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Improving Double Bench Face Performance at the Ekati Diamond MineTM

Site

Jody K. Todd James I. Mathis

BHP Billiton Diamonds Inc Ursa Engineering

Abstract

Double benching has been utilized as an excavation technique in open pit mines for many years. This processinvolves drilling, blasting, and excavating the material for the design bench height. A second bench is then

excavated without the creation of a catch bench, i.e. only minimal offset from the first bench face is allowed when

drilling off the second bench. The result is a double high bench face or double bench.

When drilling off the second bench, the offset of the drill from the bench face projects the lower bench face into the

pit up to an extra five meters. This reduces the maximum attainable bench angle, enhances the probability of rock

escaping the catch bench and increases the overall strip ratio. The generally utilized process of trim blasting also can

impact on the final bench profile by contributing to blast damage of the face.

BHP Billiton Diamonds Inc., Ekati Mine Diamond MineTM, has pioneered the usage of a technique that eliminates

this offset. Double benching is accomplished by drilling a single, 30m pre-shear (two 15m benches) to define and

protect the final bench profile.

This article provides a comparison of the attained face angles obtained in the field utilizing both of the above

methods, as well as a comparison with the analytically predicted angles prior to excavation.

1 Introduction

The BHP Billiton Diamonds Inc., Ekati Diamond MineTMis the largest producer of gem grade diamonds in North

America. It is currently an open pit operation located approximately 300km northeast of Yellowknife in theNorthwest Territories in Canada (Figure 1). This article references the Panda pit, which was the first open pit

diamond producer in Canada.

The Panda pit is centered on a diamondiferous kimberlite pipe. The pit walls are located in the country rock which

consists of granite/granodiorite exhibiting varying grades of metamorphism. The country rock is quite strong and

displays well-developed, relatively widely spaced, discontinuity (joint) sets. These joint sets have been characterized

as to their spatial characteristics both through oriented core drilling and area mapping in the open pit. Strengthparameters of the intact rock and discontinuity sets have been determined through laboratory testing.

At present, the open pit is near circular (Figure 2), with a diameter of about 650m. It has an attained a depth of

220m with a planned depth of 300m. During the pre-stripping phase and early production, drilling and blasting

utilized Driltech DK90 single pass rigs with 31cm diameter holes loaded with 30/70 emulsion. Later some of theresults of the blasting improvement studies saw these rigs modified to drill smaller 27cm diameter holes, resulting in

better fragmentation due to improved emulsion/column distribution. Excavation of the blasted ore and waste was

conducted utilizing a Komatsu Demag 655SP diesel hydraulic shovel together with Caterpillar 793 trucks. Later

mining was conducted with a CAT 5130 excavator, CAT 994 and 992 loaders matched with CAT 777 haul trucks.Approximately 80,000 - 100,000 tonnes of material were hauled per day from the Panda pit

This pit is located in an arctic climate. As such, even though the kimberlite pipe was originally located under a lake,approximately half of the depth of the pit walls was excavated in rock at permafrost temperatures. This reduces the

effects of groundwater on the pit walls, with the exception of the few months when thaw occurs, and a 5-10 meter

active zone on surface melts and releases any groundwater that was retained as ice.

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Figure 1 - Location of the Ekati Diamond Mine Figure 2 - Aerial view of the Panda pit

The economics of the pit are driven by the slope angles attainable in the country rock. The steeper the overall slope,

the greater the depth to which the kimberlite pipe can be extracted using open pit methods. As the pit slopes are

primarily controlled by catch bench requirements, the attainable bench face angles play a critical roll in the overall

slope angle. In addition, this was the first operation utilizing double benching in the Northwest Territories. As such,considerable importance was placed on maximizing the bench face angles and ascertaining that the required catch

bench widths were being maintained.

