I FUNDAMENTALS OF SLOPE DESIGN Peter Stacey

1 .I Introduction For an open pit mine, the design of the slopes is one of the major challenges at every stage of planning and operation. It requires specialised knowledge of the geology, which is often complex in the vicinity of orebodies where structure and/or alteration may be key factors, and of the material properties, which are frequently highly variable. It also requires an understanding of the practical aspects of design implementation.

This chapter discusses the fundamentals of creating slope designs in terms of the expectations of the various stakeholders in the mining operation, which includes the owners, management, the workforce and the regulators. It is intended to provide a framework for the detailed chapters that follow. It sets out the elements of slope design, the terminology in common usage, and the typical approaches and levels of effort to support the design requirements at different stages in the development of an open pit. Most of these elements are common to any open pit mining operation, regardless of the material to be recovered or the size of the open pit slopes.

1.2 Pit slope designs The aim of any open pit mine design is to provide an optimal excavation configuration in the context of safety, ore recovery and financial return. Investors and operators expect the slope design to establish walls that will be stable for the life of the open pit, which may extend beyond closure. At the very least, any instability must be manageable. This applies at every scale of the walls, from the individual benches to the overall slopes.

It is essential that a degree of stability is ensured for the slopes in large open pit mines to minimise the risks related to the safety of operating personnel and equipment, and economic risks to the reserves. At the same time, to address the economic needs of the owners ore recovery must be

maximised and waste stripping kept to a minimum throughout the mine life. The resulting compromise is typically a balance between formulating designs that can be safely and practicably implemented in the operating environment and establishing slope angles that are as steep as possible.

As outlined in Figure 1.1, the slope designs form an essential input in the design of an open pit at every stage of the evaluation of a mineral deposit, from the initial conceptual designs that assess the value of further work on an exploration discovery through to the short- and long-term designs for an operating pit. At each project level through this process other key components include the requirements of all stakeholders.

Unlike civil slopes, where the emphasis is on reliability and the performance of the design and cost/benefit is less of an issue, open pit slopes are normally constructed to lower levels of stability, recognising the shorter operating life spans involved and the high level of monitoring, both in terms of accuracy and frequency, that is typically available in the mine. Although this approach is fully recognised both by the mining industry and by the regulatory authorities, risk tolerance may vary between companies and between mining jurisdictions.

have many ramifications including: Uncontrolled instability, in effect failure of a slope, can

Safety/social factors + loss of life or injury; + loss of worker income; + loss of worker confidence; + loss of corporate credibility, both externally and

Economic factors + disruption of operations; + lossofore; + loss of equipment; + increased stripping;

with shareholders.

2 I Guidelines for Open Pit Slope Design

Mineral

Increase I eve I Recycle

designs design I I I

Reject Review Accept

(& Figure 1.1: Project development flowchart

+ cost of cleanup; + loss of markets. Environmental/regulatory factors + environmental impacts; + increased regulation; + closure considerations.

1.2.1 Safety/social factors Safe operating conditions that protect against the danger of death or injury to personnel working in the open pit are fundamental moral and legal requirements.

instability due to the complexity of mining environments, since the adoption of formal slope design methodology in the early 1970s the number of failures has generally decreased. Even so, in recent years there have been several large failures in open pits around the world. Tragically, some of these have resulted in loss of life; most have had severe economic consequences for the operation. These failures have attracted the attention of regulators and the public. Consequently, it is becoming increasingly common for management (including executives) and technical staff to face criminal proceedings when mining codes are violated, in either the design or the operation of a mine.

smaller failures, often rockfall at a bench scale, that typically result in the majority of deaths and injuries. For the mining industry to be sustainable, safety is a prime

While open pits have always been prone to wall

While the major failures attract wide attention, it is the

objective and must therefore be addressed at all scales of slope stability.

1.2.2 Economic factors The main economic incentive in most open pits is to achieve the maximum slope angle commensurate with the accepted level of stability. In a large open pit, steepening a wall by only a few degrees can have a major impact on the return of the operation through increased ore recovery and/or reduced stripping (Figure 1.2).

expansion cuts may be flatter than the optimum, either to provide additional operating width or to ensure stability where data to support the designs are limited. However, this flexibility, which must be adopted with the understanding and consent of all stakeholders, almost always has negative economic consequences.

the mine but, for example, it has been shown that an increase in slope angle of 1" in a 50" wall 500 m high results in a reduction of approximately 3600 m3 (9000 t) of stripping per metre length of face.

Increasing the slope angle will generally reduce the level of stability of the slope, assuming that other factors remain constant. The degree to which steepening can be accomplished without compromising corporate and regulatory acceptance criteria, which usually reflect the safety requirements for both personnel and ore reserves,

In some instances, 'operating slopes' in initial

The impact of slope steepening will vary depending on

1 3 Fundamentals of Slope Design

Steepened Slope

Current Slope

Figure 1.2: Potential impacts of slope steepening

must be the subject of stability analyses and ultimately risk assessments.

It is often no longer sufficient to present slope designs in deterministic (factor of safety) terms to a mine planner who accepts them uncritically. Increasingly, the requirement is that they be proposed within the framework of risk levels related to safety and economic outcomes for a decision-maker who may not be a technical expert in the mining field. The proposed design must be presented in a form that allows mine executives to establish acceptable levels of risk for the company and other stakeholders. In this process the slope designers must play a major role.

