MINE SUBSIDENCE BOARD

IN ASSOCIATION WITH

P U T T I N G S E R V I C E A N D T H E N E E D S O F P E O P L E F I R S T

TRANSGRIDPOWER SUPPLY AUTHORITIES

DEPARTMENT OF MINERAL RESOURCESDEPARTMENT OF URBAN AFFAIRS AND PLANNING

NSW MINERALS COUNCIL

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CONTENTSFOREWORD TO THIRD EDITION ............................................................... 4

FOREWORD TO SECOND EDITION ........................................................... 5

FOREWORD .................................................................................................. 5

POLICY STATEMENT ................................................................................... 6

EFFECTS OF COAL MINING ON TRANSMISSION LINES ..................... 7

PROCEDURES .............................................................................................. 8

GRANT OF MINING LEASES AND APPROVALS TO EXTRACT PILLARS/LONGWALLS ............................................................................... 8

TRANSMISSION LINE ROUTE SELECTION ............................................. 8

FLOW CHART FOR GRANT OF MINING LEASE ...................................... 9

CO-ORDINATING MINING AND TRANSMISSION LINE ACTIVITIES ........................................................ 10

CHOOSING BETWEEN TRANSMISSION LINE MODIFICATIONS AND COAL STERILISATION ..................................................................... 14

LEGISLATION ............................................................................................. 16

APPENDIX A: COAL MINING AND ITS EFFECTS ................................. 17

OVERVIEW OF MINING METHODS ........................................................ 17

UNDERGROUND MINING ........................................................................ 17

FIGURE 1 - SUBSIDENCE FROM BORD AND PILLAR MINING .......... 19

SUBSIDENCE EFFECTS ............................................................................ 20

FIGURE 2 - SUBSIDENCE FROM PANEL AND PILLAR MINING ........ 21

FIGURE 3 - LONGWALL MINING ............................................................ 22

OPEN CUT OPERATIONS ......................................................................... 22

FIGURE 4 - TERMINOLOGY ASSOCIATED WITH SUBSIDENCE ........ 23

APPENDIX B: THE IMPACT OF COAL MINING ON TRANSMISSION LINES ...... 24

FIGURE 5 - TOWER DEFORMATION FROM HORIZONTAL LEG DISPLACEMENTS ............................................................................. 25

FIGURE 6 - RIGID BODY ROTATION MODEL ......................................... 25

STRUCTURE TYPES AND FUNCTIONS .................................................. 25

FIGURE 7 - TYPICAL DOUBLE CIRCUIT STEEL LATTICE TOWERS .... 26

APPENDIX C: DESIGN MEASURES TO PROVIDE FOR SUBSIDENCE .... 27

LARGE STEEL LATTICE STRUCTURES ................................................... 27

WOOD POLE AND SIMILAR TRANSMISSION LINES ......................... 28

CONDUCTOR CONTROL ........................................................................... 28

SUBSTATIONS ........................................................................................... 28

APPENDIX D: CASE STUDIES ................................................................. 29

INITIAL CONTACTS FOR ENQUIRIES RELATING TO MATTERS COVERED BY THE GUIDELINES ..............................................Back Cover

First Published August 1988Second Edition, December 1990

Published by the Mine Subsidence Board,117 Bull Street, Newcastle West, N.S.W. 2302

Copyright by the Mine Subsidence Board

Apart from any fair dealing for the purpose of private study, research, criticism or review, as permitted under the Copyright Act, no part may be reproduced by any process without the written permission of the Publisher.

ISBN O 646 34359 9

Production - Helen Duncan Communications and MediaDesign & Artwork - Curzon Creative ServicesPrinting - NCP Printing

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FOREWORDThe preparation of this document was originally commissioned by the Mine Subsidence Board in October 1983, following incidents where conflicts occurred between coal mining and transmission line construction. These conflicts, if not resolved, might result in substantial damages to the transmission lines concerned, or sterilisation of mineable coal resources. The Mine Subsidence Board then set up a Committee, consisting of all interested parties, to draft a much needed document. The Committee was not intended to be a mandatory one, but would make recommendations to the Mine Subsidence Board, which would then determine the adoption of the Committees recommendations. Committee recommendations were generally arrived at by consensus and this consensus process has proven to be more exhaustive and justifiably time consuming.

It was not until early 1987, after some very active participation by members of the Committee, particularly the Electricity Commission, that the draft document progressed significantly to the format and contents agreeable to all parties.

After considerable discussions, the Committee agreed that the electrical effects of transmission lines on coal mines, which could be hazardous, should not be dealt with by the Committee, but reference is to be made to the Electricity Commission.

One significant consensus arrived at by the Committee is that this document should be endorsed by all organisations represented on the Committee.

These organisations, as at September 1987, were:

Mine Subsidence Board NSW Electricity Commission of NSW Department of Mineral Resources NSW Department of Industrial Relations NSW Department of Environment and Planning NSW NSW Coal Association

Endorsement was requested in February 1988, and received from all organisations on 4th July 1988.

As a result of a change in Government in March 1988, the Chief Inspector of Coal Mines now comes under the Department of Mineral Resources, and not the Department of Industrial Relations. The Department of Environment and Planning changed to the Department of Planning. Further editorial works had to be carried out following these changes.

Even with continuous editorial works, I have found it quite a task to keep up with the changing environment which this document attempts to address. Some text and statistics would have now been dated; but it would not be beneficial to delay printing of this document on grounds of these minor deficiencies.

It is intended, therefore, that this document will be reviewed and updated by the Committee every two years. Any comments and suggestions by users of this document are welcome. Where up to date information is required by the user, enquiries may be directed to relevant Departments and Authorities.

I take this opportunity to thank all Committee Members, past and present, for their individual contributions of expertise, hard work and perseverance to the success of this document.

Special thanks are due to Mrs J Rose, our Minute Secretary, for her hard work and patience in typing and revising the many drafts and manuscripts.

E M To BE, MBdgSc, FIEAust, FASCEChairman Committee on Transmission Lines

July 1988

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FOREWORD TO THE THIRD EDITIONThe preparation of a document to examine the undermining of transmission lines was originally commissioned by the Mine Subsidence Board in 1983. The information in this Third Edition has been updated to include current references, more comprehensive details and case histories not available in previous editions. Following the success of earlier versions of this document, a similar set of Guidelines has been developed for Roads and Coal Mining with Respect to Mine Subsidence.

The availability of such reference documents relevant to Australian conditions is important to our understanding of mine subsidence and the ability to maximise extraction of coal reserves.

As coal reserves are further developed there will be a need to undermine an increasing number of transmission towers, whilst still maintaining continuity of supply. The relevant agencies will continue to liaise on such matters and investigate any new support methods which allow transmission towers to be undermined. Accordingly, it is intended that the Guidelines will evolve as new information becomes available and be reviewed in the future.

I take this opportunity to thank all Committee Members, past and present, for their valuable contribution to the Guidelines and acknowledge the efforts of our Minute Secretary, Mrs J Rose, in typing and revising the many drafts.

G Cole-ClarkChairman

Committee on Transmission Lines

September 1997

FOREWORD TO SECOND EDITIONFeedback from users has indicated general acceptance of this document as a useful and effective guideline for the industries and departments concerned, and fulfilled a role in bridging the planning gap in coal mining and electricity transmission.

Due to changes to departments and internal procedures, minor amendments to the First Edition were considered necessary. Furthermore, supplies of this document had diminished to a small number of copies, and reprinting was obviously warranted.

Committee Members were requested to re-nominate and forward suggested amendments. As a result, the Committee has three new members, and the members organisations are:

Mine Subsidence Board NSW Electricity Commission of NSW Department of Minerals and Energy NSW Department of Planning NSW NSW Coal Association

The Electricity Commission has advised that ownership of many transmission lines up to 132 kV have been or are being transferred to various county councils. Users of these Guidelines should refer to appropriate county councils in place of the Electricity Commission in such cases.

Amendments are in Sections 2, 3.1.2, 3.2.3, 3.3.2, 3.3.3 and 3.4.2, 3.5, Appendix A, Figures 6 and 7, and Flow Charts A and B. I carried out the final editing of these amendments from suggestions provided by Committee Members, to whom I am much in debt for their contributions.

