Risk Management Open Pit Slopes by fmb31949


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Herath, A. K
BHP Billiton-Nickel West – Mt Keith Operation, Mt Keith, Western Australia, Australia

ABSTRACT: Optimistion of geotechnical design within acceptable risk criteria enables the exploitation of marginal ore sources.
Increasingly very large deep open pits having relatively steep slopes are developed to achieve maximum economic benefit from
these projects. They generally require very stringent slope management strategies. Mt Keith Nickel operation may be considered to
fall into this category, with a planned depth of 550m, developed in a series of staged cutbacks. Major considerations of the slope
management process are an unfavourable structural regime, time dependent deformation of weak host ultramafic rocks and steep
wall designs. MKO has prudently implemented a comprehensive slope management strategy for the early detection of potential
failure, this comprises automated prism monitoring and slope stability radar for displacement monitoring, smooth wall blasting and
various risk management procedures. It is expected that this paper will provide information that will be relevant to many large
scale open pits where slope management is a key requirement.

Keywords: Deep pits, Geotechnical hazards, Risk management, Monitoring

With the base metal prices at an all time high open
pits are being developed larger and deeper than ever
before. The viability of a (marginal) project or the
profitability of an existing operation is directly
related and sensitive to the pit wall angles. The
slopes are increasingly designed as steep as
geotechnically feasible, within risk benefit
framework acceptable to the mine operations.
Generally the risk-benefit criteria are based on the
probability of failure and estimation of the
consequential cleanup costs, without the need for
drastic modifications to a design. However if
unaccounted geotechnical factors come into the
equation, the failure potential and the economic risk
can increase dramatically.

As the pit walls become higher and steeper, the
probability of failure also increases.
This reality is reflected in the relatively common
occurrence of slope failures in Australian open pit
mines. Between June 1994 and October 2004 a total
of 859 significant wall failures were reported in
Western Australia alone1. They have resulted in two                 Figure 1. MKO location plan and regional geology
fatalities and 16 personal injuries.        Therefore
instability of slopes is one of the most critical risks             The Mt Keith nickel mine, owned and operated by
to be managed in open pit mining.                                   BHP Billiton Nickel West is situated approximately
                                                                    720km North East of Perth in the Northern region
                                                                    of the Eastern goldfields district of Western
                                                                    Australia, 80km south of Wiluna. The deposit
 Personal communications with the Department of industry            occurs in the Agnew-Wiluna Greenstone belt
and resources of Western Australia

(Figure1), a highly metalliferous section of Western            The footwall of the MKD5 orebody comprises
Australia’s Yilgran Craton. Started in 1994, it is a            fragmented, dacite to andecitic, phenocryst-rich
low grade, high tonnage operation and is mined                  lavas, with minor gabbro intrusions.
using a conventional drill, blast, load and haul
system, with plans for another 20 or more years of
production. The pit is being developed in a series of
cutbacks and the ultimate pit (Stage–J) is envisaged
to be 3.2km long, 1.5km wide and 570m deep. At
present the Stage-F pit development (2200 x 1250 x
350) m is nearing completion and the Stage-G
cutback is underway. Figure 2 shows the various pit
shells in a cross section at 31500m N.

Figure2. Cross sections of different pit shells at 32500N

                                                                Figure3. General geology at Mt Keith nickel mine
The general geology of Mt Keith is shown in
Figure3. The lithologies are grouped into three                 3. GEOTECHNICAL MODEL
packages from west to east, being the hangingwall,
the MKD5 orebody and the footwall sequence.                     In the Mt Keith pit area, different rock units; the
                                                                mafic, ultramafic and felsic rocks show varying
The MKD5 orebody is located in the Eastern                      degrees of fracturing and weathering. In general, the
Ultramafic unit of the Northern Agnew-Wiluna                    felsic rocks that comprise the majority of the
greenstone belt, where a disseminated Ni-Fe                     footwall are weathered to depths in excess of 150m,
                                                  o             particularly in the south. The depth of weathering in
sulphide mineralisation strikes NNW, dipping 70
                                                                the ultramafic ore body is relatively shallow, about
to the west over a length of 2km and a depth of
                                                                80m. The rock units dip steeply towards the West
about 600m. Pentalandite and millerite occur within
                                                                and strike North-South within the pit area. The
olivine     adcumulate    sequences     comprising
                                                                major structures trend NW-SE and NE-SW and
serpentinised and locally carbonate-talc altered
                                                                there are a few EW trending major fault structures.
dunite sills with intercalated mesocumulate
                                                                Ubiquitous NE and SW dipping major structures
                                                                exist in the host ultramafics that severely influence
                                                                the stability of the current East wall. Some of the
Fine grained foliated mafic volcanic and intrusive
                                                                lithological contacts are extensively sheared,
assemblages, together with thin graphitic schists
                                                                especially those of the ultramafic orebody. The
and structurally emplaced pyritic chert bands
                                                                ultramafic rocks are generally massive and less
comprise the hangingwall sequence.
                                                                intensely fractured than the other rock units.

