Transcript
Geotechnical Challenges in a Lower Margin Underground Coal Industry
(Or Back to The Future 4)
Russell Frith
Mine Advice Pty Ltd
Introduction
• in September 2012, coal industry changed (forever most likely)
• coal prices of US$200 - 300 per tonne may not ever come back
• Australian $ is a “safe haven” for overseas investors in
preference to the US$ (could remain high for some time yet)
• knew something fundamental had occurred when Martin
Ferguson (Federal Minister for Resources) and Marius Kloppers
(BHPB CEO) were agreeing with each other
• industry is now (again) unit cost rather than volume (coal at any
price) driven – geotechnical challenges AND OPPORTUNITIES
Some Experience Since 2007
• 5 Megabolts per m in MG Belt Roads that had previously been adequately
stable at 3 Megabolts per 2 m.
• Hardly a well-engineered system: symptomatic of coal at any price.
• MG roof did not collapse (for a long way into goaf).
• 19% O2 behind the longwall goaf seals (spon com risk).
• Pumping Rocsil foam behind the MG shields to isolate goaf from panel
ventilation ($$)
• Extraction delayed as tendon installation struggled to keep pace with
longwall rate of retreat
• THERE IS ALWAYS A DOWNSIDE OF BEING GEOTECHNICALLY RISK
ADVERSE – WHERE WE HAVE GOT TO TO A LARGE DEGREE
Where Were We: 1990’s Early 2000’s?
• use ACARP Projects to gauge focus of industry
• C6033: Improving the Up-Time Efficiency of Roadway Development
Units by the Use of Reduced Primary Bolting Densities and Routine
Secondary Support
• C6036: Chain Pillar Design. Calibration of ALPS
• C3032: Roadway Roof Stability and Its Attainment through Pre-
Tensioned Bolting
• C8019: Application of 50 to 60 Tonne Cable Pre-Loads in Difficult
Ground Conditions
• C1107: Investigation of Pillar Extraction Goaf Edge Formation for
Improved Safety
Where Were We: Late 90’s Early 2000’s?
• C9017: Rapid Roadway Development
• C11027: ADRS – Rib Support Design Methodology for
Australian Collieries
• C1445: Optimisation of Powered Support Performance in
Relation to Strata Loading and Engineering Criteria
• C7019: Optimisation of Longwall Mining Layouts Under Massive
Strata Conditions and Management of the Associated Safety
and Ground Control Problems
• C9018: Systems Approach to Pillar Design
Where Were We: Late 90’s Early 2000’s?
• Improved or maintaining mine safety
• Faster roadway development
• Optimum coal pillar design
• More effective/value for money support hardware
• Improved geotechnical design
• More reliable longwall production
• AS PER TODAY IN FACT
• Therefore, we can perhaps use this history to help define the
future
What are the Main Differences Today?
• UNSW Graduate Diploma since 2003 – qualified (and mostly competent)
geotechnical engineers on most mine sites
• credible design methodologies for many of the geotechnical problems
linked to underground coal mining
• well-established Strata Management Systems including workforce
training, monitoring systems, TARPS etc.
• in hindsight, in late 90’s and early 2000’s, we were 10 years too early to
fully benefit from the research work that had been done in industry since
mid-1980’s
• 2013 – timing is right (necessity, knowledge and the ability to implement)
PERFECT STORM
What are the Benefits of Making Geotechnical Changes at All?
• Newton’s 3rd Law states that an object’s condition will
not change unless acted upon by an external force
• ENGINEERING INTERPRETATION: if you don’t
touch it, you cannot break it!!
• So..is it worth interfering with our current strata
control systems (should we wake the sleeping dogs)?
• OPINION: Yes, as will now attempt to demonstrate
Why Change at All?
• I was mentored many years ago by several wise men
• if roadway development costs you money, do as little of it as
possible,
• maximise the efficiency of what roadway development you actually
have to do,
• utilise secondary support according to value for money and not
just cost (they are not the same thing), and
• flexibility in mining is not to be under-estimated
• FUNDAMENTAL 30 YEARS AGO AND STILL ARE TODAY
What Are the Barriers to Change?