2

Geologic/Geotechnical Background

The Panda pit is located in the Slave Structural Province, an Archean craton that totals approximately 210,000 km2

.It consists of, in approximate chronological order of deposition/emplacement:

basement rocks composed of intrusives and metasedimentary gneisses;

mafic to intermediate volcanic rocks;

diabase dikes, and;

inland sea sediments (shale, siltstone, mudstone, possibly sandstone)

The Slave craton is bordered by two prominent faults, the Bathurst and McDonnell. The Bathurst fault is sinistral

and trends 340 while the McDonnell fault is dextral and trends 060. The resulting fabric induced in the host rock

by the associated stress regimes can be seen in the accompanying stereonets (Figure 3).

Within the alkaline granite-diorite assemblage (hereafter referred to as granite) in which the pipes have been

emplaced, some gneissic texture is present, with segregation and alignment of biotite micas (or chlorite). The

presence and magnitude of the foliation varies throughout the rock mass.

Pegmatite dikes are occasionally encountered in the granite, as are diabase dikes. While not having the same

composition as the surrounding rocks, they are limited in size and sufficiently similar in characteristics as to behave

similarly tothe granite.

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Figure 3 - Typical stereonet, Panda granite Figure 4 - Discontinuity shear strength, granite

2.1 Rock fabric

Rock fabric orientation data utilized in the Panda pit design was collected from oriented drillholes as well as area

(cell) mapping on rock faces exposed during the pit pre-strip. Both of the above techniques provided information

regarding joint orientations and surface conditions (large and small scale roughness as well as infill). Only cell

mapping of the exposed pit walls provided information regarding the continuity (length) and joint density (spacing)of the fabric sets.

As stated previously, rock fabric within the country rock tends to sub-parallel faulting in the area. The Panda pit graniteexhibits discontinuities that are very continuous and planar, with relatively wide spacing between individual

structures. Discontinuity infill consists primarily of chlorite, clay, epidote, calcite, quartz, and hematite.

Discontinuity sets were chosen by manually examining and determining sets for the total structure orientation

database within a potential geologic domain. A typical stereonet for the Panda pit rock fabric is shown as Figure 3.

There are essentially three primary groupings of discontinuity sets: sub-vertical, intermediate dipping, and sub-

horizontal. The sub-vertical sets appear to be related to the Bathurst and McDonnell regional fault systems. These sets

strike northeast and northwest. At times, and dependent upon location, these generalized groupings may be broken

down into two joint sets, as shown for the northeast striking sets of Figure 3, with an offset of about 30 degrees. Thisis apparently a function of a shift in prehistoric stress fields. Two intermediate dipping sets, striking to the northeast

and the northwest are also present. These appear to be somehow related to accommodation of the sub-vertical

features, becoming more frequent, with greater length, as major faults are neared. A variety of sub-horizontal jointsand faults are also found in the Panda area. Their genesis is unknown, but at times they exhibit a shear component,

indicating that they are not strictly related to glacial unloading. These sub-horizontal features have not yet had a

substantial impact upon slope stability but are one of the leading conduits of blast gases into the pit wall.

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2.2 Material properties

The material properties of the rocks composing the pit walls are required for geotechnical analysis of bench stability.

The properties that were determined for rocks composing the benches were:

density (mean: 2.71, deviation: 0.06)

intact rock compressive strength (mean: 150 MPa, deviation: 40 MPa)

discontinuity shear strength (peak and residual, power curve, see Figure 4)

As can be seen, this material may be classed as strong rock. Bench scale failures will be restricted to blocks defined

by discontinuities, whether pre-existing or induced by excavation. Rigid block displacement along geologic

structures with very minor corner crushing is the primary failure mode.

3

Bench Analytical Design

A bench design for a cut slope in rock incorporates two components (Figure 5):

catch bench width and catch characteristics

bench face height and face angle

These two components interact in terms of design. For example, the bench face height, face angle, and face

irregularities determine, to a large extent, the horizontal velocity component of rock falling from the face above.

