Wessel Oo Paper

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1Slope Stability Santiago Chile, November 2009

Rock mass damage in hard rockopen pit mine slopes

In the design of hard rock slopes, characterising the strength behaviour of the rock mass is still one of the major deficiencies in

slope design practice. Conventional approaches generally ignore the role of the brittle fracture process and the stress regime in

open pit slope b ehaviour and substitute them with duc tile, shear models with shear parameters taking only gravitational loading

into account. A better understanding of rock mass behaviour can only be achieved through an understanding of the brittle fracture

process taking place in the rock slope. This paper discusses some of the concepts related to this brittle fracture process in light of

the experience gained from microseismic monitoring of slopes. The use of seismic monitoring in slopes is invaluable in advancing

our knowledge about the true behaviour of brittle rock slopes and forms a valuable part of the suite of monitoring methods for

managing the risk of slope failure.

Abstract

J. Wesseloo and P.M. Dight

Australian Centre for

Geomechanics

The University of Western

Australia

INTRODUCTION

The greatest risk to a successful open pit is that the ore is less than stated. The second greatest risk is the geotechnical risk associatedwith slope failure. With a continued drive to slope optimization, steepening and a long term trend in the increase in the economic depthof open pit mines, the geomechanical considerations for slope design and the uncertainties associated with it, forms a critically importantcomponent of the uncertainty in the performance of the slope and, ultimately, the risk in not achieving the mine plan, and hence notrecovering the reserve.

In 2000, Hoek et al. (1) considered that the determination of the rock mass strength was a major deficiency in rock slope designpractice. This sentiment was echoed by Stacey (2) when he concluded that conventional approaches for the evaluation of the stability

of rock slopes in high stress, hard rock environments are of questionable validity. The reason is that conventional approachesgenerally ignore both the role of the brittle fracture process and the stress regime in open pit slope behaviour and substitute it wi thinappropriate ductile, shear models with equally inappropriate shear parameters taking only gravitational loading into account.

In general literature on rock slope stability it is often stated that stresses in slopes are low and in design practice, sometimes, onlygravitational stresses are taken into account. We agree with Sjberg (3) when he calls this a dangerous view, as it only applies to theregions close to the pit wall and at the toe of small-scale slopes (Sjberg, 3). The stresses and the stress changes during the miningprocess are much more complex with zones of comparatively low and high stresses. Slope failure in hard rock slopes are often onlythrough the fracturing of rock bridges to create kinematically feasible failure modes. The driver behind this breakdown of rock bridges isthe change in the stress behind and below the slope resulting from mining.

STRESSES IN ROCK SLOPES

Generally around the world, near surface horizontal in situ stresses are higher than the vertical stress with the horizontal stress rotating

and concentrating around the pit bottom and toe of the slope being the larger stresses. What is often regarded as a destressed slopemay in fact be a slope with comparatively low 3, but comparatively high 2 (or circumferential stress) and 1. The influence of 2 isimportant under bi-axial loading conditions and it is therefore important to develop some appreciation of the true three dimensional stressfield around the open pit. This can only be achieved by full three dimensional analyses.

An excavation process in rock involves a disruption and redistribution of the in situ stress field. For underground openings this usuallyinvolves relatively small exposed surface areas and volumes, where there is continued three dimensional confinement of the rock massclose to the excavation. Mining of open pits, in contrast, exposes very extensive surfaces, and the rock adjacent to the pit walls thereforebecomes unconfined in the direction normal to the slope face over large areas. The rock mass in the zone of low confinement adjacentto the slope face will be subjected to varying stress changes as mining progresses and to the effect of the excavation processes.Understanding this stress path is critical to understanding the fracture processes in open pits.

Stacey (2) refers to several examples of slopes where stress conditions appears to have played a major part in the development ofinstability. He also pointed out that there is considerable evidence to show that stress is already playing an important role in the behaviour

of slopes with low heights that are well within the range of slope heights in the existing experience base and provides a summary of severalsuch failures.