• Discussion with a mine operator in 1998 (ACARP Project C6033)
• Cannot reduce primary bolting density as we don’t have labour to
install routine secondary support. WHY?
• Don’t make enough profit to pay for the extra labour. WHY?
• Don’t meet longwall budgeted tonnes. WHY?
• Have production outages between successive panels. WHY?
• Cannot drive roadways fast enough. WHY?
• Install too many roof bolts at development face!!!
• WE NEED TO CHALLENGE AND BREAK THESE ENTRENCHED
POSITIONS
Subject Areas Discussed Today
• Roof bolt lengths
• Top-down or bottom-up grouting of tendons?
• Application of cavity fill to longwall face
recovery
• Pillar extraction (it still has a role –including
QLD?)
Primary Roof Bolt Lengths
• Increased bolt length increases cost, drilling time (particularly
double- pass drilling: self-drilling bolts!) and slows roadway
development
• Australian industry – 1.8 m to 2.4 m long bolts
• US industry – 1.2 m to 1.8 m long bolts
• Why?
• Low development heights are certainly a restriction in the US but is
there anything else?
• Examine the known issue of gloving/ resin un-mixing and potential
solutions that are available now (ACARP Project C21023)
Basis of the Problem (1) • the upper portion of a roof bolt can be affected by both “gloving” and
“resin un-mixing” – has been endemic to our industry here for many
years
• both act to reduce or corrupt the resin bond between bolt and rock and
so reduce the effective length of the roof bolt
• influenced by:
1. the properties of the plastic film used in the resin capsule,
2. the properties of the mastic/filler and
3. the relative proportions of mastic and catalyst in the resin cartridge
– now explained
Basis of the Problem (2)
• problem is evident by uncured resin and/or a resin
colour variation
Basis of the Problem (3)
• DSI video showing the “shredding” of their patented film and large granule mastic
as compared to other products FILM SHRED SHORT VIDEO CD.mpg
Severity of the Problem
• laboratory tests
• bolts 1-3: non-gloved
• bolts 4-6: gloved
• in situ tests
• gloved/resin unmixed bolts
perform no better than 10% of a
properly encapsulated bolt
Previous Research (1)
• undertaken in NZ by
Solid Energy and SCT
• evaluated different bolt
profiles, bolting rigs,
installation methods
etc for 15:1 resins
Previous Research (2)
Previous Research (3)
• “an average 450 mm of bolt length is typically effected by gloving and/or un-
mixing (range 30 mm to 790 mm). 65% had in excess of 500 mm gloved
length”
Previous Research (4)
• top 400 mm of the bolt is
gloved
• effective bolt length is reduced
by 400 mm
• a reliable solution to this
problem should allow bolt
lengths to be reduced without
geotechnical risk
Available Products
• DSI, Jenmar and Minova all have “US type” 2:1 resins
available for use in the Australian coal industry – majority of
mines still use 15:1 resin though?
• not all identical as the DSI resin also contains the more brittle
plastic film and larger limestone fragments in the mastic (see
earlier video)
• ACARP project was proposed on the basis of evaluating the
DSI US resin product
Previous Testing (DSI)
• grout filled pipes prior to bolt
installation
• placing grout filled pipe
above surface bolting rig
Previous Testing (DSI)
• installing an AX bolt into the
grout filled pipe
• cutting the pipe open
Previous Testing (DSI)
• removing the bolt surrounded by grout
• breaking grout off the resin encapsulated bolt
Testing Results (DSI)
• ungloved and mixed resin at top of bolt (2:1 resin)
• gloved and unmixed resin at top of bolt (15:1 resin)
Grouting – Tendons and Strata Consolidation
• ACARP Project C18022 examined the development of and potential
strata control benefits of adhesive bonds between injected materials
(grout or resins) and roof strata
• Emanated from anecdotal mining experiences that once a conventional
cable bolt was installed in very poor roof conditions as the last TARP
response and bottom-up grouted, roof stability was never a problem
after that
• Raised the question as to why?
