This section describes how to:

• scale and control excavations to prevent rockfall or slope instability
• monitor slopes to detect any instability
• prevent or put right ground instability
• excavate safely under water.

On this page

• 10.1 Planning and design
• 10.2 Excavation rules
• 10.3 Excavation control and scaling
• 10.4 Slope movement monitoring programmes
• 10.5 Working near slopes
• 10.6 Remedial measures
• 10.7 Preventing falls from highwalls or faces
• 10.8 Historic underground workings

10.1 Planning and design

Before any excavation begins, the responsible person should carry out an appraisal to identify principal hazards. If ground or strata instability is already identified as a principal hazard, a geotechnical assessment of the site ground conditions should be undertaken by a competent person to determine all factors likely to affect the stability of the ground and the limitations that should be imposed on the excavation site design.

This should be documented. The assessment should be reviewed and revised where necessary 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 such as surveying, installation, maintenance and inspections.

Following appraisal of ground conditions, a design should be prepared setting out the measures to control ground instability. Where an existing design has already been proved, it may be used as the basis for the design of a new excavation, if the ground conditions at both sites are not significantly different.

During planning and design, there is usually a relative lack of data available when the slope design is first developed. It is essential that geotechnical information obtained during operations is consolidated with information in the geotechnical model and continually used to assess the suitability of the slope design in relation to ground stability.

Implementing the design typically involves considering suitable ground control strategies, such as minimising unnecessary damage to slopes during blasting, excavation control and scaling, and installation of ground support and reinforcement.

For more information on excavation design, see Section 6.

10.2 Excavation rules

Excavation procedures should be reviewed and revised regularly. The control measures specified in these procedures are essential for the proper management of excavations, ensuring the people working in and around them are safe.

Consider the following in your procedures:

• the way excavation activities should be carried out, specifying the type and reach of excavators
• the physical dimensions of the excavation including slope, height of faces, width of benches, position of catch-berms and gradient, position and protection of access ramps
• the way in which material should be removed from the excavation
• the sequence in which material should be removed
• maintenance of faces (including scaling of crest lines)
• the nature and frequency of supervision
• the response to defects.

10.3 Excavation control and scaling

Good excavation control, scaling, and equipment selection help create and maintain safe slopes.

In soils and weak and weathered rock, batters can be excavated by free digging using hydraulic excavators. It is critical slopes are not under-cut so the as-built slope is steeper than the as-designed slope, as it could result in instability. Provide adequate surface runoff control measures to the benches separating the batters to minimise water infiltration and slope erosion.

In strong rocks, drilling and blasting is needed to fragment the rock mass before the final preparation of the slope. Care should be taken to prevent over-digging of the face, particularly where there is blast damage or fractured rock.

Scaling of the batter crest and face following excavation is an important component of the implementation of the design. Scaling is intended to remove loose blocks and slabs that may form rock falls or small failures. Scaling also helps preserve the catch capacity of benches needed to retain loose rock material rilling from above. In soils and weak and weathered rock, experienced mobile plant operators can construct slopes with smooth surfaces, so scaling is not generally required.

Scaling from the bench above is normally done by chaining the face using a large chain (ship’s anchor chain) with or without attached dozer track plates. The chain can be dragged along the face by a dozer or backhoe. Do not use a backhoe to scale the face from the bench above, as large rocks may pull the plant off balance.

Scaling from the bench below is generally performed by an excavator configured as a backhoe. Most manufacturers offer specialised units equipped with long booms holding small buckets or rock picks.

The debris accumulated at the toe of the batter after scaling should be removed before access to the toe is lost. This will make sure adequate catchment volume on the safety bench is maintained. Supplementary bench cleaning will depend on access and the service life of a slope. Periodic bench inspections should identify sections that require cleaning.

