Russell frith, mine advice

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

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


• “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”


• 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


• installing an AX bolt into the

grout filled pipe

• cutting the pipe open


• 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)

Russell frith, mine advice

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