Table of Contents
- Introduction and purpose of this document
- 1. Declared dams
- 2. Dams Safety NSW may direct actions to ensure safety of a declared dam
- 3. Notification areas
- 3.1 Review of notification areas
- 4 General
- 4.1 Mining near a prescribed dam before November 2019 – under the Mining Act 1992
- 4.2 Mining near a declared dam after November 2019
- 4.2.1 Continuation of existing lease conditions
- 4.2.2 Continuation of notification and consultation requirements in relation to dams safety in the Mining Act 1992
- 4.2.3 Consents applied for after 1 November 2019 – under the Dams Safety Act 2015 (the Act)
- 5. Mining in notification areas – consideration by Dams Safety NSW
- 5.1 Applicant information guidance
- 5.2 Modifications to consents
- 5.3 Repair of mining damage to declared dams - agreement with the dam owner
- Appendix 1 Risk analysis by the applicant
- Risk assessment
- Risk criteria
- Hazard Identification
- Consequences to the dam
- Geological/hydro-geological analysis
- Ground movement predictions
- Estimate the risk
- Evaluate the risks
- Treat the risks
- Management and monitoring
- Contingency plan
- Closure plan
- Risk assessment
- Appendix 2 Monitoring and management
- Monitoring management plan
- Visual inspection of workings
- Water monitoring
- Geological mapping - surface & underground
- Surface geological mapping
- Underground geological mapping
- Surface subsidence and strain surveys
- Vibration monitoring
- Drilling ahead of the workings
- Water fingerprinting
- Additional monitoring
- Appendix 3 Contingency plans
- Possible triggers (action indicators)
- Triggers for inflow into mine workings
- Contingency actions
- Appendix 4 Abbreviations
Introduction and purpose of this document
Dams Safety NSW is an independent regulator established under the Dams Safety Act 2015 (the Act). Dams Safety NSW is responsible for ensuring that dam owners achieve compliance with the Act and the Dams Safety Regulation 2019 (the regulation).
Dams Safety NSW ‘declares’ those dams which have a potential to threaten downstream life, or cause major property, environmental, or public welfare damage. Owners of declared dams must comply with the requirements of the Act and regulation.
Dams Safety NSW publishes guidelines to clarify legislative requirements and to provide information to assist dam owners and others.
This guideline is not a legislative document, dam owners and others are not required to adopt the suggested techniques or other material contained in it and a dam owner may achieve compliance with the legislation by adopting other methods if they fit better with a dam owner’s systems.
Dam owners and others are responsible for ensuring compliance with the legislation and this guideline is not intended to provide an acceptable means of compliance.
When referring to the requirements of the legislation the guideline uses the words ‘need to’, ‘required to’, ‘requires’ or ‘must’. Otherwise, it uses the words ‘may’ or ‘should’ when clarifying requirements or suggesting actions.
The Act includes requirements for consent authorities to consult with Dams Safety NSW before granting development consent for the carrying out of any mining operations under the Mining Act 1992 in notification areas.
This guideline describes how the Act provisions for mining near a declared dam are administered and suggests methods to reduce risks to declared dams from mining activities. It should assist persons who are mining (or who plan to mine) in the vicinity of declared dams.
1 Declared dams
Dams Safety NSW declares those dams that meet the criteria listed in section 4 of the regulation:
- a dam having a dam wall that is more than 15 metres high
- an existing or proposed dam that Dams Safety NSW is reasonably satisfied would endanger a life or result in a major or catastrophic level of severity of damage or loss were there to be a failure of the dam.
Dams Safety NSW publishes a list of declared dams on the website www.damsafety.nsw.gov.au. Dams that were ‘prescribed’ under the old Dams Safety Act 1978 became declared dams upon commencement of the new Act.
2 Dams Safety NSW may direct actions to ensure safety of a declared dam
Section 19 of the Act allows Dams Safety NSW to direct a person to do such things as may be reasonably necessary to ensure the safety of a declared dam.
Dams Safety NSW may do this if it is of the opinion that anything done or proposed to be done by a person (A person includes an individual, a corporation and a body corporate or politic) to, or in relation to, a declared dam (including the water or other material impounded by the dam) or in the vicinity of a declared dam may endanger the safety of the dam.
To ensure the safety of declared dams, regardless of the consent process described in this guideline, Dams Safety NSW may request specific information regarding proposed mining activity near a declared dam. Dams Safety NSW may also direct that a mine owner carries out actions to mitigate the risks to the declared dam. If necessary, these may include activities that are described in the appendices of this guideline.
3 Notification areas
In accordance with Section 48 of the Act, Dams Safety NSW may declare a notification area surrounding, or in the vicinity of, a declared dam.
Dams Safety NSW will consider the impact of mining on the declared dam (A reference to a dam in this guideline includes the embankment, foundation, abutments, spillways and other appurtenant structures and components) itself when declaring notification areas, not the impact of mining on the contents impounded by the dam.
The notification area is defined by straight lines joining a series of coordinates and is also identified on a map. The notification area is set out in an order published in the NSW State Government Gazette. Not all declared dams have notification areas, only those dams which Dams Safety NSW considers may be affected by mining. Declared dams with notification areas are listed on the Dams Safety NSW website. The information on the website includes the gazette notice together with a notification area map.
