Longwall geomechanics

Question 1

what is one of the key elements in achieving high production rates

Answer 1

operational excellence

Question 2

what are the critical geotechnical considerations

Answer 2

*face stability
*tip to face roof stability
*floor stability
*caving behaviour

Question 3

how are geotechnical consideration managed

Answer 3

panel design, support design, good face management and operational practices

Question 4

what are ideal face conditions

Answer 4

*flat and intact roof
*face needs to stay vertical
*caving of immediate roof behind the shields
*competent floor is important

Question 5

what are some examples of good and bad conditions

Answer 5

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

Question 6

why were longwall supports changed to two legs from the old four

Answer 6

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

Question 7

what was the detached block model and theory

Answer 7

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

Question 8

Why is the block model not seen in practice? what is seen instead?

Answer 8

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

Question 9

What did the simplified near seam model improve upon

Answer 9

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

Question 10

what is the time dependent closure driven by

Answer 10

from the weight of the block there is a surcharge loading (abutment loading) and is what drives the supports down

Question 11

Why is retreat rate key

Answer 11

it allows you to get through any heavy loading events before they cause a problem

Question 12

how big should the rating of the longwall sheilds be

Answer 12

as high as possible whilst being practical to capital and mining costs

Question 13

what is needed in addition to set:yeild

Answer 13

a high yeild loading

Question 14

what is tip to face distance

Answer 14

the most forward point of the roof canopy contact

Question 15

what are methods of face working

Answer 15

conventional
one web back

Question 16

what happens with a faster retreat rate

Answer 16

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

Question 17

what does support effectiveness rely on

Answer 17

hydraulic stiffness

Question 18

Is canopy ratio important

Answer 18

no it is meaningless

Question 19

what increases with the face spalling?

Answer 19

tip to face distance increases

Question 20

when is the face sub critical

Answer 20

depth of cover > face width (w/h < 1)

Question 21

what is periodic weighting mechanisms

Answer 21

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

Question 22

what happens as a massive strata unit gets thicker

Answer 22

*stored load increases
*centre of gravity moves further behind the face
*face must travel further to progress beyond limits of weighting

Question 23

what is the cure for roof falls

Answer 23

numerous weighting roof falls occur because the wall was not moving or was moving to slowly during the critical stage of weighting

Question 24

what is the cure for roof falls

Answer 24

keeping the wall moving quickly through the critical stage of weighting

Question 25

What is hydrofacking used for

Answer 25

to induce caving by jetting high pressure water into the massive strata and may also use inflatable packers

Question 26

what is the critical stages of massive strata and caving

Answer 26

anything in the fist 50 m can be a problem but the first 20 m are the most critical

Question 27

what is the best control for aggressive periodic weighting

Answer 27

panel width change
(reduction of panel width increases the pillars)

Question 28

What are some pros of increasing face width and panel length

Answer 28

*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

Question 29

What are some cons of increasing face width and panel length

Answer 29

*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

Question 30

what are other geological consideration affecting the longwall

Answer 30

faults and dykes