Targeted buffer blasting to control

movement along bedding plane shears

John Latilla – Thiess (Formerly – AMC Consultants)

Batdelger Tumur-Ochir – Energy Resources, Mongolia

Coal 2016

University of Wollongong, 10-12 February 2016

Ukhaa Khudag Coking Coal Mine, Mongolia

Locality

Infrastructure

CHPP (annual washing capacity of 15 million tonnes coal)

Power plant (18 MW)

250 km Road to Gants Mod on the Chinese border

Camp and Aimag

Airfield

Planned rail link

Infrastructure

Tavantolgoi 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 dips

High quality hard coking coal plus thermal

Multiple seams dipping generally 3° to 17° into highwall

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

Production rate

Mining contractor Thiess (Asia)

Terrace mining (truck and shovel)

Ex-pit waste dumping

Coal – 4.6Mt (2014) capacity of 15M Tonnes per annum

Overburden removal – 26.3M BCM per annum (2014)

Production fleet capable of meeting the demands of 15Mtpa CHPP

Pit dimensions (end 2015) ±2.2km E-W, ±2.0km N-S and ±170m deep

Overview of structure and major stability

issues

Mine plan (current pit shell with faults)

E-W structure

Strata generally dipping into highwall along the main direction of advance

(±3 times vertical exaggeration on cross section)

W E

N-S structure

Generally dipping into the pit. There has been a significant amount for

folding and thrust faulting resulting in bedding shears

Bedding plane shears are common in coal seams and more common

in higher quality seams

Other shear zones are also present and can be up to tens of metres

wide, they often cut through bedding and are associated mainly with

thrust faulting. Not part of this presentation.

N-S structure (LOM pit design with current pit shell)

S N

N-S structure (LOM pit design with current pit shell)

NS

Geological structure summary

Complex structure with multiple major disturbance phases, faulting,

folding, and shearing. Dominant environment is compression.

Main northern and southern fault zones (boundary faults) plus

numerous other faulted zones (fault corridors) which still contain coal

seams but where it is difficult to model and predict their structure

Influence of bedding plane shears

Major driver of significant sliding failures

Structure rather than rock strength defines slope behaviour

Approx. 90% of all significant failures are classified as sliding failures

along bedding shears

Zero cohesion and friction angle (13°) assumed – confirmed by back

analysis and very similar to quoted values in literature

Recorded instances of nearby production blasts initiating failure as well

as re-mobilising failures along bedding shears

Failures along faults – have occurred but not common

South endwall slope failure (July 2013)

Buffer blast strip overrun by the leading edge of the failure by between around 10 and 15m

10m wide by 6m high waste buttress proposed for on top of the buffer blast but not placed

Overall about 20m of displacement – the entire section of slope moved as one unit along beddingplane shear 2m below roof of 0C seam (Large areas moved as solid blocks with only occasionalcracks visible)

Site geotechs measured accelerating opening of cracks (monitored crack meters) and gave warning

Failure 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 major tensile crack along sub-vertical shear visible after failure. An earlier blast (left inplace) lay along the line of the shear

Considerable work done on site to quantify slope damage due to blast vibrations (not the subject ofthis presentation). Intact rock expected to be damaged for a distance of up to 150 m from blast edgeand single blast estimated to be enough to cause failure up to about 60 m away

South endwall slope failure (July 2013)

Approx. position

of major tensile

crack

South endwall slope failure (July 2013)

South endwall slope failure (July 2013)

Toe position with reference to buffer blast

strip after failure

Bedding shear

Failed section of slope

Buffer blast strip

No heave evident down

dip of buffer blasted strip

Galena analysis – static FOS

Galena analysis – pseudo static earthquake

option

North endwall detached slope (August 2013)

Cracking first observed near crest at dispatch office

Crack monitoring indicated opening up associated with production

blasting

Crest was unloaded 10m high by 50m wide in mid Sept. This assisted

the longer term stability

Coal was recovered from beneath the failure and buffer blasting was

frequently used to anchor the toe to enable coal extraction

North endwall detached slope (August 2013)

0C coal recovery below north endwall

detached slope

Buffer blast &

waste buttress

at toe

Potential solutions

Buffer

blast

Waste

buttress

Managing bedding plane shears

Annual external assessment of proposed pit wall designs. Low FOS

slopes identified (may be the entire slope, or more commonly, a portion

of the slope or an individual batter stack)

