging Curvature (km -1 ) Longitudinal Hogging Curvature (km -1 ) Transverse Sagging Curvature (km -1 ) Longitudinal Sagging Curvature (km -1 ) Limitations This Management Plan is based on the predictions of the effects of mining on surface infrastructure as provided in Report No. MSEC157 by Mine Subsidence Engineering Consultants. Predictions are based on the planned configuration of longwalls at Tahmoor Colliery (as shown in Drawing No. MSEC ), along with available geological information and data from numerous subsidence studies for longwalls previously mined in the area. Infrastructure considered in this Plan has been identified from aerial photographs, regional maps and from discussions between Tahmoor Colliery representatives and Sydney Water personnel. The impacts of mining on surface and sub-surface features have been assessed in detail. However, it is recognised that the prediction and assessment of subsidence can be relied upon only to a certain extent. The limitations of the prediction and assessment of mine subsidence are discussed in report MSEC157 by Mine Subsidence Engineering Consultants. As discussed in the report, there is a low probability that ground movements and their impacts could exceed the predictions and assessments. However, if these potentially higher impacts are considered prior to mining, they can be managed. This Surface Safety and Serviceability Management Plan will not necessarily prevent impacts from longwall mining, but will limit the impacts by establishing appropriate procedures that can be followed should evidence of increased impacts emerge Objectives The objectives of this Surface Safety and Serviceability Management Plan (SSSMP) are to establish procedures to measure, control, mitigate and repair potential impacts that might occur on surface infrastructure owned by Sydney Water. The objectives of the SSSMP have been developed to:- Ensure the safe and serviceable operation of all surface infrastructure. Public and workplace safety is paramount. Disruption and inconvenience should be kept to minimal levels. Monitor ground movements and the condition of surface infrastructure during mining. Initiate action to mitigate or remedy potential significant impacts that are expected to occur on the surface. Provide a plan of action in the event that the impacts of mine subsidence are greater than those that are predicted. Provide a forum to report, discuss and record impacts to the surface. This will involve Tahmoor Colliery, Sydney Water, Mine Subsidence Board, Department of Mineral Resources, and consultants as required. Establish lines of communication and emergency contacts. Mine Subsidence Engineering Consultants 2 Tahmoor Colliery

7 1.5. Scope The SSSMP is to be used to protect and monitor the condition of the items of infrastructure identified to be at risk due to mine subsidence. The major items at risk are:- Sewer Pumping Stations Gravity Sewer Systems Rising Mains The SSSMP only covers infrastructure that is located within the general application area, which defines the extent of land that may be affected by mine subsidence as a result of mining Longwalls 24 to 26. The management plan does not include other property owned by Sydney Water which lies outside the extent of the general application area. The Plan also applies to persons employed or engaged by Tahmoor Colliery requiring them to carry out activities described by this Plan. Impacts are considered in terms of mining directly affecting infrastructure, rather than the effects from loss of services Proposed Mining Schedule It is planned that each longwall will extract coal working northwest from the southeastern ends. This Management Plan covers longwall mining until completion of mining in Longwall 26 and for sufficient time thereafter to allow for completion of subsidence effects. The current schedule of mining is shown in Table 1.3. Table 1.3 Schedule of Mining Longwall Start Date Completion Date Longwall 25 August 2008 August 2009 Longwall 26 October 2009 October 2010 Mine Subsidence Engineering Consultants 3 Tahmoor Colliery

8 1.7. Definition of Active Subsidence Zone As a longwall progresses, subsidence begins to develop at a point in front of the longwall face and continues to develop after the longwall passes. The majority of subsidence movement typically occurs within an area 150 metres in front of the longwall face to an area 450 metres behind the longwall face. This is termed the active subsidence zone for the purposes of this SSSMP, where surface monitoring is generally conducted. The active subsidence zone for each longwall is defined by the area bounded by the predicted 20 mm subsidence contour for the active longwall and a distance of 150 metres in front and 450 metres behind the active longwall face, as shown by Fig Predicted 20mm Subsidence Contour LEGEND Active Longwall Weekly Inspection (450m Behind Longwall Face) Weekly Inspection (150m Ahead of Longwall Face) The Active Subsidence Zone shall be defined with reference to the extraction face position at the date of each survey/inspection. Fig. 1.1 Diagrammatic Representation of Active Subsidence Zone Mine Subsidence Engineering Consultants 4 Tahmoor Colliery

