Slope monitoring practices at open pit porphyry mines in BritishColumbia, Canada

Samuel Nunooa*, Dwayne D. Tannanta and H. Warren Newcomenb

aSchool of Engineering, University of British Columbia, Kelowna, Canada; bBGC EngineeringInc., Kamloops, Canada

(Received 27 August 2014; accepted 15 September 2014)

A survey of pit wall monitoring practices was conducted at all large open pit cop-per–molybdenum mines in British Columbia. The survey found that these minestend to rely on visual inspection and prism surveys as the primary techniques for pitwall management. Open standpipe piezometers and wire-line extensometers are alsoused. Newer monitoring technologies are slow to be adopted by the mines. Oneexception is slope stability radar, which is used at two mines. All mines have estab-lished movement thresholds that trigger different operational responses. Thesethresholds do not appear to be based on the pit depth or rock mass conditions.

Keywords: BC open pits; pit wall monitoring; movement limits; total station;prisms; lidar; radar; extensometers

1. Introduction

There were seven operating open pit copper–molybdenum (Cu–Mo) operations inBritish Columbia (BC), Canada, in 2014. Figure 1 shows that these pits are scattered ina 700 km long band along the centre of the province. All the mines exploit large por-phyry ore deposits with low Cu–Mo grades. These open pit mines range in age fromroughly 50 years for Highland Valley Copper (HVC) and Endako mines to just oneyear of production for Mt. Milligan mine, which is a copper and gold mine. HVC’sValley pit is Canada’s largest and deepest open pit with depth of approximately 800 m.Rock slope movements are monitored at these mines to help manage the stability ofthe pit walls. Unanticipated pit wall failures can have serious consequences on thesafety and economics of an open pit operation. This was recently highlighted by thelarge pit wall failure at the Bingham Canyon mine in Utah in April 2013 where signifi-cant production and economic losses were caused by the failure [1].

Many new monitoring technologies have been developed over the operating lives ofthe BC mines. While BC’s open pit Cu–Mo mines have often tested and evaluatednewer monitoring techniques, the mining industry has generally been slow to acceptand routinely use new technologies. The purpose of this paper is to summarise and dis-cuss current rock slope monitoring practices used by open pit Cu–Mo porphyry minesin BC.

*Corresponding author. Email: samuel.nunoo@ubc.ca

© 2015 Taylor & Francis

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2. Survey of current practices in BC mines

A questionnaire with 50 questions was designed to collect data about current miningconditions and pit wall monitoring practices at BC’s Cu–Mo mines. The questionnairewas distributed to six operating open pit mines in the summer of 2013 (Mt. Milliganwas excluded because it had just started operations). The questionnaires returned by thesix mines varied in terms of their level of detail and completeness. Follow-up site visitsto the four of the mines (HVC, Gibraltar, Endako and Copper Mountain) were con-ducted to verify the data provided, and to inspect the pit walls and slope monitoringsystems. The data from the questionnaires were used to understand each mine’s opera-tions and to determine the similarities and differences between pit wall monitoringpractices in use at the mines.

Figure 1. Operating open pit Cu–Mo mines in BC.

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A summary of the open pit conditions for the surveyed mines is presented inTable 1. The pits range in depth from less than 100 m (Copper Mountain) to more than800 m (HVC Valley pit). Most pits use 15 m high single benches, with some double-bench configurations. Overall pit slope angles range from a low of 34° in the Lornexpit to 55° in Pit 3 at Copper Mountain mine. Pit wall angles are typically 40° ± 2°.The overall rock mass quality as quantified in terms of rock mass rating (RMR) islisted in Table 1. The overall rock mass quality for the porphyry ore bodies is generally‘Fair’ to ‘Good’ for most of the Cu–Mo mines in BC. The Lornex pit is the exceptionwith large areas consisting of ‘poor’ quality rock, generally associated with the regionalLornex fault. The surveyed mines have fault and shear zones with reduced rock massquality. An example is seen with the south basal fault (SBF) at Endako mine where therock mass quality is significantly lower beside the fault. In general, the pits walls nearlarger faults and shear zones are often designed with a shallower slope, and dewateringmay be used.