3.1 Bench face design

Stability analyses for bench scale slopes were conducted utilizing computer models written by J. Mathis (Ursa

Engineering) for simple wedge and plane shear failure. These models incorporate the statistical rock fabric data and

rock discontinuity shear strengths presented previously. Simply put, the models generate a number of geologic

structures within a rectangular solid representing the bench. The structures are represented as circular discs

following the orientation, length, and density distributions attained from fabric mapping. Strengths are assigned toeach of these structures and the designated failure geometry tested for a probability of failure. A Monte-Carlo

simulation is run to determine actual failures for wedge and plane shear geometries and the generated failures

overlapped. The backbreak distribution for the bench may then be calculated, and the expected bench face angle

distribution assessed from this (Figure 6). Additional details regarding this technique may be found in the bench face

design course at http://www.edumine.com.

Figure 5 - Sectional schematic of bench Figure 6 - Bench face angle reliability curve

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3.2 Catch bench design

As rock falls from a bench face, it converts potential to kinetic energy. In addition if the rock strikes the bench face

on the way down, angular momentum is imparted to the falling rock. Thus, when the rock strikes the catch bench it

will have a tendency to roll away from the wall.

A catch bench is simply a cut bench on the rock slope. Its sole purpose is to "catch" the aforementioned rolling rocks

so that they do not continue unhindered to the toe of the slope. The bench is designed with a specific width relativeto its height so that rocks will come to rest before falling off the next crest. In addition, a "backbreak" distance is

incorporated into design (Figure 5) since the bench crest will generally fail from its (usual) vertically blasted

orientation. The design catch bench width is thus always wider than the bench width required for safety (Figure 5).

For the Panda pit, the required catch bench width was 11m for a 30m high double bench. The actual catch bench

width as laid out for the trim, or pre-shear, shot was always greater than this required width, based on the bench face

angle that was considered to be attainable in the field (Figure 5).

4 Standard Double Bench

The standard double bench is essentially the excavation of two standard bench heights without the creation of acatch bench at the toe of the first bench (Figure 7). This can result in a steeper overall bench face angle and a steeper

interramp angle as a function of the interaction between catch bench width and bench face height.

A standard double bench layout at Panda, in the early stages of pit life, is shown in section as Figure 7. In thisdiagram it can be seen that the single bench height was 15m, with a drill offset on the intermediate bench of between

5-6m from the toe of the upper bench. It s possible to drill the intermediate bench trim row closer to the toe of the

upper bench. In fact, later drill rig modifications reduced the offset to 2.0 3.0m from the upper bench toe.

However, in practice, due to safety concerns and irregularities in the toe, it is unlikely that an offset of less than 3m

could be attained on a regular basis.

The blasthole depths were greater than the 15m bench height as sub-grade drilling of 1.5m was added both for the

catch and intermediate benches. A heavy bottom charge was utilized to eliminate hard toes. The final bench facewas blasted with a standard trim blast, delayed to detonate after the main blast initiated cast away from the bench.

The resulting bench profile is seen as Figure 8. For our purposes, we can state that for the design sectors analyzedfor this article, the mean design bench face angle was around 80. Trim blasting with a standard double bench was

producing a design bench face angle (80% catch bench reliability) of 67. As this was 13below the design value

and catch bench widths required for safety were not being maintained (Figures 7 and 8), action was required.

Figure 7 - Trim shot double bench, as planned Figure 8 - Trim shot double bench, attained

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5 Perceived Problems with the Standard Double Bench

Perceived problems with the standard double bench included the following:

gas was being injected from the main pattern blast into the wall. This resulted in damage to the rock

composing the bench face (joint extension/dilation, block motion, reduction in shear strengths, fractureinitiation) resulting in reduced bench face angle. Although less gaseous explosives could be utilized, gas

production is still substantial and not remediated with trim blast techniques. drill offset on the intermediate bench automatically added a minimum of 3m to the effective design. This

resulted in a reduced bench face angle, creation of a second source of loose, raveling material and

development of a bounce ledge for any material falling from the crest above. This bounce ledge couldeasily render the catch bench ineffective. The solution to this problem could not be addressed with standard

double benching and trim blast techniques.

sub-grade drilling for the intermediate bench blast was being conducted in the bench crest of the underlying

bench. This badly damaged the rock that would compose the bench crest below. In addition, sub-grade

drilling for the upper bench blast damaged the wall rock, inducing structural failures of greater size than

would have occurred without this damage. Again the overall result was a reduced bench face angle.Elimination of sub-grade drilling over the underlying bench crest eliminated this.

vibration may or may not have enhanced the aforementioned stability issues. Monitoring of the production

and trim blasts did, however, give reasonable peak particle velocities.