Mobile plant working on faces

Faces that have potential for instability should be worked within the reach height of the equipment used, whether they are working in sand or hard rock (see Figure 40). Typically, wheel loaders can reach 6-8m and excavators 9–12m. Larger mining shovels (120t or more) are capable of reaching 18–20m depending on how they are used.

If mobile plant is at risk of being engulfed in a face collapse, a trench or rock trap should be used to maintain a safe operating distance (see Figure 41).

Post excavation inspection of blasted sections

When the excavator reaches the batter face following a blast, the designed toe and crest should be achieved, and no blast-induced damage should be visible of the face. After excavation is completed, the face should be inspected and analysed for excessive over break. The damage should be classified into the categories shown in Table 10 to help guide design refinement.

A detailed record should be made of the post excavation performance of the batter face.

Indications of failure

Even the most carefully designed slopes may be subject to instability. Some of the more common indications of failure are listed below.

TENSION CRACKS

Cracks forming at the top of a slope are an obvious sign of instability. Cracks form when slope material has moved toward the floor. Since this displacement cannot be detected from the floor, it is extremely important to frequently inspect the crests of slopes above active work sites. Safe access should always be maintained to the regions immediately above the active excavation. Frequent inspections may be necessary during periods of heavy rain or spring run-off, and after large blasts.

The simplest method for monitoring tension cracks is to spray paint or flag the ends so that new cracks or propagation along existing cracks can be easily identified on subsequent inspections. Measurement of tension cracks may also be as simple as driving two stakes on either side of the crack and using a survey tape to measure the separations.

SCARPS

Scarps occur where material has moved down in a vertical or nearly vertical fashion.

Material that has moved vertically and the face of the scarp may be unstable and should be monitored accordingly.

ABNORMAL WATER FLOWS

Sudden changes in rainfall or water flow may also precede slope failures. Spring run-off from snow melt or after periods of heavy rain is one of the most obvious examples of increased water flow which may have adverse effects on slopes. But changes in steady flow from dewatering wells or unexplained changes in piezometer readings may also show subsurface movement that has cut through a perched water table or hit a water-bearing structure. Changes in water pressure from drain channel blockage can also trigger slope failures.

Water can also penetrate fractures and accelerate weathering processes. Freeze-thaw cycles cause water-filled joints to expand and loosen slope material. Increased scaling may be necessary during cold weather.

CREEP OR SLOW SUBSURFACE MOVEMENT

Bulging material or ‘cattle tracks’ appearing on a slope indicate creep or slow subsurface movement of the slope. Other creep indicators can be identified by looking at vegetation in the area. While most quarries or mines do not have vegetation on slope faces, the movement of trees at the crest of a slope can indicate instability.

RUBBLE AT THE TOE

Fresh rubble at the toe of a slope or on the floor of the excavation is an obvious indicator of instability. Work out which portion of the slope failed, and whether more material may fail.

One of the most dangerous situations is an overhang. If workers are not aware that material below them has failed, they may go onto an unsupported ledge. Remedial measures such as scaling, supporting, or blasting the overhang or other hazardous rock may be necessary.

10.4 Slope movement monitoring programmes

Provide enough suitable slope movement monitoring as required by a geotechnical assessment or risk assessment to detect instability early, so safety measures can be taken in time. Monitoring ‘after the fact’ does little to undo damage caused by unexpected failures.

The ground or strata instability PHMP must cover the appropriate equipment and procedures to monitor, record, interpret and analyse data about seismic activity and its impact on the operation. It must also include collection, analysis and interpretation of relevant geotechnical data and monitoring openings and excavations where appropriate.

The purpose of a slope movement monitoring programme is to:

• maintain safe operational practices for the protection of workers, equipment, and facilities
• provide warning of instability so action can be taken to minimise the impact of slope movement
• provide crucial geotechnical information to analyse the slope failure mechanism
• design the appropriate corrective measures.

