28 April 2021 Civil Engineering

In order to ensure the safe operation of the new underground mine, the existing Pit 2 highwall was redesigned to a slope geom-etry appropriate to the projected 20-year life of the new underground mine. Exxaro appointed specialist drilling and blasting contractor, Eire Contractors, to design the new highwall and execute the drilling and blasting thereof. Eire Contractors in turn retained the collaborative services of Rockland Geoscience and ARQ Geotech to design the highwall and monitor blasting and excavation works.

GEOLOGYThe new underground mine will exploit the no. 2 and 4 coal seams hosted within the coal-bearing sandstones and siltstones of the Permian Vryheid Formation, Ecca Group, Karoo Supergroup. The rocks of the Vryheid Formation either rest con-formably on diamictites and associated glaciogenic sediments of probable Dwyka age, or unconformably on basement rocks of the Lebowa Granite Suite, which in turn is underlain by volcanic rocks of

the Loskop Formation. A stratigraphic column of the geology at the mine is presented in Figure 2.

GEOTECHNICAL INVESTIGATIONA geotechnical investigation was under-taken to generate geotechnical parameters as input to slope stability analyses. The geotechnical investigation consisted of a desktop study of all available geotechnical information pertaining to the develop-ment; 3D terrestrial laser scanning of mining slopes to obtain the dip, dip direction, spacing and persistence of dis-continuities; manual scan-line mapping of selected faces to ground truth the laser scanned data and inspect joint surface roughness profiles and infill; double tube rotary core drilling; down-hole acoustic and optical televiewer surveys of bore-holes to determine the dip and dip direc-tion of discontinuities in boreholes; and logging, sampling and laboratory testing of the borehole core.

Laboratory testing comprised uniaxial compressive strength testing including

Blasting and excavation of a new highwall for ExxaroExxaro Resources is developing a new bord and pillar underground mine at its Dorstfontein Coal Mine East (DCME), 10 km north-east of the town of Kriel in Mpumalanga. The new underground mine will be accessed via seven portals driven from the base of the existing Pit 2 open cast operation.

Gerrie van JaarsveldPrincipal Engineering GeologistRockland Geosciencegerrie@rockland.co.za

Werner BritzGeotechnical EngineerARQ Geotechwerner@arq.co.za

Thomas O’BrienGeotechnical Engineer, HoDARQ Geotechthomas@arq.co.za

Figure 1 dCme locality

Figure 2 stratigraphic column

0

Dep

th (m

)

10

20

30

40

50

60

70

80

No. 5 Seam

No. 3 SeamNo. 4 Seam

No. 1 Seam

No. 2 Seam

AlluviumSandstone

SiltstoneCoal

Limestone

Diamictite

Basement

Civil Engineering April 2021 29

the determination of Young’s modulus and Poisson’s ratio, basic friction angle tests on saw cut joints, petrographic and x-ray diffraction analyses, slake durability and Duncan swell index tests.

SLOPE DESIGNThe data obtained from the geotechnical investigation was subjected to rock mass classifications, which in turn were used to inform deterministic kinematic assessments,

2D limit equilibrium (LE) and finite ele-ment (FE) analyses, rockfall analyses and various probabilistic assessments. These analyses were primarily conducted utilising Rocscience’s and Midas’ suites of software.

Figure 3 (a) 3d terrestrial laser scanning of mining slopes, (b) laser scanned point cloud data of mining slope showing bedding traces (with permission from maptek), (c) rotary core drilling in progress, (d) down-hole borehole survey in progress, (e) typical down-hole survey log, (f) stereonet presenting poles of main joint sets

N

S

W E

Equal AreaLower Hemisphere

650 Poles650 Entries Poles

NGAM Depth TIMM AMPM OTVM PIKT CALIGamma Ray

1m:50mTime Reflected Amplitude Photographic Image Structures wrt TN Three-Arm Caliper

API MM0° 90° 180° 270 60° 0 60 170 700° 90° 180° 270 60°0 300

2

3

4

5

6

7

All Events

Polar PlotRose DiagramBedding

Azimuth – Percent Interval (Count) Schmidt Plot – LH – WWS Standard

AZIVAzimuth Vector Plot

Vector Azimuth (Base to Top) – WWS Standard

(e)Legend: NGAM: Total natural gamma ray, CALI: Three-arm caliper, TIMM: Sonic travel time, AMPM: Sonic reflection amplitude, OTVM: Optical image

PIKM: Structures Picked an Classified wrt Magnetic North, PIKT: Structures Picked and Classified wrt True North, Rose Diagram: Sum of Azimuths at Horizontal Plot: Schmidt Plot, Equal Area, AZIV: Azimuth Vector Plot, TILT: Borehole tilt from vertical, AZIM: Borehole azimuth from magnetic north

0° 90° 180° 270 60°

(f)

(a) (b)

(c) (d)

30 April 2021 Civil Engineering

Two slope designs were evaluated:1. A three-tier design consisting of

15 m high vertical bench faces, and horizontal bench widths of 10 m on the upper bench, and 20 m to 30 m on the lower bench.