1.2.3 Environmental and regulatory factors Most open pits are located in jurisdictions where there are mining regulations that specify safety and environmental requirements, including those for mine closure. The regulations may be federal, as in the case of the Mine Safety and Health Administration (MSHA) in the USA and the SNiP Codes in Russia, or local, for example the

provincial mining codes in Canada and state regulations in Australia.

The regulations related to open pit slopes vary considerably between jurisdictions, as do the degrees of flexibility to modify slope configurations from those specified in the codes. However, regardless of the type of code, in most if not all jurisdictions it is the ultimate responsibility of the registered Mine Manager to maintain the standard of care and regular reviews by a competent person that are required.

Levels of requirements in codes can be summarised as follows.

1 Duty of Care, e.g. Western Australia, which place accountability on the registered Mine Manager to maintain appropriate design levels and safe operating procedures. General Directives, e.g. MSHA, which are general in nature and do not specify minimum design criteria, although they may include definitive performance

2

4 I Guidelines for Open Pit Slope Design criteria for catch benches and stable bench faces. Mines Inspectors enforce these regulations and are therefore responsible for approving the operation of a pit in terms of slope performance. General Guidelines, e.g. 'Geotechnical Guidelines in Open Pit Mines - Guidelines', Western Australia, which outline the legislated background for safety in the context of the geotechnical factors that must be considered in the design and operation of open pit mines. Defined General Criteria, e.g. British Columbia, Canada, which define minimum bench widths as well as maximum operating bench height, both of which are related to the capacity of the excavating equipment. Detailed Criteria, e.g. the Russian SNiP Codes, which define methodologies to be used at different project levels for investigation and design of excavations.

In most jurisdictions it is possible to obtain authorisation for variations from the mining code, e.g. the use of multiple bench stacks between catch berms, provided that a clear engineering case can be presented and/or precedence for such a variation in similar conditions can be shown. For slope design practitioners, this means staying abreast of regulatory changes.

Mine closure considerations depend on regulatory requirements, company standards and/or other stakeholder interests.

1.3 Terminology of slope design This section introduces the terminology typically used in the slope design process and presents a case for standardising this terminology, particularly with relation to slope movements and instability.

1.3.1 Slope configurations The standard terminology used to describe the geometric arrangement of the benches and haul road ramps on the pit wall is illustrated in Figure 1.3. The terms relevant to open pit slope design as used in the manual are given in the Glossary.

elements varies by geographic regions. Some important examples include the following.

It should be noted that terminology related to the slope

Bench face (North America) = batter (Australia). Bench (North America) = berm (Australia). The flat area between bench faces used for rockfall catchment. The adjective 'catch' or 'safety' is often added in front of the term in either area. Berm (North America) = windrow (Australia). Rock piles placed along the toe of a bench face to increase rockfall catchment and/or along the crest of benches to prevent personnel and equipment falling over the face

below. Note the potential confusion with the use of the term 'berm' for a flat surface. Bench stack. A group of benches between wider horizontal areas, e.g. ramps or wider berms left for geotechnical purposes.

Another aspect of terminology that can cause confusion is the definition of slope orientations. Slope designers usually work on the basis of the direction that the slope faces (dip direction), as this is the basis of kinematic analyses. On the other hand, mine planning programs usually require input in terms of the wall sector azimuth, which is at 180" to the direction that the slope faces, i.e. a slope facing/dipping toward 270" has an azimuth of 090" (inset, Figure 1.3). It is important that the convention adopted is clearly understood by all users and is applied consistently.

Note that the bench face angles are defined between the toe and crest of each bench, whereas the inter-ramp slope angles between the haul roads/ramps are defined by the line of the bench toes. The overall slope angle is always measured from the toe of the slope to the topmost crest (Figure 1.3).

1.3.2 Instability Increased ability to detect small movements in slopes and manage instability gives rise to a need for greater precision in terminology. Previously, significant movement in a slope was frequently referred to in somewhat alarmist terms as 'failure', e.g. failure mode, even if the movement could be managed. It is now appropriate to be more specific about the level of movement and instability, using the definitions that recognise progression of slope movement in the following order of severity.

Unloading response.

Initial movements in the slope are often associated with stress relaxation of the slope as it is excavated and the confinement provided by the rock has been lifted. This type of movement is linear elastic deformation. It occurs in every excavated slope and is not necessarily symptomatic of instability. It is typically small relative to the size of the slope and, although it can be detected by instruments, does not necessarily exhibit surface cracking. The deformation is generally responsive to mining, slowing or stopping when mining is suspended. In itself, unloading response does not lead to instability or large- scale movement.

Movement or dilation.

This is considered to be the first clear evidence of instability, with associated formation of cracks and other visible signs, e.g. heaving at the toe (base) of the slope. In stronger rock, the movement generally results from

1 5 Fundamentals of Slope Design

t Bench Height

BENCH DETAIL

Crest 1- ,,Inter Ramp Angle

(bench-toe to-toe)

Benches

Overall Angle (wall crest-to-toe)

Sector Azrmuth 360 1

Sector Azimuih 090

Sector Azimuth 270

Sectarhimuth 180

EXPLANATION OF WALL SECTOR AZIMUTH

Figure 1.3: Pit wall terminology

sliding along a surface or surfaces, which may be formed by geological structures (e.g. bedding plane, fault), or a combination of these with a zone of weakness in the material forming the slope.