9 CHAPTER 2. RISK MANAGEMENT METHOD 2.1. General The Australian/New Zealand standard for Risk Management defines the terms used in the risk management process, which includes the identification, analysis, assessment, treatment and monitoring of risk. In this context: Consequence The outcome of an event expressed qualitatively or quantitatively, being a loss, injury, disadvantage or gain. There may be a range of possible outcomes associated with an event. 1 The consequences of a hazard are rated from very slight to very severe Likelihood Used as a qualitative description of probability or frequency. 2 The likelihood can range from very rare to almost certain Hazard A source of potential harm or a situation with a potential to cause loss Risk The chance of something happening that will have an impact upon objectives. It is measured in terms of consequences and likelihood. 4 The risk combines the likelihood of an impact occurring with the consequence of the impact occurring. The risk is rated from very low to extreme. In this study, the likelihood and consequence are combined via the qualitative risk analysis matrix shown in Table 2.1, to determine an estimated level of risk for particular events or situations. The Risk Analysis Matrix is similar to the example provided in AS/NZS 4360:1995, Appendix D, p.25. Table 2.1 Qualitative Risk Analysis Matrix LIKELIHOOD CONSEQUENCES Very Slight Slight Moderate Severe Very Severe Almost Certain Low Moderate High Extreme Extreme Likely Low Moderate High Very High Extreme Moderate Low Low Moderate High Very High Unlikely Very Low Low Moderate High High Rare Very Low Very Low Low Moderate High Very Rare Very Low Very Low Low Moderate Moderate This SSSMP adopts a common system of nomenclature to summarise each risk analysis, which is LIKELIHOOD / CONSEQUENCE LEVEL OF RISK. For example, if the likelihood of a risk is assessed as UNLIKELY, and the consequence of a risk is assessed as SEVERE, the risk analysis would be summarised as UNLIKELY / SEVERE HIGH. 1 AS/NZS 4360:1999 Risk Management pp2 2 AS/NZS 4360:1999 Risk Management pp2 3 AS/NZS 4360:1999 Risk Management pp2 4 AS/NZS 4360:1999 Risk Management pp3 Mine Subsidence Engineering Consultants 5 Tahmoor Colliery

10 CHAPTER 3. RISK ASSESSMENT The original risk assessment was conducted jointly by Sydney Water and Tahmoor Colliery in May The risks have been most recently reviewed jointly by Sydney Water and Tahmoor Colliery in September 2008 in light of experience gained during the mining of Longwalls 22 to 24A Sewage Disposal Infrastructure There are approximately 39 kilometres of sewer pipes within the SMP Area, which are owned by Sydney Water. The Picton Regional Sewerage Scheme collects sewage from the urban areas of Tahmoor and Thirlmere and transports it by gravity to the Picton Sewage Treatment Plant in Picton, which lies outside the SMP Area. The sewer pipes were installed in With the exception of the rising mains, all of the sewer pipes are designed at self-cleansing grades. The average grade of the sewer pipes within the SMP Area is approximately 32 mm/m. The design for the gravity sewer system was approved by the Mine Subsidence Board, on the condition that the sewers were installed at least 3 mm/m greater than the minimum grade required for the pipes to be self-cleansing. It has been found, on examination of information provided by Sydney Water, that some of the pipes may have been installed at grades less than self-cleansing, or installed at grades less than the Mine Subsidence Board requirements. The design for the rising mains are certified in accordance with the requirement of the Mine Subsidence Board, which specified maximum vertical subsidence of 750 mm, maximum tensile strain of 1.5 mm/m, maximum compressive strain of 2.5 mm/m and maximum curvature of 8 kilometres. The sewer pipes are constructed from PVC and have extra length sockets and rubber ring joints, which will allow them to accommodate tensile and compressive ground strains and curvature. The pipe sections are typically 3 metres long. The majority of the sewer mains are laid on sand and buried in trenches. A CCTV investigation of the pipeline was undertaken by Tahmoor Colliery in Milne Street, in consultation with Sydney Water and the Mine Subsidence Board prior to the development of subsidence movements during the extraction of Longwall 22. Sydney Water has reviewed the video and has stated that the existing pipe work is in fair to good condition. Minor ponding was observed, with minor undulation in the line. Occasional sections of pipe work were damaged or partially compressed. A blocked junction was also observed. A number of sections were observed to be unusually dirty. Sydney Water indicated that the issues found during the investigation were of a minor maintenance nature or due to the sewer not being cleaned correctly after the installation of the pipe work in The sewer pipelines within the SMP Area are shown according to their pipe sizes in Drawing No. MSEC It can be seen from this drawing that the sewer mains range in diameter between 100 mm and 375 mm. The larger sewer mains are located adjacent to the Main Southern Railway, York Street and between Remembrance Drive and Myrtle Creek. A summary of the total lengths of each diameter of sewer pipe is provided in Table 3.1. Mine Subsidence Engineering Consultants 6 Tahmoor Colliery