All open pit Cu–Mo mines in BC operate with a 2-shift per day schedule. Theapproximate average daily production for the six mines at the time of the study is listedin Table 2. A range of production rates and stripping ratios can be observed betweenthe different mines. The typical stripping ratio is roughly 3:1 (waste to ore) for mostmines although HVC is currently operating at a 1:1 ratio while extracting relativelylow-grade ore.

Table 3 summarises the monitoring techniques and frequency of monitoring typi-cally used at the mines that were surveyed. In general, all the mines use a similarapproach to pit wall management involving reliance on visual inspection and prismmonitoring. The pit wall monitoring methods can be divided into three categories: (1)visual inspection, (2) surface measurements and (3) subsurface measurements. Thereare differences in the technologies that are used at the different mines and these arediscussed in the following sections.

Table 1. General pit geometries as of 2013.

Mine Slope (°) Depth (m) Length (m) Width (m) RMR (approx.)

Copper MountainPit 2 44–51 90 1000 300 60Pit 3 37–55 75 900 800

Endako 39 350 1940 830 60–80 above SBF25–45 beside SBF40–60 below SBF65–75 at depth

Gibraltar 37–43 245 1830 765 60HVC 39–42 800 2800 2300 65Valley 34–40 530 2400 1400 40–50Lornex 40–45 155 1000 400 75Highmont

Huckleberrytop 39 125 1000 800 60bottom 400 200

Mount Polley 43–49 270 1000 600 60

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3. Visual inspection

All mines routinely use visual inspections. The Health, Safety and Reclamation Codefor Mines in BC [2] places a requirement upon operating mines that all work shall becarried out without undue risk to the health or safety of any person (Section 1.1.2 ofthe Code). Pit wall monitoring by visual inspection can be used to mitigate risk to peo-ple working in mines. Section 6.5.1 of the Code states that no work shall be carriedon, at, or below a face or wall of a surface mine until that face or wall has been exam-ined and declared safe by the shift boss. At a minimum, visual monitoring of the pitwalls is implied by this statement. Section 6.23.2 of the Code states that loose rock andsoil shall not be allowed to accumulate on a bench or catchment berms in a mannerthat endangers any person working on a lower bench. An initial assessment of theaccumulation of rock and soil on a bench requires a visual inspection.

Visual monitoring should be an important component of a pit wall monitoring pro-gramme. Typically, some instrumentation is used in addition to visual monitoring.Visual monitoring is necessary but not necessarily sufficient to meet the needs of anappropriate pit wall monitoring programme. A visual monitoring programme should berestricted to low risk pit walls and have the following attributes:

• written standard procedures including frequency of observations, personnelresponsible, areas of concern, visual cues, record keeping, etc.

• written documentation and a photographic record, and• should be re-evaluated if significant instabilities are observed or anticipated.

Table 2. Average daily ore and waste production in 2013.

Mine Ore (kt) Waste (kt)

Copper Mountain 35 140Endako 52 combinedGibraltar 85 265HVC 150 150Huckleberry 18 to 19 65 to 75Mount Polley 22 63

Table 3. Monitoring frequency for typically used instrumentation.

Mine VisualTotal station andprisms Radar

Wire-lineextensometer Piezometers

CopperMountain

Weekly Daily – Daily to weekly –

Endako Weekly 3 times per week – – MonthlyGibraltar Weekly Daily – Daily to weekly Weekly to

monthlyHVC Weekly Hourly Every

10 minDaily to weekly Weekly to

monthlyHuckleberry Weekly Daily Hourly Daily to weekly Weekly to

monthlyMount Polley Weekly Daily – Daily to weekly Weekly to

monthly

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Visual inspections focus on identification of features such as tension cracks andravelling of materials from the pit walls. Mine operations personnel working in the pittypically watch for evidence of unusual or potentially unsafe pit wall behaviour.Geotechnical engineers, mining engineers, and technicians (in some operations) gener-ally perform formal inspections. However, some of the mines in BC have implementedformal hazard recognition programmes so that other mine personnel can assist withvisual monitoring to provide more comprehensive monitoring. Photographs are usuallytaken as part of the visual inspection record. Records of visits and observations of anychanges in the rock mass behaviour over time are recorded. The occurrence of rockfalls or presence of new cracks generally triggers the need for more formal and morefrequent visual inspections and often leads to the implementation of other monitoringtechniques.