E M To CPEngChairman Committee on Transmission Lines

December 1990

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EFFECTS OF COAL MINING ON TRANSMISSION LINESUnderground coal mining may result in transmission towers being subjected to vertical and horizontal displacements and tilt. Vertical displacements may reduce clearance from ground surface and roads, and lead to infringement of statutory requirements for clearance of the transmission lines. Horizontal displacement and tilt may affect the alignment and tension of the transmission lines. Subsidence effects on a tower structure may render it unserviceable or lead to collapse.

The effects of open cut coal mining on transmission lines are different, in that relocation or deviation of a transmission line is generally possible, but may involve substantial costs and a suitable alternative route must be found. Open cut coal mines in proximity to transmission lines may produce similar effects to those caused by underground mines. Such ground movements can result from relaxation of the sides of the excavations. Other aspects of open cut mining that are not related to subsidence can affect transmission lines, eg, vibrations from blasting, dust contamination of insulators.

Further details of the impact of coal mining on transmission lines are included in Appendix B.

Throughout this document the term supply authority has been adopted to denote an organisation that manages a transmission and distribution network and associated infrastructure. In some cases, two or more authorities could be involved in the various processes and they may be involved at State or local levels.

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POLICYSTATEMENTThe extraction of coal and the supply of electricity are both vital to the NSW economy.

The coal mining and electricity supply industries are interdependent in that coal is the major energy source for electricity generation and electricity is the main energy source for coal extraction.

The coal mining companies and the power supply authority each own and operate extensive infrastructure, which must be maintained and developed.

There can be some conflict of interests between coal mining and siting of power transmission lines and associated infrastructure.

Maintenance and development of infrastructure can be achieved and conflict resolved if effective liaison and communication exists between the power supply authority, the collieries, and regulatory authorities.

This can be facilitated by the Department of Mineral Resources and the Mine Subsidence Board.

It is the belief of the Mine Subsidence Board that its decision in such cases, within the guidelines of the Mine Subsidence Compensation Act 1961, can provide the protection so essential to all interests.

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PROCEDURES3.1

GRANT OF MINING LEASES AND APPROVALS TO EXTRACT PILLARS/LONGWALLS

3.1.1 Grant of Mining Leases

Leases to mine coal (mining leases), either for the establishment of new coal mines or for additions to existing coal mines, are granted pursuant to the Mining Act 1992.

Before the Minister for Mineral Resources grants a mining lease, the Mining Act requires notices of the proposal to be served, by the Department of Mineral Resources, on each government agency that would be materially affected by the grant of the lease. The Act gives the government agency, upon whom a notice has been served, a statutory right to object to the granting of the lease or to require conditions to be included in the lease. Provision is made in that Act for the resolution of any dispute. Subject to the grant of development consent, mining leases are granted by the Minister for Mineral Resources - Refer Flow Chart.

Under its charter, the Mine Subsidence Board is not bound to object on grounds of damage compensation, but may advise the Chief Inspector of Coal Mines of the risk of damage, if any.

Similarly, before inviting tenders for a mining lease, the Mining Act requires the Minister for Mineral Resources to follow the above procedures.

Reference: Schedule 1, Part 2, Division 1, Sections 5-10, Mining Act 1992.

3.1.2 Approvals to Extract Pillars/Longwalls

Section 138 of the Coal Mines Regulation Act 1982, provides that no method of mining other than the bord and pillar system shall be used except with the approval of the Minister. Pillar extraction and longwall mining applications are normally approved by the Chief Inspector of Coal Mines under delegation from the Minister for Mineral Resources and subject to such conditions as may be imposed.

In accordance with approved practice, the Chief Inspector of Coal Mines examines all applications for approval under Section 138 of the Coal Mines Regulation Act and processes them in accordance with Section 3.3.2 - Undermining Existing Transmission Lines.

3.2

TRANSMISSION LINE ROUTE SELECTION

3.2.1 General

The selection of a route for a transmission line involves consideration of environmental, economic and technical constraints. On many occasions, the various constraints conflict and it is necessary to achieve a compromise which optimises the public interest and also recognises the legitimate rights of all persons concerned.

FLOW CHART FOR GRANT OF MINING LEASE1. DMR approves Conceptual Project Development Plan2. DMR organises Planning Focus Meeting - Designed to

provide early interface between Govt. and company to as-sist in preparation of EIS and Schedule 1,

Part 2, MA referencing

Application for Mining Lease lodged

DMR makes reference to PSA Sched-ule 1, Part 2, Division 1,

Section 5 of MA

DMR advises applicant to apply for Development Consent

DMR provides comments on EIS to DUAP

Development Consent determined by Consent Authority

PSA objects to or proposes condi-tions to be included in the mining

lease Schedule 1, Part 2, Division 1, Section 9 of MA

DMR follows up any objections/proposals with Schedule 1, Part 2,

Division 1, Section 10 (1) of MA

DMR refers any unresolvedobjections/proposals to Premier

Schedule 1, Part 2, Division 1,Section 10 (1) of MA

Premier determines anyunresolved objections/proposals

Schedule 1, Part 2, Division 1,Section 10 (2) of MA

Replies with possible requests for special conditions

The Minister grants Mining Lease Section 63 Mining Act

Legend The Minister - Minister for Mineral Resources MA - Mining Act, 1992 PSA - Power Supply Authority DMR - Department of Mineral Resources DUAP - Department of Urban Affairs and Planning

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The liaison meetings will consider the costs of proposed design solutions and make recommendations to the Department of Mineral Resources and the Mine Subsidence Board regarding the allocation of costs, particularly when structures are designed to make provision for future protective and mitigatory works.

(d) Design Approvals/ParametersThe above procedure will be modified slightly depending on whether the proposed route lies in or outside a declared Mine Subsidence District.

(1) In a Mine Subsidence DistrictOnce the route has been selected and the EIS process completed, the power supply authority should submit a formal building application to the Mine Subsidence Board for the transmission towers and any associated infrastructure. Any approval granted by the Mine Subsidence Board will be subject to conditions. Typically these include the submission of detailed engineering drawings certifying that the structures have been designed to accommodate subsidence parameters attached to the approval, or detailing any provision for future mitigatory works. Normally such an application need only be accompanied by plans showing the location and outline of the structures. The Boards approval should be obtained prior to any detailed design works being undertaken.

As part of the approval process, the Mine Subsidence Board will obtain current mine subsidence parameters from the Department of Mineral Resources and include them in the approval conditions.

(2) Not in a Mine Subsidence DistrictBefore detailed design is commenced, the power supply authority should obtain formal subsidence parameters from the Department of Mineral Resources.

The power supply authority should notify the Mine Subsidence Board of the subsidence parameters adopted for the design, to assist the Mine Subsidence Board with administration of claims in the future.

To ensure that the transmission network and coal mining operations in the area can co-exist, it is important that effective liaison is maintained during the entire planning process. Participants must acknowledge that considerable lead times are involved in the various stages of the process, and must be allowed for in any time scheduling.

3.3.2 Undermining Existing Transmission Lines

Pillar extraction and longwall mining applications are normally approved by the Chief Inspector of Coal Mines under delegation from the Minister for Mineral Resources under the provisions of the Coal Mines Regulation Act 1982.

(a) Pillar/Longwall Extraction ApprovalsAs part of the application process for approval for pillar or longwall extraction of coal by underground methods, collieries are required to address issues associated with the mining plan and its effect on the ground surface and existing structures. They are required to identify all structures affected by the proposed mining, and to assess the effect of resulting

subsidence on them. In this context, subsidence means all horizontal and vertical ground movements resulting from mining, and when major services and infrastructure are involved, the colliery should adopt a conservative value of 35 for the angle of draw, even though subsidence calculations and assessments may be based on lesser values.

The whole of any easement for transmission lines that falls either partially or completely within the zone of influence of any proposed mining based on a 35 angle of draw, shall be included in the identification process.

This will ensure that the owners/operators of major services are made aware of mining in the vicinity of their installations and will allow the effects of subsidence on them to be assessed independently.

