structural and stability assessment for ex-pit dump toe (final)

Structural and stability assessment for ex-pit dump toe locality optimisation: A case study John Latilla

26 November 2015

BBUGS / BOHOGS Supplier’s day - Moranbah

www.amcconsultants.com

Ukhaa Khudag coal mine, Mongolia


Project location


Project infrastructure

15 Mtpa CHPP 18 MW Power Station

Office and mess hall Mine Workshops


Some key facts

• Multiple seams dipping generally 3° to 17° into highwall

• Flanks (endwalls) dipping 5° to 40° out of pit wall

• Terrace mining (truck and excavator)

• Mid volatile hard coking coal with thermal coal by-product

• Ex-pit waste dumping, at least for next 3 to 5 years

• Nameplate capacity of 15 Mtpa ROM coal to CHPP

• Life-of-Mine stripping ratio of 5.6:1


The challenge Optimise the locality of the northern ex-pit waste dump on order to reduce dump truck round trip distances.

Earlier limit equilibrium analyses of cross-sections for pit walls below the dump indicated the potential to steepen the OSA by as much as 4° through buffer blasting the entire slope

Insufficient geotechnical and geological data available to properly identify buffer blasting target zones

The phreatic surface used is based on John Simmons’ work from the Bowen Basin (far wetter conditions)


The challenge in a nutshell

7

Possible structural interp. with additional data

LOM structural interp.

New “optimised” crest locality

Potential gain

50m stand-off distance not assessed – remains constant


Plan of action

Drill a series of six geotechnical boreholes sited to maximise data from the potential buffer blasted endwall area. Boreholes required to be oriented to enable structural and kinematic analyses

Drill ten percussion holes adjacent to and on top of the waste dump to determine actual phreatic surface

Review geological structure of the ground containing the final endwall slope beneath the waste dump

Conduct kinematic and limit equilibrium analyses to design optimal OSA beneath the waste dump using the updated structural and phreatic surface data


Modifications No acoustic televiewer available in country. Changed to drilling oriented cored boreholes

60° inclined core boreholes, oriented by means of Ezy-Mark tool (Reflex). Relies on leaving a crayon impression and a pin profile of the bottom of the hole

Soft and friable coal and mudstone areas - too much fine and loose material at bottom of hole to obtain an impression. Oriented core data unavoidably biased to the stronger units

Inability to orient core in softer zones judged not to be a show stopper. Overall structural orientations expected to be similar in hard and softer rock


Modifications continued

Two oriented holes close to the dump toe intersected basement rock (dolerite and basalt) and “replacement” holes were drilled about 100 m further (South) from the dump toe

Info on the basement contacts and structure was nevertheless very useful


Oriented core drilling

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Coring through the waste dump

Local exploration drilling contractor used for the dump hydrology holes. Did not have a percussion rig available so drilled the holes with a coring rig

Core was not boxed or logged but simply laid out on the ground near the rig. Recovery of core not called for in the work order

With an air drilled hole the expectation was that water level monitoring would track the gradual climb of water level back up to equilibrium. With water flushed coring the opposite was expected


Waste dump core

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Structural data collection

Oriented core holes logged and photographed by the site geotechnical team to CoreLog standard (ACARP 2015)

Individual defects identified and measured using CoreProfiler (CSIRO). Builds a composite photograph of the core column from core photographed in trays. From this, dip and dip direction are determined for each defect and plotted on stereonet. Data was then exported to Dips6.0 for further manipulation and assessment

CoreProfiler also produces RQD and fracture frequency data

CoreProfiler is still in development but was found to be a useful tool


CoreProfiler – core box photo From this:

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CoreProfiler work page example To this:

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Structural assessment

Structural data analysed in Dips6.0 as follows:

• All defects in all boreholes, then split per endwall sector (NEW2 and NEW4) and finally split into individual boreholes

• Each of the above were then sub-divided into:

All structure above the lowermost coal seam (BOC) and that below BOC – the latter being the key strata for endwall design

Bedding planes only split as above to determine structural trends in sedimentary rock

• Shears and possible shears analysed separately – insufficient numbers to analyse shears on a hole for hole basis.


Selected stereonet plots

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All shears – all b/hs

All defects – all b/hs


Structural analysis summary

Description Dip and Dip direction

(°) Set 1 Set 2 Set 3

All defects for all boreholes 13/207 50/174 – All defects above BOC 11/211 50/167 – All defects below BOC 13/204 49/180 – Bedding planes only for all boreholes 13/208 49/179 –

Bedding planes only above BOC 14/213 52/173 – Bedding planes only below BOC 12/203 47/183 – All defects for basement rocks 11/214 51/180 – Shears only for all boreholes (orientation low to medium confidence)

64/172 73/193 23/217


Kinematic assessment

Critical Surface Limits

• Batters

Final endwalls – 10%

Interim endwalls – 15%

Highwalls – 20%

• Overall slope angles

Below critical infrastructure – 5%

All others – 10%


Kinematic assessment

Planar and wedge sliding are the most likely, kinematically admissible, failure types