The frequency of the visual inspection depends on the rock mass conditions and therisk associated with the slope instability. Typically, the BC mines conduct a formalvisual inspection on a weekly basis. However, informal monitoring is performed by allpersonnel working in the open pit on a continuous basis.

4. Surface measurements

The techniques used to measure the rock slope geometry and/or rock movementsinclude total stations and prisms, radar, laser scanners, terrestrial lidar (light detectionand ranging), and wire-line extensometers. Satellite synthetic aperture radar inter-ferometry (InSAR), aerial lidar, and photogrammetric mapping are other technologiesthat have been evaluated for use at the BC mines, but our survey of current practiceshows that these techniques are not in routine use at any of the BC open pit porphyrymines. Terrestrial photogrammetry has been used by consultants to the BC mines tomeasure as-built pit geometries and map geological structures [3]. In the future, pho-togrammetry will likely find greater use in these open pits as has been occuring atmines elsewhere [4–6].

HVC mine has used surface-installed tilt metres to measure pit wall movements.

4.1. Total station and prisms

The use of a total station and prisms for pit wall monitoring is the most frequently usedmeasurement technique. Small prisms are installed in potentially unstable zones orareas of interest. One or more base stations with back-sight prisms installed in stableground are established around the rim of the open pit depending on access and visibil-ity of the pit walls. The number of prisms installed in potentially unstable areas mea-sured from any one base station may range from 10 to 100 s. To minimise the manualeffort of reading the prism coordinates and to improve the frequency and the accuracyof the measurements, HVC uses robotic total stations operating from within climate-controlled huts. These total stations have wireless technology for transmission of datato the engineering office for fast data processing and analysis thereby saving time andreducing human errors. A description of the automated system used by HVC can befound in various publications [7–10].

It is noteworthy that the mines other than HVC have elected to rely on manual sur-veying with a total station carried to pre-installed base stations. The base station is typi-cally a concrete filled pipe with a permanently installed tribrach. Manual surveying islabour intensive and exposes survey personnel (and the total station) to inclement

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weather conditions, which probably reduces the frequency and accuracy of the mea-surements. There are several manufacturers of total stations (e.g. Leica, Sokkia, Topconand Trimble); however, all the open pit Cu–Mo mines in BC use Trimble S8 total sta-tions.

Best practices for pit slope monitoring have been established [11,12], and mostmines treat prism data as being fairly robust and reliable. The highest theoretical preci-sion that can be achieved for an operating pit environment may be 1-s angle precisionand 1 mm ± 1 ppm distance accuracy. Actual accuracies obtained under field measure-ment conditions are less, typically in the range of ±5–10 mm. One advantage of prismdata is that the 3D displacement vector for the prism location is measured, which mayprovide insight into incipient failure mechanisms, e.g. toppling versus planar sliding.

4.2. Radar

Some mines in BC are now using synthetic aperture radar monitoring devices to mea-sure pit wall movements. Slope stability radar (SSR) and InSAR systems are examplesof new technologies that work to continuously scan and compare high-resolution mea-surements of a slope face. SSR can detect real-time, sub-millimetre movements in thedirection parallel to the line-of-sight of the radar unit.

The reliability and relatively high precision of a radar unit can help mine operationsto manage unstable pit walls by quickly identifying the location and surface area of anemerging failure [13–16]. However, the radar systems are expensive (ranging from$250,000 to $500,000) and some mines prefer to rely on total stations and visual moni-toring due to the lower cost, proven reliability and ease of analysing data for decision-making.