(b) Liaison MeetingsCollieries preparing such an application should arrange a liaison meeting with the power supply authority, the Department of Mineral Resources and the Mine Subsidence Board to allow:

(1) discussion of the mining plan prior to formal application being made to the Chief Inspector of Coal Mines. The colliery will provide the following information:

Depth of the extraction; Thickness of the seam to be extracted; Types of extraction (eg, pillar or longwall); Proposed start and finish dates; Location of proposed starting point; A scale drawing (1:8,000 or less) locating the

proposed mining and showing appropriate easting and northing co-ordinates and location of affected transmission line structures;

The maximum subsidence, strains and tilts at each of the structures;

Details of any known geological anomalies that may affect subsidence.

(2) the Department of Mineral Resources to assess and ratify the collierys subsidence predictions

(3) the power supply authority to assess what, if any, mitigatory works and/or monitoring is appropriate, and to submit any special requirements that should be attached to the approval to the Chief Inspector of Coal Mines

(4) the Mine Subsidence Board to determine if the affected structures are eligible for compensation under the Mine Subsidence Compensation Act, to assess the cost of mitigatory works and advise the Chief Inspector of Coal Mines if appropriate

(5) the power supply authority to negotiate with the colliery regarding the allocation of costs that may not be compensable under the Mine Subsidence Compensation Act

(6) the colliery to finalise details of the application(7) the power supply authority to submit a claim for

compensation to the Mine Subsidence Board and to obtain

3.2.2 Constraints

In general, route selection involves the determination of the most suitable route between two nominated end points, but from time to time, there are complicating factors such as the desirability or necessity for the line to pass via a third locality which may be a centre of growing demand for electricity.

The range of factors which may affect the final selection of a transmission line route include:

Topography National Parks and Nature Reserves State Forests Mineral Prospects and Mining Operations Airstrips Urban and Industrial Areas Areas of Significant Historical or Archaeological Interest Scientific Establishments Pipelines Railway Lines Road Systems Communication Networks Irrigation Areas Property Owner Requirements Flood Prone Areas Native Title

3.2.3 Consultation with Public Authorities

As a standard procedure, the power supply authority exchanges correspondence with numerous Commonwealth, State and Local Government authorities regarding its transmission line proposals. The Department of Mineral Resources and the Mine Subsidence Board are included in this process. All authorities potentially affected are requested to comment on the proposals and, where appropriate, to provide any information that may be relevant to the detailed route location, line design, and environmental assessment of the project.

Where lines are to be constructed through Mine Subsidence Districts, this process of consultation normally leads to the accommodation of the Boards requirements concerning the siting and design of the transmission line structures. Formal application and approval to construct is required under the Mine Subsidence Compensation Act. Formal application before detailed design will facilitate meeting the Boards requirements without delay or change to design. Application should be made as soon as the particular route is proposed.

3.2.4 Procedures under the Environmental Planning and Assessment Act

Transmission lines are generally covered by the provisions of Part V of the Environmental Planning and Assessment Act, 1979, which makes the board of the authority the Determining Authority for its transmission line projects. This requires the authority to carry out an environmental assessment of all its transmission line proposals.

If it is determined that a particular transmission line will have a significant impact on the environment, an Environmental Impact Statement is prepared by the power supply authority, placed on public display, and submissions from interested parties are received and assessed.

Finally, a determination report recommending a particular route for the line is prepared and also made public. In general, it may be assumed that all major steel tower lines require Environmental Impact Statements, as well as some steel, wood and concrete pole lines, depending on their proposed locations.

3.3

CO-ORDINATING MINING AND TRANSMISSION LINE ACTIVITIES

3.3.1 Planning New Transmission Lines

(a) GeneralWhen the power supply authority is planning routes for new or replacement transmission lines and/or associated infrastructure, the Department of Mineral Resources and the Mine Subsidence Board will be notified of the proposed route as part of the power supply authoritys planning procedures. The proposed route would be assessed in terms of its interaction with proposed or existing coal mining in the area traversed.

In order to facilitate these assessments, the power supply authority will, in conjunction with the Department of Mineral Resources and Mine Subsidence Board, arrange liaison meetings where the proposed route can be considered in relation to any mining (planned or existing). The Department of Mineral Resources will arrange for the appropriate collieries to attend or be represented at these liaison meetings.

(b) Liaison MeetingsThe goals of the liaison meetings are to:

(1) identify coal resources and their mining status(2) establish realistic subsidence design parameters(3) identify the most appropriate design solution (including

deviation and relocation) for the transmission line and structures, and the associated costs

(4) determine the type and degree of monitoring that may be appropriate for the structures.

(c) Design SolutionsWhen identifying the appropriate design solution, consideration will be given to designing towers and structures to:

(1) accommodate the predicted subsidence parameters(2) make provisions for mitigatory or preventative works in

the future when and if appropriate. In selecting the appropriate design solution, consideration will be given to designs that provide the optimal economic solution for the community.

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In order to relocate a section of an existing transmission line, the power supply authority would generally need at least three (3) years notice. (Note: This assumes that an Environmental Impact Statement for the line deviation is not required.) In general the power supply authority has agreed to deviate transmission lines wherever an acceptable alternative route could be obtained and agreement could be reached in respect of costs.

Ground settlement in rehabilitated open cut mines occurs over a long period of time and this must be addressed when routes are selected and structures designed for rehabilitated areas.

The power supply authority will select any route in proximity to an existing or proposed open cut mine, or crossing an area formerly mined, in the light of appropriate geotechnical reports, with a view to ensuring that the transmission line is not damaged by mine subsidence precipitated by the mining. Where the settled route is in a Mine Subsidence District, the Mine Subsidence Boards approval will be explicitly obtained in the usual way.

Where operations of an open cut mine are being planned to take place adjacent to an existing transmission line, typical conditions that the supply authority may request the Minister for Mineral Resouces to attach to any approval are:

(1) prohibition of excavation within the transmission line easement

(2) provision of appropriate geotechnical report detailing estimated ground movements. The geotechnical report should also include a stability analysis of the high wall and take into consideration any adverse effects on the slope stability due to blasting of the overburden

(3) requirement to perform inclinometer and survey monitoring of the ground movement at appropriate intervals

(4) requirement that areas of open cut mines adjacent to existing transmission lines be backfilled as soon as mining in the vicinity of the affected structures is complete

3.3.5 Forward Mining Plans

The Mine Subsidence Board has current programmes to monitor mining under major structures subject to potential subsidence damage. Transmission lines and associated infrastructure are recognised as major structures within these programmes.

Under these programmes, annual liaison meetings are convened between the Department of Mineral Resources, the Mine Subsidence Board and collieries. Coal mining companies table forward mining plans at these meetings. There is a requirement under the Mine Subsidence Compensation Act for coal mining companies to provide such planning information when requested. The Board examines the forward mining plans and undertakes to notify the power supply authority promptly on becoming aware of a plan affecting any of the power supply authoritys structures.

3.3.6 Mine Subsidence Board Approval Process

The Board approves building applications received from the power supply authority for proposed transmission lines in Mine Subsidence Districts, subject to conditions.

The approval process has been designed so that developers can submit building applications at the planning stage, including conceptual plans with their application. This allows subsidence design parameters to be determined prior to detailed design and planning being commenced.

Typically, conditions of approval may require that:

(a) Final engineering drawings are submitted to the Board prior to the commencement of construction.

(b) Final drawings are certified by an appropriately qualified engineer that improvements constructed in accordance with the specifications and drawings will be safe, serviceable and repairable, taking into account specific mine subsidence parameters for the site(s) involved. (Site specific parameters would normally be specified as part of the approval.)

c) If appropriate other special conditions are to be complied with, eg:

- A geotechnical report for the site be supplied - Work as executed drawings be submitted on

completion - Special features be catered for or provided for in the

design (eg, provision of platforms and access for relevelling jacks)

- Progress reports or specific events to be reported to the Board (eg, notice of concrete pours, etc)

- Specified design floor level to be adopted in flood prone areas

- Undertake geotechnical exploration to define limits and extent of old workings

Improvements constructed in a Mine Subsidence District that do not satisfy the conditions of approval, are not eligible for compensation under the Mine Subsidence Compensation Act.

