This section describes how to:

• excavate safely under water
• safely access floating plant.

On this page

• 12.1 Planning and design
• 12.2 Excavation procedures
• 12.3 Extracting beneath water
• 12.4 Floating plant and boats

12.1 Planning and design

Before any excavation, a competent person should assess the ground conditions to determine all factors likely to affect the stability of the ground and the limitations that should be imposed on the excavation design. See Section 6 for details on planning safe excavations.

This assessment should be documented, so it can be reviewed and revised when a material change has occurred in the ground conditions or the excavation methods. Effective ground control relies on geotechnical information obtained at different stages of the life of the site – during planning and design, at implementation of the design and through day-to-day operations.

There may be a relative lack of data available during the initial planning and design. It is essential to consolidate information gathered during operations with information in the geotechnical model and to continually assess the suitability of the design in relation to ground stability.

If your site is using a dredge or floating plant, consider whether it is a principal hazard (for example, if multiple people could be on the dredge, there could be a risk of an incident resulting in multiple deaths). If it is determined to be a principal hazard, then there must be a principal hazard management plan (PHMP). It may be more suitable to manage the use of the dredge or floating plant via a PHMP to have all the risk control measures managed in one place. Where practicable, consider remote control operation of dredges to eliminate the risk of people drowning.

12.2 Excavation procedures

Excavation procedures rules should be drawn up, setting out:

• how excavation activities should be carried out, specifying the type and reach of excavators
• the physical dimensions of the excavation, including slope, depth, height of free faces, width of benches, and position of windrows
• how material should be removed from the excavation
• restricted access to the edge of the waterline
• the nature and frequency of supervision
• response to defects.

12.3 Extracting beneath water

Excavations should be kept stable even if you cannot see them. When extracting beneath water, slopes will be saturated.

Draglines, clam shells and long-reach hydraulic excavators may over-steepen the slope they stand on and cause failure. These slopes should be treated as a significant hazard. Working methods should be based on the geotechnical assessment of the material being excavated, allowing for any variation of submerged materials.

The working bench should be kept flat and clear of equipment or material to enable a rapid exit in the event of instability of the face. The front edge of the bench should always remain visible to the operator. Tracks of surface equipment should face the excavation, or be no more than a 45° angle, with track motors facing away from the face (see Figures 56 to 58).

Edge protection, barriers, warning signs and other suitable control measures should be placed around any water filled excavation to keep people away from any hazardous zones and to stop mobile plant driving into the water. Edge protection, barriers or signs should be moved as the excavation progresses and the hazardous area changes. Means of rescuing trapped or injured people must be provided (see Section 3).

If there is any doubt about the safety of excavations, operations should be stopped and remedial control measures undertaken.

When loading floating plant, there should be clear signals or communication between the excavator operator and the floating plant operator so feeding can stop if required. Where trommel screens are used, a visual or audible warning device should be used to alert the excavator operator if the trommel has stalled. Such an occurrence can cause the screen to become overloaded and could compromise the stability of the floating plant if loading continues.

Emergency procedures must be in place. This may include equipping mobile plant with features or tools for use in an emergency, such as push-out windows or window breaking tools.

12.4 Floating plant and boats

Floating plant or boats (including those used on settling ponds) may be governed by the requirements set out in the New Zealand Maritime Transport Act 1994 and Maritime Rules. Nothing in this section precludes you from complying with the requirements of the Maritime Transport Act 1994 or Maritime Rules where it applies to your vessel. Maritime New Zealand is the national marine safety regulator.

Floating plant should be designed, manufactured and maintained to the required standard, to ensure the floating plant will:

• not become unstable due to shifting loads or being overloaded
• remain stable while being towed
• remain waterworthy in operating conditions.

Floating plant and vessels should have periodical structural checks done by an engineer or a surveyor.