11 Table 3.1 Pipe Diameter (mm) Distribution of Sewer Pipes by Pipe Diameter Total Length within SMP Area (m) Total The pipes are also shown according to their type in Drawing No. MSEC The majority of pipes are sideline and reticulation pipes that transport sewage to the rising mains and carrier pipes. A summary of the total lengths of each type of pipe is provided in Table 3.2. Table 3.2 Distribution of Sewer Pipes by Pipe Type Pipe Type (mm) Total Length within SMP Area (m) Sideline Reticulation Rising Main Carrier Overflow Total It can be seen from the above table that the total length of self-cleansing gravity sewers is approximately 39 kilometres. The majority of the sideline and reticulation pipes are 100 mm and 150 mm in diameter, although some are 225 mm and 300 mm in diameter. The rising mains are 150 mm, 250 mm or 375 mm in diameter, and the carriers are 300 mm or 375 mm in diameter. There are two rising mains within the SMP Area. The sewage pumping station (Ref. SP0919) near the corner of George and Bronzewing Streets pumps sewage along George Street towards a local high point north of Thirlmere Way, as shown in Drawing No. MSEC The sewage pumping station (Ref. SP1045) on Castlereagh Street at Myrtle Creek pumps sewage to a high point at Park Street. Only one of the pumping stations (Ref. SP1045) is located within the SMP Area. It lies within the limit of subsidence and will be directly mined beneath by Longwall 25, as shown in Drawing No. MSEC There are two carrier pipes within the SMP Area. The Tahmoor carrier pipe transports the sewage from a point immediately south of Myrtle Creek near the Main Southern Railway, to a point immediately south of Myrtle Creek near Suffolk Place. The pipe first crosses underneath the Main Southern Railway and then travels along York Street and the banks of Myrtle Creek. The Thirlmere carrier pipe transports the sewage from the end of Windeyer Street towards Picton along Bridge Street. % % Mine Subsidence Engineering Consultants 7 Tahmoor Colliery

12 3.2. Hazard Identification Eight hazards have been identified that are associated with mine subsidence impacts on the sewer mains and associated infrastructure:- 1. The hazard that joints and pipes are damaged as a result of mining induced ground strains. 2. The hazard that sewer main grades are reduced below the minimum for self-cleansing as a result of transient or final mining induced ground tilts. 3. The hazard that rising mains are damages as a result of mining-induced ground strains. 4. The hazard that there are blockages or overflows to sewer. 5. The hazard that pumping station will be damaged. 6. The hazard that the power line to the pumping station is damaged. 7. The hazard that impacts may occur to concrete encased sewers or horizontal bores 8. The hazard that impact may occur to sewer creek crossing as a result of valley upsidence and closure. The likelihood and consequence of each hazard and the associated level of risk are discussed in the following sections Hazard 1 Damaged Joints and Pipes The joints between each section of pipe consist of an extra length socket with a rubber ring joint. The joints have been designed to accommodate tensile and compressive ground strains, as well as ground curvatures. Given that the sewer pipes are typically 3 metres long, it is unlikely that the sewer pipes will be damaged as a result of mine subsidence. Any ground movements along the pipes are likely to be transferred to the pipe joints. The maximum predicted strain of 1.8 mm/m will result in movements of up to 5.4 mm at the joints, if the strain is applied along the full length of a 3 metre pipe. In reality, the peak strain is localised and the differential movement will be taken up by a number of joints, reducing the maximum movement at the joints. It is also unlikely that all of the ground strains will transfer into the pipes as the sand bedding will allow the pipes to slide as the ground moves beneath them. The maximum predicted ground curvature of 8.3 kilometres will result in a lateral deviation at the joints of less than 0.2 mm for a 450 mm diameter pipe. Standard rubber ringed joints in PVC pipes can typically tolerate a longitudinal movement of up to 20 mm and an angular deviation of up to 1 o without experiencing adverse impacts. If the sewer has been constructed with extra length sockets, the joints are likely to tolerate a longitudinal movement of up to 40 mm and an angular deviation of up to 3 o without experiencing adverse impacts. The ability of the pipe joints to withstand subsidence impacts will depend on how they were installed. If the pipe sections are not correctly joined at mid-socket length, or have been laid with angular deviations, the maximum allowable movement at the joints will be reduced. It is considered likely, however, that some tolerance will be available at the pipe joints. The CCTV investigation along Milne Street indicated that the majority of pipe joints had been installed correctly. It is therefore concluded that it is unlikely that the pipe joints will be broken by systematic mine subsidence movements. Non-systematic localised ground strains and curvatures higher than predicted can occur where compressive ground strains cause the underlying strata to buckle. It is noted that no impacts have been observed above Longwalls 22 to 24A, even though non-systematic movements have developed near sewer pipes. Based on the experiences gained to date, the likelihood of joints being damaged along correctly installed straight lengths of pipe can be considered UNLIKELY. The joints in sections of the sewer main that are not correctly installed or joints along curved sections of main or joints located where non-systematic ground movements are considered more likely to be damaged. As the peak predicted ground movements Mine Subsidence Engineering Consultants 8 Tahmoor Colliery

