Mining engineer questions

[tomferry]

Hello all I no we have lez and a couple other old mining engineers on here, I was wondering someone could help us out with a couple questions .

Maths question highlighted in green this has caused large amounts of conversation in our mining Mondays club . You will need to view the second image also .

Other question is I believe with wooden roof tells you hammer them in loosely , when passing if you feel the need you wiggle them to check they are not tight . If they are tight this means the roof is dipping and time to insert a prop . Other people in our club say they will drop onto the floor !! Myself I have never heard of this can some one knowledgeable teach us about this ??

I can see with the steel pins there is a chance if the crack grows they would drop out ? But surely this would mean the roof would dip and pinch it tight on the other hand

We need some one of knowledge!

 

[mikem]

Pretty much all engineering formulae include large fudge factors to try & overestimate the safety margin required.

If the roof is dipping from flat then the surface area is getting bigger, so cracks will widen.

 

[tomferry]

For the formula we are trying to work out / understand.

O is 15ft “ distance from rib of passage to pillar”
W is 7ft “pillar width”
Depth is 30ft

I assume D is 150 as really we have sandstone with a clay mixture above us this will easily cover us .

What do we do next then to calculate the width of our pillar required ?

 

[Ouan]

(I will regret trying to work this out...)

Is the depth below surface only 30ft?

in the formula:- d is the depth below the surface = 30 ft. 160 is the estimated overburden density which you say should be 150 as you have mainly sandstone. O is the passage width, which if I understand correctly, will be 30 ft (2 x “distance from rib of passage to pillar”), while W is the 7 ft pillar width.
Roof pressure on the pillar L = 150*30*((7+30)/7) = 23,785 lb/sq ft = 165 psi

Need to know the crushing strength of the coal to be able to calculate minimum pillar width (w), but if depth is only 30 ft, pillars only 1 ft wide result in a roof pressure of 1,156 psi, which is below the crushing strength of the anthracite samples in the book.

This is a very simplified formula, which shouldn't be used to design a modern mine - for an example of paper giving a more complicated method see

https://www.researchgate.net/publication/240362856_Coal_pillar_load_calculation_by_pressure_arch_theory_and_near_field_extraction_ratio

 

[Fjell]

“d” is depth in feet, correct me if I am wrong. 160 is the rule of thumb for whatever area this applies to (it will vary). The last bit just ratios the void.

For a 1000 feet with no void that implies 160,000lb per ft2. Which is an effective overburden density of 2.5sg. Sounds about right.

All voids induce an arching effect which diminishes with relative size (it becomes an edge effect). This arching adjusts the stress around the void, Without it most deep horizontal tunnels would collapse using classical rock mechanics, but in reality they form an oval in proportion to the ratio between vertical and horizontal stress. Very shallow there is no confining horizontal stress, so it simplifies. If you look at deep tunnels in places like SA you will seem them “plastically” deforming to a non-round shape and rock bolted to death.

 

[tomferry]

(I will regret trying to work this out...)

Is the depth below surface only 30ft?

in the formula:- d is the depth below the surface = 30 ft. 160 is the estimated overburden density which you say should be 150 as you have mainly sandstone. O is the passage width, which if I understand correctly, will be 30 ft (2 x “distance from rib of passage to pillar”), while W is the 7 ft pillar width.
Roof pressure on the pillar L = 150*30*((7+30)/7) = 23,785 lb/sq ft = 165 psi

Need to know the crushing strength of the coal to be able to calculate minimum pillar width (w), but if depth is only 30 ft, pillars only 1 ft wide result in a roof pressure of 1,156 psi, which is below the crushing strength of the anthracite samples in the book.

This is a very simplified formula, which shouldn't be used to design a modern mine - for an example of paper giving a more complicated method see

https://www.researchgate.net/publication/240362856_Coal_pillar_load_calculation_by_pressure_arch_theory_and_near_field_extraction_ratio

This is the same as how I worked it out myself .

We are not opening a mine sadly just looking into the extraction areas where pillar robbing has begun , also passages have holes coming through from the other side where the tunnel ribs are so thin the have inter linked .

 

[tomferry]

“d” is depth in feet, correct me if I am wrong. 160 is the rule of thumb for whatever area this applies to (it will vary). The last bit just ratios the void.

For a 1000 feet with no void that implies 160,000lb per ft2. Which is an effective overburden density of 2.5sg. Sounds about right.

All voids induce an arching effect which diminishes with relative size (it becomes an edge effect). This arching adjusts the stress around the void, Without it most deep horizontal tunnels would collapse using classical rock mechanics, but in reality they form an oval in proportion to the ratio between vertical and horizontal stress. Very shallow there is no confining horizontal stress, so it simplifies. If you look at deep tunnels in places like SA you will seem them “plastically” deforming to a non-round shape and rock bolted to death.

Yes this is exactly what I read earlier in the book , This is why coffin levels are good, I believe .

Where they have been extracting the ore in similar iron mines, we have a lot of collapses, because the tunnels have no walls and the openings are so wide just on pillars the roof dips sags and drops, most likely due to the props now being rotten.

 

[Fjell]

Hence about the only old room and pillar mine I would take my nearest and dearest into is the Loch Aline sand mine. Basalt shield roof. I believe it’s open to visitors now. Lovely white sand. When I was very ickle I did an internship there.