Groundwater at MKO has caused pit wall stability              4.1. Slope Hazard Management
issues in localised areas within the pit. The
groundwater volume flowing into the pit is small              At MKO the slope hazard management has two
and most water occurs along the transition zone               broad aspects:
between the fresh and weathered rock units and
along some of the major structures. The major                    •   The geotechnical knowledge and experience
permeability appears to occur along the strike of the                from investigations, analysis, visual
formations, while permeability across rock units                     observation, monitoring and performance
represents the lowest hydraulic conductivity in the                  reconciliation are rigorously interpreted and
system. Some rock units are more fractured and                       applied to design as an on going process.
hence more permeable than others, especially the                 •   Suitable controls are implemented to ensure
hangingwall mafics; where as the ultramafic host                     the safety of mining personnel and
has a relatively low permeability. Regular pit wall                  equipment from geotechnical slope hazards.
dewatering programs have been conducted at MKO,
with limited success in terms of water strikes due to          4.1.1. Responsibilities
the low permeable rocks and low conductivity.
                                                              At MKO a range of control measures have been
4. SLOPE STABILITY                                            implemented to manage the risks associated with
                                                              geotechnical hazards, in order to provide a safe
The Mt Keith pit has experienced numerous small               work environment in compliance with the BHP
scale failures and several large scale instabilities,         Billiton Nickel West ‘Zero Harm’ policy. The
mainly in the present F-stage development, which is           registered quarry manager shall ensure that all
nearing completion. Apart from one large failure in           mining geotechnical activities and issues are
the deeply weathered felsic rocks in the upper                adequately addressed in compliance with the
section of the footwall, all other significant failures       requirements of the ‘Mines Inspection and Safety
have occurred in the fresh ultramafic host rock,              act WA-1995, [1]. The geotechnical department is
which forms the basal slopes.                                 responsible for the identification, monitoring and
                                                              management of the geotechnical hazards in
The ultramafic host is not a weak rock in terms of            collaboration with the planning, drill and blast and
intact rock strength, averaging about 100MPa. It is           production      departments.      Several    important
serpentinised and contains varying degrees of water           procedures have been implemented and are strictly
absorbing minerals such as magnesite and                      adhered to in order to prevent injury and process
pyroaurite; and hence exhibits transient strength             loss from geotechnical hazards. Training of all
effects. It is generally massive in nature but there          mining personnel in basic slope stability awareness
are two or more ubiquitous joint sets having high             has been introduced to the induction process of new
continuity and weak talc, serpentine and pyroaurite           employees. A monthly updated geotechnical hazard
infill, and on the F-Stage east wall they dip towards         plan is circulated to all pit personnel. The workforce
the pit. Although the intact rock strength is                 is encouraged to express their concerns on stability
moderately high, the weak and unfavourable                    hazards and as a result a very appropriate
structural set up results in the rock mass quality            geotechnical safety culture has developed among
being generally poor in the ultramafic host at MKO.           the workforce over the years
There are also sub vertical fault structures with
weak talc/serpentine alterations that can facilitate          4.2. Current Practices
side and back release planes for large scale failure.
As such, the ultramafic basal slopes of MKO could             The slope hazard management is a structured and
be vulnerable to the development of large scale               continuous process as shown in Figure 4.
failures. Poor rock mass quality and low shear
strength of unfavourably oriented structures, with
time dependent strength deterioration, act as the
main contributing factors influencing potential
slope instability, and to a lesser extent the
groundwater pressure.