Typically, the following subsidence parameters would be specified in the conditions of approval:

Vertical Subsidence x mm Compressive Strains y mm/m Tensile Strains z mm/m Tilts w mm/m Radius of Curvature v kmThe Board requires the designer to certify on the drawings that improvements covered by the approval will remain safe, serviceable and repairable in the event of the structures being subjected to the specified subsidence. The designer is to state on the drawings the subsidence parameters that were included in the design. This would normally be included with statements detailing design loads (ie, dead, wind, live and any special loads).

their approval for any design and mitigatory works that may be necessary when the application is approved by the Chief Inspector of Coal Mines

(c) Lead TimesThe existence of considerable lead times that can be involved in mitigatory works must be acknowledged by all concerned parties. As the power supply authority has limitations placed upon it regarding the number and duration of outages that it can impose on transmission lines at certain times of the year, access to lines and towers to carry out mitigatory works is restricted. Some details of the type of works that may be required and the times involved are given in Section 3.3.3.

(d) Monitoring

It should also be noted that survey and visual monitoring may be required in cases where no physical mitigatory works are necessary. It will need to commence prior to mining affecting structures and to continue until, for practical purposes, mine subsidence has ceased.

3.3.3 Typical Protective Measures

(a) Lines Constructed to Normal Design StandardsFor transmission lines which have not been designed to withstand ground subsidence, the power supply authority would generally require at least 12 months notice to be able to organise the carrying out of any necessary precautionary measures to mitigate the subsidence effect on the affected structures and to enable suitable outages to be arranged to allow such work to take place.

If the expected ground subsidence effects are so severe that the transmission line structures and/or foundations are inadequate, then major redesigning or relocation of the line may be required. Alternatively, restriction of the undermining to partial extraction may be considered if that is the less costly option. In this regard, tension structures are far more sensitive to ground subsidence effects, and far more difficult to protect from them, than are suspension structures.

If a section of transmission line needs to be relocated, the power supply authority would generally need at least three (3) years notice. (Note: It is assumed that an Environmental Impact Statement for the line deviation is not required.)

Steel, wood and concrete pole transmission lines are more tolerant of ground movement than steel tower lines. However, preparatory work and, in some cases, relocation, may still be necessary and the lead time required is only a little shorter than for steel tower lines.

(b) Lines Constructed to Withstand SubsidenceIf the transmission line which is to be undermined has been designed to withstand the effects of a predetermined amount of ground subsidence, the liaison committee would consider the proposal referred to it bearing in mind the design parameters which have been incorporated in the design of the transmission line structures, and the predicted subsidence parameters for the proposed mining.

Although a transmission line has been designed for ground subsidence effects, it may still be necessary to do some work

on the line, such as survey monitoring and unloading the towers by placing the conductors and overhead earthwires in sheaves. This work would require outages of the line and arrangements would have to be made to have the line taken out of service at a time when system operating conditions are favourable. In such circumstances, the power supply authority would generally require at least 12 months notice before the undermining was to take place.

(c) GeneralNotwithstanding any other modifications or design features, it may also be necessary to take a line out of service during the actual period of undermining. At the very least, sufficient system back up would need to be available in case the line was damaged during undermining. In addition, the power supply authority would probably arrange for its own survey monitoring of the line before, during, and after the undermining of the line structures. Again, the power supply authority would generally require considerable lead time before the undermining is to take place.

All costs of mitigatory works, adjustments, repairs, etc approved by the Mine Subsidence Board will be paid by the Board subject to the provisions of the Mine Subsidence Compensation Act.

3.3.4 Open Cut Mining

It is not possible for transmission line structures to remain within open cut mining sites. It is therefore a basic requirement in route selection that transmission line routes be located away from areas where there are firm plans to mine by open cut, and where suitable alternatives are available, away from areas likely to be mined by open cut within the life of the transmission line.

Particularly where prospective open cut mining areas are large and mining plans are not firm, it may not be possible to avoid such areas altogether. In such a case the power supply authority will acquire the most practical route having regard to all constraints including the possible future mining.

Open cut mining operations are matters which will be approved or refused by the Minister for Mineral Resources under the provisions of the Mining Act and the conditions of coal leases granted thereunder.

When planned or currently operating open cut coal mines affect an existing transmission line, the matter of selection of a satisfactory deviation for the line shall be subject to negotiation between the power supply authority and the mining company concerned, having regard to the legal rights of both parties. Generally, a similar liaison process is to be followed as outlined for approval of extraction by underground mines. The Mine Subsidence Board is not normally involved, except that its approval (with design requirements) would be necessary for any new structures, that are in a Mine Subsidence District.

In cases where open cut mines operate in proximity to transmission lines, adverse effects may result from ground movements near the highwall, vibrations from blasting and excessive dust pollution on the insulators of the line.

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As well as coal being sterilised directly by subsidence restrictions, a further quantity is likely to be lost due to the geometric constraints placed on the mine layout. For example, if a line of transmission towers cuts across one end of a proposed longwall panel at an oblique angle, the whole panel, and possibly an adjacent one also, may be sterilised because of the practical inability to rotate panels or to extract irregular pockets of coal by modern, high production longwalls.

As a typical longwall panel can contain in excess of 1,000,000 tonnes of coal, the quantity lost due to indirect sterilisation can quickly become very significant.

Once the total quantity of coal to be sterilised, both directly and indirectly, has been determined, the next step is to assign a value to it. For the purposes of these guidelines, the value per tonne of coal sterilised will be taken as being the difference between the market price (at the mine gate) and the incremental cost per tonne of winning the coal. This value of coal might be higher if other factors have stronger influences, such as export demands, etc. Users should check this value at time of estimate.

From the communitys viewpoint, it is desirable that non-renewable resources such as coal be exploited to the fullest extent possible. Similarly, from the viewpoint of mining communities, it is desirable to extend the life of each mine for as long as possible by ensuring that the greatest possible percentage of the coal is mined. However, as with the community cost of transmission line works, it is not possible to define a simple approach to the assignment of value to this factor. Nevertheless, when evaluating a proposal which will involve sterilisation of coal, every effort should be made to determine whether the community cost is significant enough to warrant departure from a purely economic evaluation.

3.4.4 Procedural Matters

In order to afford the maximum flexibility in modifying mine layouts, transmission line locations and tower designs, it is important that consultation and the assessment of the relative merits of coal sterilisation and transmission line modification be carried out at the earliest possible time.

Accordingly, as soon as it becomes apparent that a coal mining proposal is affected by an existing transmission line, or a transmission line proposal is affected by an existing coal mining operation, consultation should commence. This will be initiated by any of the involved parties depending on circumstances, eg, by the Board when the transmission line is in a Mine Subsidence District; by the Department of Mineral Resources when the transmission line is not in a Mine Subsidence District; by the power supply authority when a transmission line is involved; by the mining company when required by condition of approval of the particular extraction.

Once a course of action has been determined in accordance with the principles outlined in 3.3.1, 3.3.2 and 3.3.3, it is likely that compensation or reimbursement will be payable by one or other party. For new transmission line construction, the matter of compensation for sterilised coal and/or the cost of transmission line modifications shall be negotiated between

the power supply authority and the mining company at the time.

Where an existing transmission line is involved, the costs associated with precautionary and/or remedial measures by the power supply authority, including the cost of survey monitoring, will be reimbursed by the Mine Subsidence Board subject to the provisions of the Mine Subsidence Compensation Act.

Where a condition of approval requires certification of plans by designer:

- After subsidence, any damage will be repaired under the provisions of the Mine Subsidence Compensation Act, but the damage is expected to be consistent with design requirements.

- Serviceable means that the improvement must be able to continue to be used for the purpose for which it was designed.

- Safe means that occupants and users of the improvement must not be at risk from loss or reduction of asset integrity.

- Repairable means that materials and their method of utilisation allows damage after subsidence to be repaired/replaced economically.

3.4

CHOOSING BETWEEN TRANSMISSION LINE MODIFICATIONS AND COAL STERILISATION

3.4.1 The Least Community Cost Principle

It is anticipated that, from time to time, situations will arise where, as a result of a transmission line being affected by a mining proposal, it is necessary to choose between sterilising coal reserves and relocating or modifying the transmission line. Whilst it is not practicable to quantify in general terms the relative economic merits of sterilising amounts of coal against relocating or modifying specific transmission line structures, it is possible to assess all situations in terms of an agreed guiding principle.