Slope dilation may take the form of a constant creep in which the rate of displacement is slow and constant. More frequently, there can be acceleration as the strength on the sliding surface is reduced. In certain cases the displacement may decrease with time as influencing factors (slope configuration, groundwater pressures) change. Even though it is moving, the slope retains its general original configuration, although there may be varying degrees of cracking.

monitoring program is established to manage the slope performance, particularly if the movement rates are low and the causes of instability can be clearly defined. However, if there is no intervention, such as depressurisation of the slope, modification of the slope configuration or cessation of mining, the movement can

Mining can often continue safely if a detailed

lead to eventual failure. This could occur as strengths along the sliding surface reduce to residual levels or if additional external factors, such as rainfall, negatively affect the stress distribution in the slope.

Failure.

A slope can be considered to have failed when displacement has reached a level where it is no longer safe to operate or the intended function cannot be met, e.g. when ramp access across the slope is no longer possible.

The terms failure and collapse have been used synonymously when referring to open pit slopes, particularly when the failure occurs rapidly. In the case of a progressive failure model, failure of a pit slope occurs when the displacement will continue to accelerate to a point of collapse (or greatly accelerated movement) (Call et al. 2000). During and after failure or collapse of the slope, the original design configuration is normally completely destroyed. Continued mining almost always involves modification of the slope configuration, either

6 I Guidelines for Open Pit Slope Design through flattening of the wall from the crest or by stepping out at the toe. This typically results in increased stripping (removal) of waste and/or loss of ore, with significant financial repercussions.

The application of a consistent terminology such as that outlined above will also help to establish a more precise explanation of the condition of a slope for non- practitioners such as management and other stakeholders.

1.3.3 Rockfall The term rockfall is typically used for loose material that either falls or rolls from the faces. As such it is primarily a safety issue, although it could possibly be a precursor to larger-scale instability.

implementation, i.e. poor blasting and/or scaling practices. However, it may also result from degradation of the slope as a result of weathering or from freeze-thaw action.

Rockfall can be a symptom of poor design

1.4 Formulation of slope designs 1.4.1 Introduction The process of pit slope design formulation has been developed over the past 25 years and is relatively standard, although some of the methodologies vary between practioners. This section presents the general framework as an introduction to the detailed methodologies, which are discussed in the chapters that follow.

The basic process for the design of open pit slopes, regardless of size or materials, is summarised in Figure 1.4. Following this approach, the slope design process at any level of a project essentially involves the following steps:

formulation of a geotechnical model for the pit area; population of the model with relevant data; division of the model into geotechnical domains; subdivision of the domains into design sectors; design of the slope elements in the respective sectors of the domains; assessment of the stability of the resulting slopes in terms of the project acceptance criteria; definition of implementation and monitoring require- ments for the designs.

The resulting slope designs must not only be technically sound, they must also address the broader context of the mining operation as a whole, taking into account safety, the equipment available to implement the designs, mining rates and the acceptable risk levels.

the mine executives, who are ultimately responsible, and the operators, who implement the designs, to fully understand the basis and any shortcomings of the designs, as well as the implications of deviation from any

The designs must be presented in a way that will allow

constraints defined by the designer. In this context, a key element in the designs is the acceptance criteria against which the designs are formulated. These must be clearly defined by management working in consultation with the slope designers and mine planners.

and hence the level of confidence in the resulting designs generally improve with each successive stage in the development of an open pit mining project. However, the basic design procedures are essentially the same for all projects, with minor modification depending upon such factors as geology, groundwater conditions and proposed mine life.

The following points describe the basic elements of each step. They are discussed in following chapters, cited in parentheses.

As discussed in the following section, the available data

1.4.2 Geotechnical model The geotechnical model (Chapter 7), is the fundamental basis for all slope designs and is compiled from four component models:

the geological model; the structural model; the rock mass model (material properties); the hydrogeological model.

These models also have applications for other aspects of the mining operation, for example in ore reserves and mining operations. However, particular aspects of each are critical for the slope design process.

can be important in specific cases, for example in situ stress, particularly in relation to very high slopes, the presence of extensive underground openings and seismic loading.

discussed in detail in Chapter 2.

There are other aspects of the geotechnical model that

Methods for collecting the data for each model are

1.4.2.1 Geological model (Chapter 3) The geological model presents a 3D distribution of the material types that will be involved in the pit walls. The material type categories can relate not only to lithology but also to the degree and type of alteration, which can significantly change material properties, either positively (silicification) or negatively (argillisation).

In some deposits, notably those located in the tropics, geomorphology may also play a significant role in slope designs.

It is important to understand the regional geological setting and the genesis of the mineralisation. This often involves an appreciation that differs somewhat from that required by the mine geologists, who typically focus primarily on the mineralisation. Slope design studies must take a broader view of the geology of the deposit, including

1 7 Fundamentals of Slope Design

Figure 1

MODELS Ceotechnical Model

1 Ceotechnical

Domains

DOMAINS Strength Structure

/ - Design Sectors

1 Bench

Configurations

Inter-Ramp Angles

Mine Plannin

F Z Overall Slopes -

DESIGN

ANALYSES

Groundwater

Final Designs

-7 Blasting

Closure Design Model

.4: Slope design process

the surrounding waste rock, focusing on the engineering aspects.