13 are localised and likely to be shared across adjacent joints, the likelihood of damage to these joints can also be considered UNLIKELY. The result of damaged joints is the leakage of sewage into the surrounding area. The leaked sewage can be readily cleaned up and damaged joints can be repaired at a relatively low financial cost. This includes the sewer pipes within the rail corridor. The consequence can therefore be considered SLIGHT. The general level of risk can therefore be considered UNLIKELY / SLIGHT --> LOW. If sewage leaks into a watercourse, however, the EPA may impose significant fines on Sydney Water. The consequence can therefore be considered MODERATE. The level of risk for pipes near watercourses can therefore be considered UNLIKELY / MODERATE -- > MODERATE Hazard 2 Sewer Main Grades Reduced Below the Minimum for Self-Cleansing There are approximately 38 kilometres of self-cleansing gravity sewers within the SMP Area. The pipes ability to self-cleanse is dependent on their gradients, which vary depending on the diameter of the pipe. The minimum grades for self cleansing are provided in the Picton Regional Sewerage Scheme Design and Construction Plan (Issue A, Rev. 1), and are shown in Table 3.3. The design for the sewer system was approved by the Mine Subsidence Board, on the condition that the sewers were installed at least 3 mm/m (0.3%) greater than the minimum grade required for the pipes to be self-cleansing, which are also shown in Table 3.3. The grades of each section of pipe were provided in GIS format by Sydney Water, which divided the sewer network into 1511 pipe sections within the SMP Area. It is noted that 3031 metres (8%) of sewer pipes did not have any recorded grades in the GIS. All of these pipes are, however, small in diameter (100 mm or 150 mm) and short in length, with an average length of approximately 5 metres. Upon examination of this information, it was found that the majority of these pipe sections have been installed to grades greater than those required for self-cleansing or the MSB requirements. The average grade of all pipe sections within the SMP Area was approximately 3.2%, or 32 mm/m. However, approximately 353 metres of 100 diameter pipes have existing grades that are less than the minimum grade for self-cleansing, and these are shown in Drawing No. MSEC It was also found that approximately 1,503 metres of pipe have grades that are less than the minimum grades to comply with MSB requirements, and these are shown in Drawing No. MSEC It is presumed that it was impractical for these pipe sections to be installed at steeper grades due to the topography of the area. Mine Subsidence Engineering Consultants 9 Tahmoor Colliery

14 A summary of pipes with pre-mining grades less than self-cleansing grade or not complying with MSB requirements is provided in Table 3.3. Table 3.3 Sewers with Pre-Mining Grades less than Self-Cleansing Grade or MSB Requirements Pipe Diameter Minimum Grade for Self- Cleansing (%) Minimum Grade to comply with MSB Requirements (%) Total Length of Pipes less than Self- Cleansing Grade prior to mining (m) Total Length of Pipes with Grades that do not comply with MSB Requirements (m) Total Each pipeline section consists of two end node points and some also contain some intermediate node points. Predictions of subsidence were made at each node point along each pipeline section within the sewerage network and mining-induced tilts were calculated between each connecting node. The worst tilt between nodes along each pipeline section was used for future analyses. It was also possible to calculate the grades of each pipe section after each stage of mining as the direction of flow could be determined from the GIS information Drawing No. MSEC illustrates which pipe sections within the SMP Area are predicted to experience a change in grade after extraction of Longwalls 22 to 26. Any tilts predicted to be less than 0.5 mm/m (0.05%) were considered as being a negligible change in grade for the purposes of the analysis. A summary of pipes with changes in grade due to mining each longwall is provided in Table 3.4. Table 3.4 Summary of Predicted Change in Grade of Sewers due to Mining Stage of Mining After LW 22 After LW 23 After LW 24 After LW 25 After LW 26 Total Length of Pipes with Decrease in Grade greater than 0.05% (m) 1,327 (3.5%) 3,897 (10.2%) 8,068 (21.2%) 11,412 (30.0%) 13,472 (35.4%) Total Length of Pipes with Increase in Grade greater than 0.05% (m) 776 (2.0%) 1,132 (3.0%) 3,474 (9.1%) 4,971 (13.0%) 6,226 (16.4%) Total Length of Pipes with change in Grade of less than 0.05% It can be seen from the above table that approximately twice as many sewer pipes are predicted to experience a decrease in grade than an increase in grade after each stage of mining. This result can be explained by the design of the sewers in relation to the longwall layout. The majority of sewers in Tahmoor flow towards the rising mains and carrier pipes, which are located along George St, which is not being directly undermined by the proposed longwalls. This means that the majority of sewers will be flowing from areas of maximum subsidence to an area of less subsidence, which will decrease the grades in the pipes. Mine Subsidence Engineering Consultants 10 Tahmoor Colliery (m) 35,976 (94.5%) 33,050 (86.80%) 26,537 (69.7%) 21,695 (57.0%) 18,380 (48.2%)

