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 CHPP18 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
11

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
13

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:
15

CoreProfiler work page example
To this:
16

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
18
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
22
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
25
Cross section lines
Geotech boreholes
Arrow denotes hole azimuth
Hydrology boreholes

Structural interp. CS 20a
26

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
28

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
30
Buffer blast
Waste buttress

Cross-section 28
31
GALEN A Version 6.10
Project UHG 2014 Slope Stability Analysis - Cross Section 28
File: C:UsersJlatillaDocumentsNumerical modellingGalenaUHG2015 NEW4 Buffered endwall below dump CS 28 (extended).gmf
Edited:
Processed:
21 Aug 2015
21 Aug 2015AMC ConsultantsLicensed to:
Analysis
Results
4
Multiple Stability Analysis
Method:
Surface:
Spencer-Wright
Circular
Critical Factor of Safety: 1.24
Interslice Force (Final) Angle: 28.9 °
Material K eys
WOB (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)
WR (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

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

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