As pit slopes become higher, the potential for impact by in situ stresses, particularly acting in combination with the high stresses created at the toe of the walls, must be considered. In situ stress assessment must be included in the geological model.

1.4.2.2 Structural model (Chapter 4) A structural model for slope designs is typically developed at two levels:

major structures (folds, inter-ramp and mine scale faults); structural fabric (joints, bench scale faults).

This differentiation relates largely to continuity of the features and the resultant impact with respect to the slope design elements. Major faults are likely to be continuous, both along strike and down dip, although they may be relatively widely spaced. Hence they could be expected to influence the design on an inter-ramp or overall slope scale. On the other hand, the structural fabric typically has limited continuity but close spacing, and therefore

8 I Guidelines for Open Pit Slope Design becomes a major consideration in design at a bench scale and possibly for inter-ramp bench stacks.

1.4.2.3 Rock mass model (Chapter 5) The properties of the materials in which the slope will be excavated define probable performance and therefore the design approach. In strong rocks, structure is likely to be the controlling factor, even in relatively high slopes. In weaker materials and for very high slopes, the rock mass strength could be expected to play an important role, either alone or in combination with structures.

be given to the possible changes in behaviour with time. This particularly applies where there has been argillic alteration involving smectities (swelling clays) or in clay-rich shales, since the strength properties and behaviour of the material can change after exposure.

In determining the material properties, the slope designer can also provide important data for other aspects of the mining operation, for example in blast designs (Chapter 11, section 11.3). This should not be overlooked when designing the testing programs.

Back-analysis of failures and even of stable slopes can play a significant role in the determination of material properties. Detailed records of the performance of phase slopes and the initial stages of ultimate slopes can provide large-scale assessments of properties that can normally only be determined through small-scale laboratory tests during the feasibility and earlier stages of design. This is discussed in detail in Chapter 12.

1.4.2.4 Hydrogeology model (Chapter 6) Both the groundwater pressure and the surface water flow aspects of the hydrogeological regime may have significant negative effects on the stability of a slope, and must therefore be fully understood.

design that can be readily modified by artificial intervention, particularly at a large (inter-ramp and greater) scale. However, dewatering and depressurisation measures require operator commitment to be implemented effectively, and usually need significant lead time for design and implementation. Identification and characterisation of the hydrogeological regime in the early stages of any project are therefore of paramount importance.

In defining the material properties, consideration must

These aspects are usually the only elements in a slope

1.4.3 Data uncertainty (Chapter 8) With the move towards probability-based slope design methodology the need to define the reliability of the data in the geotechnical model has increased significantly. At the early stages of project development the available data are limited and hence the reliability of various model aspects will be low. This frequently leads to a situation where the

Level 1

Level 2

Level 3

Level 4

Level 5

4

7

Ore Reserve: r Probable

Proved

I

Figure 1.5: Geotechnical levels of confidence relative to the IORC code

uncertainties dominate the probabilistic results and a more deterministic approach must be used.

A high degree of uncertainty can exist even at the feasibility level, particularly where high (greater than 500 m) slopes are involved and the only available data are from drill holes and surface exposure. In this situation, either additional information obtained to reduce the uncertainties or the potential impacts must be made clear to the decision-makers.

In parallel with the introduction of codes for reporting exploration results, mineral resources and ore reserves in several countries (e.g. JORC in Australia, SAMREC in South Africa and 43-101 in Canada), the increased need to define data reliability has generated a requirement for a geotechnical reporting system related to the slope designs for the pits that define the reserves. Accordingly, a system of reporting the level of uncertainty in the geotechnical data is discussed in Chapters 8 and 9. The system is linked to the levels of effort at the various stages in the life of an open pit, outlined in section 1.5 and Table 1.2. It uses terminology to describe the different levels of uncertainty equivalent to the inferred, indicated and measured levels of confidence used by JORC (2004) to define the level of confidence in mineral resources and ore reserves (Figure 1.5).

1.4.4 Acceptance criteria (Chapter 9) The definition of acceptance criteria allows the stakeholders, normally management or regulators, to define the level of performance required of a slope against instability and/or failure. The criteria were initially expressed in terms of a factor of safety (FoS), which compared the slope capacity (resisting forces) with the driving forces acting on the slope (gravity and water pressures). More recently, the probability of failure (PoF), i.e. the probability that the FoS will be 1 or less, has been introduced as a statistically based criterion.

The level of acceptance in either term may vary, depending upon the importance of the slope. For example, pit slopes that have no major facilities (ramps, tunnel portals, crushers) on the wall or immediately behind the

1 9 Fundamentals of Slope Design

Table 1.1: Typical FoS and PoF acceptance criteria values

Acceptance criteriaa

FoS (min) FoS (min) PoF (max) Slope scale Consequences of failure (static) (dynamic) P[FoS < I] Bench Low-highb 1 .I NA 25-50% Inter-ramp Low 1 .I 5-1.2 1.0 25%

Moderate 1 .2 1.0 20% High 1.2-1.3 1 .I 10%

Moderate 1.3 1.05 10% High 1.3-1.5 1 .I 5%

Overall Low 1.2-1.3 1.0 15-20%

a: Needs to meet all acceptance criteria b: Semi-quantitatively evaluated, see Figure 13.9

crest might have an acceptable FoS of 1.2 or 1.3, or a PoF in the 10-15% range. For more critical slopes these values might be raised to 1.5 and less than 5%, respectively. Typical values are shown in Table 1.1.