Manufacturers of the radar technologies for slope monitoring include Reutech,GroundProbe and IDS [17–19]. Copper Mountain, Endako, Gibraltar and Mount Polleymines have not conducted field trials of any radar systems. Huckleberry mine and HVCmine are the only mines that own and operate a radar system for slope monitoring;both mines use a GroundProbe system. In 2011, HVC rented and tested an IBIS radarsystem for approximately five months. At the end of the test, HVC realised that thesystem did not meet their needs because the as-built geometry of the pit needed to beupdated in the IBIS system any time the pit geometry changed. The radar monitoringtechnology is evolving and improving and has been found effective at other mines[20].

4.3. Lidar

Terrestrial laser scanners (lidar systems) can be used by the mining industry fordetailed surveying of as-built geometries of pit walls. The current lidar technology isknown for its long-range measurement performance of up to 4000 m and the ability tooperate in poor visibility. Riegl [21] and Optech [22] are two manufacturers of terres-trial lidar systems. Lidar systems have been used in open pit mines to monitor slopemovement and alert mine workers to potential instabilitites [23]. It is noteworthy thatno BC porphyry mines routinely use lidar systems to monitor pit wall movements eventhough this technology is also useful for other purposes such as waste dumpmonitoring.

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4.4. Wire-line extensometer

Wire-line extensometers are frequently used to measure changes in tension crack widthsin active areas of instability. Typically, an inexpensive extensometer is established bydriving a steel peg into the ground on the downslope side of an observed crack. A thinsteel wire is attached to the peg and is extended across the crack to a pulley attachedto a tripod and connected to a hanging weight. Changes in the crack width are mea-sured visually from the position of the hanging weight relative to a ruler fixed to thetripod. A combination of simple in-house constructed wire-line extensometers and com-mercially available extensometers with alarm triggering capabilities are used by themines in BC.

Extensometers are installed once a tension crack is detected by visual monitoring.Subsequently, an extensometer can be read multiple times per day, daily or weeklydepending on the risk associated with the observed instability. Extensometer readingsare useful for tracking changes in displacement rates and for making operational deci-sions concerning mining activities in the area affected by the tension crack.

5. Subsurface measurements

Subsurface measurements typically involve some form of instrumentation placed withina borehole. A wide range of instrumentation is available including time domain reflec-tometry (TDR) cables, inclinometers, micro seismic sensors and borehole extensometers[14,15,24–26].

HVC mine is the only mine to use a range of borehole instrumentation to measurerock mass movements. HVC uses TDR, inclinometers and borehole extensometers tocollect subsurface measurements. TDR can detect the location of damage along a coax-ial cable grouted into a borehole. As deformation occurs in the rock around the bore-hole, the cable is pinched or sheared off and a TDR instrument can detect the distancealong the cable or borehole where deformation is occurring [27].

Aside from HVC, the only other mine using borehole instrumentation is Huckle-berry mine, which uses inclinometers to measure pit wall deformations.

The stability of a rock slope is affected by the presence of ground water within thesurrounding rock mass [28]. Piezometers are frequently used to measure pore pressureand the efficiency of mine dewatering programmes [25]. The most common types ofpiezometers are open standpipe piezometers, vibrating wire piezometers, pneumaticpiezometers and multi-point piezometers [29]. All mines except for Copper Mountainmine use piezometers to monitor groundwater pressures in critical sections of their pitwalls. Where piezometers are used, the mines have elected to use simple open stand-pipe piezometers. These piezometers typically require manual reading of water levelswithin the standpipe and readings are taken at weekly to monthly intervals. In addition,the mines surveyed use visual inspection of their pit walls for new signs of seepage orchanges in flow rates as these are often precursors to rock slope instability [30].

