Mechanisms of instability & failure in the walls of open pit mines
By Malcolm Bridges - Principal Geomechanics Geologist

There is a common belief that instability and failures in an open pit's wall occur predominantly by sliding on geological structures or on shear surfaces through a weak rock or soil mass. Toppling and ravelling mechanisms are regarded as minor and operate at only small scales. Management of instability in pit walls is often based on the principles of sliding.

But, evidence from instability and failures in pit walls does not appear to support sliding mechanisms, except for rare and special circumstances. Rather, evidence appears to support a predominance of toppling, ravelling and creep-subsidence mechanisms. Traditional methods of analysis may be inappropriate and their application may be unreliable.

Reappraisal of the evidence

Hoek and Bray, in their widely-accepted book “Rock Slope Engineering”, first published in 1974, identified four basic mechanisms of failure:

1) sliding on geological structures, subdivided into plane sliding and wedge sliding; 

2) sliding on a shear failure surface generated through a weak rock mass;

3) toppling, influenced by steep-dipping geologic structures; and 

4) ravelling, where small pieces of rock fall individually to form a scree pile.

The “Pit Slope Manual”, published in Canada in 1977, set out a similar classification. Sliding mechanisms, both on geologic structures and through rock materials, included complex and multi-plane modes. Additionally, “block flow sliding”, typified by landslides, was identified.

Recently, the book “Rock Slope Stability” by Kliche (1999) set out a similar classification of mechanisms of failure, concentrated on sliding mechanisms of failure.

In contrast, evidence from experience in open pits and cuttings that has accumulated in published papers, articles and personal experience indicates that toppling-ravelling-creep-subsidence modes of instability and failure have been dominant. There appears to be an emergent consensus on this amongst geomechanics practitioners, judged by conclusions in published papers and articles.

At the least, this evidence demonstrates that instability and failure are complex phenomena, occurring within complex geological conditions.

Two examples serve to illustrate different types of complex failure, in different geological conditions. Each are common types of failure for the conditions.

One example (Figure 1) is a failure within the upper part of a pit wall where rock material was weathered – effectively a soil-like material. Failing material was intensely cracked and dilated. It toppled outward (toward the pit) at the top and subsided down-slope.

Figure 1 - Failure of a section of a pit wall in a weathered rock mass.

The other example (Figure 2) is a failure within a fractured rock mass where the failed material appeared to have dilated on exposure, broken-down and collapsed. Geologic structures appeared to have influenced the process.

Figure 2 - Failure of a section of a pit wall in a strong rock mass (adapted from the photograph on the cover of the book by Kliche, 1999).

Experience from observed failures

The proper basis for development of theory and analytical methods is the observed behaviour of pit walls and cuttings. Naturally, more can be learned from a failure than from a stable wall or cutting.

There is now a range of published examples of failures, along with many more unpublished examples for which some useful information is available. For balance, there is a much larger number of cases where no failure occurred.

Following sections describe the apparent development of instability and failure in pit walls that has emerged from this experience.

Initial dilation and cracking

Typically, the first visible sign of a potential failure is the development of extension cracks.

They preferentially develop at the surface behind a pit wall's crest (Figure 3) and on benches. Initial opening is sub-horizontal, although there may be a slight downward component on the pit side. Cracks tend to be located near the mid-length of longer pit walls, and clustered in arcuate-shaped zones. Typically, they are aligned with pervasive geological structures, such as bedding, foliation or veins, where they are present. They have been reported to have developed out to many tens of metres behind a pit's crest.

As well, there are reports of cracking, bulging, or distress being observed or measured in the vicinity of the toe of an eventual failure. Such bulging, or at least sub-horizontal displacement, appears to be an early stage of dilation and subsidence in a pit wall.

Progression of instability

In some instances, instability continues to develop. Cracks continue to develop, and there is increasing displacement and progression of instability over larger areas of a pit wall. Small failures may develop over a pitwall, especially at the crests of batters.

At this stage, there tends to be a divergent style of instability for the various types of geological conditions, such as weak surficial sediments, weathered and altered rock materials, and strong unweathered or unaltered rock materials. Many common elements remain, though.

Displacements measured at the crest and upper areas of instability in a pit wall generally show a transition from principally sub-horizontal to down-slope. Displacements are greater at the crest than at the toe. Magnitudes and rates of displacements may be influenced by rainfall, dewatering of groundwater and progressive exposure of the underlying pit wall.

The combination of visible cracking and measured displacements show an overall outward rotation of the mass of unstable material. Especially, the crest of a failure rotates outward from the pit wall (toward the pit). Measurements of displacements within an unstable pit wall, by tiltmeters for example, show an outward rotation of the unstable mass. This may be regarded as toppling.

Instability may progress over time, in response to down-cutting of the pit wall. Initial small-scale ravelling or sloughing failures – mostly at a bench scale – may extend through the area of instability.

In some instances, typically in a weak rock material, a surficial zone of failed material may creep down-slope, more or less at a steady rate, with little or no visible sign of cracking or break-down.