Neither approach to stability assessment takes into account the consequences of instability or eventual failure or, conversely, the impacts of mitigative measures. Risk- based designs, which combine the PoF with the consequences (section 9.5), allow management to assess a slope design in terms of acceptance criteria that can easily incorporate risk in terms of safety and economic impacts, as well as societal views and legislated requirements.

1.4.5 Slope design methods (Chapter 10) The formulation of slope design criteria fundamentally involves analysis against the predicted failure modes that could affect the slope at bench, inter-ramp and overall scales. The level of stability is assessed and compared with the acceptance criteria nominated at the various levels by the owners and/or regulators for safety levels and economic risk.

The process of slope design starts with dividing the geotechnical model for the proposed pit area into geotechnical domains with similar geological, structural and material property characteristics. For each domain, potential failure modes are assessed and designs at the respective scales (bench, inter-ramp, overall) are based on the required acceptance levels (FoS or PoF) against instability.

Once domains have been defined, their characteristics can be used to formulate the basic design approach. This involves evaluating the critical factors that will determine the potential instability mode(s) against which the slope elements will be designed. A fundamental division relates to the rock properties in that, for stronger rocks, structure is likely to be the primary control, whereas for weaker rocks strength can be the controlling factor, even down to the bench scale.

Where structure is expected to be a controlling factor, the slope orientation may exert an influence on the design criteria. In this case a subdivision of a domain into design sectors is normally required, based upon kinematic considerations related to the potential for undercutting structures (planar) or combinations (wedges), or toppling on controlling features. The sectorisation can reflect controls at all levels, from bench scale, where fabric provides the main control for bench face angles, up to the overall slope, where particular major structures may be anticipated to influence a range of slope orientations with a domain.

For pits in weak rocks, where the rock mass strength is expected to be the controlling factor in slope designs, the design process commences with analyses to establish the overall and inter-ramp slope angle ranges that meet the acceptance criteria for stability. These angles are then translated down in scale into bench face configurations.

slope design depends on several factors, including: The type of stability analysis performed to support the

the project stage (available data); the scale of slope under consideration; the properties of the materials that will form the slopes.

The main analysis types used for design include:

kinematic analyses for bench designs in strong rock; limit equilibrium analysis applied to: + structurally controlled failures in bench and

+ inter-ramp and overall slopes where stability is inter-ramp design,

controlled by rock mass strength, with or without structural anisotropy;

numerical analyses for assessing failure modes and potential deformation levels in inter-ramp and overall slopes.

It should be stressed that stability analyses are tools that help formulate slope designs. The results must be

10 I Guidelines for Open Pit Slope Design evaluated in terms of other factors before they are finalised. These other factors include the mining methods and equipment that will be used to excavate the slopes, as well as the operators' capability to consistently implement such aspects as controlled blasting, surface water control and slope depressurisation.

planners as the basic slope design criteria. Only when ramps have been added does the overall slope angle become apparent. Thus, for initial mine design and evaluation work, an overall slope angle involving the inter-ramp angle, flattened by 2-3" to account for ramps, may be used for Whittle cone analyses and other similar studies. This is discussed further in section 11.2.

The inter-ramp angles are normally provided to mine

1.4.6 Design implementation (Chapter 11) Incorporating the slope design into the mine plan and implementing it requires clear understanding between all involved parties. This involves careful communication of the assumptions inherent in the design, plus the uncertainties and anticipated constraints on the construction of the slope. For the communication to be effective, the slope designer must understand the requirements and constraints influencing the other parties.

1.4.6.1 Mine planning (section 11.2) The requirements from a slope design into the mine planning process, including the level of accuracy, depend on the project stage. At the early stages of evaluation, inter-ramp or overall angles suffice but as the project advances into the feasibility study and detailed design, more information about bench configurations and operating considerations are required. This is discussed further in section 1.5 of this chapter.

It is important at all stages that the slope designer and mine planner understand such aspects as the basis of the design, the level of accuracy, constraints and terminology. It is critical that there be regular communication between the two parties and that the slope designs be fully documented.

1.4.6.2 Operational aspects Implementation of the slope designs typically requires the use of operating procedures that ensure minimum risk in terms of safety of personnel and recovery of reserves, including:

the consistent application of effective controlled blasting (section 11.3); excavation control and face scaling (section 11.4); artificial support (section 11.5).

These requirements should be a fundamental part of the design definition and must be within the capability of the operators who will implement the design.

It may also be necessary to consider the potential impact on production factors such as mining rate and excavation efficiency.

Where specific operating practices are required for implementing the slope design, it is critical that additional costs be incorporated into the budgets and recognised in terms of associated potential benefits to the overall revenue. For example, a mine superintendent will have little interest in implementing a controlled blasting program that allows steeper slopes unless corporate management recognises that the associated costs will be more than offset by reduced stripping costs or increased ore recovery.