15 While the above predictions indicate that 35.4% of sewers within the SMP Area will experience a reduction in grade, only a small percentage of sewers will have grades less than those required for selfcleansing after mining. A summary of pipes with grades less than self-cleansing grade after Longwall 26 is provided in Table 3.5, and these are shown in Drawing No. MSEC The calculations did not include pipeline sections with no reported grades in the GIS information. Table 3.5 Sewers with Predicted Grades less than Self-Cleansing Grade Pipe Diameter Minimum Grade for Self- Cleansing (%) Total Length of Pipes less than Self-Cleansing Grade Initial (m) LW 22 (m) LW 23 (m) LW 24 (m) LW 25 (m) LW 26 (m) Total It can be seen from the above table that 674 metres of sewers are predicted to have grades less than those required for self-cleansing, which represents 1.8% of the total length of sewers within the SMP Area. This result is relatively small given that 353 metres of sewers have grades less than self-cleansing prior to mining, and 1,503 metres of sewers have grades that do not comply with MSB requirements. It is noted, however, that some MSB compliant sewers are predicted to have grades less than self-cleansing after the extraction of Longwall 26. It is further noted that no sewer pipes are predicted to experience reverse grades due to the extraction of the proposed longwalls. It can be seen from Drawing No. MSEC that the pipeline sections predicted to have grades less than self-cleansing are spread over the SMP Area and are relatively short in length. The longest section of sewer affected is approximately 59 metres long. The risk associated with sewers that have grades less than self-cleansing is that a blockage may develop in the pipes over time. This risk can be reduced by periodic cleaning of the sewer, which incurs an increase in the cost of maintenance of the system, by re-laying the pipes to higher grades, which may not be practically possible due to the topography of the land. It is noted, however, that the frequency of incidences above the previously extracted Longwalls 22 and 23A are similar to areas that have not been affected by mine subsidence. If the predicted tilts were to be exceeded by 25% to 500%, the number of potentially affected sewers would increase. The remedial measures would be similar, except that monitoring of sewer pipe levels would be required over a more extensive area. Mine Subsidence Engineering Consultants 11 Tahmoor Colliery

16 A summary of the predicted number of sewers affected if the predictions were exceeded are shown in Table 3.6. Table 3.6 Total Length of Sewers if Predictions exceeded Increased Prediction Total Length of Pipes less than Self- Cleansing Grade after LW 26 (m) Predictions x Predictions x Predictions x Predictions x Predictions x Predictions x The above information has been used to develop the risk assessment below. The majority of sections of sewer main that have a pre-mining grade less than the minimum for selfcleansing also have a predicted grade less than the minimum grade for self-cleansing after mining. The likelihood of these sections of sewer having a grade less than the minimum for self-cleansing can therefore be considered ALMOST CERTAIN. There are some sections of sewer main which have pre-mining grades that are greater than the minimum required for self-cleansing but are predicted to have grades less than the minimum for self-cleansing, during or after mining. The likelihood of these pipes having a grade less than the minimum for selfcleansing can therefore be considered LIKELY. The majority of sewer mains have pre-mining grades that are greater than the minimum required for selfcleansing and are predicted to have grades greater than the minimum for self-cleansing during and after mining. The likelihood of these pipes having a grade less than the minimum for self-cleansing can therefore be considered UNLIKELY, but not rare, as it possible that the predictions could be exceeded. The result of a sewer main having a grade less than the minimum required for self-cleansing is the increased likelihood of blockage in the sewer. The consequence of a blockage is dependant on the number of properties that are affected, the cost to remove the blockage and, if required, the cost to repair the main. The consequence is therefore dependent on the size of sewer main and has been summarised in Table 3.7. Table 3.7 Consequence of a Sewer Main Grade Being Less than the Minimum for Self-Cleansing Pipe Diameter (mm) Minimum Number of Properties Affected Consequence Consequence if near a watercourse SLIGHT MODERATE SLIGHT MODERATE SLIGHT MODERATE MODERATE SEVERE 375 Carrier Pipe almost whole town SEVERE SEVERE Mine Subsidence Engineering Consultants 12 Tahmoor Colliery