2. A steeper three-tier design, consisting of 15 m high 80° bench faces, and 5 m bench widths.

Both designs were found to offer accept-able long-term global stability, and poten-tial instability was found to be governed by structurally controlled mechanisms on the face, such as toppling rotation, planar and wedge sliding.

The design with the shallower overall stack angle (design 1 above) was selected and taken forward as the additional width on the benches afforded compliance with mine safety protocols, which impose 5 m and 3 m stand-off distances from bench faces and bench crests respectively. This design thus provides pedestrian access to the intermediate bench and vehicle access to the lowest bench for inspection and maintenance purposes. The design slope is shown in Figures 4 and 5.

EXCAVATIONThe reactionary forces produced by blasting cause a deterioration of rock mass quality in faces adjacent to blasting, which is greatly exacerbated by poor control and the selection of inappropriate drill and blast methods and materials. Cost, pro-gramme, and slope life need to be taken into consideration when selecting drill and blast methods for final wall control. Although the temptation to apply lowest cost rapid drill and blast methods is great, these often impart substantial damage

to final slopes which then require costly remediation.

The optimal approach is best arrived at through multidisciplinary collabora-tion between the client, drill and blast contractor, load and haul contractor, and slope and blast designers. Given the influ-ence of geology on blasting outcomes, it is important that ground conditions are monitored as drilling and blasting

progresses to allow appropriate alterations to blast designs where required. This approach was successfully adopted during the project.

Excavation of the design profile, shown in Figure 5, was achieved through presplitting and trim blasting techniques. Presplitting consists of a single row of closely spaced holes which are left un-stemmed i.e. without backfilling the void

Figure 5 isometric view presenting slope design and transverse and longitudinal gradients

41

Portals

1 56

01

550

1 54

01

530

1 52

01

510

–10 0 10 20 30 40 50 60 70 80 90 100 110 120

Safety factor0.000

6.000+5.5005.0004.5004.0003.5003.0002.5002.0001.5001.0000.500

2.255

Figure 4 2d Le and Fe analyses to assess the global stability of the design slope; (a) shows results of a Le analysis and (b) shows the results of a Fe analysis

1 56

01

500

1 51

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520

1 53

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540

1 55

0

–20 –10 0 10 20 30 40 50 60 70 80 90 100 110

Maximum Shear Strainmin (stage): 0.000

0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0010.0010.0010.0010.0010.0010.0010.0010.0010.0010.001

max (stage): 0.001

Critical srF: 2.77

(a)

(b)

Civil Engineering April 2021 31

between the top of explosives columns and the collar of holes, loaded with decoupled explosives and detonated si-multaneously in advance of trim and bulk blasts. Presplitting provides a preferential fracture plane to intercept cracking pro-duced by adjacent trim and bulk blasting, while trim blasting reduces the rate of energy release against presplit planes.

In an effort to limit backbreak and the dilation of sub-vertical jointing, some presplits were divided into separately fired groups. Trim blasts were taken together with bulk blasts, but only initiated after the detonation of bulk holes. In order to limit damage to presplit planes, due to high vibration levels and back thrust, trim and bulk blasts were initiated with electronic delay detonators, which provide for precise detonation timing and flexible sequencing.

A robust quality control system, comprising as-drilled surveys, hole depth measurements, and charging and initia-tion inspections, were adopted to ensure adherence to blast designs. All blasts were captured via unmanned aerial vehicle (drone) for record keeping and analysis purposes. Blasting results were subjected to rigorous review to allow optimisation of subsequent blast designs. There was significant adaptation of blast design in the light of developing an understanding of the blasting results, with close commu-nication between the client, drill and blast contractor, explosives supplier, load and haul contractor, and consultants.

Eire Contractors supplied drilling and blasting services, Enaex/Sasol supplied explosives and Andru Mining undertook load and haul of blasted materials.

Total excavation quantity amounted to some 1.5 million m3 with the following approximate divisions between various types of blasting:1. Presplitting: 60 000 m2

2. Trim blasting: 337 000 m3

3. Bulk blasting: 1 177 000 m3

LATERAL SUPPORTAs mentioned previously, global slope stability of the rock mass over the larger stack, and on a bench scale, was adjudi-cated as satisfactory and did not require stabilising measures. However, localised structurally controlled instabilities were expected on the final highwall face. It was necessary to introduce adequate stability to the face such that safe access could be

Figure 6 Various phases of the excavation process: (a) drilling of blast holes, (b) blast drilled and charged-up, (c) initiation of blast, (d) load and haul of blasted material

(a)

(b)

(c)

(d)

32 April 2021 Civil Engineering

afforded at the portal entrances and in areas of the platform below the highwall which would be utilised for mining activi-ties. A two-tiered approach was employed to achieve this.