6. Movement thresholds and monitoring frequency

A primary purpose for measuring pit wall movements is to allow mines to manage thepit wall behaviour for the safety of personnel working in the open pits. For this pur-pose, it is important for the mines to establish movement thresholds that trigger specificoperational responses in the mine when a movement threshold is exceeded. In general,

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a mine can use four movement ranges to make operational decisions: (1) normal, (2)caution, (3) alert and (4) stop. Geotechnical consulting engineers and expert judgementare typically used to provide recommendations to the mines for appropriate movementthresholds defining the transition between the movement ranges. Normal movementsare those which are expected to occur in a pit in response to mining activities. Thesemovements involve elastic and some plastic pit wall displacements. The caution thresh-old signifies movements (velocities) beyond those that would be normally expected.There are few reliable analytical or numerical methods for predicting normal pit wallmovements, thus, the threshold values are primarily based on past experience at the pitin question or other similar pits elsewhere.

The alert threshold occurs when pit walls movements begin to accelerate or ifmovements continue unabated at high velocities. Pit wall movements in the alert rangetypically trigger requirements for more monitoring instrumentation and increased moni-toring frequency. The stop threshold is assigned to situations where the pit walls areprogressing toward failure and mining in the affected area should stop.

After mining stops in an area where high movements were occurring there are twopossible outcomes: (1) the movements gradually decrease and stabilise at levels thatallows mining to restart in the area or (2) the movements continue to accelerate untilpit wall failure occurs. Thus, some mines have a movement threshold for resumingmining activities in the affected area. The mining method may be altered and/or actionslike dewatering may be implemented to help stabilise the pit wall movements oncemining is resumed.

The pit wall movement thresholds used at the BC mines were obtained from thequesitionnaire and then confirmed with the personnel in charge of the slope monitoringat each mine by means of telephone interviews. Table 4 summarises the thresholds inuse at the different mines. Experience with prism measurements and defining movementthresholds at the BC mines has been built up over many decades. The expert knowl-edge often does not reside at the mine itself but comes from geotechnical consultantsthat have examined and analysed prism data from multiple mines over many years.Nevertheless, it is interesting that there are differences between the movement rates orthe response options used by the different mines. Possible reasons for different thresh-olds include differences between the mines in terms of:

• monitoring coverage and ability to increase monitoring in areas experiencingunexpected movements,

• production rates and size of the pit walls, and• rock mass quality and influence of structural geology.

As a pit becomes deeper, a larger volume of rock behind the pit wall experiencesstrain generated by stress relief and thus the resulting pit wall displacements are larger.Thus, one might expect that the movement thresholds for expected pit wall behaviourmight be related to the depth of the open pit. However, an examination of the thresh-olds used by the BC mines shows little link between pit depth and the thresholds inuse.

When radar systems are used (HVC and Huckleberry mines), the movement thresh-olds that trigger a given response are much higher than for prisms. For example, atHVC a prism movement rate of 15–25 mm/day triggers a caution response, whereaswhen a radar system is used, the trigger threshold is 4 mm/h or nearly 100 mm/day.Compared to prism monitoring the radar systems provide more rapid, real-time

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monitoring and better knowledge of the area/volume of pit wall that is deforming. Itappears that the mines using radar systems are taking advantage of this knowledge bycontinuing to mine in areas that would have previously been shut down if only prismdata were available. The movement thresholds used with radar systems will likelychange as further experience is gained using this technology in the open pits in BC.

The appropriate monitoring frequency depends on factors such as the pit walldeformation velocity and the size and consequences of a potential failure. From apractical perspective, the monitoring frequency also depends on the method(s) used tomonitor the pit walls. In addition, the failure mechanism can affect the monitoringrequirements. For example, a sudden brittle failure mechanism may result in a highlymobile rock avalanche with little response time whereas a progressive creeping earth-flow provides ample time for appropriate operational responses with low risk to person-nel working in the mine. Some of these issues have been discussed by others[11,31,32]. For example, a diamond mine in South Africa [31] uses weekly monitoringwhen movements are 2–5 mm/day, daily monitoring in areas experiencing10–50 mm/day and continuous monitoring when the movements exceed 50 mm/day.