• Excluding the effect of shears:

Batters should not be formed steeper than 55° in NEW4 and 70° in NEW2

Overall slope angle (OSA) should not exceed 47°

• Taking shears into account:

Batter slope angle drops to 25°. OSA 23° for NEW2 and 30° for NEW4

• Assuming bedding shears will be buffer blasted:

Max batter angle 55° and OSA 27° to 34°


Selected stereonet plots

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NEW2 batter scale planar sliding

NEW2 batter scale wedge sliding


Kinematic analysis results summary

Slope scale Pit sector

Maximum slope dip angle and percentage of critical surfaces

Planar sliding (° / %)

Wedge sliding (CS Mod)*

(° / %)

Batters (15% critical surfaces limit)

NEW2 70/13.7 70/13.2*

NEW4 55/14.9 67/15.0*

NEW2 shears 28/14.3 25/13.2

NEW4 shears 59/7.1 33/13.2

OSA (10% critical surfaces limit)

NEW2 49/9.8 47/9.8*

NEW4 47/9.7 49/9.8*

NEW2 shears 27/7.1 23/9.9

NEW4 shears 59/7.1 30/6.6

*The critical surface results for wedge sliding have a correction factor of 0.5 applied to better match site experience. This is referred to as the modified wedge sliding critical surface value (CS Mod.).


Structural interpretation

General trend indicated steeper than originally modelled coal bearing strata along the northern boundary fault in 3 out of 4 cross-sections (CS 20a, 22 & 23)

The other cross-section (CS 28) indicated steeply dipping boundary fault cutting off the coal strata

A stronger and less disturbed conglomerate zone was identified below the BOC

Zone of steeply dipping sedimentary rocks assumed to extend along the southern face of the northern boundary fault. Based on previously exposed fault zones at UHG. Width variable


Dump toe and borehole locality

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Cross section lines

Geotech boreholes Arrow denotes hole azimuth

Hydrology boreholes


Structural interp. CS 20a

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Phreatic surface model

Locality

Distance from toe (m)

Phreatic surface below original ground surface (m)

Minimum Maximum Minimum Maximum Generic model

240 m from dump toe (on pit side) – 240 – – 28

~100m from dump toe 50 127 18 29 23

Immediately below dump toe 0 0 13 14 13

~60 from toe under dump 40 92 3 6 3

~180 from toe under dump 151 230 0 0 0

210 m from toe under dump 200 230 – – +2

[1] Note that where the pit crest is closer than 240 m the generic (or specific if known) phreatic surface model must be merged with this model


Phreatic surface interp. CS 28

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Limit equilibrium analyses

Based on oriented borehole data and observations of core recovered from hydrological drilling the interpreted structure from the 2014 model has been modified

The endwall crest can be located up to 180 m further south

Existing minimum stand-off distance between dump toe and slope crest of 50m was not investigated or altered in this study

The endwall areas represented by the two eastern most cross sections (CS 23 & 28) require no additional buttressing or dewatering

The most westerly, and deepest, endwall slope (as represented by the two most westerly cross sections, CS 20a & 22) requires extensive toe stabilisation plus dewatering


Cross-section 20a

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Buffer blast

Waste buttress


Cross-section 28

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G A L EN A Version 6.10

Project UHG 2014 Slope Stabi l i ty A naly sis - Cross Sect ion 28

Fi le: C:\Users\Jlati l la\Documents\Numerical model l ing\Galena\UHG\2015 N EW4 Buf fered endw al l below dump CS 28 (extended).gmf

Edited:Processed:

21 A ug 201521 A ug 2015A MC Consul tantsL icensed to:

A nalysis

Resul ts

4Mul t iple Stabi l i ty A nalysisMethod:Surface:

Spencer-WrightCircular

Cri tical Factor of Safety : 1.24I ntersl ice Force (Final ) A ngle: 28.9 °

Material K eysW OB (c:27 ø:22.4 g:22)

MWOB (c:115 ø:31.7 g:23)

FOB (c:358 ø:37.2 g:24.5)

CF (c:40 ø:23 g:14.4)

BPS (c:0 ø:14 g:19.4)

W R (c:27 ø:26.8 g:18.3)

FSZ (c:24 ø:18 g:21.8)

CG (c:785 ø:42.9 g:24.7)

BW (c:612 ø:34.1 g:29)

BF (c:828 ø:41.1 g:29)

800 900 1000 1100 1200 1300

1200

1250

1300

1350

1400

1450

1500

1550

1600

1650

Buttress of flat to shallow dipping strata


Conclusions

This extensive study has resulted in far better understanding of the structure and strength of critical strata beneath the waste dump toe

This improved knowledge has enabled the dump toe locality to be moved significantly closer to the pit crest thereby significantly reducing dump truck round trip distances

Thank you

Energy Resources LLC are thanked for their assistance in preparing this presentation and for permission to share this experience