6

Development of Double Bench Pre-shear Blast

The development and implementation of the single shot, double bench pre-shear was not completed overnight.

Instead, the process was long and involved.

6.1 Theory

It was deemed unlikely that the problems associated with trim blasting utilizing standard double benching could be

addressed sufficiently to meet bench design criteria. Thus, thinking outside the box resulted in the following:

a properly designed and initiated pre-shear, would protect the bench face against gas intrusion from the

main blast. Blast gases would vent along the pre-shear line and up the pre-shear blastholes. In addition, if a

high detonation velocity, low gas product was utilized for the pre-shear, gas damage would be minimized

from the pre-shear as well.

the pre-shear would also assist in reducing blast vibration damage to the bench face, as it would function asa reflector. While this may or may not be important, it was a consideration.

a normal pre-shear could have been utilized with standard double benching techniques, i.e. with the drilloffset for the intermediate bench. However, the bench face angles would still be flattened by 5-10from

design values as the drill offset would reduce the bench face angle. The pre-shear could be theoretically

drilled and blasted in one pass for the full 30m double bench height, something which would be very

difficult to do with trim blasting.

The resulting concept was a single shot pre-shear that would protect the bench face and crest from gas intrusion and

reduce vibration damage. The elimination of the drill offset alone on the intermediate bench would immediately

result in a steeper attained bench face angle.

6.2 Equipment

Small diameter pre-shear holes were initially tested using a contractors Ingersoll-Rand DM-45 blasthole rigequipped to drill 165mm holes in excess of 31.5 meters. Polar Explosives Ltd., Ekatis explosives supplier assisted

in selecting a pre-shear product that had a high velocity of detonation, low gas development, and would allow a fully

decoupled charge within the pre-shear line. The product chosen was DYNOSPLIT C, a 44mm diameter, water-gel

based explosive with the detonation cord incorporated into the string of explosive chubs.

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Figure 9 - Double benching showing "lip" Figure 10 - 30m single shot pre-shear

6.3 Blast development

The research and development that lead to the present blast design occurred over a year and a half period. This

evolution was well documented by Street (2000) and Peterson and Tannant (2001). A brief summary of the blast

development program follows:

initial testing by trim blasting using reduced loads of 460 kg/hole gave poor results. The majority of the

wall showed no blasthole half-casts and the final bench face was controlled by major joint sets, leaving abrick-like appearance occasioned by the openness of the sub-horizontal fabric.

testing of large diameter (311mm) 15m long pre-shear holes spaced 5m apart loaded with 100Kg of 30/70

emulsion also gave marginal results.

over a period of 6 months, emulsion toe loads were varied between 60Kg to 120 Kg and drillhole spacing

was varied between 3-5m with no substantial improvement of bench face conditions.

the decision was then made to test small diameter pre-shearing using the equipment and explosive

mentioned in Section 6.2 (above). A 15m high pre-shear pattern was designed with 2m spacing and loaded

with DYNOSPLIT C. This blast was the final wall to the main ramp and was shot a few days before themain ramp shot was taken. Minor cratering was noted in the collar area of the pre-shear holes. This

cratering later lead to modifications to the loading depth of the DYNOSPLIT-C in the pre-shear blasthole.

After encouraging results were obtained from the first 15m pre-shear blast, the next pattern tested was the lower

15m bench on the ramp. Again the results were promising but the drill offset or lip remained (Figure 9). Using the

DeMag shovel, this lip was scaled back. However, a 2-3 m lip of fractured granite remained that provided asource of raveling and a bounce platform. This led the company to initiate a trial 30m pre-shear on the next bench.