3.1 Review of notification areas
Dams Safety NSW may review and modify a declared dam notification area. A review may result from the receipt of additional information about risks to a declared dam from mining operations, or from a Dams Safety NSW policy determination on the extent of notification areas needed for declared dams. Dams Safety NSW will also consider requests for a review of a notification area. Requests may be sent via email to firstname.lastname@example.org.
4.1 Mining near a prescribed dam before November 2019 – under the Mining Act 1992
Prior to commencement of the Act on 1 November 2019, the requirements for mining in notification areas were covered by the Mining Act 1992, which stated that the Dams Safety Committee may recommend to the Minister that a mining lease within a notification area be amended, by the variation of the conditions of the lease or by the inclusion of additional conditions in the lease, so as to prevent or mitigate damage to a prescribed dam.
To administer these requirements the Dam Safety Committee recommended the inclusion of standard conditions in mining leases. These standard conditions required a person to notify the Minister and the Dams Safety Committee if proposing to mine within a notification area.
Subsequently, the Dams Safety Committee may have recommended to the Minister that specific conditions for mining within the notification area be included in the lease. The lease holder was required to comply with the conditions of the lease.
Upon commencement of the new Dams Safety Act 2015 in November 2019, the requirements for mining within notification areas were omitted from the Mining Act 1992.
4.2 Mining near a declared dam after November 2019
4.2.1 Continuation of existing lease conditions
Any existing lease conditions that relate to mining in notification areas continue to apply. Any reference to the Dams Safety Committee in those existing lease conditions will be taken to refer to Dams Safety NSW and the conditions remain in force.
- a lease condition that requires notification to the Minister of any plans to mine in a notification area
- a specific lease condition that requires regular reporting of mining parameters to the Dams Safety Committee - the lease holder must continue to notify and report in accordance with the lease conditions to Dams Safety NSW.
The Mining Act 1992 (Schedule 6, Part 26 clause180) states:
(1) The amendments made to this Act by the Dams Safety Act 2015 do not apply in relation to an application for a mining lease that:
- (a) was made but not granted before the commencement of this clause, or
- (b) is subsequently made after that commencement,
and accordingly this Act continues to apply in relation to any such application as if it had not been so amended.
This means that the provisions of the previous version of the Mining Act apply for mining lease applications under a consent that was in operation prior to 1 November 2019.
Dams Safety NSW may recommend to the Minister that a mining lease for a mine that comes under this provision be amended by variation of the conditions of the lease or by the inclusion of additional conditions in the lease, to prevent damage to a declared dam.
4.2.3 Consents applied for after 1 November 2019 – under the Dams Safety Act 2015 (the Act)
For applications for a development consent in a notification area after 1 November 2019, a consent authority must:
- refer the application for development consent to Dams Safety NSW, and
- take into consideration any matters that are raised by Dams Safety NSW in relation to the application.
Dams Safety NSW is primarily concerned with maintaining the safety of the dam and will consider the impact of mining on the declared dam when raising matters with the consent authority, not the impact of mining on the storage impounded by the dam.
The matters raised with the consent authority will address the risks to the declared dam from the mining activity by recommending the inclusion of conditions on the consent, to be addressed by the applicant, where appropriate.
5 Mining in notification areas – consideration by Dams Safety NSW
In accordance with the Act, the consent authority will refer an application for development consent that includes mining within a notification area (the application) to Dams Safety NSW.
Dams Safety will normally have 28 calendar days to assess the application and raise any dam safety matters with the consent authority. Dams Safety NSW may request a time extension from the consent authority if needed.
The aim is to determine that mining operations will not introduce unacceptable dam safety risks, based on the dam’s design and construction, operation and maintenance and its consequence category.
5.1 Applicant information guidance
The consent authority is responsible for making the decision on a development application but must take into consideration any matters raised by Dams Safety NSW on the application to mine within a notification area.
Dams Safety NSW will determine if there are any dam safety risks that need to be raised as matters to be considered by the consent authority.
To enable this, the applicant for the development consent (the applicant) should provide sufficient information on hazards and risks to the dam, resulting from the proposed mining activity, for Dams Safety NSW to review.
The applicant should be able to demonstrate (using a rigorous scientific approach) that the mining proposal will not introduce unacceptable risks to the dam.
The information that is needed for Dams Safety NSW to be able to objectively raise matters with the consent authority depends on the extent and type of the proposed mining and the structures affected.