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2Santiago Chile, November 2009 Slope Stability

Some illustrations of the rock mass response to stress change is shown by microseimic monitoring. Lynch and Malovichko (4) showa correlation between the rate of seismicity with the rate of mining (Figure 1). This seismicity is created in the rock mass by fracturegrowth taking place as the rock mass adjusts to its new stress state.

Figure 1 Comparison Between Seismic Activity and Mining Rate at Navachab Mine (Modified After Lynch and Malovichko, 4)

The microseismic activity is attributed to fracture growth as a result of pseudo-static stress change. This conclusion is supported byLynch and Malovichko (4) who observed an increase in the microseismic activity, about two days after a blast at the Navachab Mine.They also reported the rapid decrease in seismicity after mucking at the toe of the slope at Union Reefs, an Australian mine.

McCallum (5) also showed a change in the clustering of seismicity recorded within the array, with the migration of the mining away fromthe array, at Sunrise Dam. This change in seismic clustering is shown in Figure 2. The three dimensional views in this figure all show thefinal pit shape. This obscures the significance of the data and the illustration showing the relationship between the mining activity and theseismicity is provided for further clarity.

Figure 2 Change in Seismic Event Location with Mining at Sunrise Dam


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Wesseloo et al. (6) shows a migration of the seismic cluster towards areas of higher stress change at an Australian mine (Mine A), asmining take place. Figure 3 shows a northward migration of the seismicity was evident as a cutback was mined to the north of the installedseismic array.

Figure 3 Spatio-Temporal Distribution of Seismicity at Mine A (Wesseloo et al. 6)

a) March 2005 - June 2005 a) June 2005 - February 2006

ROCK MASS DAMAGE IN THE SLOPE

In quantifying the rock mass quality, the quantification of rock bridge geometry is generally ignored which leads to an ignorance of thetrue interaction between the intact rock and its discontinuities (Dight and Baczynski, 7). Figure 4 provides an illustration of two fracturemaps with identical fracture statistics, each with a different fracture termination style (Brown, 8).

Figure 4 The Influence of Fracture Network Parameters on Rock Mass Fabric

Imposing stress changes due to the mining of a slope on the rock mass, illustrated in Case A, will result in extensional fracturegrowth through the rock bridges while the rock mass in Case B will respond by differential rock block sliding and rotation. Generallyaccepted methods for the quantification of rock mass behaviour do not recognise the difference between the two rock masses. To developan understanding of rock mass behaviour, one needs to develop an understanding of both the mechanisms of deformation and the

mechanisms of damage that governs its behaviour and ultimately its stability.

To date, the largest part of our understanding of rock slope behaviour is derived from post mortem investigations after the total collapsehas occurred. If the fracture mechanism leading up to failure is poorly understood, and/or the forensic examination assumes the wrongbehaviour, an outcome may well be a non-conservative interpretation (i.e. an upper bound solution).

We often talk about shearing of the rock mass while the processes governing the stability of the rock mass are seldom shearing alone.Tensile failure of intact rock bridges, tensile failure due to bending, block rotation and flexure all are lumped together into what is calledrock mass shearing for which we seek to find appropriate parameters for shear strength rock models. The shearing behaviour observedin the slope is often the last stages of failure development.

Eberhardt et al. (9) describes these processes as follows: massive rock slope failure processes are largely driven by the initiationand propagation of brittle tensile fractures driven by extensional strain, which interact with natural pre-existing discontinuities to

eventually form basal and internal shear planes. Shear failure only becomes a factor once enough tensile fracture damage wasincurred to allow mobilization.


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One of the conclusions made by Wesseloo et al. (6) from the seismic monitoring data at Mine A was that most of the damage detectedwas the result of extensional cracking. These cracks are expected to develop in response to the change in stress resulting from the openpit mining. Such cracking would generally be expected to occur below the base of the pit and in the pit wall. Below the base of the pit thesefractures are expected to be low angle structures while the fracturing in the wall is expected to be at a steeper orientation. The orientationof newly generated fractures on the local scale will be determined by the local disruption of the stress field.