6.4 Present Pre-shear Pattern and Loads

The current pre-shear design for the Panda pit is a 165mm diameter drilled a total depth of 31.5m (1.5m sub-grade).Two strings of 44mm DYNOSPLIT-C are hung by poly rope in each hole. The lower string has two 3 BLASTEX

chubs tied to the bottom to reduce the formation of any hard toes at the base of the bench. The two pre-shearblasthole spacings presently in use in the Panda pit are 1.7 m and 2.0 m. The tighter pre-shear spacing is utilized inareas of the pit that either have an increased density of unfavorably oriented joint sets and fault zones or in areas

where the pre-shear line and the dominant sub-vertical joint set make an angle less than 30 degrees to the pre-shear

line. Observation has shown that if the pre-shear spacing was not reduced in the latter case, the resulting final wallhad a sawtooth appearance dictated by sub-vertical joint set control. This lead to increased raveling potential and

additional safety concerns when personnel and equipment were working close to the highwall.

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Figure 11 - 30m pre-shear, as planned Figure 12 - 30m pre-shear, attained

The pre-shear is blasted prior to the trim shot. The trim shot is defined here as a 50-70 meter wide section of rock

left in place to protect the final wall from production blasts. Normally a 35ms delay is placed between every 7-8 pre-shear holes to reduce vibration from the pre-shear blast. One benefit of the 30m pre-shear is that once the 30m bench

pre-shear is drilled and blasted the lower bench is prepared for production and trim blasting. Another benefit of the30m pre-shear is the reduced risk to the drill and blast crew. Personnel need not operate directly beneath the bench

face, as they would be when drilling the second 15m bench with standard trim shot double benching.

7 Pre-sheared vs. Standard Double Bench

The present state of the bench faces leaves little doubt in the observer's mind that converting to the 30m single shot

pre-shear was appropriate. As can be seen photographically in Figure 10, schematically in section in Figures 11and

12, and as bench face angle reliability curves in Figures 13 and 14, the evidence is overwhelming.

The verifiable increase in attained bench face angles is, on average, 14 degrees for the design sectors analyzed for

this report. While some of this can be attributed to changes in sub-grade drilling and improved main blastsequencing, these are relatively minor constituents.

This improvement in face angles, and face conditions, leads to less raveling and, as a consequence, less bench

loading. Benches remain relatively clean with only minor crest scaling required at spring thaw. In addition, the

reduction in face damage and gas injection into the rock composing the walls may increase overall slope stability aswell. Blasts have been observed where gases penetrated into the rock mass more than 100m from the blast itself. If

an established pre-shear can vent these gases, then the rock composing the wall should have higher retained shear

strengths, resulting in greater stability.

Figure 13 Case 1 - 30m bench face reliabilities Figure 14 - Case 2 - 30m bench face reliabilities

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8 Reverse Bench Slope

For the Panda pit, the bench was designed to slope from the crest back towards the toe at a ratio of 1V:10H. This is

not as effective as a catch fence, nor as effective as a berm at the bench crest, as falling rocks with a high horizontal

velocity component land near the bench crest. As only a small amount of vertical rise must be accomplished at thislocale to cascade to the next bench, more escaping rock may be found using this design than with a catch fence or

rock berm. However the latter two cases require maintenance and hinder cleanup and scaling, making the reverse

inclined bench a practical alternative.

This reverse inclined bench (dipping back from face towards toe) is a relatively novel design. It would probably not

be a good choice in areas experiencing high rainfall as it may channel water into the bench. However, it appears tofunction well in the arctic in sound rock. In addition, as blasting generally heavily damages the sub-grade on most

benches, it is uncertain if a horizontal bench promotes improved drainage in any case. For the Panda pit, the design

appeared satisfactory upon analysis and has functioned well in the field.

The results for a design catch bench width of 11m (80% reliability) specified with a reverse slope on the bench of1m vertical /10m horizontal are shown in Figure 15. Note that few rocks escape the bench. This simulation was

conducted using RocFall, a computer based falling rock simulation program provided by RocScience (Toronto,

Ontario). Although this computer program is theoretical in nature and is sensitive to the elastic and shape parametersof the falling rocks, it is considered sufficient to provide a reasonable estimate of bench catch performance.