• Refer to the outcomes of this project to explore some possible
opportunities for future use in industry
Project Basis • adhesive bonds in the order of 4 MPa for injectable material being quoted by
suppliers in 2009
• a 3 MPa adhesive bond across only 3 m of roof is equivalent to 15 x 63 tonne
tendons per m or the weight of some 120 m of roof strata!!!
• if resins or grouts injected into strata pro-actively rather than reactively once
roof is failed, theoretically they will have a significant reinforcing effect
Testing Arrangement
• samples are laterally gripped not glued – effective and quick
• top and bottom shackles are used rather than rigid platens
• basis for conducting further adhesion bond testing if required
Adhesion Test Results
Top Down (No Permeation) v Bottom-Up Grouting (Permeation)
• NSW mine – better outcome using BU
grouting when roof < 60 mm (reinforcing
effect)
• PUR injection through point-anchored
tendons when required averted several roof
falls at Crinum (makes sense given test
results)
• when using bottom-up grouting in a
deteriorated LW installation roadway, those
areas that had moved most on first pass,
moved the least upon widening
• appears that bottom-up grouting of tendons
may be worth another serious look
• data source Payne 2008
Summary
• the adhesive bonds formed in open fractures between injected
material and strata can be a significant part of the overall stability
equation in all support applications (reinforcement, consolidation
and suspension)
• in hindsight, they were largely lost when the industry moved to top-
down grouting
• hence we often tend to neglect them in support design
• needs a serious re-think (improved support effectiveness and
improved value for money) as there is great value to be liberated
Use of Cavity Fill on Longwall Faces
• currently argument in industry as to the suitability of two
different cavity fill products used in the marketplace
• one is stronger than the other (0.05 MPa v 0.025 MPa UCS)
and inevitably the lower UCS product is slightly cheaper
• argument is as to the significance of a 0.025 MPa difference
in UCS (overlooking the fact that one is 100% stronger than
the other!) – if it is judged as insignificant, two products are
essentially the same – logically use the cheaper product
• is it that simple?
Nature of the Problem
• requirements when using cavity fill are it (i) must stay in place en
masse as the face mines beneath it, (ii) must confine loose strata
around the cavity/face and prevent it falling onto the face/AFC and
(iii) allow the inclination of the canopy to be corrected
• on the above basis, why the UCS of the cavity fill is overly relevant to
its in situ performance is not obvious. SO WHAT IS?
Comments • more interested in (a) shear strength [internal cohesion and friction], (b)
its adhesive strength with rock, (c) its propensity to shrink after being
emplaced and (d) filling the entire cavity - WHY?
• Shear strength is a better indicator of a materials ability to support its own
weight when cut as a vertical face
• Many roof cavity shapes are unstable wedges – need the cavity fill to stick
to the rock to have the best chance of keeping it in place
• No point in sticking it to the rock if it then shrinks significantly - adhesive
bond is likely to be subsequently lost
• Do not want large amounts of loose rock dead-loading the fill – major
surcharge that will act to de-stabilise it
• CLUE: some cavity fills contain inert fillers: FILLERS ARE KILLERS relating to
internal shear and adhesive strengths, but have a far lesser impact on UCS
• NEED A MORE RELEVANT AND INFORMED TECHNICAL DEBATE ($ involved)
Pillar Extraction
• was the mainstay of the early Australian underground coal
industry
• gradually been phased out (almost) in favour of longwall mining
• had a poor safety record leading into the mid-1990’s
• does it still have a role to play in the future and one that could
be expanded?