Appendix 1 describes the typical analysis that would be expected when proposing to mine in a notification area. The following is the typical information resulting from that analysis and the background information that would be needed to enable Dams Safety NSW to raise any dam safety matters with the consent authority:
- plans showing the proposed mining area(s) in relation to previous mining operations, the 35° Angle of Draw plus half depth of cover from the Full Supply Level of the dam, and the dimensions of all panels, pillars, roadways and cut throughs
- detailed plans and cross-sections showing depth of proposed workings, surface and underground geological information such as lineaments, dykes, faults, prominent joints and jointing patterns and ground water flows. Fault and dyke information should be provided. Joint information should include orientation, continuity, spacing and aperture
- predictions of changes to the hydro-geological regime in the area of mining and expected inflows to mine workings under normal conditions
- records of surface subsidence and horizontal strain measurements resulting from previous mining operations by the mine in areas where conditions are relevant to the proposed mining within the Notification Area
- predictions of surface subsidence and horizontal strains for the proposed mining operations, including a description of the methods adopted for predicting subsidence and strains
- if available, information relating to in-situ stress measurements, estimates or interpretations. This should include, if available, data on stress magnitudes and directions
- for open-cut operations, a prediction of likely blast vibrations at the dam and comments from a dams engineer on the dam safety impacts of such vibrations.
- risk analysis report
- details of any proposed monitoring programs and
- details of contingency plans and Trigger Action Response Plans (TARPs) (if required).
5.2 Modifications to consents
Modifications to an existing development consent that involves mining in a notification area are treated in the same way as the initial application, in that the consent authority must refer the application for modification of the development consent to Dams Safety NSW.
5.3 Repair of mining damage to declared dams - agreement with the dam owner
In the event that the safety of a dam structure may be compromised due to mining-induced impacts, Dams Safety NSW may decide that it is necessary to issue a direction under the Act to assure the safety of the dam. This is to assure the safety of the dam and not to protect the water supply storage, or contents impounded by the dam.
The Act allows Dams Safety NSW, if it is of the opinion that a declared dam is unsafe or is in danger of becoming unsafe, to direct the owner of the dam to do such things as may be reasonably necessary to ensure the safety of the dam.
The Act also allows Dams Safety NSW, if it is of the opinion that anything done or proposed to be done by a person to or in relation to a declared dam (including the water or other material impounded by the dam) or in the vicinity of a declared dam may endanger the safety of the dam, to direct the person to do such things as may be reasonably necessary to ensure the safety of the dam.
In an emergency quick action may be required. Matters such as the roles of the various parties, access rights in the event of an emergency, the responsibility for costs and the like should be discussed and agreed before an emergency occurs. The contingency plan and TARP should consider and work together with the declared dam owner’s emergency response plan.
A written agreement between the mining company and the dam owner should be established to formalise the above.
Appendix 1 Risk analysis by the applicant
The material in this appendix has been provided to guide applicants when applying for development consent for carrying out of mining operations in a notification area.
The appendix includes guidance on the risk analysis and specific associated topics that should be addressed by the applicant. An applicant should comprehensively analyse the risks to dam safety associated with mining in a notification area and produce a report which contains the analysis and conclusions associated with the analysis.
A comprehensive risk assessment should be undertaken for any mining proposal in a notification area. Applicants should carry out a risk assessment that follows the process elements of ISO 31000:2018 – Risk Management – Guidelines. The following has been included for guidance. Other risk assessment methods may be used, such as the International Council on Mining and Metals critical control approach or a ‘bow tie’ risk approach.
The generic risk assessment process can be summarised in the following steps:
- Determine risk criteria
- Identify mining hazards that impact on the dam
- Determine the consequences
- Determine the likelihood
- Estimate the risk
- Evaluate the risks
- Treat the risks so that the risk rating is reduced (at least) below the safety threshold for the dam and so far as is reasonably practicable (Refer to section 15 of the Regulation).
These steps are discussed below in relation to assessing the impact of mining on dam safety. Each case will have unique aspects which should be considered. Experienced persons should be involved in discussions at each stage, as well as the stakeholders. These may include the dam owner; the applicant; risk analyst (mining side); risk analyst (dam safety side - dam safety risk engineers); independent peer reviewer; relevant regulators.
Setting the criteria requires a discussion with stakeholders and especially with the declared dam owner. The dam owner should agree the criteria, as any hazard introduced by mining activity will add an additional failure mode, or failure modes, for consideration in the dam owner’s dam safety risk assessment and will contribute to the assessment of the risk rating (Refer to section 15 of the Regulation) for the dam.
A comprehensive identification of the hazards to the dam, associated with mining activity, should be undertaken.
Mining hazards that may impact a dam include:
- large scale subsidence (see the section on ‘ground movement predictions’)
- vibration due to blasting * creation of open cut voids and potential highwall instability
- changes in permeability due to vertical and horizontal fracturing and collapse of strata, or opening of pre-existing fractures
- opening of a drainage point or conduit which could lead to dam instability
- changes to the stress regime, leading to changes in permeability
- changes to groundwater levels which could lead to dam instability.
The section on ‘Geological/hydro-geological analysis’ includes the expected analysis of hazards associated with the geological and hydrological features of the declared dam and its surrounds.