When planning a slope movement monitoring programme, consider:

• defining site conditions
• predicting all potential mechanisms to control instability
• setting parameters to be monitored and consequence of not keeping within set parameters
• establishing suitable monitoring systems, including instrumentation and location
• formulating measurement procedures, including frequency, data collection, processing, interpretation and reporting
• assign tasks for design, construction, and operation of systems
• plan regular calibration and maintenance
• establish trigger action response plans (TARPs) and associated accountabilities for action to minimise impacts of slope movement.

Slope monitoring methods can be surface or subsurface and either qualitative or quantitative. All have their place in specific environments and are often related to potential failure size. Select the most appropriate technique depending on site-specific conditions.

Monitoring methods

The instruments selected for a slope monitoring programme depend on the particular problems that need monitoring. A comprehensive monitoring system may include instruments capable of measuring rock mass displacement, ground water parameters, and blast vibration levels.

When selecting monitoring instruments, incorporate some level of redundancy in the system to cross-check instrument performance and eliminate errors. Redundant or over- lapping measurements also provide a back-up if an instrument fails.

Automated equipment is generally more accurate than manual equipment since some human error is removed. Automated systems also provide added flexibility in the sampling rate and can therefore monitor more frequently than manual readings. Another advantage is their ability to trigger alarms if certain thresholds are reached.

Instruments should be placed where they will be the most effective. Estimating the movement expected in a particular area should help make sure the instrument’s limits are not exceeded. There may also be environmental limitations (such as extreme heat or cold) that determine whether a particular instrument will work at a specific site. All these factors should be evaluated against the primary objectives of the monitoring programme.

VISUAL INSPECTION

A basic element of a slope movement monitoring programme should be visual inspection by a competent person, with observation by all workers. Maintain this qualitative, but extremely important aspect of the programme throughout the life of the operation.

Workers should report rock falls, be involved in slope inspections and regular detailed inspections.

Any visual monitoring programme should be supported by instruments to provide a quantitative basis for defining any movement.

Develop and implement a procedure for the regular inspection of faces above every place of work and every road used by workers.

You must make sure:

• a competent person examines every area of the operation where a worker is present, or will be present, before every shift, and at suitable times during the shift
• every accessible area of the site (including areas containing barriers, machinery and infrastructure) is examined at least weekly.

It must also make sure that written procedures are included in the HSMS, setting out:

• what to examine
• when to examine
• how to record inspections
• how to action results.

Practical information and advice on actions to take when defects are identified may include:

• if it is safe to work below and above a face
• if there is any loose material on the face
• if there is potential for instability
• whether maintenance is required to the face before starting work
• when further advice is needed, such as from the geotechnical specialist.

SURFACE EXTENSOMETERS AND CRACK MONITORING

If evidence of movement is detected from visual inspection, the first step to increase the monitoring programme could be simple crack monitoring. Results of visual inspections and crack monitoring are useful when selecting additional secondary monitoring points for detailed survey assessments.

Crack monitoring techniques typically consist of:

• regular detailed mapping of location, depth, width of cracks, rate of extension and opening
• installation of targets on opposite sides of cracks to monitor rates of opening
• installation of surface (wireline) extensometers
• installation of picket lines or a line of targets to monitor using theodolites or precise levels to detect changes in alignment, and location of elevation along a given crack or the crest of the slope.

Surface extensometers for monitoring local wall movement or tip movement can be easily constructed. They provide a rugged practical system of monitoring that can be inspected and interpreted regularly by operational workers. They can also be equipped with automatic devices such as lights or sirens to provide excessive movement warnings. More sophisticated units can provide real-time indications of movement to remote locations (such as offices) through a telemetric link.

TERRESTRIAL GEODETIC SURVEYS

The most reliable and complete measurements associated with initial movement can be obtained from conventional geodetic survey techniques, using precise theodolite and electro-optic distance measuring (EDM) combinations or total stations. These systems can be installed by survey workers, generally with survey equipment in regular use at a site.

