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• 7.1 Identifying tips, ponds, voids and dams as principal hazards
• 7.2 PHMP for tips, ponds or voids
• 7.3 Planning and design criteria for tips
• 7.4 Planning and design criteria for ponds or dams
• 7.5 Construction of a tip or pond
• 7.6 Rehabilitation of tips

7.1 Identifying tips, ponds, voids and dams as principal hazards

At a mining operation, an A-grade quarrying and alluvial mining operation, the responsible person for the operation must carry out an appraisal of the operation to identify principal hazards. The responsible person should seek technical advice from a competent person, as required.

To determine whether tips, ponds, voids or dams are a principal hazard, the responsible person should consider how they might fail, and the likely consequences if they do.

The consequences will depend on:

• the size of the failure and the area affected
• whether people are likely to be killed.

Typically, a principal hazard exists where:

• the tip is, or will be, in a wholly or mainly solid state and not in solution or suspension (that is, not likely to flow if it is not contained), and:
  • the area of the tip exceeds 10,000m², or
  • the height of the tip exceeds 15m, or
  • the average gradient of the land covered by the tip exceeds 1 in 12 (see Figure 15)
• the tip or pond contains, or will contain, any liquid or material wholly or mainly in solution or suspension (that is, likely to flow if not contained), and:
  • the contents of the tip or pond is more than 4m above the level of any land which is within 50m of its perimeter, or
  • the contents of the pond exceeds 20,000m³ (see Figure 16)
• regardless of the size of the tip, pond or void, other factors (for example the geology, location of or proximity to an excavation) means there is a principal hazard
• vehicles operate near the edge of the tip, pond, void or dam.

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7.2 PHMP for tips, ponds or voids

If the risk appraisal finds that tips, ponds or voids are a principal hazard at your site, the responsible person must ensure there is a PHMP. For general information about PHMPs, see Section 2.6.

A description of how the risk assessments will be conducted, as well as the results of the risk assessment, must be included in the tips, ponds and voids PHMP.

In addition, the responsible person must make sure a reassessment of the stability of the tip, pond or void is carried out by a competent person:

• at least once every two years after the date the PHMP is made
• when the construction of a tip, pond or void deviates from the geotechnical design
• if a new tip, pond or void is created.

WHEN THE RESPONSIBLE PERSON MUST PUT A PHMP IN PLACE

For mining operations, a PHMP must be in place, if a tip is or will be:

• located on a slope
• greater than 15m in height, and
• greater than 100,000m³ in volume.

WHAT THE RESPONSIBLE PERSON MUST INCLUDE IN THE PHMP

PHMPs for tips, ponds or voids must contain the general PHMP requirements listed in Section 2.6 of these guidelines and Regulation 68 of the MOQO Regulations. It must also include:

• procedures and processes to ensure the safe design, construction and maintenance of tips, ponds or voids
• a geotechnical assessment
• road design and traffic movement
• tipping rules
• records of tipped material
• an inspection and monitoring regime.

For a full list of requirements for tips, ponds or voids PHMPs, see Regulation 81 of the MOQO Regulations.

Where the PHMP requires regular inspections, the PHMP must specify:

• the nature and interval of inspections
• the appointment of a competent person to supervise tipping operations, including a requirement that this person supervises every tip inspection.

Develop the PHMP in the context of your entire HSMS. This helps identify any gaps and overlaps when putting control measures in place for tips, ponds and voids.

Geotechnical assessment

If you identify a tip, pond or voids as a principal hazard at your site, you must arrange for a geotechnical assessment.

This assessment must be proportionate with the type and scale of tipping operations and consider the:

• underlying geotechnical structure at the tip site
• properties of the material being tipped
• creation of any ponds or voids.

The geotechnical assessment should tell you the type of foundation and surface treatment needed, which may include:

• removing unsuitable, weak material from the foundation
• benching of the foundation
• installing under-drains and final slope toe drains
• installing surface cut-off drains.

