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

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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