Numerical analyses were performed to determine the stress state at the location of each of the seismic events at the time of occurrence

for the data from Mine A. The stress state at each of the seismic locations at the date of its occurrence is shown in Figure 5. Fracturing inthe slope occurs at stress levels well below the estimated HoekBrown rock mass strength. The relationship between the estimated stressat fracturing and the HoekBrown strength estimate is similar to the experience from underground tunnelling presented by Martin (10).

Figure 5 Estimated Stress State at Occurrence of Seismic Event

Kaiser et al. (11) and Diederichs (12) presented a conceptual model of rock mass strength behaviour consisting of a damage thresholdshear failure and spalling limits (Figure 6). The in situ strength behaviour of the rock mass will depend on the mode of failure, which inturn depends on the stress regime.

The stress at which damage occurred at Mine A is lower than what these theories predict. This is a result of the influence of pre-existingdiscontinuities. The effect of the discontinuities is to disrupt the stress flow and create localised stress concentration at the edges in rock

bridges. Locally, therefore, the stress is amplified and fracture growth occurs when this level exceeds the rock strength. When this occurs,

a disruption of the stress field will also occur locally and this may result in further fracture growth in other areas. At the early stages of

damage accumulation, this process will be a stable process, although only small changes in the stress state may be required to create

new fractures or extend existing ones. The resulting fracture pattern may then appear random to some extent, depending on the temporallocation of the events.

1

1

2

2

3

3

4

4

Intact Rock

Rock Mass D = 0

Rock Mass D = 1

Data

Figure 6 Conceptual Strength Envelopes (Kaiser et al., 11; Diederichs et al., 13)


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The upper shear strength envelope shown in Figure 6 is of lesser importance in rock slopes and may only come into play for small rock

bridges preventing failure of large daylighting structures in large slopes. The spalling limit also has little application in slope stability. In

the comparatively low stress environment of slopes, stability is only compromised when damage accumulation in key locations results in

kinematically feasible mechanisms on low strength structures.

For slope stability in hard rock slopes the UCS, triaxial strength and stiffness of the intact rock, and rock mass is of secondary

importance, while the tensile strength of intact rock and the geometry of rock bridging and the geometry of large structures are of

primary importance.

The conceptual failure development processes described previously can, from a continuum rock mass perspective, be described

as a process of cohesion loss and frictional increase with increased damage. These concepts have been incorporated in material

models like the Cohesion-Softening-Friction-Hardening model where cohesion and friction are parameters dependent on the shear

strain (Hajiabdolmajid et al., 14, 15; Hajiabdolmajid and Kaiser, 16, 17). Models like these, however, still rely on continuum plasticity to

represent the brittle fracturing process. Alternative approaches based on hybrid and discontinuum methods attempt to model the brittle

failure process by modelling the development, growth and interaction of fractures (Stead et al., 18), which show great promise.

As part of their development, these new methods need field verification. Even after these methods have reached maturity, the complexities

in geomechanics will still necessitate field observation to verify design, according to the established principal of Terzaghis observational

method of geotechnical design. The use of microseismic (MS) and acoustic emission (AE) methods in this regard are invaluable as they

provide a method for quantifying the extent, timing and mechanism of rock mass damage accumulating within the slope.

Our knowledge of the damage accumulation process under a low stress environment is inadequate for the future challenges of open

pit mining. It is also our conviction that data from MS/AE monitoring is a key component in advancing the state-of-the-art in rock slope

design and analysis beyond the current empirical methods which are based on experience with shallower pits and developed within

a paradigm of ductile shear behaviour. MS/AE monitoring also provides valuable information in tactical monitoring approaches as it

provides information on the location and rate of damage accumulation at small strains levels, outside of the practical working strain

levels of some of the other monitoring methods.

MEASURING DAMAGE WITH MICROSEISMICITY AND ACOUSTIC EMISSION

Passive seismic monitoring in geosciences comprises the detection and recording of transient pulses of elastic wave energy rapidly

released in the rock mass as fresh fractures are generated or deformation takes place on existing ones, i.e. detecting the sound of rockbreaking. Only a small portion of the energy is released as an elastic wave.