On this basis, it is agreed that each individual situation will be assessed on its own merits and the course of action which represents the least cost to the community will be adopted. That is, if the cost to the community of coal sterilisation is less than the cost of modifying or relocating specific transmission line structures, then the coal will be sterilised. If the cost to the community of coal sterilisation is greater than the cost of transmission line modification, then the transmission line will be modified.

3.4.2 Transmission Costs

In determining the cost of any transmission line modifications which may be necessary to allow undermining to proceed, three factors need to be considered:

(a) Actual Direct Cost of Modifications or Precautionary Measures

The actual cost of any particular proposal is readily definable in that the power supply authority can provide budget estimates for the various types of line construction. If necessary, these estimates can be verified by the calling of tenders for the work involved. As discussed in previous sections, these costs will normally be borne by the Mine Subsidence Board for undermining of existing transmission lines or by the mine owner for other cases.

(b) Cost of Non-optimal Operating ConditionsThe NSW electricity transmission system operates as a competitive market, with all generators bidding in prices on a half-hourly basis. The cheapest units then supply the electrical demand at the time, and all are paid the price bid by the most expensive of these.

When taking a transmission line out of service for modification or reconstruction, the resultant changes to system operating conditions may limit the output available from cheaper generators, requiring more expensive generators to be used. This may raise the price paid by all customers and may also expose the transmission authority to commercial sanctions for limiting the market access for the cheaper generator(s). The actual cost of these effects depends on the nature of the line and the generation and load conditions at the time and estimates may be obtained from the authority. Taking a line out of service also results in higher electrical loads on other lines, which normally increases overall electrical losses (due to line heating) on the network. The incremental cost of system losses for a typical 330 kV line outage in the Hunter Valley would be $300 per hour based on the cost of coal into the power station boilers. However, such costs vary widely with system conditions and the line involved and should be obtained from the power supply authority for particular cases.

In addition, if, as a result of having an important interconnector out of service, additional system disturbances were to result in load shedding, the cost of an unplanned loss of supply to the community is much greater than the supply tariff. There is a great deal of variation in this cost, which includes loss of production, spoilage of goods, and disruption of essential services. This cost may be significant and should be obtained from the authority in the first instance.

(c) Community CostsThe environmental considerations associated with the modification or relocation of a transmission line can vary and each case must be assessed on its own merits. Some of the factors which merit consideration are visual impact, affectation of natural amenity and the availability/desirability of alternative routes.

3.4.3 Coal Mining Sterilisation Costs

In determining the cost of sterilisation of coal due to the need to protect surface structures, three factors need to be considered.

(a) The value of coal directly sterilised;(b) The value of coal indirectly sterilised due to

subsidence restrictions; (c) The community cost arising from the loss of a non-

renewable resource and the shortening of mine life.

Coal is sterilised directly when it is left in place in order to support a surface structure. The two normal methods used to provide such support are total sterilisation within a nominated angle of draw or the use of a method that provides some control of subsidence, such as panel and pillar which may reduce the percentage of coal recovered from 90% to less than 50%.

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COAL MINING AND ITS EFFECTSA1

OVERVIEW OF MINING METHODS

In 1982-83, nearly 70% of coal in NSW was won by underground mining methods and the remaining 30% by open cut methods; longwall mining provided approximately 12% of the underground coal production. In 1995-96, 51% of coal was won by underground mining methods, 69% of which was by longwall. The total production in NSW has increased from 67 million tonnes in 1982-83 to 92 million tonnes in 1995-96. More recent figures are available from Joint Coal Board publications, Australian Black Coal Statistics and NSW Coal Year Book.

Underground extraction of coal in NSW is generally carried out at present at depths of mostly less than 500 metres. Nearly 50% of total coal production in 1982-83 was at depths less than 200 metres. Mining at depths greater than 200 metres is increasing, especially in longwalls.

For coal mineable by underground methods, a practical minimum seam thickness is 1 metre. The seams mined in the past mostly fall within the thickness range of 2.5 to 3.5 metres and most underground coal production was derived from seams within this range.

Open cut coal extraction in NSW is generally confined to linear overburden to coal ratios not greater than 10:1 and a maximum mining depth of 200 metres. In practice, however, coal by open cut methods is mined currently at much shallower depths. During 1982-83 the greatest depth worked was 88.5 metres and the weighted average overburden to coal ratio was 4.6:1. These figures have not changed significantly.

A2

UNDERGROUND MINING

Underground mining practices followed in NSW fall into three main categories: bord and pillar mining (first workings), partial extraction, and total extraction.

Development Headings are roadways (tunnels) that are driven to provide access to areas of virgin coal. Normally driven in groups of up to four roadways with cut-throughs between them, they form a development panel and are used to block out large areas of coal that will be extracted by longwall or pillar extraction methods.

Bord and Pillar methods of mining are well established and vary in detail. They comprise a network of underground roadways (tunnels), interconnected as shown in Figure 1, leaving blocks of coal, generally square or rectangular in plan view. These are termed pillars. Historically, the sizes of pillars and roadways formed in this system of mining have been specified in mining lease conditions and under the provisions of the Coal Mines Regulation Act, 1982. In more recent years, pillars have been required to be designed with long term stability to ensure mine safety.

LEGISLATIONLegislation referred to in the Guidelines includes:

Electricity Transmission Authority Act 1994 - under which the Electricity Transmission Authority (trading as TransGrid) is responsible for the transmission of bulk electricity throughout NSW.

Electricity Act 1945 Electricity (Overhead Line Safety) Regulation 1991 - which specifies the conditions under which overhead lines may be designed, constructed, operated and maintained.

Environmental Planning and Assessment Act 1979 - which embodies the general provisions relating to land use planning, zoning, preparation and exhibition of environmental planning instruments, local environment plans, and environmental impact statements.

Public Works Act 1912 - under which TransGrid is empowered to resume land for its various undertakings.

Mining Act 1992 - covering the issue of titles to mine for and/or prospect for minerals and coal, and also includes provisions for including in the Register of Colliery Holdings new Colliery Holdings or making amendments to existing Holdings. Approvals for open cut coal mining activities are dealt with under this Act.

Coal Mines Regulation Act 1982 - which controls, for example, pillar extraction and longwall methods of mining in underground coal mines. This Act also provides for the preparation of mine plans and record tracings showing coal mine workings.

Mine Subsidence Compensation Act 1961 - under which the Mine Subsidence Board controls the establishment of improvements, including transmission lines in Mine Subsidence Districts and considers claims for compensation for damages to improvements as a result of underground coal mining.

Other legislation which can affect the siting of transmission lines includes various Acts covering the operation of other public authorities such as:

National Parks and Wildlife Service State Forests of NSWHeritage Council of NSW National Trust of Australia (NSW)Sydney Water CorporationHunter Water CorporationDepartment of Public Works and ServicesDepartment of Land and Water ConservationTelstraDepartment of Transport NSWState Rail Authority of NSWRoads and Traffic Authority NSW

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FIGURE 1SUBSIDENCE FROM BORD AND PILLAR MINING

ZONE OF INFLUENCE

WORKED AREA

SMALL SUBSIDENCE

ORIGINAL SURFACE

COAL SEAM COAL SEAM

SECTION

A. Bords or RoomsB. Pillars

A

B

(Not to scale. Diagrammatic only)

CUTAWAY VIEW OF TYPICAL MODERN COLLIERY

Longwall mining unit

Bord and pillar development

1 Drift for men and materials access2 Shaft winder house3 Bathhouse and administration building4 Workshops5 Coal preparation plant6 Coal storage bins7 Gas drainage system8 Longwall mining unit9 Coal seam10 Continuous miner11 Coal pillar12 Underground coal bin13 Main roadway14 Coal skips bring coal to the surface

(Not to scale. Diagrammatic only)

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1112

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FIGURE 2SUBSIDENCE FROM PANEL AND PILLAR MINING

ZONE OF INFLUENCE

WORKED AREA

SUBSIDENCE IF CRITICAL EXTRACTION OCCURRED

ORIGINAL SURFACE

COAL SEAM COAL SEAM

RESULTING SUBSIDENCE:*PANELS 1+2

PANEL 1 PANEL 2

Subsidence from extracting Panel 1Subsidence from extracting Panel 2Amount of resulting subsidence will depend on stability of pillars

SECTION

A. Panel 1B. Panel 2C. Standing Pillars

B

AC

Pillar sizes and the number of roadways in any particular panel (mining region) vary considerably depending on the purpose of the panel. Any extraction system (discussed below) requires access to the coal to be extracted, and so bord and pillar workings are used as the means of blocking out the coal for this purpose. In this sense, the bord and pillar workings are also referred to as first workings, or development workings.