Data from the geotechnical assessment should be used to develop a geotechnical design that establishes an appropriate foundation for your site. The assessment should also recommend maximum lift heights, depths, volumes and maximum overall tip height.

Records to be kept

If you identify a tip, pond or void as a principal hazard at your operation, the PHMP must include record keeping of materials being tipped.

7.3 Planning and design criteria for tips

You should consider tipping and storage of waste material at your site. This should occur during the project’s planning phase, not towards the end or after the site design is finalised.

When choosing control measures to manage risks at tips, consider:

• the geology of the area (particularly the foundation of the tip, foundation materials and tip materials)
• the quantity, type, and rehabilitation of overburden
• the type and size of mobile plant to be used
• access roads for vehicles and pedestrians
• preparation of the receiving area
• settling requirements, drainage and runoff controls
• stabilising methods, including inspections
• the risk of spontaneous combustion
• controlling public access
• any other hazards (for example, overhead power lines).

Tip sites should be able to safely receive waste material. This includes removing vegetation and topsoil, and keying into the substrata to ensure the stability of the material placed above.

Tree felling

Where tree felling is needed, only competent people with appropriate tree felling qualifications should undertake the work. WorkSafe must, so far as is reasonably practicable, be notified at least 24 hours before you intend to undertake tree-felling.

The following guidelines from Safetree may be useful Safetree(external link)

Subsoil drainage

Subsoil drainage should be considered to ensure there can be no liquefaction of the material placed there.

This can be as simple as placing large rocks to allow moisture to ‘wick’ through, or using drain coils and piping to capture and carry moisture through the waste material to a controlled discharge located below the tip.

Water diversion and drainage structures should be designed and implemented according to acceptable engineering standards and be functional for the life of the tip.

If tip rehabilitation is needed, it should be completed as soon as possible to prevent scouring and water damage through erosion.

Access roads

Access roads and other vehicle operation areas should be designed according to engineering standards. Criteria should include:

• road width
• road gradient
• edge protection
• signage
• speed limits
• lighting
• overhead hazards
• passing rules.

For more information on roads, see Section 8.

Adjacent stockpiles

Adjacent stockpiles can influence each other (for example, stability may be altered when they overlap). The adequacy of vehicle routes should also be considered when planning the position and size of stockpiles. In particular, the risk of collision can be minimised by making sure drivers have a clear field of view. For more information, see Section 14.10.

7.4 Planning and design criteria for ponds or dams

You must eliminate risks at your pond or tailings dam, so far as is reasonably practicable. If the risks cannot be eliminated, you must minimise them, so far as is reasonably practicable. This includes during operations, decommissioning and after the pond or dam has been abandoned. Some common hazards include:

• seepage
• dust generation
• exposure to chemicals or hazardous particulates
• erosion
• overtopping
• abrupt failure of a retaining structure
• impediment of surface water flows
• pollution.

When designing and selecting the site for a pond or dam, some things to consider include:

• hydrology (potential for flooding and catchment area characteristics)
• topography (influence of watershed, streams and creek systems)
• foundation material (water tightness, strength and liquefaction potential)
• foundation conditions (physical, geochemical and geotechnical properties)
• characteristics of construction (suitability, availability and proximity)
• characteristics of tailings material (physical, geochemical and geotechnical properties)
• climate (rainfall patterns, evaporation rates and prevailing winds)
• geology (faults, fractures, shear zones and areas of instability)
• hydrogeology (potential impact on ground water resources)
• seismicity
• minimum freeboard
• seepage control methods
• characteristics of embankment or other retaining structures (stability, erosion resistance, resistance to dynamic or static liquefaction and integrated waste landform)
• operating strategy
• access requirements
• characteristics and availability of cover and rehabilitation methods
• whether there are any populated areas downstream which may be adversely affected in the event of a failure.

Planning should ensure the pond disposal area is left in a condition where it will:

• maintain an acceptable level of risk control (for example, for dust control or access)
• remain structurally stable
• resist deterioration through erosion and decay
• prevent loss of containment.

