Slope Stability 2011: International Symposium on Rock Slope Stability in Open Pit Mining and Civil Engineering, Vancouver, Canada (September 18-21, 2011)

Geotechnical Design of the Jwaneng Mine Cut 8

A.B. Tunono Debswana Diamond Company, Gaborone, Botswana

M. Ruest De Beers Global Mining Services, Johannesburg, Republic of South Africa

L. Dimbungu Debswana Diamond Company, Gaborone, Botswana M. Brook Debswana Diamond Company, Gaborone, Botswana

Abstract Jwaneng Mine in Botswana is the richest diamond mine in the world by value and a major contributor to the Gross Domestic Product of the country. The mine was established in 1981 and has a capacity to process 10.5 Mtonnes of ore per annum. Ore is exposed by mining a series of waste cut backs following a split shell strategy with each cut exposing enough ore for 3 to 5 years of supply. The Cut 8 project follows the current Cut 6 and 7 push backs and will deepen the mine to 622 meters extending the life of Jwaneng Mine to 2024. The project represents a major historical step for the operation as the stripping ratio increases from the current 4:1 to 10:1. Waste stripping will be 713 million tonnes and will expose 75million tonnes of ore containing 95million carats.

This paper describes the rock mechanics and hydrogeological investigations and analyses required for designing a stable Cut 8 slope. Strict acceptance criterion for safety factors and probability of failures to ensure an optimized design and stability of plant infrastructure that will be located in the vicinity of the proposed pit crest was followed whilst maintaining industry accepted guidelines on risk. Given the importance of Jwaneng Mine to Debswana Diamond Company and to the country of Botswana as a whole, a rigorous design and review process was followed.

1 Introduction and context Jwaneng Mine is located in the southern part of Botswana about 130km due west from Gaborone. Figure 1 shows the location of Jwaneng Mine and other Debswana Diamond Companys mine sites.

Figure 1. Location of Jwaneng Mine and other Debswana Diamond Company (Pty) mine operations.

Jwaneng mine is owned by Debswana Diamond Company (Debswana) which is part owned by the Botswana government and De Beers Socity Anonyme. It is the largest producer of diamonds by value for Debswana and produces the worlds best gem diamonds. Diamonds account for 40% of Botswana foreign reserves earning, hence the importance of the mine to the company and the country. It has been in operation since 1981.

The pit dimensions are approximately 2.4km in the long axis (NE-SW) and 1.8km wide in the short axis (NW-SE). The open pit is developed using the split shell mining sequence. The current pit depth is at 360m and is intended to reach 536m in the Cut 7 pushback. The next mining pushback will be Cut 8 and this will go down to a depth of 622m with waste mining increasing from the current 40Mtpa to a peak of over 122Mtpa.

The Cut 8 pushback will encroach on existing plant infrastructure that will need to be relocated. The Main Treatment Plant (MTP) will be located at about 220m from the proposed Cut 8 pit crest and hence the geotechnical design will need to ensure its stability. Figure 2 below shows the Cut 8 pit crest and location of plant infrastructure.

Cut 8 Pit Crest

Affected InfrastructureRe-crush plant beltsSlimes pipelineIn-pit crusherBelts to surface crusherPrimary and secondary stockpilesWorkshops (service equipment)

Figure 2. Jwaneng Mine Cut 8 pit crest showing affected plant infrastructure.

2 Geology There are four diamondiferous kimberlite pipes at Jwaneng Mine that have intruded package of sedimentary rock formations comprised of dolomites, shales and quartzites of the Pretoria group of the Transvaal super group, deposited during Proterozoic times. The sequence is broken up by extensional shears, thrusts and steep block faults. Dolerite sills and dykes also intrude the country rock. Due to this complex geology, the dip angle of the strata in the vicinity of the mine varies between 10- 40 towards the northwest and is bounded by major faults that dip steeply into the south east wall.