Options:

• Mine 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

routinely

• Targeted 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=1.13

FOS=1.22

What happens

Blasting disturbs the bedding plane shears and results in disrupted

continuity along the bedding shear planes

This results in an increase of cohesion and friction angle

To achieve best results, the blast must only be strong enough to disturb

the ground and not completely pulverise it

Note – this is not a new idea:

• Earliest identified example Civils in Tennessee “shot in place buttress” late

1960’s (Moore 1986)

• Softwall blasting is a similar concept with the aim being to obliterate

structure (Kelso 2011)

Buffer blasts for different purposes

Targeted buffer blast strip ( the subject of this presentation):

• Utilised where a target zone (typically a coal seam containing

bedding plane shears) has been identified

• The intention is to disrupt the bedding plane shears at the seam

level and then displace the rest of the overlying strata without

completely fragmenting it

• Strata dip generally 5° to 20°

• Also referred to as shot-in-place buttressing (Moore 1986)

Buffer blasts for different purposes

Bench buffer blast (not covered here):

• Entire batter, plus the bench behind, it is identified as being so

structurally disturbed that it is better to blast it and obliterate all

structure

• The 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 45°

• Also referred to as softwall blasting (Kelso 2011)

• Strata dip generally >20°

Material properties of buffer blasted rock?

For UHG the following Mohr Coulomb material properties have evolved with

time: Unit weight 21.4 kN/m3, c=60 kPa and ϕ=33°

Based on:

• Unsaturated Cat 4 Spoil (Simmons and McManus, 2004) c=50 kPa and ϕ=35°

• Softwall paper: (Kelso) ϕ=30°

• Bowen Basin softwall: c= 50 to 100 kPa and ϕ=35°

• Tennessee, in jointed sandstone with shale bands, application - civils for road

(Moore 1986 - in Xanthakos 1994) ϕ=38°, unit weight 22kN/m3

Phreatic surface

Assumed the buffer blast material acts as a “drain” thereby dropping the

phreatic surface

Current UHG phreatic surface model derived from dipping water levels in blast

holes prior to charging up

The following simplified model for the depth of the average in-pit water level is

suggested:

At surface 23m

Below bench / batter crests 15m

Below bench / batter toes 6 m

Below overall slope toe and under pit floor 1m

Below buffer blast areas Surface conforms to base of buffer blast

Limit equilibrium analyses of buffer blasting

potential

FOS=1.48

FOS=0.94

Limit equilibrium

Galena is the preferred software used for LE analyses – site geotechs

also use Galena

Models usually built by tracing cross sections from the geological

model – some are fairly complex and a few have used up the

50 allowed material profiles

Buffer blasts are limited to max 40 m in design stage due to drilling

constraints – aim is to intersect known zones of bedding plane shears

Buffer 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 FOS

Limit equilibrium

Targeted buffer blasting to control movement along bedding plane

shears is considered a practical option within a strata dip range of

5° to 20°

Minimum – no sliding along bedding plane shears is expected where

the dip is less than 5°

Maximum – 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 down

Alternatively, BB the entire slope in a series of 50m batters (bench

buffer blasting)

Practical implementation

Identification of areas requiring buffer

blasting

Bench or targeted buffer blast required?

Galena analysis of selected cross sections as supplied by mine

geotech team.

If FOS <1.2 then:

• Reduce slope angle

• Place buttress

• Carry out buffer blast at toe

Depth, width and length of buffer blast block

Depth depends on locality of suspected bedding plane shear zones

and is also limited by drill capability (generally ≤ 40m but one BB of

50m has been blasted

Width determined by Galena analysis – incrementally increased in size

until FOS≥1.2

Length of buffer blast strip is derived by considering a set of cross

sections (Galena is a 2D code)

Buffer blast layout and design

Some issues:

• Drilling capacity

• Depth of bedding shear zones

• Slope of berm or bench must be suitable for safe drilling – wide

enough and flat enough

• Angled buffer holes have been considered but not yet used

• Sometimes it’s too late to buffer the area because the OB is

production blasted or there is no OB left on top of coal seam etc.

