PowerPoint Presentation
Targeted buffer blasting to control movement along bedding plane
shearsJohn Latilla AMC ConsultantsBatdelger Tumur-Ochir Energy
Resources, Mongolia(Chris Oldroyd stood in as presenter on the
day)26 November 2014BOHOGS / BBUGS Suppliers day - MoranbahUkhaa
Khudag Coking Coal Mine, Mongolia
Locality
InfrastructureCHPP (annual washing capacity of 15 million tonnes
coal)Power plant (18 MW)250 km Road to Gants Mod on the Chinese
borderCamp and AimagAirfieldPlanned rail
linkInfrastructureTavantolgoi coal basin and UHG infrastructure map
(proposed railway line is in red)
CHPP (with 3 modules)
Power plant
Central office at mine site
Workshops
Camp
Residential village for workers
Coal seams qualities and dipsIn 2012, total mined coal 8.6Mt
(3.65Mt hard coking and 1.6Mt thermal product by CHPP) In 2013,
total mined coal 9.2Mt (5.2Mt hard coking and 2.2Mt thermal product
by CHPP) Multiple seams dipping generally 3 to 17 into
highwallFlanks (endwalls) dipping 5 to 40 out of pit wallProduction
rateTerrace mining (truck and shovel)Ex-pit waste dumpingCoal 9.2Mt
(2013) capacity of 15M Tonnes per annumOverburden removal 52.9M BCM
per annum (2013)Overview of structure and major stability
issues
Mine plan (current pit shell with faults)
E-W structureStrata dipping into highwall along the main
direction of advance
(3 times vertical exaggeration on cross section)N-S
structureGenerally dipping into the pit. There has been a
significant amount for folding and thrust faulting resulting in
bedding shearsBedding plane shears are common in coal seams and
more common in higher quality seams Other shear zones also present
can be up to tens of metres wide. Not part of this study
N-S structure (LOM pit design with current pit shell)
N-S structure (LOM pit design with current pit shell)
Geological structure summaryComplex structure with multiple
major disturbance phases, faulting, folding, and shearingMain
northern and southern fault zones (boundary faults) plus numerous
other faulted zones (fault corridors) which still contain coal
seams but are difficult to model and predict their structure
20
Influence of bedding plane shears
Major driver of significant sliding failuresStructure rather
than rock strength defines slope behaviourApprox. 90% of all
significant failures are classified as sliding failures along
bedding shearsVery low cohesion (1.5 kPa) and friction angle (13)
assumed confirmed by back analysis and very similar to quoted
values in literatureRecorded instances of nearby production blasts
initiating and further driving failures along bedding
shearsFailures along faults have occurred but not commonSouth
endwall slope failure (July 2013)Buffer blast strip overrun by the
leading edge of the failure by between around 10 and 15m10m wide by
6m high waste buttress proposed for on top of the buffer
blastOverall about 20m of displacement the entire section of slope
moved as one unit along bedding plane shear 2m below roof of 0C
seam (Large areas moved as solid blocks with only occasional cracks
visible)Site geotechs measured accelerating opening of cracks
(monitored crack meters) and gave warningFailure triggered by
confined, high energy production blast in box cut sited on SW
corner of failure (Shot # 547 maximum instantaneous charge
4,604kg)Long straight fissure along sub vertical shear visible
after failure. An earlier blast (left in place) lay along the line
of the line of the fissureConsiderable work done on slope damage
due to blast vibrations (not the subject of this presentation).