The application of artificial support, either as part of the design or to stabilise a moving slope, has been in use for several decades. At a bench scale, rock bolts, mesh, shotcrete, straps and dowels are used to ensure stability or reduce degradation of the faces. Support also has a significant application where a pit slope is being mined through underground workings. These methods have largely been adapted from the underground mining environment, where the technology is well-developed. Cable bolts have been used successfully for inter-ramp slopes up to approximately 100 m in height. However, the 30 m practical length of cables is a major restriction and there have been several instances near the limit where the support has simply acted to tie together a larger mass, which subsequently failed. It is therefore important that any artificial support is carefully designed to the appropriate acceptance level, which will be partly dictated by the intended life of the supported slope and its overall importance.

1.4.7 Slope evaluation and monitoring (Chapter 12) The performance of the slope during and after excavation must be monitored for unexpected instability and/or the potential for significant instability. Monitoring programs, which must continue throughout the life of the slope and often into closure, typically involve:

slope performance assessment (section 12.1); slope displacement detection and warning (section 12.2); ground control management plans (section 12.3).

Assessment of slope performance focuses on validating the design model and ensuring that the operational methods for implementing the designs are appropriate and consistently applied.

It is important to validate the design model through geotechnical mapping and evaluating slope performance, particularly during the initial stages of mining. When the slope designs have been formulated on the basis of drill hole data alone, validation should include confirmation of the continuity of structures and the interpolation of geological data between holes.

I l1 Fundamentals of Slope Design Slope displacement monitoring is particularly

important where instability exists and is being managed as part of the ongoing operation. A monitoring program may still be required after completion of mining, particularly if the open pit void is to be used for other purposes such as industrial (e.g. waste landfill) or recreational, where the public will have access to or below the slopes.

define responsibilities and outline the monitoring procedures and trigger points for the initiation of specified remedial measures if movement/instability is detected. It should form an integral part of the slope engineering program and the basis for the design of any required remedial measures.

The ground control management plan for a pit should

1.4.8 Risk management (Chapter 13) Certain degrees of safety, economic and financial risk have always been implicit in mining operations. In open pit mines, slope instability is one of the major sources of risk, largely due to data uncertainties, as well as the generally modest levels of stability accepted for the designs.

field of soil mechanics, is the traditional and widely practised slope design criterion. The uncertainty and variability of geology and rock mass properties led to increasing use of probability techniques rather than the deterministic FoS method; these provide the advantage of a linear scale for interpretation of the risks associated with slope designs. However, the concept of probability in a geotechnical sense is not easily understood by non- technical persons.

With the increasing requirement for management to be involved in the decision-making process for slope designs, a requirement for the quantification of risks has developed. To address this, risk assessment and management processes have been applied to slope designs.

Risk assessment methods range from qualitative failure modes and effects analysis (FMEA) to detailed quantitative risk/consequence analysis, depending on the level of definition favoured by management, regulators or practitioners. A fundamental requirement of all methods is that management defines acceptable levels of corporate risk against which the slope designs can be assessed. The assessment process can then be operated retroactively, with a design reviewed in relation to the acceptance criteria. Alternatively, the slope designer can proactively design a slope to meet the corporate risk profile, and the potential impacts of design variations can be assessed in terms of economic impact.

management with quantitative information for:

Factor of safety determination, which originated in the

The objective of risk-based design is to provide

defining acceptable risks in terms of safety and economics;

assessing relative risk levels for different slope configurations; benchmarking risks against industry norms and the corporate mission statement.

The risk-based design approach has been successfully applied to the design of slopes in several large open pit mines.

1.4.9 Closure (Chapter 14) Current legislation in many jurisdictions requires mines to be designed with a view to closure and that a closure plan be in place before a mining permit is issued. Discussing the environmental aspects of closure as they relate to factors such as pit lake chemistry is outside the scope of this book, but is a critical consideration in closure.

In open pits, the closure plan should include long-term stability, particularly if the public is to have direct access to the area, for example as a recreational lake. Alternatively, if a pit lake is to be formed with outflow through a controlled surface channel, the potential for slope failures to cause waves that would overtop the channel and create a downstream flood must be considered. Other factors include aesthetics, particularly where the pit is located close to populated areas.

Stability during the closure process, for example while the pit lake is forming, could also be an issue that requires consideration and continued monitoring, particularly if slope stability has been achieved through an active slope depressurisation program. In this case, rapid repressurisation of the slopes relative to the formation of the lake could result in wall instability. This can generally be prevented by maintaining the depressurisation system until equilibrium is established.

continue through the initial closure and in many cases on a continuing basis post closure, particularly if the public has access to the open pit area.

Monitoring of slope stability can be expected to

1.5 Design requirements by project I eve I Guidelines for the typical level of investigation and design effort expected at various stages of project development are presented in this section. It should be noted that the actual required effort can vary significantly, depending on the degree of complexity in the geotechnical model and the level of risk assurance required by the owner (sections 1.4.3. and 1.4.4).

1.5.1 Project development There are six main levels in the development and execution of a mining project at which slope design input is required. These are:

12 I Guidelines for Open Pit Slope Design conceptual study (Level 1); pre-feasibility (Level 2); feasibility (Level 3); design and construction (Level 4); operations (Level 5); closure (Level 6).

The mine planning requirements at these levels, which are discussed in detail in section 11.2, can be summarised as follows.

At the conceptual study level, various mining methods are assessed. At this early stage the viability of open pit mining may be based on judgment or experience in similar environments. Cost estimates and slope designs are at the order of magnitude level.