Primary monitoring points should be surveyed at regular intervals, consistent with the type of rock and expected rates of movement.

Surveillance monitoring frequencies vary from weekly to quarterly depending on conditions such as the stage of mining or extraction, mining or extraction rate, changes in piezometric surface, and climatic variations.

The individual aspects of a typical system are:

• Control points for the system should consist of the instrument stations near the crest of a slope and reference stations located away (100m to 3km) from mining or quarrying activities. Control points are usually established by conducting a first-order survey, using conventional survey techniques such as triangulation, trilateration or triangulateration, or GPS.
• GPS is much more efficient, accurate, and less labour-intensive than the conventional survey techniques when used for control surveys, especially when the network covers a relatively large area. The main requirements are that the system used picks up variances and good quality equipment.
• The stability of instrument stations can be checked by resurveying the control network or reference stations each time the instrument station is used. Make sure reference stations are observed regularly.

Plot and assess data from the survey after each set of readings. If movement is detected, monitoring frequency of secondary points will depend on the size of the failure and movement rates, and could be hourly to weekly.

GPS STATIONS

Global positioning systems (GPS) can be used for real-time positioning at any location 24 hours a day, in any weather. Positioning is accomplished using timing signals transmitted by the satellites to ground receivers.

With two or more receivers working simultaneously in a so-called differential mode, you can measure relative positions (3D coordinate differences) between the receivers. This will be accurate from a few millimetres to about 20mm, over distances up to several kilometres.

Unlike conventional survey techniques (such as those using EDM, total stations, and levels) GPS does not need a direct line of sight between survey stations. If the GPS base line length is within 1km, it is not affected by local atmospheric conditions. So, they are usually more efficient and accurate and require less labour than conventional survey techniques. Therefore, GPS was adopted as the general surveying technique at many extractives sites. They are also an ideal tool for setting up control surveys to monitor slopes.

RADAR

Where more extensive areas of movement are detected, radar enables real-time monitoring of the movements to help make sure workers remain safe below the slope. Radar units used in conjunction with geodetic surveying can effectively provide real-time warning of movements and accelerations.

It is important that radar is not the sole basis for monitoring. It is also essential to maintain a degree of conservatism when deciding to withdraw workers from below a moving slope, even if it is being monitored by radar. Even small rock falls from the deformation can have serious safety consequences and may not be detected by radar.

SUBSURFACE TECHNIQUES

Costs of subsurface techniques are greater than those for surface instrumentation. These costs can be modest, if available drilling equipment is used, and workers perform the installation after instruction from specialists or the instrument supplier. For example, inclinometers and TDR cables, give very valuable and precise information on the locations of deep-seated slide surfaces, and on rates of movement needed to properly plan remedial work cannot be adequately planned.

MICRO-SEISMIC MONITORING

Routine, real-time micro-seismic monitoring in opencast environments can provide 3D data where rock breakage or movement is occurring. The data can be used to enhance surface monitoring systems to identify potential instability and the associated failure mode. The technique is commonly used in underground mining operations and has recently been applied in opencast environments.

MONITORING OF GROUNDWATER PRESSURE

If the slope design is based on achieving a given future pore pressure profile, it is important year-by-year pore pressure targets are developed to make sure depressurisation is occurring at the desired rate.

Include piezometer installations in the most critical areas for slope performance in the final slope design. Target pressures are then developed for each piezometer, for each year of operation.

The components of a groundwater monitoring system could include:

• data acquisition systems
• piezometers
• horizontal drain flows
• dewatering well discharges
• monitoring of slope conditions.

Instrumentation data

A detailed draft of monitoring and reporting procedures should be prepared during the planning phase, then finalised after the instruments are installed. Then, responsible workers will be familiar with instrument operation and specific site considerations.

These procedures should include:

• a data collection list
• equipment specifications, including servicing requirements
• processing and presentation procedures
• interpretation procedures, including alarm levels.