Consequences to the dam
The risk assessment should focus on the consequences of the mining activity to the safety of the dam, including, but not limited to:
- subsidence or settlement, which may reduce freeboard, leading to a reduction in flood capacity and possibly overtopping
- changes in slope, causing problems with drains and hydraulic structures. In extreme case instability in the upstream or downstream face due to a steepening of slope
- differential movement between the embankment, abutments, spillway, conduits, and other components, possibly leading to physical damage of components, increasing the likelihood of leakage and piping failure, or a failure of particular components (e.g. blocked conduits, burst conduits)
- instability of slopes around the storage resulting in large-scale land-sliding and seiche wave generation, or damage to the dam or its associated structures, abutments and foundations
- differential movement on geological structures present in the foundation
- strain concentration, in particular, over long structures, leading to cracking and leakage, and potentially piping
- dynamic loading, which may result in settlement, liquefaction of foundations or (for tailings dams) of the storage
- changes in groundwater, leading to ground movements or changing the groundwater pressure regime at the dam wall
- the potential for geological discontinuities to act as, or become conduits for, groundwater flow and thereby causing pressure changes at the dam wall
- groundwater depressurisation leading to groundwater pressure changes at the dam wall
- development of transverse cracks in the crest of an embankment dam as a result of settlement or stress concentration, leading to an increased risk of piping failure during a flood
- disturbance of the dam foundations, abutments and storage rim in the vicinity of the dam site leading to the opening of voids, uncontrolled leakage and the potential for the development of foundation piping failure
- effect on pre-existing geological structures or in-situ stresses in the foundations leading to a change in permeabilities and potential kinematic failure mechanisms.
As part of the hazard identification and risk assessment process, an applicant should analyse the influence of the geology of the dam’s surrounds on the potential for mining impacts on the dam. Where the mining may impact a major water supply storage dam, geology and hydrogeology play a critical role and should be given a suitable priority.
Geology can influence:
- the stability of the workings;
- the development of the goaf and other sub-surface impacts;
- subsidence and other surface ground movements; and
- groundwater flow paths.
The most credible models will be based on inspections of all the available sources: surface, underground, and boreholes. In particular, the combination of surface and underground mapping may permit an assessment of the likelihood that structural features are continuous from the surface to the workings.
The highest level of detail would be needed for any higher risk applications. The analysis should integrate all available relevant data, such as:
- general surface mapping to determine contacts between geological units, overall structure, domains, structural fabric, etc.
- lineament analyses
- satellite and airborne geophysical data
- detailed surface mapping in the vicinity of significant structural geological features or areas of uncertainty
- data from boreholes, including lithology, structure, piezometric data, and the results from permeability testing
- data from existing workings nearby
- in-situ stress measurements.
This data will provide important inputs to the hydro-geological modelling to predict mining effects on the groundwater regime and the source and magnitude of potential inflows to the mining operation, which could impact the safety of the dam.
As it integrates a number of data sources, care should be taken when presenting the geological/hydro-geological model. The primary model will be a series of plans and cross sections which aim to summarise this information, supported by explanatory notes. An appendix containing all the source documents should be provided.
Ground movement predictions
Predictions of surface ground movements are critical to analysing risks to a dam.
The prediction of ground movements should consider but not be limited to:
- large scale movements: These usually occur over the mine workings and in the immediate vicinity of the workings, perhaps defined by an angle of draw to 20 mm of vertical movement (which is the common criterion in NSW). Angles of draw vary significantly, but an average of 35 degrees from vertical at the underground edge of an extraction is often adopted in NSW. In this realm “normal” or “systematic” mining induced subsidence and strain occur
- far-field movements: These are predominantly horizontal movements which increasingly dominate the total ground movements further away from the “limit” of the large scale movements. They have been recorded up to 1.5 km from mine workings
- response of the mine/dam site: ground movements (large scale and far-field) may be modified locally by the characteristics of the site. Geological features can also modify the locally recorded movements
- response of the dam structure; dams tend to be long structures and may therefore be more susceptible to certain ground movements. The nature of the dam will also contribute to its response. For example, rigid construction (e.g. concrete) will be more susceptible to concentration of strain at discrete points than flexible construction, such as an earth or rockfill embankment. For applications relating to open cut mining, blasting predictions should be determined for the maximum likely peak particle velocities and accelerations at critical points on the dam, together with the prediction of the impacts of these blasting effects.
Determinations of likelihood should take into account:
- the probability of sources of risk (e.g. ground movements due to subsidence, vibrations from blasting) being within predictions, and exceeding predictions
- the probability that the workings may not perform as designed or that the geology is not as theorised (e.g. the probability of pillar failure, or the probability of encountering a major fault)
- the ability of a dam structure to accommodate ground movements. Generous filter zones, conservative design, and flexible components should improve the capability of a dam to withstand ground movements, whereas brittle components such as a concrete core or face, limited or no filter zones, or poor overall design will lead to greater concern for the safety of the structure
- the level of detail, or degree of uncertainty in the available data, models, analyses etc..
Estimate the risk
An appropriate methodology for determining the resultant risk level (combination of likelihood and consequences) should be adopted.
Evaluate the risks
Risks may be assessed both quantitatively and qualitatively, but quantified risks are preferred, where available, as they can be included in calculations for the dam risk rating.
Treat the risks
Risks can be avoided, the likelihood can be reduced, or consequences reduced. Risks which cause the safety rating to be above the dam safety threshold will need treatment. For each treatment option the likelihood, consequence and risk should be re-estimated, and the risk re-evaluated.
Management and monitoring
All mining activities in notification areas should have some form of monitoring. These should be implemented and procedures for monitoring integrated with a TARP. Refer to Appendix 2 for more guidance.
