A Significant Step Towards

Removing the Roadway

Development Constraint Due to the

Installation of High Bolting and Long

Tendon Densities

Russell Frith

Principal Geotechnical Engineer

Mine Advice Pty Ltd

Statements of “Fact” Up-time efficiency of roadway development units is at least in part

constrained by high bolting densities and the ever more common use

of long tendons in the primary support cycle

In years gone by this what not the case – many mining engineers over

the age of 50 lament this on a regular basis

Our industry is “addicted” to the use of long tendons as a first

response to deteriorating roof conditions at the CM and in gate roads

for secondary extraction – this wasn’t always the case

Roof bolting hardware has changed little in 20 years whereas a new

tendon seem to arrive regularly with a new “brand name”

Self-drilling bolts have not yet proven to be the answer to

development woes (e.g. two-speed resin, single-pass drilling/long

bolts and cost)

Lack of roadway development constrains longwalls and increases cost

Problem is not getting better with time – or so it seems

Take a Step Back for a Second In late 2011, ACARP organised an industry workshop addressing

the issue of installing long tendons from the CM (Gary Gibson)

All manner of presentations (including the author) and in effect, it

summarised where we were at, not necessarily why we were there

SO…why do we install long tendons and high bolt densities as part

of development?

Are we risk averse? Safety not negotiable, but this is not the cause

Is it simply easier than separating support out to less primary and

more secondary? Some of this but more complicated (next slide)

Main reason – logically our roof bolts don’t work as well as we

would like or need them to. No option but to use high densities and

cables.

NEED SUBSTANTIALLY MORE EFFECTIVE PRIMARY ROOF

BOLTS

Actual Discussion

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!!!

VICIOUS CIRCLE – Q. How to Break It?

Ensham Late 2013

Received a call from Tony Crozier – somewhat

troubled by what he was observing

Cut and flit using 14 to 15 m cut-outs in 6.5 m wide

roadways (highly stable extended cuts)

BUT: install roof support and noticed roof often

cracking and starting to deteriorate

Q. Aren’t roof bolts supposed to improve roof

stability? YES – something must not be right

Very rare (most development units are CM’s) but as

it turned out, critical observation.

One Year Ago I spoke at this conference and mentioned some roof

bolt development work that was being done by MA

and DSI

Focused on gloving and resin un-mixing of resin

associated with 15:1 resins

12 months later that work is complete, has

uncovered some other “problems” and a commercial

product addressing each of them is now available

Briefly describe (very brief in fact as there is now a

whole new area of roof bolt science) and then

discuss initial roll-out experience

Substantial Problems with Our

Current Bolting Systems current systems (fully encapsulated bolts) have been in use for

many years – established technology and a commodity

recent research studies have shown that it contains a number of

significant problems that reduce bolt effectiveness

gloving and resin un-mixing at top of bolt – reduced effective bolt

length

very high resin back-pressures during installation that tend to

hydro-fracture roof and rib strata

less than maximum use of bolt pre-tension, if at all

tendency for stress-corrosion cracking

PEAK System addresses all of the above (DSI and Mine Advice

initiative – DECLARING COMMERCIAL INTEREST)

Gloving and Resin Un-Mixing at

the Top of the Bolt

ungloved and mixed resin at top of bolt

gloved and unmixed resin at top of bolt

Gloving of

Plastic Film

gloving occurs as the plastic film does not “shred” during bolt

installation

PEAK system resin uses a more brittle plastic and larger

limestone fragments

RESULT: plastic shreds into much smaller pieces

Resin

Un-Mixing

when resin pressurises (towards top of bolt), catalyst

becomes forced to circumference of bolt hole

can be easily missed with a 15:1 resin, not so with a 2:1

resin

PEAK system uses a 2:1 resin, current system is a 15:1

RESULT: significantly improved resin mixing with a 2:1

particularly as hole diameter increases

PEAK System v Current Resin

ungloved and mixed resin at top of bolt (PEAK)

gloved and unmixed resin at top of bolt (current)

Resin Back Pressure

as a bolt is spun into a hole, if the resin cannot migrate along the

bolt at the same rate as the bolt is being inserted into the hole,

pressure inevitably builds up in the resin

use of longer bolts, more resin, small diameter holes, fast

insertion rate, powerful bolting rigs and a bar profile that pumps

the resin up the hole, all contribute to the generation of resin

back pressure

has the potential to fracture the roof strata in the top section of

the bolted interval and cause resin loss into the strata so that full

encapsulation is not achieved

common response is to use a smaller hole and more resin –

encapsulation generally gets worse as back pressure increases

as a direct consequence!

US Test Data

5000 psi (34 MPa) is slightly above the setting pressure of

longwall shields

maximum pressure ever measured is 68 MPa (twice the

setting pressure of shields)

Local Test Data

Pump efficiency can reduce by 1% for every 0.025 mm increase in

clearance between an impellor and casing

This is why a 1 mm increase in annulus thickness (2 mm hole

diameter increase) has such a profound reduction in resin back

pressure development

Wongawilli Mine

Thick coal roof in Wongawilli Seam – CMRR of 35

Old roadways supported on 5 bolts/m → hand-held

bolting, not fully encapsulated and 6 m cut-outs

But resultant roof conditions generally favourable

Current drivages in same area – 6 x 2.1 m fully

encapsulated bolts per m installed from CM near face

Roof conditions commonly broken and deteriorated

Why would a theoretically better bolting system in use

today installed close to the face result in less stable and

more deteriorated roof conditions?