Intact rock expected to be damaged for a distance of up to 150 m
from blast edge and single blast estimated to be enough to cause
failure up to about 60 m awaySouth endwall slope failure (July
2013)
24South endwall slope failure (July 2013)
South endwall slope failure (July 2013)
Galena analysis static FOS
Galena analysis pseudo static earthquake option
North endwall detached slope (August 2013)Cracking first
observed near crest at dispatch officeCrack monitoring indicated
opening up associated with production blastingCrest was unloaded
10m high by 50m wide in mid Sept. This assisted the longer term
stabilityCoal was recovered from beneath the failure and buffer
blasting was frequently used to anchor the toe to enable coal
extractionNorth endwall detached slope (August 2013)
0C coal recovery below north endwall detached slope
Potential solutions
Buffer blastWaste buttress
Managing bedding plane shearsMine coal out following dip slope
angle same as dip or shallower. In many cases this is not optimal
for coal recovery especially in tough financial times (targeting
most advantageous stripping ratio)Waste buttressing issues with
in-pit dumping at present so not used routinelyTargeted buffer
blasting relatively light charges to rough-up zones containing
bedding plane shears. Increasing cohesion and friction angle. This
approach is analysed in this presentation
How does buffer blasting work?
FOS=1.13FOS=1.22What happensBlasting disturbs the bedding plane
shears and results in disrupted continuity along the bedding shear
planesThis results in an increase of cohesion and friction angleTo
achieve best results, the blast must only be strong enough to
disturb the ground and not completely pulverise itBuffer blasts for
different purposesTargeted buffer blast strip ( the subject of this
presentation):Utilised where a target zone (typically a coal seam
containing bedding plane shears) has been identifiedThe intention
is to disrupt the bedding plane shears at the seam level and then
displace the rest of the overlying strata without completely
fragmenting itStrata dip generally 5 to 20
Buffer blasts for different purposesBench buffer blast:Entire
batter, plus the bench behind, it is identified as being so
structurally disturbed that it is better to blast it and obliterate
all structureThe entire batter and bench are blasted with a similar
charge weight as a normal production blast and the blasted material
is then excavated at a slope angle of between 40 and 45Also
referred to as softwall blasting or shot-in-place buttressingStrata
dip generally >20
Material properties of buffer blasted rock?For UHG the following
Mohr Coulomb material properties have evolved with time: Unit
weight 21kN/m3, c=70 kPa and =32Based on:Unsaturated Cat 4 Spoil
(Simmons and McManus, 2004) c=50 kPa and =35Softwall paper: (Kelso)
=30Bowen basin softwall: c=100 kPa and =35Phreatic surface:Assumed
the buffer blast material acts as a drain thereby dropping the
phreatic surfaceCurrent UHG phreatic surface model derived from
dipping water levels in blast holes prior to charging upPhreatic
surfaceAssumed the buffer blast material acts as a drain thereby
dropping the phreatic surfaceCurrent UHG phreatic surface model
derived from dipping water levels in blast holes prior to charging
upThe following simplified model for the depth of the average
in-pit water level is suggested:At surface 23mBelow bench / batter
crests 15mBelow bench / batter toes6 m Below overall slope toe and
under pit floor1mBelow buffer blast areas Surface conforms to base
of buffer blast
Limit equilibrium analyses of buffer blasting potential
FOS=1.48FOS=0.94Limit equilibriumGalena is the preferred
software used for LE analyses site geotechs also use GalenaModels
usually built by tracing cross sections from the geological model
some are fairly complex and a few have used up the 50 allowed
material profilesBuffer blasts are limited to max 40 m in design
stage due to drilling constraints aim is to intersect known zones
of bedding plane shearsBuffer blast width is arrived at iteratively
with target FOS of 1.2. In some cases a supplementary waste
buttress is needed to achieve the target FOSLimit
equilibriumTargeted buffer blasting to control movement along
bedding plane shears is considered a practical option within a
strata dip range of 5 to 20Minimum no sliding along bedding plane
shears is expected where the dip is less than 5Maximum practical
limitation indicates that targeted BB will be difficult at dip
>20. At steeper dip it is considered best to extract coal along
dip mining from the top downAlternatively, BB the entire slope in a
series of 50m batters (bench buffer blasting)Practical
implementation
Identification of areas requiring buffer blastingBench or
targeted buffer blast required?Galena analysis of selected cross
sections as supplied by mine geotech team.If FOS = 40m
1010kgIntermediate holes 20-40m 332kgShallow holes < 20m
316kgPowder factoravg. 0.36kg/bcm (0.14 to 0.52)Maximum
instantaneous charge (MIC (8ms))2532kgResults
Summary of results4 successful, 2 possibly successful and 1
unsuccessful
Blast block IDPit sectorDateRemarks586aNEW13/09/2013Unnecessary
in retrospect - flat seam dip identified in subsequent (closer)
cross section. Indicated dip at time of design
6605NEW125/09/2013Successful (without subsequent placement of waste
buttress)397ELW3/12/2012Successful (ramp operating on top of buffer
block - no cracks
observed)433NEW14/02/2013Successful480NEW123/03/2013Successful512SEW1A6/05/2013Unsuccessful
(major endwall failure, triggered by box cut blast, overran buffer
strip). Buffer narrower than planned and waste buttress not placed
on top. Buffer may have prevented the failure from extending
further down slope.675SEW1A29/11/2013Possibly successful - slope
behind buffer stable but narrow strip between buffer and toe is
unstable (where they overlap) - floor heave at toe. Part of floor
heave and toe instability area is not in front of the buffer
blast.343SEW112/10/2012Probably successful (slope stable but
exposed buffer portion of slope does not appear very
disrupted)Unsuccessful caseShot# 512 Outcome - Probably helped but
major south endwall failure overran (pushed?) this buffered area.