At the pre-feasibility level, preliminary slope designs are required to determine if the ore body is technically and economically viable to mine so that reserves and associated mining method can be defined.

picture of the anticipated costs of mine development and operation. At the completion of the study alternative interpretations may be possible, but in the view of a competent person these would be unlikely to affect the potential economic viability of the project. To achieve this level of accuracy, overall slope designs in the order of f5 are necessary.

At the design and construction level, the ore body has been shown to be potentially economic and financing has been secured for production. Confidence in the pit slope design should be increased at this stage, particularly for open pits with marginal rates of return. This stage may be skipped and initial mining may be based upon the feasibility level slope designs.

During the operations level, pit slope optimisation may be possible, based on additional data collected from the pit walls and incorporating operating experience with slope performance to refine the geotechnical model and provide revised slope design criteria for future cutbacks.

Increasingly, the slope designs must also address long-term stability associated with landforms required at closure and potential uses of the open pit void. Closure designs should be established during the operating phase, when mine staff will have experience of slope performance that may not be available post closure.

The feasibility level is typically used to establish a clear

1.5.2 Study requirements Most mining companies have specific requirements for the level of effort required to achieve the mine design at various project levels. Table 1.2 presents a summary of suggested levels of effort from the Level 1 conceptual stage through to operations (Level 5). Mine closure (Level 6) is addressed in Chapter 14. Requirements vary between

companies and even between projects, therefore the table is only a guide.

The responsibility for collecting, compiling and analysing the data to establish the slope designs depends on the in-house capabilities of the mining company and on the project level. In larger companies the initial level evaluations and slope management in operating mines are typically performed by in-house staff. For larger studies (Level 3 ) , and for most work in smaller mines, consultants play a significant role. There is an increasing requirement for independent review at the pre-feasibility and subsequent project levels (discussed further in section 1.6).

1.6 Review 1.6.1 Overview Slope designs are increasingly subject to formal reviews, both prior to commencement of mining and during the operating phase. These reviews, which may be undertaken by in-house specialists, an external review consultant or a board of specialists, are conducted for a number of reasons. At the feasibility and mine financing stages, a review gives management and potential financiers confirmation of the viability of the proposed project. At the operating stage a review, which may involve a board addressing all geotechnical and hydrogeological aspects of the mine, gives management an independent assessment and additional confidence in the designs and the implementation procedures.

suggested the following guidelines. If a board is to be used, Hoek and Imrie (1995)

A Review Board should be composed of a small number of internationally recognised authorities in fields relevant to the principal problems encountered on the mine. The purpose of the Board should be to provide an objective, balanced and impartial view of the overall geotechnical activities on a mine. The Board should not be used as a substitute for normal consulting services since members do not have the time to acquire all the detailed knowledge necessary to provide direct consulting opinions.

The function of the Board should be to act as the technical review agency for the Mine Management. Ideally, a Board should ask the geotechnical team and associated mine planning staff have you considered this alternative? rather than be asked to respond to a request such as please provide recommendations on a safe slope angle.

very small (2 to 4 members) and are carefully chosen to cover each of the major disciplines involved in the

In my experience, the most effective Boards are

Table 1.2: Levels of geotechnical effort by project stage

PROJECT STAGE

Project level status Conceptual Pre-feasibility Feasibility

Design and Construction Operations

Geotechnical level status Level 1 Level 2 Level 3 Level 4 Level 5

Geological model Regional literature; advanced exploration mapping and core logging; database established; initial country rock model

Aerial photos and initial ground proofing

Mine scale outcrop mapping and core logging, enhancement of geological database; initial 3D geological model

lnfill drilling and Targeted drilling and mapping; refinement of geological database and 3D model

Ongoing pit mapping and drilling; further refinement of geological database and 3D model

Structural mapping on all pit benches; further refinement of 3D model

Structural mapping on all pit benches; further refinement of fabric data and structural domains

mapping, further enhancement of geological database and 3D model

Structural model (major features)

Mine scale outcrop mapping; targeted oriented drilling; initial structural model

Trench mapping; infill oriented drilling; 3D structural model

Refined interpretation of 3D structural model

Structural model (fabric)

Regional outcrop mapping

Mine scale outcrop mapping; targeted oriented drilling; database established; initial stereographic assessment of fabric data; initial structural domains established

Mine scale airlift, pumping and packer testing to establish initial hydrogeological parameters; initial hydrogeological database and model established

lnfill trench mapping and oriented drilling; enhancement of database; advanced stereographic assessment of fabric data; confirmation of structural domains

Targeted pumping and airlift testing; piezometer installation ; enhancement of hydrogeological database and 3D model; initial assessment of depressurisation and dewatering requirements

Targeted drilling and detailed sampling and laboratory testing; enhancement of database; detailed assessment and establishment of geotechnical units for 3D geotechnical model

Targeted sampling and laboratory testing; enhancement of database; detailed assessment and establishment of defect strengths within structural domains

Refined interpretation of fabric data and structural domains

Hydrogeological model

Regional groundwater survey

Installation of piezometers and dewatering wells; refinement of hydrogeological database, 3D model, depressurisation and dewatering reauirements

Ongoing management of piezometer and dewatering well network; continued refinement of hydrogeological database and 3D model