7.5 Construction of a tip or pond

Develop and put in place a construction plan to make sure the tip or pond construction meets design specifications and tolerances. This should include quality assurance procedures.

The plan should also include systems of work and procedures to make sure the proposed construction can be carried out safely.

Use a competent person to ensure construction of tips or ponds meet design specifications and tolerances. They should document:

• the conditions encountered during construction (including field and laboratory testing), verified against the design assumptions
• corrective measures taken where conditions did not meet the original design or specifications
• all changes required that deviated from the original design
• the testing and measurement regime used to validate the design parameters
• survey data and drawings of the tip or pond construction.

The design should show the true locations of:

• borrow pits and embankments
• drains and seeping trenches
• topsoil stockpiles and capping material sources
• process water and return water ponds
• monitoring instrumentation
• decant towers
• buried pipework and cables.

The construction records and monitoring data form the basis for the design of subsequent stages. If construction is staged, prepare a separate construction report for each stage.

Your tip or pond may also be considered a dam under the Building Act 2004(external link)

For more detailed information on dam notifications, dam classification, dam safety assurance programmes or dam compliance certificates, see Dam safety requirements(external link)

Drainage of a tip

If sufficient water is present, either from heavy rainfall or other sources, some or all of a tip can become saturated. In this case the water in the saturated portion has a buoyant effect and reduces the strength of the material, making the tip more prone to sliding. Therefore, measures should be taken to make sure water drains away.

Water should never be allowed to accumulate against or on any part of the tip, unless it is specifically designed as a dam or pond. Seal off the surface of a tip, using mobile plant to compact the surface and minimise the water penetrating into the fill material.

If tips are constructed above an existing water course, the water course should be diverted or culverts of sufficient size provided to channel the water through the tip area.

The tip should have internal drainage to deal with expected rainfall. This is usually provided by under-tip drains or coarse, permeable layers positioned at appropriate levels. Internal drainage systems should be designed by a suitably competent engineer or hydrologist.

Drainage systems should be maintained.

Dam safety scheme

For storing water or another fluid under constant pressure you must comply with the requirements of a dam, as set out in the Building Act 2004.

Structures at extractives sites that may fit the definition of a dam include settling ponds, tailing dams and reservoirs.

7.6 Rehabilitation of tips

When the site is temporarily (suspended) or permanently closed (abandoned), it should be left in a safe condition.

Typically, rehabilitation is carried out progressively. This means parts of the site can be abandoned, while other parts are still operational. For example, rehabilitation of overburden tips that have reached capacity.

The objectives of abandonment of all or part of a site are:

• to make sure the public is safe by preventing inadvertent access to site infrastructure
• to provide for the stable, long-term storage of overburden and tailings
• to make sure the site is self-sustaining and prevent or minimise environmental impacts
• to rehabilitate disturbed areas for a land use (for example, returning disturbed areas to a natural state or other acceptable land use).

Rehabilitation should address water runoff, air quality, stability of material, erosion control, and treatment and containment of all possible hazardous substances.

Stability of material and control of water runoff are the most important factors as they will be the first indicators of any problems in the rehabilitated area. Stability should be monitored by a study of the toe area of any overburden tip. Make sure the toe area is well compacted and not bulging or moving out from its original placement. Another indicator of movement would be cracks appearing around the crest or top of the rehabilitated tip.

Rehabilitation tends to be a condition for all new resource consents and most current resource consents.

Rehabilitation should be considered and incorporated into all aspects of site planning, construction, and operation. This allows key aspects of the abandonment to be planned for throughout the site’s life cycle. Plans should identify measures to be undertaken during the operations phase, aimed at progressive rehabilitation of disturbed or developed areas of the site.

Review and revise rehabilitation plans as necessary throughout the site’s life cycle. The plans may become more detailed, incorporating more site-related activities and consideration of other site conditions.