Bench and inter-ramp scale stability on the south east wall is therefore predominantly controlled by the foliation/bedding which daylights into the mine slopes and forms planar failures. In the more fissile units of the sedimentary packages, breakback occurs along foliation planes as mining progresses whilst in the more massive units the deterioration progresses with the aid of blasting. This causes rock falls which are hazardous to operating conditions as mining progresses to lower benches (Figures 3).

Figure 3. Cut 8 Pushback south east wall rock fall hazard due to undercut foliation/bedding.

2.1 Regional geology The stratigraphy of Jwaneng Mine begins with a cover of about 60m of Kalahari sequence comprised of sand and calcrete followed by Laminated and Carbonaceous Shales of the Timeball Hill Formation. The Laminated Shales are thinly bedded and occasionally have intercalations of Sandstone. The Carbonaceous Shale is a 30m thick unit that consists of graphitic shale interbedded with felsic volcanic tuff.

The Rooihoogte Quartzites beneath the shales form the bulk of the Jwaneng Cut 8 slopes. They are as thick as 375m and consist of Siltstone and Sandstone beds. A thin marker horizon of conglomerates called the Bevets splits the quartzites into two units. The lower Quartzitic Shale is almost the same as that above though it is more ferruginous and coarser grained. The lower Rooihoogte Formation is another Carbonaceous Shales unit that is less than 10m thick.

Dolomites of the Malmani Subgroup are at the bottom of the mine sequence and though there has been less intersection from drill holes they are thought to be massive. They comprise of a greyish conglomerate with clasts of Dolomite and this has a sharp contact with the underlying cherty dolomites. Lenses of Carbonaceous Shale occur in places.

Intrusions of Syenites, Dolerites and Quartz-porphyry also cut into the sedimentary sequence to form sills and dykes. A stratigraphic column of the Jwaneng Mine is shown in Table 1.

2.2 Structural geology The structural setting of the Jwaneng Mine is a result of four phases of deformations. The D1 phase was extensional and resulted in the formation of north east to south west striking normal faults that are down thrown to the south east. The D2 phase was a compressional strike slip phase that led to the north east to south west steep structures. This has caused tectonic shortening resulting in duplication of the stratigraphic and local variations of the rock fabric within the domains.

The third phase (D3) was that of dolerite intrusions and the D4 phase was the re-activation of the D2 and D3 phases. These phases of deformations have led to a complex structural geologic framework that is characterised by major faults that have compartmentalised the bounding blocks into domains each with a unique rock fabric. The isometric view of the Jwaneng Mine country rock model is shown in Figure 4.

Table 1. Jwaneng Mine Stratigraphic Column.

Stratigraphic Name Rock Type (Mine Rock Code) Typical Thickness (m)

Kalahari Sequence Sand & Calcrete 55-60

Timeball Hill Formation Laminated Shale (LS) Residual

Lower Timeball Hill Formation Carbonaceous Shale (CS) 30

Rooihoogte Formation Quartzitic Shale (QS) 135

Chert Pebble Conglomerate Bevets (BVT) 0-4

Rooihoogte Formation Quartzitic Shale (QS) 375

Lower Rooihoogte Formation Carbonaceous Shale (CS) 10

Malmani Subgroup Dolomite (DM) Residual

Figure 4. Isometric view of the Jwaneng Mine County Rock Model (SRK, 2009).

Of consequence to the geotechnical design therefore is the rock fabric (foliation) and its relation to the south east wall slopes. Rock fabric data has been collected from face mapping, oriented core and acoustic televiewing (ATV). It has been observed in the existing slopes that stability and the risk of rock falls increases as the foliation dip gets steeper due to planar failures resulting from undercut the planes. Stability analysis have been conducted to determine the critical foliation dip at which the planes cannot be undercut without affecting stability of the slope. The results of this analysis have been used as design guides for the mine planners.

The faults in Cut 8 south east wall pushback generally dip away from the pit wall and they are not anticipated to cause any stability related concerns apart from acting as conduits of water into the slope.

3 Geotechnical model 3.1 Rock mass properties The rock mass at Jwaneng Mine can be classified mostly as fair to good. The Quartzitic Shale that forms the bulk of the slope is massive. The laminated shale is thinly bedded whilst the carbonaceous shales are fissile and graphitic along the main contacts. Tables 2 and 3 below give a summary of the rock mass and the deformation properties for the rock units that comprise the south east wall slopes of Cut 8.