As a general guide, you will need the documents outlined in Table 11 to legally operate your floating plant or boat (hereafter referred to as a vessel):

For more information on the Maritime Transport Act 1994, Maritime Rules and maritime safety systems contact Maritime New Zealand or visit maritimenz.govt.nz(external link)

Safe means of access and egress

Safe means of access (for example, a gangway) should be provided to vessels, floating processing platforms, draw off points or submersible pumps where people have to access them for work purposes (see Figure 59).

Where using jetties, gangways, platforms, bridges or walkways they should be fitted with suitable handrails or other means to stop people falling in the water (see Figure 60).

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Separate the cables and pipes or store them away from walkways to avoid tripping (for example, in cable trays). Surfaces of walkways should be slip-resistant.

Sufficient lighting should be provided if jetties, gangways, platforms, bridges, walkways, stairs or ladders might be accessed when it is dark.

Where ponds and floating processing plants are used in alluvial mining, take precautions at the edge of the excavation. While emphasis should be on the stability of large excavators and unstable ground conditions, precautions should also include pedestrians accessing floating platforms from the excavation edge.

A person should not be lifted in plant, if the plant is not specifically designed to lift or suspend a person. Excavator buckets should never be used to transfer people to floating screens or plants. Do a risk assessment to identify a reasonably practicable method of access. For example, using a drawbridge or moving the floating platform to the shore (see Figure 61).

Where reasonably practicable, avoid having people on board floating plant while it is fed by an excavator.

Consider providing remotely operated rope winch systems and power wash systems during the design stage (safety by design). This eliminates hazards associated with workers making frequent visits to floating processing platforms.

For more information on construction and installation of platforms, walkways, stairways and ladders refer to Clause D1 Access Routes of the New Zealand Building Code, or see the Building Performance website(external link) at the Ministry for Business, Innovation and Employment (MBIE).

Design and modifications to vessels

You should not submerge dredge or floating plant decks under any circumstances. If the freeboard of a dredge or floating plant appears to be insufficient, a competent person should be engaged to evaluate and rectify the buoyancy. Make sure dredge or floating plant decks do not become submerged under any operating conditions. This is particularly important when sludge builds up on the cutter head and when the cutter is driven into the bottom of the pond or into a working face during mining operations.

Equipment installed on the dredge or floating plant should be secure so that it will not shift and destabilise the dredge or floating plant.

Where trommel screens are used, an automatic tripping device or warning should be installed to stop the trommel screen if the tailings discharge belt stalls. This can cause the screen to get overloaded and affect the stability of the plant.

Modifications can cause vessels to capsize due to additional weight or the effects modifications can have on the balance of the vessel. Establish procedures to ensure modifications to the original design do not exceed the design capacity set by the manufacturer.

Procedures should also consider examination and maintenance of safety control measures provided by the manufacturer to make sure modifications to the original design do not reduce the in-process weight safety margin. For example, ensure dredge overload and full hopper alarm switches are functioning within the specifications of the manufacturer to maintain freeboard levels.

For more information on barge stability, see Maritime New Zealand Barge Stability Guidelines.

Repairs and maintenance of vessels

To ensure the integrity of vessels, you should establish maintenance and repair programmes. These may include:

• regularly checking decks and hulls for cracks and holes
• sealing all covers over hatches in the deck with continuous excess marine sealant to ensure water tightness
• regularly checking all hull compartment bulkheads are watertight to isolate water flow if water ingress occurs in any individual compartment
• providing a sounding tube for each hull compartment that extends to near the bottom of the compartment so the compartments can be sounded daily for water ingress.
• procedures to make sure repairs are undertaken in pontoon cells when leaks develop
• regularly checking ropes and rigging for signs of wear.

Dredges should also have an adequate capacity pump with a non-collapsible suction pipe long enough to reach the bottom of any hull compartment. When water ingress is detected, the water can be quickly and efficiently removed from the hull before the buoyancy of the dredge is seriously affected.

The use of polyurethane or polystyrene in hull compartments does not ensure buoyancy of dredges. These materials are not recommended because they deteriorate over time, becoming porous and water absorbent, and they do not allow for regular inspection of the hull compartment surfaces.