When bord and pillar workings are developed, they cause negligible surface subsidence and consequently no mining-induced ground strain (see Section in Figure 1).

Partial Extraction can take various forms depending on mining conditions, production requirements and constraints such as subsidence control. Essentially, partial extraction refers to any mining method where, within a block of coal, large quantities of coal are removed, but some remains. This may take the form of partial pillar extraction, or pillar splitting, within a panel of bord and pillar workings, or it may be on a larger scale where alternate panels are fully extracted while the intervening blocks of coal are left intact.

This latter method, sometimes referred to as panel and pillar mining (Figure 2), is a variation of selective partial pillar extraction methods and is used to prevent or control surface subsidence. In this method, panels are separated by long barrier pillars of sufficient width to support the overlying strata, even if the immediate roof over the extracted panels sags considerably or fails (caves). By careful selection of the panel and pillar geometry relative to mining height, depth of workings and strata conditions, surface subsidence can be controlled successfully and the level of differential subsidence or lateral strain affecting surface features or structures, can be restricted.

Total Extraction is an extension of partial extraction whereby as much coal as can safely and economically be mined is removed from each panel, leaving only small remnant pillars or narrow barrier pillars and pillar regions. These often crush out as the major overlying roof strata settle and the surface subsides. Total extraction is achieved either by large scale pillar extraction from bord and pillar workings over a large area, or by mining of wide blocks of coal between narrow development panels using the longwall method (see Figure 3).

Total extraction is the favoured mining method where subsidence is not restricted, as it leads to maximum coal recovery, and minimum sterilisation of resources. Where the coal can be totally extracted prior to construction of surface structures, they can be built on stable ground which has already undergone complete subsidence. Total extraction results in more predictable and uniform strains and subsidences which simplifies the control of surface development.

A3

SUBSIDENCE EFFECTS

A3.1 General

A subsidence basin forms on the surface when coal is extracted over a wide area. The amount and extent of subsidence depends on many factors which include the geometry of the extracted area, the layout of unmined pillars, the number of seams mined, the coal recovery from each seam, the nature of the superincumbent strata and other geological factors.

When development headings are driven, no significant subsidence occurs. Generally, for bord and pillar workings, subsidence can be around 20 mm. Over a large area of pillar or longwall extraction where the critical extraction width is exceeded, subsidence can be up to 65% of seam thickness.

The main elements of a subsidence profile are the vertical displacement (subsidence), the change in ground slope and the curvature of the ground surface which determines the amount of surface strain. These elements can be calculated from field observations of level of, and distance between, monitoring points using standard survey techniques. Profiles of subsidence and associated characteristics are derived from the field data. Some of the terminology used in defining surface subsidence is shown in Figure 4.

In the case of longwall mining, most of the subsidence generally takes place soon after mining. In a virgin area, up to 10% of maximum subsidence can occur as delayed subsidence. In the case of pillar extraction with standing unmined pillars in the goaf, subsidence can continue for a long time due to the collapse or gradual failure of any unmined pillars over time.

Dykes or fault planes can affect subsidence and strain profiles. They can provide a plane of weakness in the strata which under certain circumstances could facilitate subsidence movements. Surface cracking need not necessarily occur when such geological features are present. The occurrence of surface cracking and its extent, generally depends on the amount of strain.

The effects of ground movements are important in areas when there are structures or surface features which require protection. Different types of structures have different tolerances to subsidence.

Construction of new structures can be deferred until mining has taken place and subsidence is complete. It may be possible to modify existing structures to enable them to withstand anticipated subsidence. Proposed structures can be built with allowances for subsidence. Whether subsidence movements occur or are allowed to occur under structures or surface features depends on several factors. The importance of the structure and any socio-economic consequences of damage will need to be carefully considered in relation to the coal which could otherwise be sterilised.

A3.2 Underground Extraction

(a) Bord and Pillar Mining (First Workings)Studies in NSW have observed subsidence values over bord and pillar workings without pillar extraction of approximately 20 mm.

Determining values of mine subsidence for values of this magnitude is complicated by other factors. Standards and tolerances of surveys have to be considered and it must be remembered that changes in the ground surface level resulting from natural groundwater movement or swelling/contraction of surface soils/clays in excess of 50 mm are not uncommon. This ground movement occurs quite independently of any mining influence.

Undetected geological anomalies or poor mining practice may lead to unusual subsidence.

Limits can be placed with reasonable confidence on surface

(Not to scale. Diagrammatic only)

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the slope. In clayey material, the movements are presumed to result from elastic recovery following lateral and vertical stress relief. In the coal measures, the movements are frequently traced to shear failure along discontinuities such as bedding planes.

Displacements after backfilling are due to the consolidation of the backfill material and are a function of the type of material, the depth of the fill and the backfilling process. Observations have been made over a period of between three (3) and six (6) years when backfill material was placed in an open cut. The maximum consolidation of 40 metres of overburden was

FIGURE 4(Source: National Coal Board 1975)

TERMINOLOGY ASSOCIATED WITH SUBSIDENCE

CRITICAL WIDTH (W)

SURFACE

COAL SEAM

SUPER CRITICAL WIDTH

Miningdepth

ssm

STRAIN CURVE FORCRITICAL WIDTH OFEXTRACTION

WORKINGS

SUBSIDENCEPROFILES

20mmSUBSIDENCE

LIMIT

(M)

ANGLEOF

DRAW

ANGLE OF DRAW - Angle between vertical at edge of workings and line joining edge of workings to 20mm subsidence point at surface

sm MAXIMUM SUBSIDENCE - Occurs at the centre of the panel when work-ings reach critical width and length

E STRAIN - Lengthening or shortening in subsidence trough per unit length

M SEAM THICKNESS

W WIDTH OF WORKINGS

(or Limit Angle)

(extracted)

observed to be 200 mm in this case. The consolidation was also found to be irregular, reflecting variations in the type of material and the degree of compaction.

Further work is required to predict subsidence in rehabilitated areas of open cut operations. Structures in the vicinity of open cut mines could suffer damage if these are located within the zone of influence of the workings. The development of rehabilitated lands should take account of ground movements due to the consolidation of the backfill for a number of years.

movements due to mining as empirical and mathematical methods have been proved reliable.

In summary, given good mining design and practice, bord and pillar (first) workings should cause no damaging surface subsidence.

(b) Partial ExtractionThe subsidence effects of irregular partial extraction methods can be quite significant on the surface in terms of both vertical displacement and strain. Where the partial extraction involves pillar splitting or extensive pillar extraction there can be a significant time delay in the development of total subsidence.

In a panel and pillar extraction operation, it is possible to carefully control and limit the amount of surface subsidence. This can be achieved by designing the widths of both panels to be extracted and the barrier pillars between them to suit strata conditions and depth. Of more importance, the differential subsidence over pillars and panels can be carefully evened out so that the surface strains are negligible, thereby protecting any sensitive surface features.

(c) Total ExtractionTotal extraction generally results in 80%-90% recovery, by plan, of the coal seam. In NSW strata conditions, maximum surface subsidence can amount to approximately 65% of the extracted seam thickness once the total extraction area exceeds a critical width.

Over the extremities of the extracted area, peak strains occur at the surface (see Figure 4). The magnitude of these strains is dependent on depth and strata conditions. The angle of draw defining the limiting lateral extent of subsidence is also very dependent on strata conditions and can vary from 0 to 35.

In total extraction, as with all mining methods, given a combination of empirical guidelines based on measured subsidence data, structural and geotechnical strata properties and modern numerical design techniques, it is possible to design mining layouts to predict and hence control surface subsidence with a reasonable degree of reliability.