               Staged                                           digital photogrammetry; namely the Siro-Vision
             Development                                        photogrammetry technique. SiroVision is a digital
                                    SiroVision Mapping          photogrammetry system developed by CSIRO-
                                    Defect Sets
           Geotechnical Data
                                    Continuity of Defects
                                                                Australia that enables safe and comprehensive
            Collection and
                                    Shear strength              mapping of structures in potentially hazardous and
                                    Rock Strength
                                                                inaccessible rock slopes. The important aspect of
            Identification of
                                    Groundwater                 shear strength of defects is however obtained by
              Potentially                                       laboratory testing and physical mapping, with
            Unstable Areas                                      particular reference to their infill minerals and
                                                                surface characteristics. The structural data thus
                                                                obtained is used in the stability analysis of current
    Design               Risk Management
  Modifications             Procedures                          slopes and for the design of future cutbacks.
                                    Visual Inspection
                                    Crack/Extensometer           4.2.1. Reconciliation of Design Parameters
                                    Slope Indicator
                                    Micro Seismic               The performance of recent slope excavations is
                                    Radar (SSR)                 verified through the comparison of the in situ
            Identification of                                   geotechnical characteristics to the estimates used in
            Critical Hazards
                                                                the original slope design. This is an ongoing process
                                                                and is done through routine collection and update of
               Critical                                         geotechnical data and the back analysis of slope
            SSR Monitoring
                                                                deformation and any failures that occur, as
                                                                described in the previous section.
              Isolation and
                                                                The verified key parameters are:
Figure 4. Slope hazard management process
                                                                   •   Existence of major structures
                                                                   •   Orientation, distribution and characteristics
The current practices at MKO, as given in the MKO
                                                                       of major defects
Geotechnical Management Manual [2], are:
                                                                   •   Overall rock mass quality
    •   Ongoing geotechnical data collection in                    •   Groundwater influence
        actual mine excavations                                    •   Transient effects
    •   Regular data analysis, back analysis of all
        slope failures and slope deformations,                  If one or more of these factors significantly differ
        evaluation of potential slope hazards and               from the original design estimates then a new
        recommendation of necessary design                      risk/opportunity will be quantified and presented to
        changes                                                 planning/management for appropriate consideration.
    •   Groundwater depressurisation and
                                                                 4.2.2. Slope depressurization
    •   Risk management procedures for working
                                                                The MKO wall design assumes that minimal
        near crests and slopes
                                                                groundwater pressure acts on the slopes. Hence
    •   Post blast procedure to work near a                     adequate depressurisation of the slopes is important
        potentially unstable slope                              and achieved through the systematic installation of
    •   Quality control on slope formation                      horizontal drainage holes.
    •   Rockfall response
                                                                The geotechnical       department     assumes    the
Geotechnical Data Collection                                    responsibility of:
Important structural/defect data is recorded on a                  •   Reviewing the hydrogeology model and
routine basis as any pit shell is developed. This is                   depressurisation planning and monitoring
done by the physical mapping of excavations and by

    •   Plan and arrange horizontal drainage hole                  4.3.2. Working Near Slopes
        programs to fit with the mining sequence
    •   Supervision of drilling and installation of              Apart from potential instability imposed by
        horizontal    drains,    piezometers     and             structurally controlled blocks, loose blocks (scats)
        monitoring instrumentation.                              on the batter faces cause rockfall hazards. The
                                                                 development of loose scats is a continual issue
4.3. Risk Management Work Procedures                             associated with the ongoing deformation of the
                                                                 walls, especially if the rock mass is highly
A Number of work procedures have been                            fractured. Smooth wall blasting techniques
implemented to manage risk from geotechnical                     including pre-splitting, trim blasts adjacent to the
hazards [3]. They are;                                           walls, zero or minus sub drill on the crest zones, are
                                                                 practiced to minimise this rockfall risk at MKO.
  4.3.1. Working Near Crests – Edge procedure                    Batter scaling is also performed to a very high
  Zone (EPZ)                                                     standard. Standoff distances from the base of a wall
                                                                 are specified depending upon the quality of the
An edge procedure has been developed and allows                  slope, and a risk assessment should be conducted
control of activities conducted in the edge zones and            and approved by the geotechnical department before
ensures that the slope below is monitored and                    any work can be undertaken adjacent to a slope.
appropriate alarms set while any activity is
occurring inside the edge procedure zone (EPZ).                    4.3.3. Designated Rockfall Area
This is particularly relevant to MKO as the pit is
developed in a series of staged cutbacks where very              Potential rockfall areas are identified and delineated
high slope interfaces exist (Figure 5). The EPZ is               by the geotechnical department. These ‘rockfall
delineated by the geotechnical department and the                designated areas’ are no entry zones for personnel
zone width depends upon the geotechnical                         or equipment, unless strict controls are put in place
conditions. The standoff distance is adjusted to                 to eliminate the rockfall risk. These areas are shown
include any structural elements considered unstable.             on the geotechnical hazard plan, and updated
The emergency response ensures that during an                    monthly or as required, and communicated to all pit
emergency the EPZ is clear of personnel and                      personnel.