• OPINION = Yes – now attempt to explain
Pillar Extraction
• a good example of where quality
cannot be inspected into a product
• need to look at pillar extraction design
in more detail
90
50
2113
0
20
40
60
80
100
1970 1993 1998 2009
% Bord & Pillar tonnes of total NSW ROM tonnes
Pillar Extraction Design – Required Outcomes
• all methods of pillar extraction attempt to achieve the following:
1. maximise coal extraction
2. maximise roadway development rate
3. maximise rate of extraction (wheeling distances, shuttle car
change points, minimise CM flits etc.)
4. double-sided lifting as often as possible
5. promote safe working conditions (splitting/developing near the
goaf edge, lifting off conditions (roof and rib), goaf flushing)
• inevitably some of these conflict, hence the numerous methods
developed in the attempt to optimise the extraction layout (no
universal solution, not until recently anyway)
Extraction Layout Design Basics • 4 fundamental considerations
1. SAFETY
2. RESERVE RECOVERY
3. RATE/COST OF PRODUCTION
4. SUBSIDENCE (NSW particularly)
• inevitably, at least one of these has to be
compromised to benefit the others
• safety is a given, subsidence control is often a
condition of mining and we need to stay in business –
all about leaving coal behind (planned or
unplanned)!!
Modified Old Ben
• set up similar to a longwall (gate roads plus extraction panel)
• final splitting is done towards the goaf (characteristic of Old Ben)
• lots of intersections (geotechnical downside), but close shuttle car
change point and two routes back to the boot end (productivity
upside)
Wongawilli/Modified Wongawilli
• developed in NSW in the
1950’s.
• friable roof conditions led to
the need to minimise
intersections (pre the era of
effective bolts and tendons etc.)
• less intersections (splitting
along goaf edge only) than
Modified Old Ben, but long car
change point for much of cycle
(less productive)
Origin of the Duncan Method
• Duncan Colliery in Tasmania
• high cover depth (up to 350 m)
• thick seam (up to 3 m)
• overlain by dolerite sill (up to 250 m thick)
• required a non-caving method that could work efficiently
and safely at high depth of cover (which it does)
• explain by reference to Tasman Mine (Sutherland and
McTyer 2012)
Duncan Colliery
Cornwall Coal – Duncan Colliery
Tasman Mining Lease
• works the Fassifern Seam which
outcrops on the N,E & W
boundaries of the lease
Tasman Mine Plan
1 South Panel
overlying old workings < 6 m separation
Duncan Non-Caving Extraction System
• both operational and surface
subsidence control reasons led to
the use of a modified Duncan
Method of pillar extraction
• square pillars formed (45 m
centres) and then stripped on all
four sides
• the remnant pillar is designed to
be load-bearing and also “squat”
(high w/h)
2 North Panel – four way intersection
3 North Panel extraction
Duncan Method Summary
• Duncan Method aims to reduce strata control hazards in pillar
extraction to as low as reasonably practical (ALARP) whilst
maintaining reserve recoveries and mining efficiencies at
acceptable levels
• founded on the favourable behaviour of high w/h ratio or squat
pillars, not just Factor of Safety
• leaves coal behind with a purpose rather than on an ad hoc basis
in total extraction where coal left behind works against safety
Duncan Method Summary • efficient use of development roadways
• no pillar splitting near goaf edge
• reduced abutment stresses at the goaf edge
• low extraction spans – caving minimal and often back from goaf
edge
• efficient layout in terms of production rates
• excellent subsidence and groundwater control
• good reserve recoveries
• universal operator acceptance
• impeccable safety record over the past 13 years in difficult mining
conditions at Duncan and Blackwood Collieries particularly
• as close to an optimum pillar extraction method that ticks all of the
boxes as we have ever had
Overall Presentation Summary • industry can benefit from marginal efficiency improvements in a
whole range of geotechnical areas as part of current fiscal
challenges (if it wants to)
• this is not geotechnical risk taking for the sake of improved
business performance, but optimisation of current practices and
support hardware improvements
• we have the design methods (most of them anyway), people at
mines and management systems to justify and implement them
over time
• NO SHORT CUTS (Indian anecdote)
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