17 The levels of risk associated with the grade of a sewer main being less than the minimum required for self-cleansing due to the extraction of Longwalls 24 to 26 are summarised in the table below. Table 3.8 Levels of Risk Associated with the Grade of a Sewer Main Being Less than the Minimum for Self-Cleansing due to the Extraction of Longwalls 24 to 26 Pipe Grade Pipe Diameter (mm) Sections of Pipe with a 100 Grade Less than the Minimum for Self- Cleansing Before Mining 150 Sections of Pipe with a Grade Less than the Minimum Grade for Self-Cleansing During or After Mining only Sections of Pipe with a Grade Greater than the Minimum Grade for Self-Cleansing, before, during or after mining Level of Risk ALMOST CERTAIN / SLIGHT MODERATE ALMOST CERTAIN / SLIGHT MODERATE LIKELY / SLIGHT MODERATE LIKELY / SLIGHT MODERATE UNLIKELY / SLIGHT VERY LOW UNLIKELY / SLIGHT VERY LOW UNLIKELY / SLIGHT VERY LOW UNLIKELY / MODERATE LOW UNLIKELY / SEVERE MODERATE Level of Risk (Near a Watercourse) ALMOST CERTAIN / MODERATE HIGH ALMOST CERTAIN / MODERATE HIGH LIKELY / MODERATE HIGH LIKELY / MODERATE HIGH RARE / MODERATE LOW RARE / MODERATE LOW RARE / MODERATE LOW RARE / SEVERE MODERATE RARE / SEVERE MODERATE The likelihood of any adverse impacts for sewer pipes located outside the predicted 20 mm subsidence contour have been assessed as LESS THAN VERY RARE. A risk analysis has therefore not been made for these pipes. It can be seen from the above table that the sewer pipes with an assessed level of risk of moderate or high include the following pipes:- All sewers identified on Drawing No. MSEC All carrier sewers (300 mm diameter pipe under Main Southern Railway and all 375 mm diameter pipes) within the predicted 20 mm subsidence contour on Drawings Nos. MSEC and MSEC It is noted that the above risk assessment is based upon standard subsidence predictions, which were provided in the SMP Report No. MSEC157. Following the mining of Longwalls 22 to 24A, no pipes have experienced reverse grades as predicted. However, subsidence and tilts have substantially exceeded predictions above Longwall 24A and this may also be observed above Longwall 25. This is discussed in further detail in Section 3.12 and Section Mine Subsidence Engineering Consultants 13 Tahmoor Colliery

18 3.5. Hazard 3 Rising Mains There are approximately 950 metres of rising mains within the SMP Area. There are two rising mains within the predicted limit of subsidence. The George Street rising main is not directly mined beneath by the proposed longwalls, while the Castlereagh Street rising main will be directly mined beneath by Longwall 25. The predicted subsidence and mining-induced tilts are expected to have little impact on the serviceability of the rising mains. The George Street rising main begins at Sewage Pumping Station SP0919, outside the predicted limit of subsidence and finishes north of Thirlmere Way within the predicted limit of subsidence. The head difference between the pumping station and the end of the rising main is predicted to reduce by approximately 180 mm, which should improve the serviceability of the main. The Castlereagh Street rising main runs above Longwall 25 and the head difference between the pumping station and the end of the rising main is predicted to be less than 20 mm, which is negligible and unlikely to affect the serviceability of the main. While ground strains along the rising mains have not been predicted, it is noted that the maximum predicted strains within the subsidence bowl due to the proposed longwalls are 1.4 mm/m tensile and 1.8 mm/m compressive. These predicted strains are less than the design requirements of the Mine Subsidence Board, which were 1.5 mm/m tensile and 2.5 mm/m compressive. The minimum predicted radius of curvature within the subsidence bowl is approximately 8.3 kilometres, which is greater than design requirements of the Mine Subsidence Board. The designer of the rising mains has certified that the designs for the rising main are in accordance with the Mine Subsidence Board s requirements. Non-systematic movements may occur in the vicinity of rising mains, and these movements may result in impacts to the rising main, particularly the rising main along Castlereagh Street above Longwall 25, which is approximately 200 metres long. Non-systematic movements have been observed above Longwalls 22 to 24A and no impacts have been observed to sewer pipes. Furthermore, longwalls have mined directly beneath 960 metres of DICL and 2700 metres of older CICL pressured water mains and impacts have been observed to one CICL pipe to date. It is considered therefore, that the likelihood of impacts occurring to the rising main pipes is RARE. The result of leakage of a rising main is leakage of sewage into the surrounding area, stoppage of the pumping station, and build-up of sewage at the pumping station, which backs onto Myrtle Creek. The rising main contains a number of automated alarms that immediately alert Sydney Water in the event of a fault occurring. The pumping station at Castlereagh Street has a 4 hour capacity to store sewage before any overflows into Myrtle Creek. If the length of time to repair the leakage exceeds four hours, it is possible that sewage may pollute Myrtle Creek. The consequence can therefore be considered MODERATE. The level of risk can therefore be considered RARE / MODERATE --> LOW Hazard 4 - Blockages or Overflows Blockages or overflows can occur outside mine subsidence areas due to the condition of the sewer main. It is possible that blockages or overflows could occur during mine subsidence events. While these blockages and/ or overflows may not be due to mine subsidence it is important for Sydney Water and Tahmoor Colliery to manage these issues together, to reduce the impacts. A CCTV investigation of Milne Street indicated that minor ponding, and one blockage at a drop junction had occurred prior to mining. Some debris was also found in the pipe network. Due to the condition of the pipes in Milne Street, the likelihood of blockages or overflows occurring is therefore considered UNLIKELY. The result of a blockage or overflow is leakage of sewage into the surrounding area. The leaked sewage can be readily cleaned up and damaged joints can be repaired at a relatively low financial cost. The level of risk can therefore be considered UNLIKELY/ SLIGHT --> LOW. If sewage leaks into a watercourse, however, the EPA may impose significant fines on Sydney Water. The consequence can therefore be considered MODERATE. The level of risk near watercourses can therefore be considered UNLIKELY / MODERATE --> MODERATE. Mine Subsidence Engineering Consultants 14 Tahmoor Colliery

