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

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)