The nature of mining is such that unexpected conditions are not uncommon. Given the possibility of serious consequences occurring in the event of encountering these conditions, applicants should develop a contingency plan, consistent with the mine TARP, for mining in major water supply dam notification areas. Contingency plans are aimed at minimising damage to the dam in the event that a major connection from the workings to the storage develops. The plans should be reviewed regularly (typically, annually). See Appendix 3 for more details.
The life of a typical major water supply dam is probably an order of magnitude greater than a typical mining operation. Mine workings remain for a very long time after mining has ceased and constitute an ongoing dam safety hazard.
Applicants should develop a plan for mine closure which takes into account:
- the need for stabilisation or sealing of the workings in order to achieve acceptable levels of risk in the long-term
- the likelihood that deterioration of the workings after completion will lead to additional mining impacts
- the combined effect of any future mining together with that under current consideration.
Closure plans are particularly important where layouts are used which provide some support to the strata, (e.g. first workings or partial extraction layouts) as these have components which can deteriorate or fail over a long period. They are also important for applications that might affect a dam which has a societal risk that is already close to the safety threshold.
Applicants should consider all possible post-mining failure scenarios, determine that there are feasible technical solutions to these and that they have the capacity to implement a solution. It may be difficult to fully develop these plans at the outset, so it may be appropriate to plan to review the performance of the workings at the completion of mining in an area.
Appendix 2 Monitoring and management
Mining applicants should establish and implement a monitoring programme so that:
- the package of controls to mitigate mining risks to the safety of the dam is complete
- predictions on which the risk analysis was based are verified
- areas of uncertain knowledge are clarified
- unexpected behaviour is recognised during mining which provides triggers for action or additional monitoring
Management and monitoring programs are generally aimed at controlling residual risks, ensuring good management of activities, and facilitating the interaction between the declared dam owner and the mining company. Many competing priorities present themselves to mine management. It is important that adequate resources be applied to the dam safety aspects and a management program can help ensure that this happens. In situations where there is an extensive monitoring program the interface between the mining company and the dam owner should be well defined to ensure that the flow of information is handled efficiently by both parties.
Ongoing monitoring is important in determining the impacts of mining including the need for baseline monitoring prior to extraction (preferably for at least two years), to provide a basis against which to assess mining impacts. Where there are a number of monitoring streams, it can be a challenge to keep a program on track. An effective management plan will assist in keeping track of what has to be done, who does it, where the resources come from etc..
Monitoring management plan
The monitoring management plan (developed in conjunction with the declared dam owner) should include details of monitoring trigger levels and actions to be taken, including all roles and responsibilities.
The plan should specify at least:
- what is required to be done
- what is required to be delivered and when
- who is responsible for each task.
Some more common components include:
- inspection of workings;
- water monitoring;
- geological mapping;
- surface subsidence and strain surveys;
- vibration monitoring;
- drilling ahead of the workings;
- water fingerprinting and
- other monitoring conditions.
Visual inspection of workings
A record of any features encountered or events which occurred should be kept (e.g. if significant floor heave or pillar spalling occurs, or if a dyke or joint zone is encountered). For open cut mining it would include the location of the pit with respect to the dam, the levels in the pit, and areas of active mining. Face location records are also useful in determine how the mine is progressing with respect to the predicted locations of identified hazards.
The safety of a dam structure may be compromised due to a rapid reduction in water storage or groundwater causing stability issues for the dam itself.
Therefore, the presence or rate of water flow into the workings can be a useful, readily measured indicator of the performance of the workings. Unexpected water inflows can provide a useful trigger for further investigation or increased surveillance in accordance with the TARP where these might be of a magnitude that could affect the structural integrity of the dam.
There are two types of programs: routine and emergency. As a general guide, if the workings are unlikely to have an effect at the surface (e.g bord and pillar workings with a reasonable depth of cover and no significant geological structures) then only "emergency water monitoring" will apply. Otherwise both routine and emergency monitoring should be included.
A declared dam owner must report an incident to Dams Safety NSW that results in a significant amount of groundwater entering the mine (clause 19 (1) (h) of the Dams Safety Regulation 2019).
Note that a code of practice is available for reference at: https://www.resourcesregulator.nsw.gov.au/__data/assets/pdf_file/0011/543935/NSW-codeof-practice-Inundation-and-inrush-hazard-management.pdf
Regular measurement (typically daily or weekly) and reporting (typically monthly) of water entering and leaving an area of the mine is recommended. The detail recommended in these surveys varies depending on the importance of the dam, the type of workings, the history of water make in the seam to be extracted, and on the geological conditions in the mine in general. If full monitoring is considered to be necessary, the mining company should monitor all water entering and leaving the approval area. This may involve regularly measuring water coming in and out by ventilation, leaving in the coal or ore, or pumped out or in; that is, a complete water balance for the approval area. In some cases, water fingerprinting may also be needed.
