Longwall geomechanics Flashcards
what is one of the key elements in achieving high production rates
operational excellence
what are the critical geotechnical considerations
*face stability
*tip to face roof stability
*floor stability
*caving behaviour
how are geotechnical consideration managed
panel design, support design, good face management and operational practices
what are ideal face conditions
*flat and intact roof
*face needs to stay vertical
*caving of immediate roof behind the shields
*competent floor is important
what are some examples of good and bad conditions
good - face sprags folded back as the shearer goes past
bad - large cavities form above the face where large rock falls occur at the face
why were longwall supports changed to two legs from the old four
two legs support and apply load closer to the face this is where is is most beneficial. The back legs of four also tend to push the support down at the back and can place the tip at the face down.
two legs also provide some form of horizontal confinement
what was the detached block model and theory
it was the first way to determine the amount of strength the support needed. The theory was based about the weight above the block. It determined the greater mining height to need greater support
Why is the block model not seen in practice? what is seen instead?
in practice it is seen that the strata deforms and the behaviour of coal is time dependent. Detached block model does not deal with issues of guttering failure and without the support of the issue
What did the simplified near seam model improve upon
it recognised that the ground being supported goes into the face. It identified that there was a cantilever behaviour.
it accounts for the time dependent closure
what is the time dependent closure driven by
from the weight of the block there is a surcharge loading (abutment loading) and is what drives the supports down
Why is retreat rate key
it allows you to get through any heavy loading events before they cause a problem
how big should the rating of the longwall sheilds be
as high as possible whilst being practical to capital and mining costs
what is needed in addition to set:yeild
a high yeild loading
what is tip to face distance
the most forward point of the roof canopy contact
what are methods of face working
conventional
one web back
what happens with a faster retreat rate
there is a lower roof convergence above the power supports this led to the positive of a lower amount of bending in the immediate roof
there is also less time for weighting induced fractures
what does support effectiveness rely on
hydraulic stiffness
Is canopy ratio important
no it is meaningless
what increases with the face spalling?
tip to face distance increases
when is the face sub critical
depth of cover > face width (w/h < 1)
what is periodic weighting mechanisms
when there is an uncontrolled rotation of massive strata about the face.
as the extraction height increases so does the void into which unstable block can rotate
what happens as a massive strata unit gets thicker
*stored load increases
*centre of gravity moves further behind the face
*face must travel further to progress beyond limits of weighting
what is the cure for roof falls
numerous weighting roof falls occur because the wall was not moving or was moving to slowly during the critical stage of weighting
what is the cure for roof falls
keeping the wall moving quickly through the critical stage of weighting
What is hydrofacking used for
to induce caving by jetting high pressure water into the massive strata and may also use inflatable packers
what is the critical stages of massive strata and caving
anything in the fist 50 m can be a problem but the first 20 m are the most critical
what is the best control for aggressive periodic weighting
panel width change
(reduction of panel width increases the pillars)
What are some pros of increasing face width and panel length
*less longwall moves reduce time and cost
*cut at a higher rate
*fewer number of gateroads
*lower capital cost per tonne
*less coal sterilised in pillars
*lower development meters per tonne
What are some cons of increasing face width and panel length
*slower face retreat
*harder to align and steer
*harder to ventilate
*harder to get power sources and water to the face as well as repair broken machinery
*higher overall inital capital cost
*takes longer to get first coal
*chain pillar size may change
what are other geological consideration affecting the longwall
faults and dykes