Table 2. Jwaneng Mine Cut 8 Pushback rock mass properties (SRK, 2009).

c(kPa) () t c(kPa) () tCS 2616 115 54 47 4.9 654 32 -173 354 37 -148

DM 214 66 54 12.6 1,467 50 -357 903 54 -315

DOL 2880 222 61 49 8.9 1,168 44 -346 694 48 -301

KIM 36 55 50 9 437 30 -46 174 36 -3.9

LS 2615 56 44 39 3.3 296 20 -118 130 23 -98

QS 2742 173 60 47 5.2 864 36 -350 508 40 -302

CODEDENSITY

(Kg/m3)

Charecterisation

UCS (Mpa) RMR GSI miDeep Rock Mass (>50m) Shallow Rock Mass (

Laboratory shear tests were conducted on both closed and open joints and the Barton-Bandis methodology was used to scale the determined strength to field properties. The Mohr-Coulomb strength parameters are then determined from the Barton-Bandis shear strength. The maximum shear strength was derived by assessing the potential load on failure surfaces. To estimate the load the lithostatic load has been used. In the case of the Jwaneng slopes, it has been determined as 0.5MPa for shallow failure planes occurring within 50m of the slope face, and up to 2MPa for deep seated rock mass failure.

A summary of the shear strength of joints in the Jwaneng Cut 8 south east slopes are shown in Table 4.

Table 4. Summary of joint strength peak values for Cut 8 foliation discontinuities (SRK , 2009).

c(kPa) () c(kPa) () c(kPa) () c(kPa) ()

CS 80.8 34 21 35 20.2 34 5.3 35

DM 25 34 6.7 36

DOL 22 37 5.7 38

KIM 20 33 5.1 34

LS 80.3 34 20.9 35 20.1 34 5.2 35

QS 84.6 36 22.1 37 21.1 36 5.5 37

StrengthFoliation Open Joints

Deep Rock Mass (>50m) Shallow Rock Mass (

Figure 5. Jwaneng Mine geotechnical section line A6 and assumed phreatic surface (Itasca, 2009).

4 Cut 8 slope design 4.1 Slope design process and acceptance criteria Given the importance of the revenue from Jwaneng Mine to Botswana and the risk of Cut 8 incapacitating the Main Treatment Plant (MTP), a rigorous design and review process was followed. Field investigations were conducted under the supervision of the mine geotechnical department. Two different design techniques (Limit Equilibrium and Distinct Element) were independently conducted and then compared and contrasted (SRK, 2009, Itasca, 2009). Internal reviews were conducted first by Debswana and then by Debswana shareholders De Beers and its majority shareholder Anglo American. External reviews were conducted on the geotechnical design by Stacey Mining Geotechnical and on the hydrogeological models by Water Management Consultants.

De Beers and its Family of Companies (FOC) have adopted the acceptance criteria for slope design outlined in the Guidelines for Open Pit Slope Design (Read & Stacey, 2009). This criterion was also used for the Jwaneng Mine Cut 8 pushback and is shown in Table 5. For the first time in the history of the FOC, the acceptance criterion was provided to the design consultant rather than the acceptance criterion recommended by the consultant to Debswana.

Table 5. Slope design acceptance criteria (adapted from Read & Stacey, 2009).

Slope Scale Consequences of Failureb Acceptance Criteriaa

FoS (min) (Static)

FoS (min) (Dynamic)

PoF (max) P[FoS1]

Bench Low - high 1.1 NA 25% - 50%

Inter-ramp Low 1.15 - 1.2 1.0 25%

Medium 1.2 1.0 20% High 1.2 1.3 1.1 10%

Overall Low 1.2 1.3 1.0 15% - 20%

Medium 1.3 1.05 5% - 10% High 1.3 1.5 1.1 5%

Notes: a. Needs to meet all acceptance criteria; b. Semi-quantitatively evaluation

In summary, the Cut 8 slope design process was intended to meet world class standards.