Emergency water monitoring
Any significant inflow of water should be reported immediately to the dam owner. “Significant” in relation to groundwater flows into the workings is defined on a case-by-case basis, however a starting point might be:
- the volume or rate of flow that could cause structural integrity issues for the dam itself,
- the volume or rate of flow, or the change in the volume or rate of flow, or the location causes surprise among persons familiar with the workings
- the volume or rate of flow doubles over a few days, or
- the volume or rate of flow is more than 3 standard deviations above the mean value (i.e. larger than about 99% of recent readings, or in the top one percentile).
Geological mapping - surface & underground
The section on Geological/hydro-geological analysis describes the analysis that should be carried out to develop a credible geological/hydrogeological model to address the hazards associated with the geology of the dam’s surrounds. Development of the model should continue during the operation of the mine. The conditions requiring underground geological mapping and additional surface mapping contribute to this.
For underground workings the geological/hydrogeological models are critical to understanding the interaction between the mine workings and the surface. The geology can influence:
- the stability of the workings
- the development of the goaf and other sub-surface impacts
- subsidence and other surface ground movements and
- groundwater flow paths.
Structural geological features may have a strong influence on the performance of the system, and consequently should be a major focus of the investigations.
The most credible models will be based on inspections of all the available sources: surface, underground, and boreholes. In particular, the combination of surface and underground mapping may permit an assessment of the likelihood that structural features are continuous from the surface to the workings. The highest level of detail is recommended for any high risk applications and medium risk applications that may impact on a declared dam.
In the case of open cut mining, geological mapping facilitates the assessment of highwall stability (where this could impact on the safety of the dam structure) and of the likelihood that a flow path will develop between the storage and the pit.
Surface geological mapping
Surface geological mapping involves developing a traditional surface geological map with additional emphasis on structural geology and significant lineaments. Lineaments are defined by an investigation of air photos or satellite images. “Significant” lineaments may be defined by their persistence, proximity to the dam or stored water, or magnitude. The significant lineaments are then a focus of surface inspection aimed at determining the conditions that gave rise to the lineament. The following features would typically be shown on the surface geological map:
- topographic contours at 10 m intervals (small intervals may obscure other detail)
- surface boundaries of major geological units
- the results of detailed surface geological mapping in the vicinity of each significant lineament, aimed at determining the surface conditions giving rise to the lineament. Features considered may include: outcrop mapping, with fracture, joint, or bedding patterns; soil cover; vegetation changes; slope changes; wet areas or seepage
- the outline of past extraction and the significant results of underground mapping in these areas. These include all faults (including major slickensided joints, sheared or crushed zones which may not show displacement) and
- the areas of proposed extraction.
These maps should integrate, as far as possible, the significant features determined from any relevant investigation undertaken as part of the application, in particular borehole data, results from geophysical investigations, and the results from in-situ stress investigations. These maps should be prepared by suitably qualified persons (i.e. geologist, etc).
Underground geological mapping
This involves the preparation of a geological map based on an inspection of the underground workings. Its focus is on the elements that may impact on the behaviour of the workings, the stability of the workings, or that may indicate a connection from the underground to the surface. These elements include:
- structural features: faults, major joints, crushed seams, and sheared zones
- igneous features: dykes, sills, other intrusions
- evidence of the impact of in-situ stress on the workings (guttering, roof falls, etc.) other features that may impact on the level of extraction (e.g. thinning or dip of seam) and
- other features that may impact on the performance of the workings (sedimentary features such as channels, changes in roof or floor conditions, etc.). These maps should be prepared by suitably qualified persons (geologist, geotechnical engineer, etc.).
Surface subsidence and strain surveys
For almost all applications relating to underground workings, predictions of surface ground movements would be needed and are critical to the decision whether to permit mining or not. Accurately predicting ground movements in valleys and at points remote from mining, where movements may be small, is difficult. Both these situations are relevant to the impact of mining on major dam structures.
Consequently, measurement of actual ground movements is usually incorporated into a monitoring program to verify the predictions, to allow for the early detection of unexpected ground movements, and to increase the overall understanding of the impact of mining on declared dams.
When designing a measurement system it is useful to characterise the type of movements to be measured. Ground movements might be characterised as:
- large scale movements: These usually occur over the mine workings and in the immediate vicinity of the workings, perhaps defined by an angle of draw to 20 mm of vertical movement (which is the common criterion in NSW). Angles of draw vary significantly, but an average of 35 degrees from vertical at the underground edge of an extraction is often adopted in NSW. This is the realm of “normal” or “systematic” mining induced subsidence and strain. This monitoring:
- allows for the verification of overall subsidence predictions
- will be closely related to the progress of mining
- contributes to the understanding of the development of the overall subsidence profile, which can then be compared or correlated with ground movements at specific locations.
- far-field movements: These are predominantly horizontal movements which increasingly dominate the total ground movements further away from the “limit” of the Large Scale Movements. Cases have been recorded up to 1.5 km from mine workings.
- response of the site: Ground movements (large scale and far-field) may be modified locally by the characteristics of the site. Geological features can also modify the locally recorded movements.
- response of the structure: Dams tend to be long structures and may therefore be more susceptible to certain ground movements. The type of dam will also contribute to its response. For example, rigid construction (e.g. concrete) will be more susceptible to concentration of strain at discrete points than flexible construction, such as earth or rockfill.