Within a stronger rock mass, cracking tends to preferentially develop along natural fractures and faults, or other geologic structures. Dilation, displacements and ravelling may even be concentrated at one or more extensive geologic structures, particularly where they occur at an acute angle to a pit wall.

Final displacements and failure

A final progression to failure may take any of several different forms, but there are still common underlying elements.

In some instances, creep-subsidence may continue. Rates of displacement may increase over time, although there may be periods of accelerating and steady displacement. Outwardly, the overall bench configuration of a pit wall may be maintained, although there may be bench-scale ravelling. There may not even be an eventual collapse of the pit wall.

In other instances, an unstable pit wall may slump, breaking-down along cracks and geologic structures, but remaining recognisably intact and (meta) stable for a time (Figure 1). This process may extend to a complete progressive break-down of the failing mass, resulting in a scree pile against the pit wall (Figure 4).

Typically, though, increasing rates of cracking and displacement lead to a final massive collapse of a pit wall (Figures 2, 5 & 6).

There are reports of observations of a few such failures, which was possible when a collapse was predicted from monitoring:

• “…. blocks …. could be seen to topple and rapidly disintegrate.”

• "Witnesses …. said the top of the east highwall leaned out toward the middle of the 4840 level and suddenly crumbled under the weight of the rock.”

• “…. large cracks were observed forming in the face at the eastern end. The failure occurred in 5 minutes ….”

A series of photographs of the failure of the Chuquicamata mine's pit wall in 1969 (which was also filmed) showed the break-down - or rather the ensuing dust - of the collapsing mass (Figure 5).

Back-surfaces of collapses, where they could be seen, are typically jagged to undulating, in an approximate arcuate profile in plan and section (Figures 2 & 6). In some instances, back-surfaces are defined in part by geologic structures.

Figure 3 - Extension cracks, representing relief dilation, developed behind the crest of a pit wall during the early stage of excavation of the pit.

Figure 4 - Extensive ravelling, with progressive cracking and break-down of wall-rock, forming a scree pile.

Figure 5 - Failure of the Chuquicamata mine's pit wall in 1969. Its aftermath was similar to that shown in Figure 6.  

Figure 6 - Aftermath of a massive failure of a pit wall. Note the exposed jagged failure surface.

Conclusions on the mechanism of failure

Instability and failure that are induced in a pit wall by underlying subsidence (eg, highwall mining, stoping, caving) have a similar style as that in a pit wall unaffected by such subsidence. There appears to be common elements in the processes and a common mechanism of instability and failure.

Generally, instability within a pit wall appears to begin where differential subsidence develops from relief dilation. This might be centred where there is a zone of locally weaker, more deformable material, or higher porewater pressure. As a pit wall is cut-down, exposing the more-deformable or pressured material, its confinement and then underlying support are reduced, inducing dilation and subsidence. The first signs of localised, or differential subsidence may be bulging of the pit wall at the bottom of the subsiding section of the wall and cracking at the top of the subsiding section of the wall. As cut-down of the pit wall progresses, differential subsidence may extend deeper if the more-deformable conditions persist, or steady differential subsidence may continue within a confined area of the pit wall. Eventually, differential subsidence within a pit wall may progress to cracking, toppling, ravelling and down-slope creep. Ultimately, it may lead to an extensive failure of the pit wall.

Interpreted this way, the evidence indicates that a single mechanism operates where instability and failure develop within a pit wall, in nearly all geologic conditions and mining circumstances, including where there is underlying subsidence. The range of observed modes of failure arise from the differing geologic conditions and circumstances.

Implications for analysis and design

Instability and failures are inherently three dimensional. They develop in roughly arcuate-shaped zones. Displacements within an unstable part of a pit wall may be greater at the top than the bottom, greater down the centre-line than down the edges and greater at the surface than within a pit wall. Displacements may be influenced by the excavation of underlying benches. Instability and failure cannot therefore be validly represented and modelled in two dimensions.

The progress from initial dilation and cracking to failure is path dependent. Valid modelling must follow the path from initial relief, through progressive displacements, to yield and to failure. Simply applying gravity or an in situ stress to an excavated pit wall does not represent an appropriate stress-displacement path, and results from modelling may be unreliable.

Limit equilibrium modelling based on circular or planar sliding does not incorporate observed loading, the nature of materials and the style of failure. Results may be unreliable or perhaps even misleading.

Numerical modelling offers the best potential to incorporate three-dimensionality, complex material behaviour, path-dependent deformation with progressive yield, and large plastic displacements at yield and failure. There are examples of such applications of numerical modelling, but appropriately complex models are generally beyond the current understanding of the behaviour of rock materials in pit walls. General validation of numerical modelling for this style of behaviour of pit walls remains to be done.

At this stage, expert professional judgement offers the most comprehensive and reliable form of analysis of pit walls for design and operations. An expert follows a logical, reproducible process, based on an understanding of the mechanism of instability and failure, and a range of precedents. Precedents are drawn from published studies and each expert's documented experience of stable and unstable pit walls.