Hull compartments are confined spaces, so use a confined space working procedure. Refer to the standard AS 2865 Confined Spaces for more information on confined space entry. For more information, see our guide for Confined spaces: planning entry and working safely in a confined space

For more information on repairs and maintenance, see Section 15.15 of this guide.

Boats

Boats, like any other equipment, should be of adequate size and power to properly perform the anticipated task. Weight capacity includes people, motor, equipment and any other haul load. If you are using a boat to retrieve an item, then the weight of the item should be taken into consideration when assessing if the boat is suitable for the retrieval task.

Boats must be operated by workers who comply with any licensing and certification requirements, and who have adequate experience or training, or who are supervised by a competent person.

For licensing and certification relating to boats, contact Maritime New Zealand(external link)

Commercial vessels will need to be entered into the Maritime Operator Safety System (MOSS). For more information on MOSS, contact Maritime New Zealand(external link)

Ropes, pulleys, winches and rigging

All navigable floating plant will require mooring. This is usually accomplished with winches and ropes.

On smaller plants with manual winches and rope, the main hazard is tripping. Larger plants may have substantial winches and large diameter wire ropes. These present additional hazards from gear failure (ropes or pulleys breaking) and whiplash as strain is exerted on rigging. One control measure is to establish exclusion zones.

Ropes, pulleys and other rigging should be covered or otherwise protected. Workers should stand well clear of any hazardous zones when the ropes are taking strain. For more detailed information on load lines see Part 47 of the Maritime Rules.

Anchoring should be firmly positioned and not prone to undermining.

The use of galvanised ropes is advisable to prevent the unseen, internal corrosion that can occur in steel wire ropes operating constantly in and around water. Regardless of the rope used, all associated equipment such as pulleys, rope clamps and sheaves should be specified based on the rope diameter and safe working load. A routine rope condition inspection procedure should be used.

Personal flotation devices

The terms personal flotation device (PFD), lifejacket, life vest, life preserver, buoyancy vest and buoyancy aid are used interchangeably, all with the same key purpose: to prevent people drowning. A PFD is a garment designed to keep a conscious person afloat and to assist with buoyancy in the water.

Generally, Type 401 open waters lifejackets are the most appropriate lifejackets for a working environment, as they are designed to keep an unconscious person face up in the water. Some are manually activated with a pull-cord and others will automatically inflate when submerged in water (‘hydrostatic’).

Check that your lifejackets are marked as meeting an approved standard. They should meet NZS 5823:2005 – Specification for buoyancy aids and marine safety harnesses and lines, or another national standard (such as Australian) that is at least equivalent or better.

Establish and enforce policies for wearing PFDs. Like seatbelts in vehicles, PFDs are effective only when they are worn. The PCBU should do a risk assessment and select the most appropriate PFD for use at the operation.

All workers working on or next to water should wear a PFD. Take into account the personal protective equipment (PPE) and equipment a worker will have on their person when considering PFDs. Subject to the operational conditions and remoteness of workers (including emergency response time), consider using PFDs with integrated safety systems that include an automated man overboard alert system if the unit becomes immersed.

Provide sufficient quality PFDs of the proper type appropriate for each worker’s weight. Inspect and maintain the PFDs in serviceable condition and replace them if they become worn or damaged. Make sure all relevant personnel are trained in the use of PFDs. Consider keeping spare PFDs on site for replacement of damaged units and for visitors.

PFDs may be supported by buoys and rescue lines (ropes) for emergency response, but should not be seen as a replacement for a PFD.

Emergency exits

Cabins should have an emergency exit in the event of a sinking or capsize, such as a push-out window or a trapdoor. The emergency exit should be large enough to allow a person to pass through wearing their PFD.

Fire systems

Dredges and vessels with enclosed engine rooms should have a means of detection and alert in the event of excess heat, smoke or fire. There are various integrated systems that are commercially available and should be powered by battery and not from a generator or mains power.

Fire detection systems and fire-fighting equipment are subject to various requirements for testing, inspection and record-keeping under the Maritime Rules.

Workers should also be trained in what to do in the event of a fire on board the dredge or vessel.