Buffer blast layout and design

Deep buffer

blast

example

Buffer blast layout and design

Targeted buffer blast charge weight typically around 40% of the normal charge

weight for a production blast of the same depth:

Burden normally 7.5m

Drill hole diameter usually 229mm

Average charge weight per blasthole

• Deep holes >= 40m 1010kg

• Intermediate holes 20-40m 332kg

• Shallow holes < 20m 316kg

Powder factor avg. 0.36kg/bcm (0.14 to 0.52)

Maximum instantaneous charge (MIC (8ms)) 2532kg

Buffer blast scheduling

The scheduling of the blast must be such that the buffer

strip is blasted before mining of the coal or other strata

down dip from it. In other words, while the planned buffer

blasted toe is still buttressed by solid rock or coal.

Drilling and blasting the buffer strip at the slope toe after

the down dip material has been removed, would entail

drilling and charging up on a bench at the toe of a sub 1.0

FOS slope.Note that this slide was added after Coal 2016 in response to a question from a

delegate. The need to schedule the blast as outlined on this slide was implied but not

specified – this additional page rectifies that shortcoming.

Results

Blast

block

ID Pit sector Date Remarks

586a NEW1 3/09/2013

Unnecessary in retrospect - flat seam dip identified in subsequent (closer)

cross section. Indicated dip at time of design 6°

605 NEW1 25/09/2013 Successful (without subsequent placement of waste buttress)

397 ELW 3/12/2012 Successful (ramp operating on top of buffer block - no cracks observed)

433 NEW1 4/02/2013 Successful

480 NEW1 23/03/2013 Successful

512 SEW1A 6/05/2013

Unsuccessful (major endwall failure, triggered by box cut blast, overran

buffer strip). Waste buttress not placed on top. Buffer may have prevented

the failure from extending further down slope.

675 SEW1A 26/11/2013

Probably 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.

343 SEW1 12/10/2012

Probably successful (slope stable but exposed buffer portion of slope does

not appear very disrupted)

Summary of results4 successful, 2 probably successful and 1 unsuccessful

Unsuccessful case

Shot# 512

Outcome - Probably helped but major south endwall failure overran (pushed?)

this buffered area. Crackmeter monitoring indicated that slope movement was

triggered by blast vibration

• Planned 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 charges

• Powder factor 17 kg/bcm

• Buffer blast overrun by ±15m by the front of the failure

• No floor heave observed on pit side of buffer strip

• 6m high by 10m wide waste buttress planned on top of buffer strip – not

placed

Other

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)

Conclusions

successful

possibly successful

unsuccessful

Summary of conclusions

This 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

probably successful in stabilising the slope

• Blast vibration has triggered movement in some cases – this has

received significant attention on site and is far better controlled now

• It appears that, on average, a slope of up to 13° above the strata dip

can be held with the aid of buffer blasting

Also note that conditions at UHG are generally quite dry – low rainfall

and no really strong aquifers. Different outcomes possible in wetter

conditions.

FOS Design dimensions

Blast

block

ID

Pit

sector Seam dip

OSA of

slope above

buffer blast Before BB After BB Depth Width Remarks

(°) (°) (m) (m)

586a NEW1 2 to 6 15 1.25 1.42 40 10 Unnecessary

605 NEW1 8 20 0.88 1.14 / 1.22* 27 40 Successful

397 ELW 15 NA 0.64 1.23 22 38 Successful

433 NEW1 5 27 1.17 1.5 50 10 Successful

480 NEW1 5 16 1.01 1.36 15 30 Successful

512 SEW1A 5 to 11 18 1.14 1.25 / 1.21^ 10 30 Unsuccessful

675 SEW1A 12 to 9 13 0.62 1.03** 32 30 Probably successful

343 SEW1 5 to 10 24 0.81 1.1^^ 22 43 Probably successful

Summary of conclusions

Perceptions

Mining personnel think that buffer blasting helps the slope stability

because of the result of the successful buffered slopes, especially the

Northern Endwall ones

Room for improvement and future

developments

Planning, planning, planning…

A more pro-active method of designing buffer blast blocks. At times the

rate of mining is such that buffer blasts are not designed in time

Improved 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 assessments

No photos of the previous buffer blasts once exposed. This will be

done in future

Buffer blast assessment data sheet to be developed to collect all

relevant data. Particularly aim at photographic record of bedding plane

shears after they have been disturbed.

Fully buffer blasted endwalls

In 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 slope

Fully buffer blasted endwalls

OSA 20°

OSA 24°

Thank you

Energy Resources, Mongolia are thanked for their

assistance in preparing this presentation and for

permission to share this experience