19 3.7. Hazard 5 - Sewage Pumping Station There is one sewage pumping station within the SMP Area, Station SP1045 on Castlereagh Street, and it is located within the predicted limit of subsidence. The maximum predicted subsidence at this pumping station is approximately 735 mm after the extraction of Longwall 26. The predicted systematic mining-induced tilts at the station are 3.7 mm/m (0.37%) after the extraction of Longwall 26. The maximum predicted systematic strains at the station are 0.3 mm/m tensile and 0.3 mm/m compressive. It is considered that the pumping station is unlikely to be adversely impacted if normal systematic subsidence movement occurs. However, it is possible that non-systematic movements may adversely impact the station structures. Pumping station SP1045 is located close to Myrtle Creek, where upsidence and closure movements will occur. In particular, it is considered that the backfilled and buried 8 metre deep chamber may experience impacts if it is subjected to bending in the vertical plane or ground compression. However, this chamber has been constructed with reinforced concrete and occupies a small footprint of 3 metres in diameter. A structural analysis suggests that cracking to the chamber may develop if curvatures exceed 1.1 x 10-6 mm -1, which equates to a radius of curvature of approximately 900 metres (JMA, 2008). This curvature is substantial compared to normal observed mining-induced curvatures. Furthermore, the tank is partially protected from ground curvature by 100 mm thick polystyrene foam, which has been placed between the external concrete faces and the excavated foundations. The likelihood of impacts occurring to Pumping station SP1045 is therefore considered VERY RARE. The pumping station contains a number of automated alarms that immediately alert Sydney Water in the event of a mechanical or electrical fault occurring to the station. The pumping station has a 4 hour capacity to store sewage before any overflows into Myrtle Creek. If the length of time to repair the fault exceeds four hours, it is possible that sewage may pollute Myrtle Creek. Sydney Water has developed robust contingency plans to respond and address failure modes. The consequence of mechanical or electrical failure can therefore be considered MODERATE. If cracking develops within the chamber, the consequence depends on the depth of the crack. Normal operating depth is the bottom 1 metre of the chamber. A possible failure point is the structural joint, which is located above the normal operating depth. If cracking occurs, the pumping rate can be increased to reduce the depth of sewage in the chamber. The cracks can also be repaired by patching. Horizontal struts can also be installed across the chamber if required (JMA, 2008). The consequence of cracking can therefore be considered MODERATE. The level of risk can therefore be considered VERY RARE / MODERATE --> LOW. Given limited access to the chamber, automated continuous tilt monitoring of the chamber to measure deflection and visual inspections (if required) has been included as part of this management plan. Trigger levels have been set following recommendations from structural engineer John Matheson & Associates (2008) Hazard 6 - Power Line to the Sewage Pumping Station The sewage pumping stations are connected via buried cables to the Integral Energy overhead reticulation network. The maximum predicted ground strains within the subsidence bowl due to the proposed longwalls are 1.4 mm/m tensile and 1.8 mm/m compressive. Standard copper power lines can typically tolerate strains up to 20 mm/m without damage. No impacts have been observed to power lines during the mining of Longwalls 22 to 24A. It is unlikely, therefore, that power supply to the pumping stations will be disrupted due to mine subsidence impacts. The likelihood of the power line being damaged by systematic mining impacts can therefore be considered RARE. The pumping station contains a number of automated alarms that immediately alert Sydney Water in the event of a fault occurring. The pumping station has a 4 hour capacity to store sewage before any Mine Subsidence Engineering Consultants 15 Tahmoor Colliery