Intact rock strength

Literature values supplemented by index tests on core from geological drilling

Index and laboratory testing on samples selected from targeted mine scale drilling; database established; initial assessment of lithological domains

lnfill drilling, sampling and laboratory testing; refinement of database and 3D geotechnical model

Ongoing maintenance of database and 3D geotechnical model

Strength of structural defects

Literature values supplemented by index tests on core from geological drilling

Laboratory direct shear tests of saw cut and defect samples selected from targeted mine scale drill holes and outcrops; database established ; assessment of defect strength within initial structural domains

Assessment and compilation of initial mine scale geotechnical data; preparation of initial geotechnical database and 3D model

Selected sampling and laboratory testing and refinement of database

Ongoing maintenance of database

Geotechnical characterisation

Pertinent regional information; geotechnical assessment of advanced exploration data

Ongoing assessment and compilation of all new mine scale geotechnical data; enhancement of geotechnical database and 3D model

Refinement of geotechnical database and 3D model

Ongoing maintenance of geotechnical database and 3D model

14 I Guidelines for Open Pit Slope Design project. For example, in the case of a large open pit mine, the board members could be:

A geologist or engineering geologist with experience in the type of geological conditions that exist on the site. This is particularly important when unusual or difficult geological conditions such as very weak altered rocks or major faults are likely to be encountered. A rock engineering specialist with experience in rock slope stability problems in the context of open pit mining. A mine planning engineer with a sound understanding of rock mechanics and a strong background in scheduling, blasting and mining equipment characteristics.

Recent experience has suggested that a hydrogeologist can also play an invaluable role where large open pit slopes are concerned, since slope depressurisation is usually required.

In large projects, it is important that the reviewers be involved from the early stages and be given regular updates on progress and changes. This should avoid complications during final presentation of the design.

1.6.2 Review levels There are three levels at which reviews are commonly performed.

1 Review at discussion level - at the discussion level the reviewer is not provided with all the relevant reports and data required for an independent assessment or independent opinion. Generally, only selective information is presented, often in meeting presentation form, and there is insufficient time to absorb and digest all the pertinent information and develop a thorough understanding of all aspects relating to the design, construction and operation. The reviewer relies on information selected by the presenter and substantially on the presenters observations, interpretation and conclusions. Review level - at this level the reviewer generally examines only key documents and carries out at least reasonableness of results checks on key analyses, design values and conclusions. The reviewer generally relies on representations made by key project personnel, provided the results and representations appear reasonable and consistent with what an experienced reviewer would expect. This level of review

2

is appropriate for all levels of project development beyond the conceptual (Level 1).

3. Audit level - an audit is a high-level review of all pertinent data and analyses in sufficient detail for an independent opinion on the general principles of design, construction and operations, and on the validity and accuracy of the key elements of the design analyses, construction control and operating methods. This level of review is often appropriate at the feasibility (Level 3) stage of investigation.

1.6.3 Geotechnically competent person Unlike the codes in use in different countries to support ore reserve estimates (JORC in Australia, 43-101 in Canada), there is no standard definition of geotechnical competence to assess and sign off slope designs for use in reserve estimate pits. However, for slope designs it is anticipated that a definition of a geotechnically competent person and/ or reviewer for slope designs will be established in the near future to complement the equivalent standards for the presentation of ore reserves. Until such a definition becomes available, the basic criteria could include:

an appropriate graduate degree in engineering or a related earth science; a minimum of 10 years post-graduate experience in pit slope geotechnical design and implementation; an appropriate professional registration.

1.7 Conclusion The following chapters expand on the design of large open pit slopes within the general framework outlined above. It must be a basic design premise that a slope design addresses the requirements of all stakeholders, from the owners through the operators to the regulators.

In delivering a design, technical soundness is the foundation. The slope designer must build on this, responding to the varying conditions in each phase of the mines life. The safety of personnel and equipment is of paramount importance in all phases, and acceptable risk levels must be carefully assessed and incorporated into the designs.

By presenting the slope designs in a manner that enables mine personnel, from executives to operators, to fully understand the basis and shortcomings of the designs, practitioners provide the means of discerning the risks associated with deviation from those designs. With greater understanding, better and safer decisions can be made.

Front MatterTable of Contents1. Fundamentals of Slope Design1.1 Introduction1.2 Pit Slope Designs1.2.1 Safety/Social Factors1.2.2 Economic Factors1.2.3 Environmental and Regulatory Factors

1.3 Terminology of Slope Design1.3.1 Slope Configurations1.3.2 Instability1.3.3 Rockfall

1.4 Formulation of Slope Designs1.4.1 Introduction1.4.2 Geotechnical Model1.4.2.1 Geological Model1.4.2.2 Structural Model1.4.2.3 Rock Mass Model1.4.2.4 Hydrogeology Model

1.4.3 Data Uncertainty1.4.4 Acceptance Criteria1.4.5 Slope Design Methods1.4.6 Design Implementation1.4.6.1 Mine Planning1.4.6.2 Operational Aspects

1.4.7 Slope Evaluation and Monitoring1.4.8 Risk Management1.4.9 Closure

1.5 Design Requirements by Project Level1.5.1 Project Development1.5.2 Study Requirements

1.6 Review1.6.1 Overview1.6.2 Review Levels1.6.3 Geotechnically Competent Person

1.7 Conclusion

AppendicesTerminology and DefinitionsReferencesIndex