The creation and deformation on very small fractures emit high frequency sound waves while the same process on a larger scale

is associated with lower frequency waves. Figure 7 provides an illustration of the spectrum of seismicity. Fracturing and deformation

on sub-metre size fractures are often associated with ultra-sonic frequencies and are referred to as acoustic emission (AE). Large

earthquake seismicity forms the low frequency end of the spectrum while mine seismicity falls in the category between these two and

is referred to as microseismicity (MS). Blasting results in frequencies in the lower end of the MS measurement window.

Figure 7 Frequency Ranges of Different Classes of Seismicity in Geosciences (Modified After Manthei et al., 19)


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AE is often associated with the detection of very small fractures in laboratory test samples and is sometimes associated with the methodof in situ stress measurement. AE have been used very successfully in several field applications aimed at monitoring the accumulation ofrock mass damage (cf. Wisecarver et al. 20; Manthei et al., 19).

The application of AE and MS monitoring of natural rock slopes and civil excavations varies widely in purpose and approach. AEmonitoring has been used in small slopes such as critical road or railway cuttings, while the large slopes have been instrumented withMS systems. The use of AE as a stability indicator in small slopes is often based on the increase in the seismic event count rate. Where

natural rock slopes have been instrumented the unstable rock masses are very large and either the systems are comparatively small orthe sensors sparse. These systems are able to detect only the very few largest events occurring within the slope, and effort is focusedat maximising the information from the good quality waveforms. For a more detailed discussion of the l iterature, the interested reader isreferred to Wesseloo and Sweby (21) and Wesseloo et al. (6).

Table I provides a list of published mine site experience of which the authors are aware. Some of the earlier work was performed inthe 1960s and 1970s relied on the rate count of events as precursors to developing instability while the later work aim to locate andfurther interpret the source parameters of the recorded seismicity.

Mine Country Reference Year

Kimbley USA Wisecarver et al. (20) 1969

Chuquicamata, Kennedy failure ChileKennedy and Niermeyer (22)

Voight and Kennedy (23)

1970

1979

Liberty and Tripp-Vetern USA Stateham and Merrill (24) 1970

Cerro de Pasco Peru Deza and Jan (25) 1979

Fimiston (Super Pit) Australia Hudyma et al. (26) 2003

Navachab Namibia Lynch et al. (27) 2005

Sandsloot South Africa Lynch et al. (27) 2005

Crosscourse Pit Union Reefs Australia Lynch et al. (27) 2005

Sunrise Dam Australia Lynch and Malovichko (4) 2006

Mount Keith Australia Sweby et al. (28), 2006

Chuquicamata Chile Trifu et al. (29) 2008

Table I Open Pit Mine Sites Referred to in Literature where Microseismic Monitoring has been Performed

The recorded moment magnitude appears generally to be between about -2 and 0, as shown in Table II. This relates to fractures of about1 m2 to 100 m2. Larger magnitudes were recorded at Chuquicamate Mine which is considerably deeper than any of the other sites. Trifuet al. (29) mentioned that a large number of the events recorded at Chuquicamata occurred below the pit bottom, i.e. > 800 m. The lowerlimit is determined by the system sensitivity which is largely determined by the inter sensor spacing, while the upper limit is determinedby the geotechnical environment.

Site ReferenceRecorded Magnitudes

Min Max

Cerro de Pasco Deza and Jan (25) -2 0

Sandsloot Lynch et al. (27) -2.5 -0.5

Navachab Lynch et al. (27) -2 0

Mine A Wesseloo et al. (6) -1.6 -0.6

Sunrise Dam McCallum (5) -2 0

Chuquicamata Trifu et al. (29) -0.7 1.4

Large natural rock slopes Brckl and Mertl (30) -2 0

Table II Recorded Moment Magnitude Values from Rock Slopes


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THE ROLE OF SLOPE MONITORING

Both strategic and tactical monitoring are important in managing the risks associated with rock slope engineering. The aim of strategicmonitoring is generally the reduction of long-term geotechnical risk by the gathering of relevant data to aid in understanding the underlyingmechanisms that govern slope behaviour. Tactical monitoring aims at reducing short-term risk by applying alarm trigger levels to initiatetactical response.