The survey method depends on the type of movements to be measured. The following notes are intended to provide some guidance only:
- large scale movements: A traditional subsidence and strain survey grid (i.e. perpendicular lines extending over the panels), or aerial methods can be used to determine these movements. Precision of not less than 1% of the maximum predicted movements should be acceptable. For traditional subsidence surveys over longwalls this equates to acceptable error bands of the order of 10 mm for vertical movement, and 1 mm for horizontal movement per nominal 20 m bay length. The lower precision of aerial surveys is acceptable when the magnitude of maximum subsidence is around 1 m or greater, and where specific points or areas are targeted with higher precision surveys.
- far field movements: The nature of these movements is such that high precision survey techniques utilising long-baseline survey grids are recommended in order to record acceptable results.
- response of the site: If the site is a valley, traditional surveying techniques across the base, or along the top of the valley may produce acceptable results, in particular if the predicted magnitude of movements is small. Alternatively, 3D survey marks can be installed on the valley walls and floor, monitored by EDM. Other sites (e.g. adjacent to a geological feature of interest) would typically be monitored by traditional surveying techniques.
- response of the structure: dams for which a dam movement monitoring program is recommended would typically already have an existing survey monitoring grid on which measurements have been taken and recorded for a number of years. Ideally, “during mining” monitoring of the dam would utilise the same grid and equipment, thus allowing the pre-mining movement history to be used as baseline data. Survey techniques on dams typically utilise high precision EDM to give 3D movement vectors.
Where mining may result in significant ground vibrations, for example as a result of blasting, peak particle velocity limits or other limits may be set at critical structures. Typically, a conservatively low limit should be applied initially, for example 10 mm/s PPV at a dam wall.
Vibration monitoring is recommended to ensure that the limits have not been exceeded. However, the information obtained from a vibration monitoring program can be used to more accurately predict ground vibrations.
The mining company should develop a plan to monitor ground vibrations. Where blasting is the source of vibrations, blasts should be designed to maintain vibrations below the limits. The involvement of a blasting specialist is recommended in this process. Monitoring is typically conducted in conjunction with an event (i.e. a blast) and at or close to the critical components. Generally, the following conditions apply:
- the dam owner is advised prior to a blast
- a management plan should be considered; this may include evacuation of persons downstream of the dam prior to blasting or inspections of the dam following blasting
- exceeding set limits may trigger:
- an immediate notification of the dam owner
- a cessation of blasting while the cause is investigated and procedures refined to ensure future blasts are within limits and
- an inspection of the affected components (the dam wall).
Drilling ahead of the workings
In-seam drilling ahead of workings has become a relatively common practice in underground coal mines in NSW. Typically, these holes are not cored. Information on geology is determined by inspection of the cuttings, consideration of the rate of advance and pressure on the drill, and roof and floor “touches”.
Drilling ahead of the workings can assist in the detection of significant geological features before the heading intersects them. Of interest are major water bearing features which might represent a flow path from the surface to the underground workings. The method may be applied as an additional check on geology when mining near dam structures.
Generally drilling ahead is not a substitute for adequately developing a geological model as part of the application, as only coarse geological features can be detected by this method.
An understanding of the source of an inflow may allow for a more focussed remedial plan if an uncontrolled inrush occurs, where this inrush could affect the structural integrity of the dam.
A water fingerprinting program should be developed with considerable care as the outcomes depend on a robust and defensible system of collection and analysis. A typical program might include:
- the collection of background samples of likely source waters (surface, workings, groundwater, etc);
- analysis of these samples to determine typical properties: chemical, algal, pH, etc; and
- collecting samples in the event of an inrush and comparing these to the database of samples to determine the likely source.
Additional monitoring should be considered in special cases; for example, when large scale extraction is proposed in much wider panels, or at much shallower depths than previously attempted within the notification area of a declared dam. In these instances, the monitoring program may include groundwater monitoring, sub-surface subsidence monitoring, pillar stress monitoring, geophysical surveys, or other techniques to assist in evaluating the impact of the mining on the surrounding strata and the affected surface feature.
Appendix 3 Contingency plans
The purpose of a contingency plan for the protection of a declared dam is to provide effective procedures that will assist in minimising damage to the dam if unexpected conditions are encountered during mining operations. The plan is not a substitute for adequate pre-mining investigation nor does the existence of a plan allow mining in areas that would otherwise have an unacceptable risk and this appendix is intended as a guide only. The plan should be consistent with the mine TARP and may be included as part of the other plans developed by the mine to mitigate risks - where contingency or emergency plans already exist for other reasons it may be more appropriate to extend one of these plans rather than develop a completely new plan.
The contingency plan is should be an ‘action plan’ and should include:
- relevant, reliable and readily measured parameters identified for use as ‘triggers’
- identified levels or events for triggers
- explanation of the monitoring or other information supporting each trigger level
- an explanation of the actions to be implemented at each trigger level. For example, for mine inflows, procedures will specify levels of inflow above which key personnel, both at the mine and in other organisations, are to be informed of the situation, and what actions are required of these personnel
- definition of the roles, responsibilities and contacts for persons involved in any trigger or action
- outlining the resources required to monitor the parameters and implement any required actions as is the process for obtaining these resources and
- arrangement such that the plan can be implemented rapidly by someone in the event of an emergency.