A3.3

OPEN CUT OPERATIONS

Ground movements can occur both during the mining operation and after rehabilitation due to the consolidation of the backfill. Very little published information is available in Australia on the ground movements associated with the consolidation and settlement of backfilled areas of open cut mines.

The area of influence behind the crest of a deep excavation slope depends upon the type of coal measures. Movements have been observed for a distance of 2.5 and 3.0 times the height of the slope behind the crest of clayey soils. In excavations in the stronger coal measures, it is generally believed that this distance reduces to less than the height of

FIGURE 3LONGWALL MINING

A

E

A. Longwall Panel Extraction CompletedB. Longwall Panel Extraction in ProgressC. Small Remnant Pillar between Longwall PanelsD. Main HeadingsE. Development Headings to create next Longwall Panel

C

B

D

(Not to scale. Diagrammatic only)

(Not to scale. Diagrammatic only)

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Deformation can readily cause failure of conventional steel towers. Horizontal relative displacements of pairs of adjacent legs tend to cause the tower to deform as indicated in Figures 5 and 6. Diagonal spread throws the legs on one diagonal into severe compression, while the legs on the other diagonal are put into tension. In this mode of deformation, which is also produced by ground curvature on the diagonal, the tower may be quickly brought to failure once the slippage of bolted splices and joints is exhausted. Rotation of the footings and secondary effects of tilt or displacement can add significantly to the tower loadings. For severe deformations of this type it is not practicable to reinforce existing towers to withstand the loads involved.

TransGrid is currently giving attention to the development of new tower and foundation designs which are less vulnerable to deformation.

Single pole structures without guy wires are not subject to the large deformations applicable to lattice towers whose legs may be 10 to 15 metres apart on each face. Two-pole structures linked by a crossarm are deformed by ground curvature or relative displacements. However, pole flexibility may relieve the seriousness of both primary and secondary effects.

Major transmission lines supplying electrical loads greater than about 100 MW (100,000 kW) are normally constructed with steel lattice towers. Current designs for self-supporting steel pole and reinforced concrete pole structures cannot carry the loads associated with large angles or terminations of major transmission lines.

THE IMPACT OF COAL MINING ON TRANSMISSION LINESB1

GENERAL

The primary effects of mining on transmission line structures are loss of height due to vertical subsidence, displacement due to ground strain and tilt and deformation of structures or footings.

The principal secondary effect is alteration of tower loadings due to movement of conductor attachment points. Such movements follow mainly from tower tilt or lateral displacements and can cause major changes of conductor tensions.

Loss of height due to subsidence applies equally to all structure types and functions. Transmission lines are required to maintain statutory clearance over ground, roads, etc. Where the effects of subsidence reduce the clearance to less than the statutory limit, it will be necessary to restore the clearance by reconstruction or structure extension. The acceptability of extension of structures will depend on the capacity of existing structures to resist the increased moments associated with the height increase. New structures can of course be designed with additional ground clearance to provide for reasonable difference of subsidence between structures and points of low conductor-to-ground clearance.

The severity of the other aspects depends not only on the subsidence parameters, but also on the structure type, structure function, and proximity of adjacent structures.

Ground strains and curvature associated with substantial subsidence can deform conventional steel lattice transmission structures sufficiently to cause failure and collapse. Even without deformations, substantial tower tilt can alter conductor tensions sufficiently to cause tower overloads or complete failure. In some circumstances the collapse of one tower could throw excessive loading on to adjacent structures causing the cascade failure of several spans of the transmission line.

Pole structures are more flexible than lattice towers and may accept tilt more readily. They also have a unitary foundation and may be relatively unaffected by ground strain. However, where guys are fitted, structures can be dramatically overloaded by movement of the guy foundation relative to the pole. Accordingly, it is unwise to assume that any transmission line structure can be safely undermined. Even if the structure has been designed in anticipation of future undermining, it may be necessary to alter conductor tensions and modify attachments prior to the event and, in any case, the provisions made must be checked for the ground movements associated with the specific pattern of coal extraction proposed.

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FIGURE 6DIAGRAMMATIC VIEW

RIDGID BODY ROTATION MODEL

DISPLACEDSHAPE

ORIGINALSHAPE

(A) SQUARE WAVEFRONT (B) DIAGONAL WAVEFRONT

Horizontal Displacement

Vertical Reactions

FIGURE 5

DIAGRAMMATIC VIEW

TOWER DEFORMATION FROM HORIZONTAL LEG DISPLACEMENTS

B2

STRUCTURE TYPES AND FUNCTIONS

Transmission line structures may be divided into two functional groups, tension structures and suspension structures, both of which are illustrated in Figure 7.

The usual functions of a tension tower include:

- separating sections of transmission lines which have differing tensions under various service conditions;

- accepting the longitudinal loading of at least one phase terminated on the tower, to facilitate conductor stringing and maintenance;

- accepting deviation angles or uplift loads withou infringing electrical clearances of conductors to tower.

The heaviest tension towers accept termination of all phases on the tower at a small to medium line angle OR accept the most extreme deviation angles likely to be required on the route. They cannot generally carry both simultaneously.

At a tension tower, conductors are cut and dead ended, terminating on the tower, with the insulation carrying full conductor tension. A slack jumper makes the electrical connection between the conductors on either side of the tower.

The usual function of a suspension tower is to support the conductor, which is continuous at the structure, and to restrain its transverse movement under wind loading. As the normal loads are wind loads and weight loads, suspension towers are usually designed for much greater transverse strength than longitudinal (along the direction of the transmission line route). These towers are therefore vulnerable to forces associated with longitudinal tilt.

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Modern helical suspension units used to grip the conductors do not allow conductor slip under longitudinal load. However, it is feasible to place the conductors into sheaves temporarily to prevent large longitudinal loads being applied to suspension towers during a subsidence event.

The effect on conductor tension of the displacement of a support clamp or tower attachment is also highly dependent on the proximity and types of adjacent structures.

For two adjacent tension towers with conductors strung at normal tension, a longitudinal displacement of about 0.8 metres will double the conductor tension of a 350 metre span, but for a 200 metre span the corresponding displacement is only 0.3 metres.

Doubling of normal tensions would bring the towers to a state where they would not be safe to work on to reduce the tensions.

Where a tension tower is separated from the next tension tower by several spans in each direction, often only the overhead earthwires would need to be placed into sheaves (because of their short suspension linkages) in order to reduce the conductor control problems to an acceptable level. When this is not the case, the phase conductors themselves may need to be placed into sheaves and conductors pre-slackened to extend the tolerable tilt.

Tower top movements of up to one metre on suspension towers will normally be manageable utilising such techniques provided that the towers are also designed for the deformations involved. In rare cases such tower top movements may be tolerable even at tension towers.

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FIGURE 7

DIAGRAMMATIC VIEW

TYPICAL DOUBLE CIRCUIT STEEL LATTICE TOWERS

SHORT SUSPENSION LINKAGE OR RIGIDCLAMP ATTACHMENT OF OVERHEAD

EARTHWIRES

STRAINATTACHMENT

OFCONDUCTORS

SUSPENSIONATTACHMENT

OF CONDUCTOR

(A) SUSPENSION TOWER (B) TENSION TOWER

SOILROCK

SOIL

ROCK

SHORT SUSPENSION LINKAGE OR RIGIDCLAMP ATTACHMENT OF OVERHEAD

EARTHWIRES

DESIGN MEASURES TO PROVIDE FOR SUBSIDENCEC1

LARGE STEEL LATTICE STRUCTURES

(a) Resisting or Avoiding DeformationIn some cases, it may be preferable to use large pole-type structures or narrow based steel lattice structures with a single foundation, in lieu of standard steel lattice structures, to minimise the effects of subsidence.

For conventional steel lattice towers, it is possible to construct a single foundation in the form of a reinforced concrete cross-beam to prevent tower deformation by ground movements. The cost of such reinforced concrete beam foundations is several times greater than the combined cost of the structure and its normal foundation.