A competent geotechnical engineer or instrumentation specialist, selected by the site, is responsible for collection of instrumentation data determined during the planning phase.

PROCESSING AND PRESENTATION OF INSTRUMENTATION DATA

The primary aim of data processing and presentation is a rapid assessment of information to detect changes that require immediate action. A secondary function is to summarise and present the data to show trends and compare observed with predicted behaviour, so any necessary action can be initiated.

Present monitoring data in a format that is easy to read and identifies problem areas quickly.

During the planning phase, decide who is responsible for processing and presenting instrumentation data. It should be directly controlled by a competent person on site, or in special cases, consultants who have immediate 24-hour access to the data.

The time needed for these tasks can be underestimated, resulting in accumulated unprocessed data and failure to take appropriate action.

Experienced geotechnical engineers may use much of their time supporting monitoring systems instead of delegating these responsibilities to technicians. This may mean they neglect the required technical analysis to minimise or manage the impacts of potential slope failures.

The time needed to process and present data is usually similar to, and may even go over, the time needed to collect it.

Data processing and presentation depends on the specific monitoring system. For surveillance monitoring and small slopes, it can often be done with standard spreadsheets. Comprehensive monitoring programmes may require commercial survey reduction and geographic information system (GIS) programmes.

INTERPRETATION OF INSTRUMENTATION DATA

Monitoring programmes have previously failed because the data was never used. A clear sense of purpose for a monitoring programme helps, guide data interpretation.

Use early data interpretation to check the accuracy of the monitoring system. For example, atmospheric changes may result in diurnal variations of several times the manufacturer’s quoted accuracy for EDM and total station units. This is common particularly in climates where there are significant temperature differences between day and night, or climates where temperature inversions can develop in a pit overnight.

Filter out these survey accuracy variations as part of the interpretation process, either by setting wider bands before alarms are triggered or by emphasising on readings taken at the same time of the day.

The purpose of subsequent data interpretation is to correlate the instrument readings with other factors (cause and effect relationships), and to study the deviation of the readings from the predicted behaviour.

RESPONDING TO DATA VARIATIONS

Interpretation of data from movement monitoring systems primarily involves assessing the onset of changes in the movement rate. This is generally reflected by acceleration but, where a slope is already moving, deceleration may also occur.

If the material does fail, the site should have a pre-planned response to the movement. This can be done by using trigger points or trigger action responses (TARPs) for each monitoring method.

The reporting procedure in the event of any TARP should be clearly defined and understood by everyone. Slope failures rarely occur without some warning, and it is important workers can recognise potential hazards and act accordingly. It is recommended TARPs also include actions to take if monitoring systems or instruments no longer operate correctly.

REPORTING CONCLUSIONS

After interpreting each set of data, report conclusions in an interim monitoring report and give it to the workers responsible for implementing remedial actions.

At the very least, supply management with a monthly summary report of monitoring results, even if no movement is detected.

A final report is often needed, and a technical paper may be prepared.

10.5 Working near slopes

Manage hazards from individual rocks falling from a slope (highwall or face) using a mix of techniques. These include:

• support or control the fall path of potentially loose rock
• scale the loose rock
• provide rock catching berms, benches, or both
• limit workers’ exposure to areas with loose rock on the slope.

Before work starts near a slope, inspect it thoroughly for hazards, including loose rock. If loose rock is identified, scale it off the slope or cordon the area beneath the loose rock. Benching and moving roadways or work areas away from the base, can also reduce exposure. Mobile plant should work perpendicular to the base of the slope to give operators a better view of the face.