20 overflows into Myrtle Creek. If the length of time to repair the fault exceeds four hours, it is possible that sewage may pollute Myrtle Creek. The consequence can therefore be considered MODERATE. The level of risk associated with a damaged power line to the pumping station can therefore be considered RARE / MODERATE --> LOW Hazard 7 - Concrete Encasements and Horizontal Bores Typical Concrete Encasements and Horizontal Bores There are some sewer pipes within the SMP Area that have been concrete encased. These encasements are typically found across driveways and structures. Concrete encased sewers are more vulnerable to mine subsidence impacts when compared to normal sewers as the pipe joints are unable to slide in response to ground strains. If the encasements are long and subjected to large ground strains, there would be a risk of sewer breakage at the joint or within the pipe. In this case, the majority of concrete encasements do not appear to be very long and the predicted ground strains are relatively low. It is therefore unlikely that concrete encased sewers will be adversely impacted by mine subsidence except for the 66m section described in section There are, however, a number of concrete encasements that cross creeks and watercourses and these are discussed in the next section. There are some horizontal bores within the SMP Area. These are typically found across roads and railways. Horizontal bores are more vulnerable to mine subsidence impacts when compared to normal sewers as there are fewer pipe joints to accommodate the mining-induced ground strains. In this case, the majority of horizontal bores do not appear to be very long and the predicted ground strains are relatively low. It is therefore unlikely that horizontal bores will be adversely impacted by mine subsidence except for the bores described in section The likelihood of a typical short length concrete encased pipe or horizontal bore being damaged by systematic mining impacts can therefore be considered RARE. If an impact occurs, the time taken to repair a leak is greater than for other pipes as access to the leakage point is more difficult. The consequence of an impact is therefore assessed as SLIGHT for small concrete encased sewer pipes (100 and 150 mm diameter) and MODERATE for larger horizontal bores, such as those under roads and the Main Southern Railway. The level of risk associated with concrete encased pipes and horizontal bores can therefore be considered RARE / SLIGHT --> LOW for small pipes and RARE / MODERATE --> LOW for large pipes mm Diameter Sewer Main between Huen Place and Castlereagh Street There is a 225 mm diameter sewer main that includes some long sections of concrete encased mains or horizontal bores. The main begins at the rear of a property on Huen Place and finishes at the Sewage Pumping Station on Castlereagh Street (SP 1045). A plan of the sewer main is shown in Fig Mine Subsidence Engineering Consultants 16 Tahmoor Colliery

21 225 MM DIA SEWER MAINS Standard Spigot & Socket Horizontal Bore Concrete Encased Sewer Creek Crossing BRIDGE ST Myrtle Creek MILNE ST HUEN PL PARK ST WINPARA CL FRASER ST PIMELIA ST ELPHIN ST MONICA PL LEIHA PL LW No.25 AMBLECOTE PL CASTLEREAGH ST HILTON PARK RD CONNOR PL SP1045 MAIN SOUTHERN RWY Fig mm dia Sewer Main between Huen Place and Castlereagh Street As shown by the above plan, there are two sections of horizontal bore. One section is approximately 43 metres long, located near the end of Elphin Street. One section is approximately 235 metres long and includes a pit which connects the main with a 100 mm dia pipe. There are three sections of concrete encased sewer. One section is approximately 8 metres long and crosses Myrtle Creek near the end of Huen Pl. One section is approximately 12 metres long and crosses Myrtle Creek at Bridge Street. One section is approximately 66 metres long and connects the two sections of horizontal bore. A long section plot is shown in Fig. 3.3, which is based on the Sydney Water Work-As-Executed plans (Ref. 3) for Tahmoor Area 6 and 7, and surface contours provided by the Department of Lands. Predictions of subsidence, change in grade and strain have been made along the entire length of the 225 mm dia sewer main at the completion of each longwall, and these are shown in Fig It can be seen from this graph that the predicted profiles appear irregular in shape, which is explained by the many changes in direction along the sewer main, relative to the longwall layouts. The sewer main is also expected to experience valley closure and upsidence movements as it crosses Myrtle Creek (which is discussed in Section 3.10) and differential horizontal movements due to valley closure as the main runs alongside the banks of Myrtle Creek. In the case of the horizontal bores, it is predicted that the bores will experience up to 900 mm of subsidence, with a maximum decrease in grade of approximately 0.35 % following the extraction of Longwall 25. This decrease in grade is expected to reduce to approximately 0.25 % following the extraction of Longwall 26. The predicted maximum strains are approximately 0.50 mm/m tensile and 0.75 mm/m compressive. In addition to these predictions, the bores will experience some transient tilts and strains as Longwall 25 passes beneath them. The predicted travelling tilts are expected to generally increase the gradients of the pipes as the longwall is travelling in a southeast to northwest direction. The maximum travelling strains are expected to be less than 0.5 mm/m as the bores are oriented in a direction that is approximately 45 degrees to the longwall panel. It is also possible that the bore will be subjected to differential horizontal movements as a result of valley closure, as it is located alongside Myrtle Creek. It is expected, however, that these movements will be Mine Subsidence Engineering Consultants 17 Tahmoor Colliery