9 Bench Cleanup and Face Scaling

Prior to the 30m, small diameter pre-shear program, the catchments required bi-yearly clean up, especially after

spring thaw. Bench clean up utilized Caterpillar D10 dozers and required that work started at the top bench and

proceed downwards, cleaning each catchment. After implementation of the pre-shear program, the catchments have

not required clean up and are virtually maintenance free.

A bench scaling program has evolved with knowledge gained over the first three years of operations and the changes

that have occurred with final wall blasting improvements. During the initial years of pre-shearing and trim blasting,

the DeMag shovel would spend appreciable time each shift rough scaling the final wall. Since the wall was notprotected from blast gases and vibration from both the production and trim shots, the wall had a stacked brick-like

appearance. In addition, potential wedge and plane shear failures developed along the continuous joint structures.

During Demag scaling, many of these structurally defined blocks would be further loosened and/or removed by theimpressive forces imparted by the shovel. This lead to further backbreak of the bench crest and left the final wallwith a rough, jagged appearance. Many structurally defined blocks were removed as they had lost their initial

cohesion as a function of blast, and scaling, displacement. Other areas of the final wall had joint orientations that

were very susceptible to over-scaling by the shovel. The combination of these factors created areas where up to 14m

of backbreak was noted on the crest of some catchments.

Figure 15 - Reverse catch bench function Figure 16 - 30m pre-sheared, scaled, bench face

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Since the institution of the 30m small diameter pre-shear, scaling is much reduced (Figure 16). Bucket breakage of

the Demag, related to scaling, has been essentially eliminated. Bucket wear has also been reduced dramatically, as

has the time devoted to wall scaling and clean up. The primary scaling activity at present, besides obvious loose

removal with the excavator, is chain scaling of the bench crests using a Caterpillar D10 prior to drilling off the

intermediate bench

10 Summary and Conclusions

In conclusion, the development of the 30m, single shot, pre-shear appears to be a worthwhile geotechnical and

operational innovation. It reduces damage to the ultimate walls of a rock excavation, allowing the full geotechnical

potential of the rock mass to be exploited. This results in steeper face angles, and ultimately, steeper pit walls. Theend result is a more economical pit with minimized stripping.

From an operational point of view, while being more expensive than standard trim blasting, it promotes safety. The

benches, and bench faces, are extremely clean. Raveling is generally associated with spring thaw and has been

relatively minor. Personnel are not operating under a broken face to drill the trim line on the intermediate bench as ithas already been drilled, and blasted, from above as a pre-shear. Bucket wear on the excavator has been reduced as

function of the reduced scaling required. So, too, has the amount of time spent in scaling and bench clean-up.

The development process continues at present. A hard toe or a ridge of unbroken rock between the pre-shear line

and the main blast still occurs occasionally on both the intermediate bench and catch bench levels. This requiressecondary blasting for removal. In order to address this problem, the following is being conducted:

the trim shot final row is being moved from 3m away from the pre-shear line to 2.5m

place additional emphasis on secondary blasting of hard toes on the intermediate bench so that this does not

hinder the intermediate bench drilling from being on the design line

use heavier loads in the final row of the trim shot for the intermediate bench. A light load will still be

used in the upper bench trim shot final row to reduce crest damage

This above described method is not proposed as a panacea for all rock slopes and all rock masses. It will function

only in certain rock masses with specific failure modes. However, the method should be evaluated if doublebenching is proposed for an excavation, especially in strong, relatively massive, rock masses.

11 References

PETERSON, J., TANNANT, D., TODD, J., 2001. Wall Control Blasting Practices at the EkatiTMDiamond Mine.

CIM Bulletin,94, 1050, pp 67-73

STREET, S., 2000., Wall Control Blasting in Panda Pit at the EkatiTMDiamond Mine, N.W.T, Canada.University

of Toronto Mining Engineering Engineering Undergraduate Thesis.