Russell Frith - Mine Advice Pty Ltd
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Transcript of Russell Frith - Mine Advice Pty Ltd
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