Put the following control measures in place when working near slopes:

• A bench in the slope above the work area. Space it so you can clean the face below (from the floor to the first bench) using mobile plant or equipment available on site.
• Workers should not be positioned between the slope and any mobile plant or equipment that could block their escape.
• Provide safe access to the top of the slope for ground condition checks.
• Clear the top of the slope of loose, hazardous material before bringing down the shot material exposing the face. Use mobile plant (such as an excavator), that can reach the edge of the wall from a safe staging point. Use the bucket’s outward force to remove loose material from the top edge of the wall.
• Keep workers a safe distance from the toe of the wall using a buffer. This may include placing the loading excavator on a rock platform with a rock trap (or trench) between the excavator and the face (see Figure 42).
• Mobile plant should operate perpendicular to the face or toe while in the impact zone.

10.6 Remedial measures

The remedial measures taken after slope movement depends on the type of instability and how it affects operations. Each case should be assessed separately for safety, site plans and cost-benefit analyses.

Stabilisation and repair methods are used when ground movement has already occurred where artificial support methods are used to prevent instability.

LET THE MATERIAL FAIL

If a failure is in a non-critical area of the excavation, the easiest response may be to leave the material in place. Work can continue at a controlled rate if the velocity of the failure is low and predictable, and the failure mechanism is well understood. However, if there are any questions about the subsequent stability, try to remove the material.

To prevent small-scale failures from reaching the floor of the excavation, both the number and width of benches can be increased. Catch fences can also be installed to contain falling material.

SUPPORT THE MATERIAL

If letting the instability fail is not an option, a solution may be to artificially support the failure. Some operations have successfully used reinforcement such as bolts, cables, mesh, and shotcrete to support rock mass. These can be very expensive but may be worthwhile if it enables a steeper batter angle or reduces clean-up costs.

A careful study of the geological structures should be done to select the proper reinforcement (such as length of bolts or cables, thickness of shotcrete). Bolts that are too short will not do much to prevent slope stability problems from continuing. In some cases, reinforcement has only served to tie several small failures together, creating a larger failure.

Another potential solution to stop or slow down ground movement is to build a buttress at the toe of the slope. The buttress offsets or counters the driving forces by increasing the resisting force. Short hauls of waste-rock often make this an effective and economical alternative for stabilising slope failures.

REMOVE THE HAZARD

If a slope keeps failing, and supporting the slope is not possible, you should remove the hazard. Flattening the slope to a more stable angle for local geology will often solve the problem. If catchment systems are not in place, use appropriate scaling methods regularly to remove hazards from small rockfalls.

Removing, or unweighting, the top of a slide may reduce the driving forces and stabilise the area. However, this approach is usually unsuccessful and, in some situations, involving high water pressure, unloading decreases the stability of the remaining material.

Water pressure often causes slope stability problems. Dewatering using horizontal or vertical wells can be a significant way of controlling slope behaviour and minimising hazards. Surface drainage and diversions should also be used to keep surface runoff away from tension cracks and open rock mass discontinuities near the slope face.

Installation of artificial ground support and reinforcement

If artificial ground support and reinforcement are part of the slope design, they must be installed correctly. The timing of installation is a key part of the design. For more detailed information on ground support and reinforcement systems see Section 6.8.

Some installation tasks, like shotcreting or drilling, can be done from a safe distance. But installing mesh and bolts, such as plating and tensioning, can expose workers to much greater rockfall hazards than usual.

These increased safety risks during installation must be clearly understood and managed. No worker should enter an area with unsupported ground unless they are installing or supervising the installation of ground support or carrying out or supervising slope stabilisation.

Temporary support must be provided to protect workers from hazards caused by unsupported ground or unstable strata when they install or supervise the ground support installation or undertake or supervise slope stabilisation.

Managers must make sure suitable ground or rock support, or slope stabilisation, is designed and put in place for all work areas. Plans showing these arrangements must be displayed where all workers can easily access them.

Consider the following when installing artificial ground support and reinforcement:

STORAGE AND HANDLING

• Store and handle artificial ground support and reinforcement products to minimise damage or deterioration.
• Clean steel components designed to be encapsulated in resin or cement grout of oil, grease, fill, loose, flaking rust, and any other materials which may damage the grout.