Firstly, a slope face protection system would be designed and implemented over the full extent of the face where failures could not be accommodated. This slope protection system was designed

to stabilise and protect against small, shallow, and widespread instabilities caused by the unfavourable intersection of natural joints and blast-induced frac-turing. These instabilities are too small and frequent to receive localised or spe-cific treatment, and it is instead optimal to protect against these by installing a light system applied uniformly over areas of interest.

A Hoek-Brown rock mass parameter set, representative of the material near the vertical face, was derived and various analyses were computed to assess mechanisms and derive appropriate slope protection measures. It was found that 20 mm diameter, 1.8 m long resin grouted Threadbar 500 rock bolts installed on a 2 m grid pattern over the portal face would be sufficient to stabilise the face.

Figure 7 Comparison between (a) original mining slope and (b) completed design slope

Figure 8 development of portals and installation of shotcrete drainage and wire mesh. note dyke and shear zone

Dyke

Shear-zone

(a)

(b)

Civil Engineering April 2021 33

It was also found that the maximum bolt forces occurred in or around the relatively weak coal layers. Figure 9 pro-vides a typical output showing the total expected displacement and the mobilised bolt forces.

Conventionally an “open” face protection system comprising a suitable draped and pinned mesh would have been appropriate to catch and hold loose rock fragments. However, as non-durable carbonaceous shale layers are susceptible to rapid weathering and deterioration, a decision was taken to fix drainage to the rock face and apply 25 MPa shotcrete/ sprayed concrete onto the meshed face. This served to protect the sensitive materials from the elements and to retain finer fragments that would otherwise have exited through the apertures of the mesh.

Secondly, and after exposure and inspection of the final highwall face, localised measures were designed on a

case-specific basis to safeguard against larger kinematic instabilities wherever these present. These localised measures are installed over and above the nominal stabilisation measures.

Upon completion of excavation, the slopes were laser scanned by Pinpoint 3D in order to update discontinuity data and kinematic analyses. A shear zone, containing unstable wedges on the north of the highwall was noted. The shear strength of joints contained within the shear zone were estimated through inspecting joint roughness and infill with the aid of a telescopic handler. Joint surfaces were found to be mostly smooth-undulating to smooth-planar with little to no infill. Photos from the joint inspection process are shown in Figure 10.

Wedge sliding and support analyses were subsequently undertaken for various localised and larger potential instabilities utilising Midas’ Soilworks deterministic

limit equilibrium software. Joint param-eters were validated by seeking a factor of safety slightly above unity under currently prevailing conditions – good correlation was found. Additional support, in the form of a series of anchors, was designed for the localised potential instabilities such that adequate stability is maintained when less favourable load conditions manifest (increased water conditions on joints, etc.). A typical analysis conducted for a large local instability is presented in Figure 11.

At the time of publication, rockfall net-ting, rock bolts and face drainage had been installed, while the installation of cable anchors and shotcrete were in progress.

CONCLUSIONThe success of the project may be at-tributed to:

Q Thorough geotechnical investigation including combining terrestrial laser

Figure 9 Fem output from generalised slope protection analysis

0.0000

0.0002

0.0004

0.0007

0.0009

0.0011

0.0013

0.0015

0.0018

0.0020

0.0022

Total Displacementmin (stage): 0.0000 m

max (stage): 0.0021 m

0 kn Axial Force

88.138 kn Axial Force

Figure 10 (a) and (b) inspection of joint roughness and infill

Figure 11 Le output with cable anchors installed

(a)

(b)

34 April 2021 Civil Engineering

scanning and down-hole geophysical and optical surveys with traditional face mapping and borehole logging techniques to characterise rock mass structure.

Q The integration of slope and blast design to balance production pres-sures against acceptable levels of slope degradation.

Q Multi-disciplinary collaboration between the client, consultants, and contractors.

Figure 12 (a) rockfall netting, rock bolts and face drainage installed, installation of cable anchors and (b) shotcreting in progress

GEOBLOCK

COMMON USES ARE:

• Road widening• Road construction over poor soil conditions• Bridge abutments• Bridge underfill• Protection of culverts, pipelines and buried structures• Rail embankments• Landscaping and vegetative green roofs• Slope stabilisation• Levees• Airport runways

Tel: 087 086 1990Email: Sales@isomoulders.co.za

The completion of the project was made possible by the following project team: Q Client: Exxaro Resources Q Drilling and blasting contractor: Eire Contractors

Q Load and haul contractor: ANDRU Mining

Q Lateral support contractor: Sentinel Q Geotechnical drilling contractor: Diabor Q Consultants: Rockland Geoscience, ARQ Geotech and Mr CVB Cunningham

ACKnOWledgeMenTs

We would like to extend our gratitude to the following Exxaro representatives, without whom the success of this project would not have been possible:

Q Eben Barnard Q Hennie Holtzhausen Q David Kobuoe

Q Rofhiwa Phadagi Q Francois van Eeden

(a) (b)