The pit at the Huckleberry mine is relatively shallow, and the mine uses a sophisti-cated SSR system to systematically monitor the pit walls. Depending on the pit walllocation, the pit wall is monitored at weekly, daily or hourly intervals. By comparison,the Endako mine monitors the pit walls with prisms three times per week, unless thereis evidence of sloughing, and relies more heavily on visual inspections. Surveys ofprisms are attempted daily for four of the six mines that were surveyed (Table 3). Themonitoring frequency is typically increased when areas of instability are identified.Only the HVC and Gibraltar mines have dedicated personnel to monitor and assess thepit walls, which increases the mine’s ability to respond quickly to unexpected pit wallmovements.

Table 4. Pit wall movement thresholds used by BC mines.

Mine Movement rate Response

Copper Mountain 0–15 mm/day Normal>50 mm/day Alert

Endako 0–25 mm/day Normal>25 mm/day Monitor movement

Gibraltar 0–25 mm/day Normal25–64 mm/day Caution64–102 mm/day Alert>102 mm/day Suspend production<102 mm/day Resume mining with restrictions

HVC 0–15 mm/day Normalprisms 15–25 mm/day Caution

25–50 mm/day Alert50–100 mm/day Stop<50 mm/day Resume mining

HVC 4 mm/hr Cautionradar 5 mm/hr AlertHuckleberry 20 mm/h on radar Alarm triggersMount Polley 0–50 mm/day Normal

>50 mm/day Pit shut down3 days of acceleration Pit shut down<50 mm/day Resume mining

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7. Discussion and conclusion

Routine visual inspection is a key part of pit wall monitoring used at all open pit por-phyry mines in BC. Visual inspection has an advantage that everyone working in themine can play an observer role. Mines offer training to personnel working in the openpits so they are better able to identify conditions that merit a notification to engineersresponsible for pit wall stability. In addition, geotechnical engineers, mining engineersand technicians conduct more formal pit wall inspections on a weekly basis.

All open pit porphyry mines in BC use total stations and prisms for pit slope moni-toring. Several mines complement the total station monitoring with other techniquessuch as radar and extensometers. It is noted that no Cu–Mo mines in BC use lidar toroutinely monitor their pit walls. Among the six open pit mines surveyed, only HVCuses fully automated total station monitoring technology. As the other open pits becomelarger and deeper, the increased time required to survey larger numbers of prisms willlikely cause a shift in monitoring methods to robotic total stations with automated dataprocessing systems. This will allow deeper pits to monitor the slope more frequentlyand at various times of the day to maintain productivity and safety, particularly as slopeinstabilities develop. The automation system will have to be robust to withstand theharsh mining environment and extreme weather conditions, and to provide timelyresponse to failures. Automated survey systems will also likely help remove potentialhuman errors, although there will still be some uncertainties regarding the survey data.

In recent years, there has been a trend towards testing and using radar-based moni-toring systems. The mines that use radar systems allow for continuation of mining inareas that experience higher movement rates than would tolerated if just prisms wereused. Thus, radar monitoring has opened up new opportunities for pit wall manage-ment. However, the large volume of data generated by these systems can create prob-lems with data processing and analysis and further experience is still needed tooptimise and define appropriate movement thresholds for different operationalresponses.

Monitoring systems are generally purchased to match the operational needs of amine and the perceived accuracy requirements. Based on observations obtained duringthis study, further assessment appears to be warranted regarding matching system capa-bilities to operational needs for many of the BC mines.

There was consensus among the mines that the use of pit wall monitoring technolo-gies gives a better understanding of the dynamics of the pit wall behaviour. Pit wallmonitoring is an essential component of pit wall management and it reduces the overallrisk to the mining operation.

AcknowledgementsWe would like to acknowledge the personnel at Copper Mountain, Endako, Gibraltar, HighlandValley Copper, Huckleberry and Mount Polley mines for providing data and assisting with thiswork. We also acknowledge the assistance from George Warnock, BC Ministry of Energy andMines. This work was supported by the Natural Sciences and Engineering Research Council ofCanada.

Disclosure statementNo potential conflict of interest was reported by the authors.

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