Geotechnical Challenges in a Lower Margin Underground Coal Industry (Or Back to The Future 4)
Mine Advice Pty Ltd
•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
•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
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
•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 (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
•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)
•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
•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
•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
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
•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?
•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)
•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
•a good example of where quality cannot be inspected into a product
•need to look at pillar extraction design in more detail 905021130204060801001970199319982009% 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
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)
•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)
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)
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)