Studies have shown that if a steel lattice suspension tower is initially loaded at significantly less than full capacity, it may survive substantial deformations without further reinforcement, particularly for certain bracing arrangements and where the tower leg extensions are longer. Each tower has to be separately studied to evaluate its response to deformation and to determine the best measures to cope with subsidence. Such studies are being carried out by the authority for cases of undermining of existing towers not designed for subsidence. In some cases a temporary reduction of the ability of a tower to resist high winds may be acceptable having regard to the low probability over a short period of the reduced ultimate wind capacity being exceeded. This evaluation must also take into consideration safety and the impact or cost to the community as a consequence of a tower failure on the particular transmission line.

(b) Innovative DesignAs the above design measures involve high initial costs, alternative designs or procedures are being investigated by TransGrid to reduce the cost of providing for subsidence.

These investigations will ultimately cover the development of innovative structure designs including designs for tension towers, which might accept much larger deformations without losing their ability to perform their design functions in the transmission line.

Design modifications to conventional towers (including existing towers) are under investigation with a view to enabling them also to accept much larger deformations.

Because of the potential for successful development of such modifications, in the medium term, it is unwise to invest large sums of money in constructing lines to resist future subsidence using existing techniques. Much cheaper techniques may be available before subsidence occurs.

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APPEN

DIX

D

1. Transmission Tower

- Cruciform footing installed prior to subsidence - Tower A11, Eraring-Kemps Creek 500 kV - Cooranbong Colliery

2. Transmission Tower

- Tower dismantled prior to subsidence - Tower 227, Liddell-Tomago 330 kV, SCN 82 - Cumnock Colliery

3. Transmission Tower

- Conductors placed in sheaves - Tower 11, Newcastle-Tomago 330 kV, SCN 94/95N - West Wallsend Colliery

4. Wood Pole Structure

- No preventative work prior to subsidence - Wood pole structures, 132 kV, T/L 950 - Stockton Borehole Colliery

5. Wood Pole Structure

- Preventative work prior to subsidence - Wood pole structures, 132 kV, T/L 95A - Newstan Colliery

6. Electricity Substation

- Preventative work prior to subsidence - Vales Road Substation - Wyee Colliery

CASE STUDIESThe attached case studies have been included in the Guidelines to illustrate the types of transmission lines that have been undermined and the type and extent of mitigatory works and monitoring that have been adopted.

The examples have been selected to illustrate the range of works that have been undertaken to date when underground mining has produced surface subsidence with the potential to affect transmission lines.

As a result of the work undertaken, the transmission lines have remained in service and underlying coal resources have been recovered.

(c) Interim Measures Against Future SubsidenceThe proven method of making steel lattice transmission towers safe against severe subsidence is to prevent tower deformation by installing large reinforced concrete cross-beam foundations. However this is not required to be implemented until the subsidence is imminent. In order to comply with the requirements of the Mine Subsidence Board, TransGrid has agreed to make provision for future installation of such beams at certain towers by installing longer standard type footing shafts and forming parts of the shafts so that the beams can be cased around them at a future time. Such provisions add 5% to 20% to the cost of the structures and foundations, and have been made on the Eraring to Newcastle 330 kV transmission line.

It is hoped that it will soon be possible to limit such provisions to zones of extremely severe future subsidence. The development of tower modifications for which advance provision is not required, is expected to be possible for the majority of cases.

C2

WOOD POLE AND SIMILAR TRANSMISSION LINES

The main design measure for transmission lines on pole type structures is to provide a margin of conductor height sufficient to make it unlikely that subsidence will cause an infringement of statutory clearances.

In addition, the structure foundation design must allow for the ground disturbance associated with subsidence and, where multi-pole structures are involved, the structure design should provide for the relative movement of the poles due to changing ground strains and tilts. It may be necessary to restrict the application of guys or bracing or to provide special load limited designs to reduce structure loads to acceptable values.

In the case of tall tension poles, it may be necessary to provide for counter-measures to excessive tilt so that conductor tensions can be alleviated in the manner discussed below.

Prior to the subsidence it may be necessary to place overhead earthwires and conductors into sheaves and to adjust conductor tensions in anticipation of the predicted pattern of structure movements.

After the subsidence is substantially completed at a structure, it may be necessary to straighten the poles, to re-adjust conductor tensions, and to restore any damaged access tracks.

C3

CONDUCTOR CONTROL

The problems of conductor control under subsidence conditions are very different at suspension and tension towers. This is well illustrated by examining the consequences of tower tilts which for shallow seams (say less than 100 metres) can exceed 1 in 10. While the massive impacts of ground strain can be overcome by appropriate tower and/or tower footing designs, it is not possible to eliminate tower tilt. Extra high voltage double circuit towers are of the order of 40 to 50 metres in height and a maximum tilt of 1 in 10 can result in a tower top movement of up to 5 metres.

On a tension tower, conductor attachment points move with the tower when it tilts. When the overhead earthwire attachments at the tower top move by 5 metres, the phase conductor attachments (at crossarms lower on the tower) would be displaced by between 2.5 and 4.2 metres (on a typical 330 kV double circuit tension tower). Longitudinal displacements in excess of about 0.5 metres have a dramatic impact on conductor tensions and can readily result in overloading of the towers.

On suspension towers, the conductor clamps or suspension units are attached to the tower by suspension insulator strings, which can swing freely in all directions (330 kV towers) or in the longitudinal direction only (500 kV Vee strings). The movement of the conductor clamp is less than the movement of the tower attachment by an amount depending on the length and resulting inclination of the insulation or earthwire suspension linkage. The effect of tilt on tower load is therefore less for a suspension tower than for a tension tower. In addition, the ability to place the conductors and overhead earthwires back into sheaves means that conductor control is more readily manageable for suspension towers than for tension towers.

C4

SUBSTATIONS

The main design measure with large substation structures and their associated outdoor switchyards is to ensure ground disturbance due to subsidence is allowed for and to provide for the relative movement of the transformers and busbar supports due to the changing ground strain and tilts.

It will be necessary in most installations to ensure that expansion joints are fitted to busbars and connections to alleviate excessive tensions and bending moments to bushings and support insulators.

Prior to subsidence, it may be necessary to fit expansion joints to the:

High voltage and low voltage busbars; Busbar connection to the high voltage and low voltage

transformer bushings; Busbar connections to the circuit breakers; Cable sealing end connections to the busbar.After the subsidence is substantially completed at the

substation, it may be necessary to re-tension the landing spans and to straighten any pole structures within or adjoining the substation.

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4.

Wood Pole Structure - No Work

Transmission Line Affected: 132 kV, T/L 950 Wood Pole StructuresType of Mining: Longwall (Stockton Borehole Colliery)Seam Depth: 300 metresStructure Affected: 7 wood pole structuresSubsidence Predictions: Subsidence 550 mm Tensile Strain 1 mm/m Compressive Strain 1 mm/m Tilt 10 mm/mAction Taken: Survey monitoring during subsidence Visual inspectionSubsidence Results: Subsidence 750 mm Tensile Strain 2 mm/m Compressive Strain 2 mm/m Tilt 5 mm/mCost: Estimated cost - $5,000Remarks: Conductors not placed in sheaves. No clearance adjustment necessary.

5.

Wood Pole Structure - Preventative Work Carried Out

Transmission Line Affected: 132 kV, T/L 95A Wood Pole StructuresType of Mining: Longwall and Pillar Extraction (Newstan Colliery)Seam Depth: 50 to 100 metresStructure Affected: 10 wood pole structuresSubsidence Predictions: Subsidence ) Tensile Strain ) No predictions Compressive Strain ) available Tilt )Action Taken: Conductors and earthwires placed in sheaves Some poles guyed Earthing structures installed under lower spans Set line to non-auto reclose Survey monitoring during subsidence periodSubsidence Results: Subsidence 2400 mm Tensile Strain 15 mm/m Compressive Strain 15 mm/m Tilt 83 mm/mCost: Cost incurred by MSB - $37,750Remarks: Following completion of subsidence, some poles were reset plumb and conductors were re-clipped in place.

6.

Electricity Substation with Preventative Work Carried Out

Transmission Line Affected: Vales Point 33/11 kV Substation Vales Road Mannering ParkType of Mining: Longwall (Wyee Colliery)Seam Depth: 170 metresStructure Affected: 33/11 kV substationSubsidence Predictions: Subsidence 1700 mm Tensile Strain 3 mm/m Compressive Strain 4 mm/m