Where the plan calls for an action which requires the use or installation of equipment, such as pumping inflow water or constructing bulkheads, the time required for deployment of the equipment should be considered in the plan. Where necessary the equipment should be stored or installed at the commencement of mining, to be readily available.
Possible triggers (action indicators)
Triggers should be based on physical parameters which are relevant, reliable, and readily measured. Inflow into the workings is usually a prime candidate for an information source as it is relevant, and usually readily measured. However, the possibility of flow paths which do not intersect the workings developing as a result of mining should be considered. Other indicative parameters are:
- development of caving towards a known aquifer or flow path
- changes in groundwater levels
- significant water loss from reservoir
- unexpected ground movements at the surface, particularly at the dam structure, or underground or
- deterioration of pillars.
These should be considered in the context of risks to the structural integrity of the dam itself. Typically, a number of trigger levels would be set. Defined in terms of the broad actions required at each level, these might include:
- Notify, increase surveillance, mitigate (pump);
- Investigate, slow mining;
- Prepare for remedial measures, stop mining; and
- Implement remedial measures progressively (there may be more than 1 stage of remedial works).
Levels for triggers may need fine tuning based on their performance in the first few months. This is particularly the case for low level triggers, such as a ‘watch’ or ‘notify’ trigger. Activating low level triggers too frequently may reduce their perceived importance. However, an occasional (say, less than 3 times in 12 months) low level trigger will provide evidence that the system is working.
Triggers for inflow into mine workings
As far as the safety of the dam is concerned, an inflow into mine workings may increase the risk of structural failure.
At the upper end of the scale, mining should cease when the rate or volume of loss of stored water approaches the set limits. At this stage, implementation of the remedial works should have begun.
Levels could be set based on the rate of flow, rate of change of flow, appearance of flow (discolouration), etc.
In order to be reliable and readily measurable, systems should be in place to determine the background levels. Usually this would involve maintaining a water balance for an area. It may also incorporate some form of water fingerprinting to allow discrimination between different sources of water. This might minimise the number of false alarm triggers.
Options for detecting water inflows include:
- an awareness program for staff and an informal visual inspection program;
- formal visual inspections; and
- review of the water balance recordings for the mine (i.e. recording volume of water pumped in an out of an area of the mine, recording volumes in holding and recording moisture lost from an area in coal or ore or from ventilation).
Typically, a program would be developed that incorporated a number of these options. The more reliability there is in a program, the smaller the chance that the operation is interrupted by false alarms.
The following table lists generic contingency actions and provides some examples relevant to the loss of stored water that could affect the structural integrity of the dam. (Note that a similar approach is recommended for responding to other unexpected mining effects on the structural integrity of dam structures). Also refer to Appendix C of the following code for guidance: https://www.resourcesregulator.nsw.gov.au/__data/assets/pdf_file/0011/543935/NSW-codeof-practice-Inundation-and-inrush-hazard-management.pdf
Table 2: Generic actions related to trigger levels
|Generic Trigger||Generic Action||Example of specific actions related to an inflow|
|Watch||Visual inspection or observation of existing monitoring data, conducted irregularly but frequently during normal operations.||Arranging to walk past and look at an area of seepage or drippers a few times during a shift.|
|Notify||Formal advice that an event has occurred or is occurring.||Contact the dam owner and advise them that there is persistent leakage from a heading, that it will continue to be monitored, and that an update will be provided.|
|Increase surveillance||Implement a higher frequency of measurement or observation of existing data streams, or implement additional monitoring aimed at improving the understanding of the behaviour of the event.||Direct seepage water to a point where it can be measured and recorded, so that a clearer understanding develops of the rate and volume of inflow from a particular point.|
|Investigate||Undertake desk or on-site investigations to determine the source, path, or cause of an event.||Drill holes to determine if a flow path from the surface to underground exists. |
Undertake water fingerprinting to determine the source.
Re-evaluation of geological/hydrogeological model. Geophysical testing to determine water bearing fissures.
|Mitigate||Implement measures to reduce the consequence of the event.||Turn on existing pumps or install additional pumps; NB this mitigates the consequence to the operation but does nothing to mitigate the threat to the dam. Stop mining. Draw down the reservoir to reduce risks to the structural integrity of the dam.|
|Remediate||Implement measures to reduce the cause.||Install a grout curtain or sheet, or attempt to grout a specific flow path.|
Seal the area of the mine where the stored water is entering.
Appendix 4 Abbreviations
Dams Safety NSW assumes that the readers of this guideline are familiar with technical terms associated with mining and geology. However, the following are provided as they are specific to the dams safety legislation.
Act - Dams Safety Act 2015
Regulation - Dams Safety Regulation 2019
Notification Area - An area of land that surrounds, or is in the vicinity of, a declared dam, within which a consent authority must, before granting development consent for the carrying out of any mining operations under the Mining Act 1992, refer the application for development consent to Dams Safety NSW. The consent authority must take into consideration any matters that are raised by Dams Safety NSW