GROUT AND OTHER ADDITIVES

• Mix grout according to the manufacturers or supplier’s instructions including cement to water ratio, correct mixing time and speed and water quality.
• Add any additives (for example, retarders, accelerators, or fluidisers) to the grout mix in the recommended amounts and at the specified time in the mixing and pumping process.
• For full ground encapsulation of steel elements, the method of grouting should show a grout return at the collar of the hole. Other methods that can demonstrate complete hole filling may also be appropriate. All grout mixing and pumping equipment should be cleaned and maintained regularly.

PROCEDURES DURING INSTALLATION

Procedures for artificial ground support and reinforcement installation should include:

• the method of work
• the support materials and equipment to use
• the layout and dimensions of the artificial ground support and reinforcement system
• any method of temporary support necessary to secure safety
• the procedures for dealing with abnormal conditions – the method and equipment for withdrawal of support
• manufacturer’s instructions relevant to the safe use of support
• information on other hazards such as known zones of weakness, or proximity to other workings or boreholes
• the area the procedures apply to, and the date they became effective.

Use correct tensioning procedures when required, for the various artificial ground supports and reinforcement. The reason for tensioning cables should be clear to decide if post or pre-tensioning is needed.

Also consider:

• the hole’s orientation should fit the shape and expected mode of failure
• plates or straps on the rock surface should be thick enough to prevent nuts being pulled through the plate or strap when loaded against the rock surrounding the hole
• shotcrete thickness should be checked regularly during placement to make sure it meets the required thickness. Marking the shotcrete surface with a depth gauge probe may help.

Samples of the shotcrete mix should be taken at set times during normal work. They should be tested in an approved concrete laboratory to check they meet the shotcrete design specifications. Tests should include the slump, the uniaxial compressive strength and toughness of the product.

PROCEDURES FOLLOWING INSTALLATION

Use monitoring arrangements to make sure the artificial ground support or reinforcement system remains effective, including monitoring for corrosion.

10.7 Preventing falls from highwalls or faces

Any person who works on or near the edges of faces or highwalls has the potential to fall. Typically these are the driller, shotfirer and person carrying out the daily inspection. Other people potentially working on or near edges include surveyors, engineers, explosives truck workers, planners, geologists, geotechnical engineers and fencers.

You must first try to eliminate a risk if this is reasonably practicable, otherwise minimise the risks as described in the hierarchy of control measures (see Figure 1). A hierarchy of control for the hazard of working near highwalls or faces is:

1. A windrow, a fence or other physical barrier capable of supporting a person’s weight if they fall against it should be in place along the edge (see Figure 43).
2. If a barrier is not practicable, you should determine a distance from the edge that is safe to work and demarcate this area with a fence (for example, parawebbing fence or waratah wire type fencing). The safe distance should be a minimum of 2m (see Figure 44).

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When installing or removing any barrier other than a windrow, provide a travel restraint system such as a harness. Connect this harness to a fixed position that restricts workers’ ability to work outside the safe area (see Figure 47).

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A risk assessment should be carried out to establish a safe system of work for any person likely to be in a position where they may fall from a face. Consider the geology and stability of the face, the ground conditions, weather, lighting equipment being used, the need to adjust burdens, marking hold positions and profiling.

Windrows are preferable to other less substantial barriers but may hide cracks or signs of instability along the edge. Windrows should be:

• constructed only after inspection of the area below. Faces need to be inspected for faults, change in appearance, loose surface, evidence of falling rocks, water seepage, joints and cracks
• constructed a metre or two from the edge where possible so any cracks or deterioration of the edge can be seen
• constructed from suitable material to avoid trip hazards
• a minimum height of 1m (for pedestrian protection only)
• regularly inspected and maintained.

Workers should be trained in the appropriate selection and use of harnesses before starting work. Make sure workers are closely supervised until assessed as competent.