Crackmeter monitoring indicated that slope movement was triggered
by blast vibrationPlanned width 30m but using 7.5m burden spacing
means outer rows of holes were 15m apart. 7.5m burden may be
optimistic for light BB chargesPowder factor 17 kg/bcmBuffer blast
overrun by 15m by the front of the failureNo floor heave observed
on pit side of buffer strip6m high by 10m wide waste buttress
planned on top of buffer strip not placedOther Site personnel
report that there were no cases where:A buffer blast was
recommended but not implemented and then the slope failed (failure
due to not being buffer blasted) Recommended buffer blast not done
but slope remained stable (stable even though not buffer
blasted)
ConclusionsSummary of conclusionsThis is a relatively small
sample of cases and as such the following conclusions should be
treated with caution:In 86% of cases studied the buffer blasts have
been successful or possibly successful in stabilising the
slopeBlast vibration has triggered movement in some cases this has
received significant attention on site and is far better controlled
nowIt appears that, on average, a slope 11 above the strata dip can
be held with the aid of buffer blastingAlso note that conditions at
UHG are generally quite dry low rainfall and no really strong
aquifersFOSDesign dimensionsBlast block IDPit sectorSeam dipOSA of
slope above buffer blastBefore BBAfter
BBDepthWidthRemarks()()(m)(m)586aNEW12 to
6151.251.424010Unnecessary605NEW18200.881.14 /
1.22*2740Successful397ELW15NA0.641.232238Successful433NEW15271.171.55010Successful480NEW15161.011.361530Successful512SEW1A5
to 11181.141.25 / 1.21^1030Unsuccessful675SEW1A12 to
9130.621.03**3230Possibly successful343SEW15 to
10240.811.1^^2243Probably successfulSummary of
conclusionsPerceptionsMining personnel think that buffer blasting
helps the slope stability because of the result of the successful
buffered slopes, especially the Northern Endwall ones56Room for
improvement and future developments
Planning, planning, planningA more pro-active method of
designing buffer blast blocks. At times the rate of mining is such
that buffer blasts are not designed in timeImproved and quicker
identification of areas requiring buffer blasting planning TARP
implementation. To pre-identify Code Red zones (where buffer
blasting is most likely to be required)Post blasting assessmentsNo
photos of the previous buffer blasts once exposed. This will be
done in futureBuffer blast assessment data sheet to be developed to
collect all relevant data Fully buffer blasted endwallsIn some
areas the strata dip in endwalls is >20 and the final slope
angle is planned to be just less than the strata dip angle by 1 or
2 (to accommodate ramps)Limit equilibrium analyses indicates that
it may be possible to steepen the OSA by as much as 4 by bench
buffer blasting all the batters. Fully buffer blasted slopeFully
buffer blasted endwalls
OSA 20OSA 24Thank you
Energy Resources, Mongolia are thanked for their assistance in
preparing this presentation and for permission to share this
experience