4.2 Limit equilibrium (SRK, 2009) Limit Equilibrium analysis was conducted using Slide (Rocscience, 2008). Rock mass properties used in the models are as shown in Section 3.1. Anisotropy was assumed for the shales to account for the lower strength along foliation. The dip of the foliations in the different domains was taken as the geotechnical mean determined from the maximum contour of poles for bench scale and inter-ramp stability whereas in the global scale analysis, the modeled dip was used. The modeled dip was the mean dip for all the formation within the domain. A phreatic surface for the 105m3/hr total abstraction scenario was used.

Before conducting slope stability analysis for the optimized pit design, a back analysis was carried out on the existing pit profiles and Factors of Safety (FoS) were found to be consistent with the Acceptance Criteria in Table 5. An optimized pit design, which was a result of extensive slope design evaluations dating back to 2003, was used for the feasibility level slope design. Bench configuration was 16m bench height, 90 bench face angles and inter-ramp angles ranging from 40 in the Laminated Shales to 60 in the Dolomites. Figure 6 shows the location of the analytical section lines used in the slope analysis and Figure 7 shows a typical section line for the slope stability model.

Stability analysis results are shown in Table 6. They indicate that the Factors of Safety (FoS) and Probabilities of Failure (PoF) are acceptable, based on the criteria outlined in Table 5.

Slope stability at Jwaneng Mine is controlled by the rock fabric. It is evident in the pit that for shallower dipping bedding/foliation, stability is generally good. In areas where the dip of bedding/foliation is steep, there is an increased risk of rock fall due to the difficult of maintaining catchment benches once the bedding/foliation planes are undercut.

Figure 6. Jwaneng mine plan showing analytical section lines.

B6A6

C6

Figure 7. Typical section for the Cut 8 Pushback. Also indicated are the geotechnical dips of foliation/bedding

in the domains (SRK, 2009).

Table 6. Stability analysis results for the Jwaneng Mine Cut 8 Pushback (SRK, 2009).

Se ction Sc ale M ater ial F oS ( Anisot ropic )PoF

(Aniso tropic )

O vera ll 1 .22 0L owe r ha lf Q S, KI M 1 .76 6U pper hal f LS, K IM 1 .32 12

O vera ll 1 .37 0L owe r thi rd Q S, CS 1 .39 10M iddle thi rd Q S 1 .58 0Up per third LS 1 .37 0

O vera ll 1.6 0L owe r thi rd Q S, K IM , D M 2 .04 0M iddle thi rd Q S, D M 1 .54 1Up per third Q S, D O L 1 .96 1

A6

B6

C6

A sensitivity analysis was conducted to determine the cut-off orientation of the bedding/foliation planes. The foliation/bedding plane dip and the stack angle were varied and the Factor of Safety determined. Figure 8 shows results of the sensitivity analysis conducted for the stack stability in the Quartzitic Shales (QS). The critical foliation dip is 40 at which the Factor of Safety is 1.2. It has therefore been recommended that for domains with orientation of bedding/foliation more than 40 the inter-ramp angle should be 40. This is the case for Domain 7a where bedding/foliation dip is at 48 .

Figure 8. Results of stack stability sensitivity analysis for the Quartzitic Shale (SRK March 2010).

4.3 Numerical modeling (Itasca, 2009) Distinct Element Models have the capability to model geological discontinuities explicitly or implicitly in spatial location and estimate anticipated deformations as mining progresses (the deformations calculated by UDEC are provided in Section 5). Hence, it is a suitable tool to determine the effect that mining of Cut 8 will have on plant infrastructure due to the adversely dipping bedding/foliation planes exposed in the pit wall below. The Itasca numerical modeling code UDEC was used to estimate the deformations along three sections through the infrastructure area and the pit wall below. Rock strength and hydrogeological input parameters were as discussed in Section 3. Figure 9 below shows a typical geotechnical model for numerical modeling of the Cut 8 pushback with the velocity contours at failure displayed. The velocity field is a meaningful indicator of the state of failure in a model because the rock mass is in a plastic condition.