Both strategic and tactical monitoring can benefit from MS/AE monitoring as part of a larger monitoring suite. The complexities in theinterpretation of MS-AE data makes it unlikely that this technology could be used on its own to fully understand the slope behaviour orprovide tactical warning. However, this is in varying degrees also true for all other monitoring methods used in rock slopes.

It is not possible from surface monitoring to determine the depth behind the surface of failure development unless there are obviousexpressions of tensile cracking behind the crest from which an estimate of the volume of failure can be guesstimated. Where movementis expected on individual structures or contacts, subsurface monitoring is required. Early movement can often be detected from changesin the pore pressure recorded by the piezometers or on extensometers/inclinometers before any surface expression can be detected. Itis hard with this approach, however, to consider volume measurements of the rock mass response as the systems are essentially pointlocations. The only way this can be done is to use MS/AE to listen to the rock. Every movement or new/existing crack extension will emitnoise. This will occur well before any subsurface or surface movement can be detected. The level of detail available from this monitoringis extensive, but it depends on the signal to be measured, the signal to noise ratio, and the likely attenuation and careful consideration tothese factors is necessary in the design of the seismic system.

Tactical seismic monitoring looks at noise counts and likely source location provided the array is well-designed. Such a system couldcomprise an acoustic emission network (often used in highway monitoring) or triaxial/uniaxial sensors located in drill holes and on thesurface of the slope. Examples of where tactical seismic monitoring could be implemented would be along or adjacent to pit entrances/haulroads, railways or adjacent to expected failure zones.

Strategic monitoring would involve triaxial sensors located in the boreholes looking for the initiation or extension of existing cracks.There is an exponential increase in the number of events of a certain size with the decrease in the magnitude. As a consequence, theresolution of the seismic system needs to be small enough to ensure that sufficient data is available to interpret the rock mass behaviour.We recommend that the seismic system should be able to detect at least 2 to 3 magnitude units of data. For the majority of the slopes inTable II, with a maximum event size of 0, this means that the minimum reliably recoded event size (mmin) must be less than -3. Dependingon the rock mass quality, which influences the attenuation of the seismic waves, this could be achieve by an inter sensor spacing ofabout 5075 m. It is recommended that the majority of the sensors are triaxial. This ensures that you achieve good source location and

waveform for subsequent analysis. With information on the type of fracturing, the location of the fracturing or movement, a much betterunderstanding of the developing mechanism can be achieved. For large volumes this, this becomes a difficult feat and economicallyunviable or at least a challenge. Careful consideration needs to be given to the aim and target volume of any seismic system to ensurethat it will be able to perform as expected.

CONCLUSIONS

Currently, the knowledge of rock mass damage accumulation at low stress and the cumulative development of rock slope instability isstill fairly limited and in our opinion, inadequate for the challenges that face open pit mining of very deep pits. As pits go deeper, beyond theexperience base there is an increasing probability that failure mechanisms rarely observed in shallower pits will become more pervasive.This conclusion is based on some of the observations made with microseismic monitoring, as well as the advances made in understandingthe damage accumulation in intact rock in recent years.

Surface monitoring provides data on the surface movement but this will not provide any data on the development of the mechanismand how far ahead of the actual failures the excavation process has been affecting the rock mass. MS/AE provides a comprehensive wayof understanding the fracture process of rock masses. The calibration of numerical methods must take into account the near and deeperrock mass degradation and dilation to provide confidence in using such tools for prediction. Until this happens and the turnaround time ofthese types of analyses reduces to time periods comparable to the current methods, current design methods will prevail.

Every opportunity to monitor slope failure should be undertaken to improve the ability to model and predict the occurrence of suchfailures in the future. The work presented here and in Wesseloo and Sweby (21) and Wesseloo et al. (6) shows that the use of AE/MS asa routine monitoring system still has a long way to go. However, careful use of MS/AE monitoring in conjunction with other methods willprovide valuable information.

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