                                           Stage -G


                   Stage E

      Stage –F
    (Final Stages)

                                                                 Figure 6. Construction of toe bunds to mitigate risk from small
Figure 5. Figure showing EPZ and various stages of MKO pit
                                                                 The controls for elimination of risks arising from
                                                                 potential rockfalls are varied, from re-scaling the
                                                                 faces to construction of toe bunds or catch fences.
                                                                 Draping certain areas of the walls with chain link

mesh was also practiced in the past, but the                height of a batter. The exposure of drill and blast
preferred method at MKO is the provision of toe             crews to rockfall hazards was significantly reduced
bunds, as shown in Figure 6. The toe bunds are              by this change in pre-split hole depth. Occasionally
constructed using finer material (soil or crushed           hydraulic scaling (high pressure water) is also used
rock) to prevent the toe bund itself becoming a             to clean up loose scats.
rockfall hazard as the slope is progressed deeper
and in the event of batter crest failure and a              In weathered rock, batters are formed and scaled
consequent reduction in berm width. Critical radar          under survey control to achieve the design angles.
monitoring will be in place for special cases and, as
previously discussed, geotechnical department               The geotechnical department inspects and signs off
approval is required to work within rockfall                all final walls that have been scaled and cleaned to
designated areas.                                           confirm that they comply with the design. Any
                                                            slope which is out of the geometric tolerance of the
 4.3.4. Post Primary Blast Standoff                         design specifications is treated as a slope hazard.

Standoff zones are declared in areas where rockfall          If any potential slope hazards are observed during
risks exist to personnel and equipment immediately          an inspection they are noted in the inspection report
following blasting operations. The standoff distance        and slope hazard management processes will be
and time is determined by the geotechnical engineer         initiated.
and approved by the quarry manager. The rule of
thumb is to stand off 30m from the toe of the slope          4.3.6. Rockfall Response
and a minimum of 12 hours after blasting. Once
placed, the standoff can be lifted only by the              All rockfalls that occur outside of designated
geotechnical engineer, where a written site                 rockfall areas are addressed under Emergency
communication to the quarry manager will be                 Response procedures. Even the smallest rockfall
required to that effect.                                    event is treated as a serious potential incident. A
                                                            rockfall event is defined as any unexpected fall of
 4.3.5. Quality Control of Slope Formation                  ground from a wall within the pit area. This applies
                                                            to either single or multiple rock events, which may
Achieving the design batter angles free of loose            affect the safety of pit activities. If the production
material on the face or crest is a priority at MKO.         supervisor deems the event is significant and cannot
Special blasting techniques are applied where               recommence operations safely, he informs the
necessary, to minimise blast impact on the slope            Quarry manager, production superintendent and the
and reduce overbreak. These measures may                    geotechnical department. An investigation will then
comprise smooth and low impact blasting                     be carried out and adequate controls put in place
techniques such as trim blasting adjacent to final          before the commencement of work.
walls, pre-splitting, minus sub-drill near batter
crests, decoupling or no stemming of buffer rows.           5. MONITORING
The geotechnical department signs off blast designs
before implementation.                                      Several large failures have occurred in the MKO pit
                                                            in the past. At present the F-Stage cutback is
The faces of excavated slopes are scaled and                nearing completion and exhibits a high incidence of
cleaned thoroughly to remove loose rock. Past               batter instability on the East wall, which in the past
observations during and after blasts and following          has resulted in major design changes.
rain events indicated that the majority of rock falls       Notwithstanding the many operational procedures
occurring on the fresh rock batters originate from          that have been implemented to mitigate/ eliminate
either the crest area or at the interface between the       slope stability hazards monitoring is central to
two 15m pre-split blasts that made up the 30m high          geotechnical risk management in this challenging
batter face. Scaling practices were improved to             environment.
remove any loose blocks at the crest level and
instead of 15m holes 30m pre-splits are now drilled         A variety of monitoring methods are employed at
to eliminate the development of loose scats at mid          MKO, comprising:

                                                             expanded monitoring network is expected to
   •   Visual inspection                                     provide early information on stress related
   •   Crack meters                                          deformation and the potential of instability.
   •   Slope indicator
   •   Automated prism monitoring                            5.4. Slope Stability Radar (SSR)
   •   High resolution seismic monitoring
   •   Groundprobe slope stability radar                     MKO implemented monitoring of pit walls with
                                                             radar (SSR) in 2003. It is used extensively, with
Of these monitoring systems, the prism monitoring            three radars on site at all times. The SSR is able to
and Slope stability radar are the primary tools in the       scan large sections of the slope with a very high
detection of overall slope deformation trends, with          degree of accuracy and can be alarmed on any
the latter being highly effective in the monitoring of       displacement criterion the operator cares to set.
an impending failure.                                        Since their implementation they have allowed the
                                                             detection and isolation of many wall failures. The
5.1. Visual Inspection                                       current limitations relate to a maximum reliable
                                                             monitoring range of 400m and, at this range, a
This experiential process is an important part of            minimum detectable failure size of 8x8m (relating
monitoring. The observation of changing                      to the smallest pixel size).
conditions, such as an apparent loss or sudden
appearance of seepage, tension cracks etc, which             Figures 7and 8 show an example of a recent large
are not picked up by other monitoring methods, can           instability, the safety aspects of which were
represent important precursory signs of an                   managed exceptionally well with the aid of the
impending instability. The frequency of inspections          slope stability radar. The instability was identified
is determined by the relative state of stability in          well in advance and the area isolated 24 hours
different parts of the pit. Daily active workplace           before the event occurred.
inspections are carried out by pit supervisors, from
which the geotechnical department may be
requested to conduct assessments if there are                                      Slope

concerns about geotechnical hazards.

5.2. Prism Monitoring

Four automated theodolites are installed on the crest
of the pit that provide complete coverage of the pit
walls. Prisms are installed on a routine basis on
safely accessible berms, typically spaced 50m
horizontally and 30m vertically. The theodolites
scan the prisms around the clock and transfer real
time data to the MKO computer system via radio
                                                             Figure 7. Displacement monitoring of the F Stage east wall
5.3. High Resolution Seismic Monitoring                      with slope stability radar

At present the high resolution micro-seismic
monitoring is only a trial network, which is
employed to obtain information that will
subsequently be used to define a final array
configuration for the future development of the
>500m deep pit. The monitoring results to date
suggest that the predominant failure mechanism is
crushing or dilatational behaviour [4].      The

                                                           1.   Department of Industry, Infrastructure and Resources,
                                                                Western Australia (1995). Mines Inspection and Safety
                                                                Act-WA 1995, -Geotechnical Considerations.
                                                           2.   Bentel, G. (2006) MKO Geotechnical Management
                                                                Manual-Revision 2, BHP Billiton-Nickel West, Internal
                                                           3.   Mt Keith Nickel Operation, (2003),              Critical
                                                                Geotechnical Operating Procedures,              Internal
                                                           4.   Sweby, G.J., Trifu, C-I., Goodchild, D. J and Morris, L.
                                                                D. (2006) High – Resolution Seismic Monitoring at Mt.
                                                                Keith Open Pit Mine, 41st U.S Symp. On Rock
                                                                Mechanics, Golden, Colorodo.

Figure 8. Photograph showing the failure


The variability of geotechnical condition within an
operation can significantly impact the efficiency of
mining cycles. MKO has recognised that
geotechnical hazard management is an integral part
of the mining operation and has invested
extensively in geotechnical expertise and state of
the art monitoring equipment. The collection and
analysis of geotechnical data, including wall
deformation and failures, is an integral part of the
on-going geotechnical management program at
MKO. The results are used in the periodic
reassessment of the geotechnical model and the
slope designs.

Smooth wall blasting techniques and strict quality
control measures have been implemented to achieve
design slope angles and eliminate the risk from
rockfall hazards.       Various safeguards and
procedures have been established and strictly
adhered to. They ensure the safety of pit personnel
from hazards due to both expected and unforeseen
geotechnical    conditions.   Geotechnical     data
collection and monitoring are central to the
successful slope management process at MKO.

The author would like to thank the Management of
BHP Billiton-Nickel West-Mt Keith Operation for
permission to prepare and publish this paper. The
assistance of the MKO geotechnical team is also
greatly appreciated.


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