Figure 9. Typical south east wall geotechnical section line showing velocity contours B6 (Itasca 2009).

Slope stability results of the Itasca numerical models for the feasibility level mine design are shown in Table 7. The Factor of Safety or the Probability of Failure for the critical anisotropic case falls within the acceptance criterion for the FOC.

Table 7. Cut 8 slope design Numerical modelling results (Itasca 2010).

FoSoverall dx (m) FoSoverall dx (m)

-

-

1.5 100

2401.4

B6

C6

>1.8

>1.8

SectionCurrent Pit Slope Cut 8 Pit Slope

A6 1.7 60 1.3 165

FoS = Factor of Safety

dx = distance of failure surface from pit crest

5 Impact of mining Cut 8 on existing infrastructure The Main Treatment Plant (MTP) is infrastructure that cannot be compromised in a diamond mine. Given the proximity of the plant to the planed crest of Cut 8, analyses were conducted to determine the risk associated with the pushback. There are two scenarios that must be considered. The first scenario is catastrophic slope failure resulting in the loss of the plant. The second is differential settlement caused by pit wall displacements.

The risk of catastrophic slope failure is assessed by determining the risk of overall slope failure. The acceptable Factor of Safety for slopes with critical infrastructure is 1.3 - 1.5 and the cut-off Probability of Failure is

Figure 10. Graphs showing horizontal strain and vertical displacements along section line B6 due to Cut 8 mining (strain is calculated as the difference in displacements of adjacent grid points divided by the original distance between the points).

6 Conclusions Jwaneng Mine is the richest diamond mine in the world by value and a major contributor to the Gross Domestic Product of Botswana. Because of its importance and because of the step change in the magnitude of waste stripping Cut 8, an exceptionally rigorous design process was undertaken to ensure mine and plant stability.

Limit equilibrium and distinct element analyses were conducted by two independent consulting houses and slope geometry was adjusted so that acceptance criteria adopted by the De Beers Family of Companies were met. The acceptance criteria are considered industry standard and for the first time in the history of the FOC, the mining house provided the design consultant with the minimum and equally important, maximum allowable Factors of Safety and Probability of Failure. Internal and External review was undertaken for assurance and corporate governance.

The stability of the Main Treatment Plant at Jwaneng was assessed for two scenarios. The risk of catastrophic failure was calculated and was found to meet the corporate acceptance criterion. The risk of debilitation from differential strain was assessed and checked against threshold values commonly used in civilian applications and it was found to meet an acceptable high safety factor.

Not discussed in this paper is the ongoing verification and validation plans implemented at Jwaneng Mine. Their focus is on assuring the validity of the inputs to the numerical models and the model results.

7 References Cavieres, P., Gaete, S., Lorig, L., Gomez, P. (2003). Three-dimensional analysis of fracturing limits induced by large scale

underground mining at El Teniente Mine. In Soil and Rock America 2003, Proceedings of the 39th U.S. Rock Mechanics Symposium, Cambridge, Massachusetts, June 2003. pp. 893-900.

Golder Associates (2009). Jwaneng Cut 8 Hydrogeological Modelling and Pore Water Pressure Distribution. Golder Associates, RSA.

Itasca Consulting Group Inc. (2006). UDEC (Universal Distinct Element Code). Version 4.0. Minneapolis, MN, USA Itasca (2009). Numerical Modelling of Cut 8 Pit- Slope Design. Itasca s.a., Santiago, Chile. Read, J., Stacey, P. (2009). Guidelines for Open Pit Slope Design. CSIRO Publishing, Collingwood. Rocscience (2009). Slide: 2D Limit Equilibrium Slope Stability Analysis. Version 5.0 Singh, M. M. (1992). Mine Subsidence. In Britton (eds.), SME Handbook. Port City Press, Baltimore, pp. 938-971. SRK (2009). Jwaneng Country Rock Model Update 2008. SRK Consulting (Canada) Inc. SRK (2010). Jwaneng Mine Cut 8 Slope Geotechnical Design Update. SRK Consulting, RSA.