An assessment of the geological conditions at a site follows hierarchical steps from the geological setting (eg, meta-sediments, meta-volcanics, sediments, coal measures, epithermal gold, porphyry copper) to the specific geological and geomechanical conditions (eg, rock units, mineralogy, geologic structures, stress) and overlays of weathering, alteration, groundwater and seismicity. In this way, experts relate design and excavated slope angles to ground conditions, a sequence of excavation, time and potential stabilisation. Estimates of slope angles include the (qualitative) probability of failure of a pit wall and the reliability of estimates. Numerical modelling may be applied to assess styles of behaviour and the influence of site-specific factors.

Effective design and operation of a pit wall requires a cooperative relationship between technical expert advisors and mine managers, regardless of whether the advisors are employees or consultants.

In future years, as more experience is documented, there is likely to be a greater reliance by experts on 'design rules' and numerical methods.

In early 2005, Mal Dorricott from the AMC backfill team visited Cerro de Pasco in the high Andes of Peru, some 200 km north east of the capital Lima, to undertake a review of the existing hydraulic backfilling operations for Volcan Compania Minera SAA, a Peruvian company. The operation comprises the Raul Rojas open pit, the Cerro de Pasco underground mine and the Paragsha concentrator, which treats about 2.5 Mtpa of high-grade lead/zinc ore.

The mining methods are overcut & fill, room & pillar, undercut & fill and sub-level stoping. The underground inspection involved many kilometres walking that ended at the lowest level of the mine, some 550m below surface but still about 4,000m above sea level.

Volcan were keen to investigate the benefits of changing to a pastefill system, so Mal also inspected the mothballed pastefill plant at Glencore's Yauliyacu mine near La Oroya, to assess its potential suitability for Cerro de Pasco. Located at an altitude of 4,800m the almost unused pastefill plant comprised a deep cone thickener and PD pump to deliver cemented pastefill to the mine. The plant was built and operated briefly in 2001, but closed due to low metal prices and the high cement content required to provide a trafficable surface for the mechanised cut & fill method. 

Hydraulic Backfill Plant at Cerro de Pasco

AMC concluded that the existing hydraulic backfill system is serving the mine well and with some modifications, can continue to do so. The scattered nature of the stoping blocks, the small size of most pours and the rather torturous reticulation routes make pastefill impractical and unlikely to deliver significant benefits.

Mal Dorricott
Principal Mining Engineer
mdorricott@amcconsultants.com.au

Message from the Managing Director

 Peter McCarthy

AMC pays for professional society memberships for its entire staff. It does so because the benefits of having professionally aware, outward-looking employees far exceed the cost of membership. If this is true for consultancy employees who may work on a dozen mining projects in the course of a year, how much more important must it be for engineers and geoscientists working at a single, remote mine? Technical interchange and exposure to their professional peers elsewhere is essential for such people. 

I owe my allegiance to the AusIMM, which I joined as a student 35 years ago. In those days membership was expected and the local management looked for my presence at branch meetings. In return, I was able to socialise with senior managers and exchange views with them outside the workplace. We all gained from technical presentations.

AMC does not differentiate between professional societies. We think it is important to be a member of an engineering, geological or other society based in Australia or elsewhere and we support staff accordingly. Most choose the AusIMM.

For employees to benefit most, senior managers must participate in and promote professional societies. These societies represent the mining professionals, as distinct from the mining companies. Sometimes professionals may have views on local developments or on the national interest that are at odds with those of their employers, the global mining companies. The professional society provides a venue for making such views heard and, if senior managers are members, professionals can make a difference. Similarly, the society may be more trusted by the local community than corporate management is trusted, and may be able to build bridges and gain support for developments.

This, then, is a plea to senior managers in mining operations to play an active part in their professional society, and to encourage employees to do likewise. At least offer to pay their subs!

Peter McCarthy
pmccarthy@amcconsultants.com.au

AMC principal consultant addresses international accounting standards board

AMC is a strong supporter of high standards of practice in the mining industry. The JORC and VALMIN Codes are recognised as world leaders in their fields, and AMC people have taken active roles in developing and maintaining the codes. 

In a recent important development, Pat Stephenson, Principal Geologist with AMC and past-Chairman of JORC, was invited by the International Accounting Standards Board (IASB) to address its Board in London on the issue of existing industry-based national and international standards for the reporting of mineral resources and ore reserves. The IASB is in the process of developing a new Accounting Standard for the extractive industries, which include the mining industry, and reporting requirements will be a key part of the new Standard. 

Niall Weatherstone, member of the UK Ore Reserves Committee, supported Pat from London, who presented by video link from Melbourne. A representative of the oil and gas industry gave a presentation on reporting standards in that industry. 

A webcast of the meeting can be viewed at www.iasb.org/meetings/webcasts.asp. The meeting was held on 19 April 2005, and was the first of several held that day by the IASB. A copy of the PowerPoint presentations can be obtained from www.jorc.org/pdf/iasbsession.pdf

Pat Stephenson
Principal Geologist
pstephenson@amcconsultants.com.au