ONLY LOGICAL ANSWER: resin back pressure

Back Pressure Solution

little doubt that resin back pressures in our current roof and rib

bolting systems act to de-stabilise the roof and rib (weak strata

most affected)

makes sense then that higher bolting densities rarely work and

long tendons in common industry use

cannot eliminate back pressure but can substantially reduce it by

making some minor changes

PEAK System: slightly larger hole diameter (28 mm bit), shorter

bolt (no more than 1.8 m), less resin (600 mm for a 1.8 m bolt)

and a neutral deformed profile on the bolt (herringbone that does

not pump resin back up the hole)

BUT WON’T A LARGER HOLE AND LESS RESIN REDUCE

BOLTING SYSTEM EFFECTIVENESS?

Hole Diameter

and Load

Transfer

ACARP Project 10022 (UNSW)

load transfer strength and stiffness did not vary between

a 28 mm and 30 mm diameter hole

BUT back pressure development is massively reduced at

30 mm compared to 28 mm (1000 mm of resin)

Pre-Tensioning of Bolts

ACARP studies in the mid 1990’s fully demonstrated

the potential benefits of increased roof bolt pre-

tension

sample extensometer data on the next slide to prove

the point

done with single speed motor hydraulic rigs stalling

at around 200 ft.lbs (8 to 10 tonnes pre-tension)

current rigs stall at around 300 ft.lbs – but we are

not making best use of this

PEAK system addresses several of the issues

Evidence in Support of Pre-Tensioned Roof

Bolts (1)

low-tensioned (2-3 tonnes) 2.1 m fully encapsulated bolts

bolted interval fully delaminates

time dependent trend is erratic and high magnitude

displacements occur

Evidence in Support of Pre-Tensioned

Support (2)

higher bolt pre-tension (8 to 10 tonnes) – same bolts

initial 0.5 m of roof now clamped together – no delamination

substantial improvement in time dependent trend by simply clamping

together the initial 0.5 m of roof

deliberately clamping the first 0.6 m or so of roof is an important part of

the PEAK System

clamped section

Current Pre-Tension Problems

main one is that operators may not always tighten nut at

prescribed point in time – attempting to improve operational

efficiency

bearing plates are relatively small due to use of mesh –

higher contact pressures with roof and loss of head loads

over time

current nut designs unable to allow increased pre-tensions

due to more powerful drill rigs (> 14 tonnes) to be reliably

achieved

an unreliable top resin anchor due to gloving and resin un-

mixing does not allow pre-tension levels to be increased

PEAK Pre-Tension Improvements

reliable top resin anchor (2:1 resin)

longer nut (1.5 D) – better conversion of drill rig torque to

bolt pre-tension (> 14 tonnes)

larger plate (200 mm) – better able to maintain pre-

tension levels at the bolt head

deliberate use of an un-encapsulated free length (0.5 m

to 0.6 m) at bottom of bolt – operators can tighten nut at

any time, allows less (single speed) resin to be used

(lowers resin back pressure)

NB use of a short free length does not then result in a

point-anchored bolting system

Summary of Our Current Bolting

Systems

substantial portion of the bolt length is not working to stabilise

the roof (or rib)

part of the bolt length is acting to de-stabilise roof (or rib)

PEAK System

this design also solves

two of the self-drilling

bolt issues – single

speed resin and shorter

bolt/single pass drilling

use of steel with

increased fracture

toughness (above

BS)

Early Experience (Both Ends of the

Geotechnical Spectrum) Ensham: 1.8 m fully encapsulated bolt to 1.5 m partially

encapsulated bolt

Improved roof control and place change units now CM rather than

bolting critical – increased production (published by the mine)

Russell Vale: 2.1 m fully encapsulated bolt to a 1.8 m partially

encapsulated PEAK bolt (slight single bolt cost reduction too!)

Substantial improvement in visible roof conditions and stability,

maintenance of minimum support far more of the time (improved up-

time efficiency and reduced cost per m of drivage), support pods

lasting much longer between change-outs etc.

Universal enthusiastic workforce acceptance at both mines

Fair to state that initial impact has been profound and significantly

impressed those operations who have trialled and evaluated the

new technology (START OF A JOURNEY)

Summing Up: Initial Experience

New bolting hardware technology – maintains the benefits of

both pre-tension and load transfer BUT eliminates or minimises

the inadvertent negative aspects within our current systems

Significantly increased individual bolt effectiveness

Lower overall primary support densities, shorter bolts (lighter

bolts) AND improved roof stability

Increased rates of development – just by staying on minimum

support more of the time (not started optimising yet)

Primary support costs per m of roadway reduced in combination

with decreased geotechnical risk

6 m and 8 m cables removed from Development TARP – use of

increased bolting as a TARP response now an option again