Vehicles should not be parked under high walls, due to the hazard of rock falls.

10.8 Historic underground workings

Sites may mine or extract materials that were mined underground before. There are serious risks that can arise when opencast mines or quarries get close to, and then mine or extract through underground workings. Voids may be hidden in the area being mined. This is a principal hazard and a PHMP for voids must be created.

Other hazards include:

• sudden and unexpected collapse of the ground or walls
• losing people or equipment into unfilled or partially filled underground workings
• losing explosives from charged blastholes that break through into the underground workings
• overcharging blastholes where explosives fill cavities connected to the blasthole
• risk of ejecta (such as fly rock) from cavities close to the floor and adjacent blastholes, particularly when explosives have entered the cavity from the blasthole during charging and the loss is not detected.

In general, these hazards significantly increase when underground workings are not backfilled during mining. Because these hazards are usually not obvious during normal operations, take extra steps to understand their nature and how far they reach.

Hazard identification of underground workings

It is essential to thoroughly review previous mine plans before development.

Old underground mine plans should be carefully checked, especially if they are copies or summaries of the originals. While this helps to assess the likelihood of abandoned underground workings near an open pit, you cannot fully rely on their accuracy.

Reviewing underground workings should be part of design and site planning to make sure, as far as reasonably practicable, that:

• all known underground workings are marked clearly on all working plans and the plans are rechecked
• there is recognition that the rock mass surrounding the underground workings may be highly variable in strength and potentially unstable
• a three-dimensional model of underground workings is developed and used in all design, planning and scheduling.

Update all plans after each exploration phase to record the revised outlines of the actual size and shape of underground workings.

If underground workings are unlikely to be large or no plans are available, it may be necessary to confirm their location.

Several detection methods which may be used to confirm the lateral extent and shape of underground workings, include:

• probe drilling
• geophysical techniques (including seismic, resistivity, conductivity, and gravity methods)
• ground probing radar
• laser-based electronic distance measurement (EDM) surveying methods
• closed-circuit television (CCTV) cameras lowered through probe holes
• where practicable, an actual physical inspection and survey.

Once the relevant hazards have been adequately defined, the PHMP must describe control measures to manage the principal hazard and the risk of harm to workers.

Risk control

Consider the following control measures to eliminate or minimise the risk of unexpected floor or wall collapse:

• place fill materials into underground workings
• leave a reasonably sized pillar between the current working bench and the underground workings by stowing or collapsing
• restrict work away from the suspect location, allowing an adequate safety margin
• blast waste rock into voids, then further back fill to stabilise the area.

If there is a risk of intersecting underground workings, a geotechnical assessment should be carried out to determine the minimum stable floor pillar or rib pillar dimensions.

Clearly mark all areas of a working bench likely to be underlain by underground workings, and control access using a specific set of procedures. These procedures should cover a range of issues including:

• minimising pedestrian movement
• identifying the workers responsible for monitoring and marking out the hazardous areas
• probe drilling procedures
• marking the extent of underground workings
• drilling and blasting
• plant and equipment movement
• placing fill materials in unfilled workings
• monitoring rock stability
• daylight and night operations
• plant and equipment specifications
• regularly sharing information and discussing concerns with those involved
• review of the procedures as the pit depth increases.

Allow for uncertainty in the exact location of underground workings and any potentially unstable ground around them. Add an extra margin of safety between work areas and suspect zones.

If using extraction approaches in operating underground mines, potential hazards may include:

• flooding of underground workings
• instability of slopes and surrounding surface areas
• negative impacts on underground mine ventilation systems
• spontaneous combustion
• collapse of unfilled stope voids
• lack of coordination, communication, and control of mining activities between the surface and underground mines.

You must identify hazards that could lead to reasonably foreseeable health and safety risks. Put control measures in place to first try to eliminate the risk if reasonably practicable, otherwise, minimise the risk so far as is reasonably practicable.