 

[Cantclimbtom]

I have no skills to share on this, I did calculate stand up time of tunnels in exams many moons ago, but I just copied the formula from the old green British Standards booklet and then only worked above ground ☹️ (very much above ground in my case)

But.. for anyone else reading this thread who isn't up to speed on how bolting works, the demo of it in this guy's video is brilliant, would've come in handy for me when I was a student (a long time before YouTube!). Hope someone enjoys this edutainment vid

 

[Ian Ball]

Grady helps with a lot of concepts and is an excellent modeller!

 

[TheBitterEnd]

I have no skills to share on this, I did calculate stand up time of tunnels in exams many moons ago, but I just copied the formula from the old green British Standards booklet and then only worked above ground ☹️ (very much above ground in my case)

But.. for anyone else reading this thread who isn't up to speed on how bolting works, the demo of it in this guy's video is brilliant, would've come in handy for me when I was a student (a long time before YouTube!). Hope someone enjoys this edutainment vid

Its a nice video but I'm not convinced that is a good example of the most common case. Generally the bolts go through the rocks, not between them and particularly with resin bonded anchors my guess is that there is not a lot of compressive force.

 

[Fjell]

Its a nice video but I'm not convinced that is a good example of the most common case. Generally the bolts go through the rocks, not between them and particularly with resin bonded anchors my guess is that there is not a lot of compressive force.

Rock bolts are tensioned after installation, often up to more than half their tensile breaking strength. The whole point of bolts (or cables more recently) is move rock from tension into compression since rock is up to 30 times stronger in compression than tension (even leaving aside jointing and loose blocks).

If you think about it, if rock strata were going to fail in compression it would already have happened, and almost all rock we encounter on land was once buried much deeper, so there is a large safety factor that can be utilised for room and pillar mining. Only in offshore delta-type environments are you going to get substantial amounts of ongoing burial and relatively newer and weaker rock for the depth. Offshore Sarawak for instance is a continuous sequence of sediment up to 15km thick which is being steadily buried. Since it includes shales it makes for interesting times as the oil cracks to gas which then starts fracturing upwards before erupting from the seabed in large shakeholes. Something you can easily see happening on the seabed if you look, there is stuff pouring out everywhere. In fact you can see it from space, which is one way of looking for seeps. Same happens in the Gulf of Mexico. Bacteria just love it. Yummy.

 

[tomferry]

Thanks everyone for the help and information!

So who knows about installing roof tells ? . I have always believed if the roof moves this makes a loose tell go tight ? So when you check you realise it’s now solid ??

Or do they drop out onto the floor ?

Who fancies trying to guess what this is . I believe it’s a form of starting handle or jack handle . Solid steel tube .

 

[mikem]

A loose tell may get tighter or it may fall out, all depends how the roof moves.

 

[LJR]

We had to install a lot of tell tales at Dragonby. They consisted of long wires up to 8metres into boreholes fixed to traffic light coloured tubes. If the strata sagged, the wire pulled tighter and the tube showed a visible warning. They are easy to see as you travel past and if there is change you then investigate further. We started a plan to install one at every junction, starting with fourway junctions and then filling in with three way junctions as funds permitted. Not a cheap exercise!

 

[john green]

Hello all I no we have lez and a couple other old mining engineers on here, I was wondering someone could help us out with a couple questions .

Maths question highlighted in green this has caused large amounts of conversation in our mining Mondays club . You will need to view the second image also .
View attachment 16352View attachment 16353

Other question is I believe with wooden roof tells you hammer them in loosely , when passing if you feel the need you wiggle them to check they are not tight . If they are tight this means the roof is dipping and time to insert a prop . Other people in our club say they will drop onto the floor !! Myself I have never heard of this can some one knowledgeable teach us about this ??

I can see with the steel pins there is a chance if the crack grows they would drop out ? But surely this would mean the roof would dip and pinch it tight on the other hand View attachment 16354

We need some one of knowledge!

The nails in the roof could also have been to hang a candle for surveying, depends on the age of the mine

 

[royfellows]

Nails in the roof were to hang a plumb line for surveying purposes. Important to determine exaxt distances and bearing.

 

[legendrider]

The vast ,majority of survey stations I've seen in N Pennines take the form of a wooden peg inserted into a drilled hole, into which a nail or ring nail is driven to hang the survey line, sometimes (ok, rarely) accompanied by a brass ID tag. The 3 nails in the crack dont look like survey stations to me, more like a tell or warning if the crack opens up.

The wriggly steel thing could be a church fete skill tester. Move the wire loop along it without setting off the buzzer to win a goldfish.

Seriously though, my first thought was Brace-and-Bit. Not a great fit I know but just chucking ideas about here

MARK

 

[john green]

The wooden peg was only used if they couldn’t get a nail in the roof, after all it is a lot of work to drill a hole in the roof to put a wooden peg in, they would do all they could to avoid that. The three nails may be to do with getting a better sighting line, I have come across wedges to tighten rocks in roofs but not nails I don’t think they would be strong enough to do anything, and never seen then used in the north Pennines as tell tales.

 

[ChrisB]

The starting handle thing: can you give an indication of the size, either overall or diameter of material?
Also, I'm finding "Solid steel tube" ambiguous - is it solid, or a tube?