1. ULTRA-CLOSE MULTIPLE SEAM MINING
-ANALYSIS AND VERIFICATION
Daniel Su, Senior Geomechanical Engineer
Luke Stull, Geologist
Maria Jaime, Geomechanical Engineer
Jun Lu, Geomechanical Engineer
31st International Conference on GroundControl in Mining
August 2, 2012
2. Ultra-close Multiple Seam Mining - Analysis and Verification
1. Mining History
2. Geological Reconnaissance
3. Purpose of the Study
4. Numerical Analyses
5. Outcome
6. Conclusions
Outline
CONSOL Energy, Inc.
3. Ultra-close Multiple Seam Mining - Analysis and Verification
1. Mining History
2. Geological Reconnaissance
3. Purpose of the Study
4. Numerical Analyses
5. Outcome
6. Conclusions
Outline
CONSOL Energy, Inc.
12. Ultra-close Multiple Seam Mining - Analysis and Verification
> 38 feet ofVery Competent Strata Strong, Non-water-sensitive
CONSOL Energy, Inc.
13. No.2 Gas Seam Overburden
Ultra-close Multiple Seam Mining - Analysis and Verification CONSOL Energy, Inc.
14. Pillar type criteria
Ultra-close Multiple Seam Mining - Analysis and Verification
Determining Factors:
Overburden depth: 650 ft @ No. 2 Gas seam
Pillar / entry dimensions
Isolated Remnant Pillar:
Wp-ir ≤ 127 ftH5Wp
Gob-solid boundary:
Wp-gs ≥ 237 ftH3.9Wp
CONSOL Energy, Inc.
15. Ultra-close Multiple Seam Mining - Analysis and Verification
1. Mining History
2. Geological Reconnaissance
3. Purpose of the Study
4. Numerical Analyses
5. Outcome
6. Conclusions
Outline
CONSOL Energy, Inc.
16. No.2 Gas seam
Powellton seam
H > 650’
Dint ≈ 42’
4’
4’
20’
Ultra-close Multiple Seam Mining - Analysis and Verification
17. Compliance & Safety Challenge
Low breakthrough potential (IC 8741 Bureau of Mines)
Undermining justified
CFR Sec. 75.388 (f)
Boreholes In Advance of Mining
“If mining is to be conducted within 50 feet above or below an
inaccessible area of another mine, boreholes shall be drilled, as
necessary, according to a plan approved by the district manager.”
Initial Drilling Plan (MSHA):
Alternate Drilling Plan ?
Strata / Mine stability ?
Ultra-close Multiple Seam Mining - Analysis and Verification
Under Pool Areas Every X-cut Into #2 Gas mine
Elsewhere Every 200 feet Into #2 Gas mine
CONSOL Energy, Inc.
18. Finite Element Analysis
Within Interburden & Pillars:
Stresses & deformation
Safety Factors
Analysis of Multiple
Seam Stability (AMSS)
On Pillars only :
Vertical Stress
Average Safety Factor
Ultra-close Multiple Seam Mining - Analysis and Verification CONSOL Energy, Inc.
19. Ultra-close Multiple Seam Mining - Analysis and Verification
1. Mining History
2. Geological Reconnaissance
3. Purpose of the Study
4. Numerical Analyses - AMSS
5. Outcome
6. Conclusions
Outline
CONSOL Energy, Inc.
20. MSHA approved software to evaluate the multiple seam
mining stability
Developed by NIOSH andWVU
Software is based on LaModel (boundary element method)
Analysis of Multiple Seam Stability
Ultra-close Multiple Seam Mining - Analysis and Verification CONSOL Energy, Inc.
21. AMSS: Input / Output
Ultra-close Multiple Seam Mining - Analysis and Verification CONSOL Energy, Inc.
22. AMSS Output: Stability Factor
Safety factor MSHA guidelines:
Under water safety zones: > 2.0
Elsewhere: > 1.5
Ultra-close Multiple Seam Mining - Analysis and Verification CONSOL Energy, Inc.
23. All Safety Factors meet MSHA’s guidelines
Ultra-close Multiple Seam Mining - Analysis and Verification CONSOL Energy, Inc.
AMSS Output: Stability Factor
Pillar Centers
24. Ultra-close Multiple Seam Mining - Analysis and Verification
1. Mining History
2. Geological Reconnaissance
3. Purpose of the Study
4. Numerical Analyses - FEM
5. Outcome
6. Conclusions
Outline
CONSOL Energy, Inc.
25. FEM: Model Input
Ultra-close Multiple Seam Mining - Analysis and Verification
Isolated Remnant Pillar: 90 ft
gobgob
Crosscut centers: 90 ft
depth = 650 ft
depth = 694 ft
Crosscut
20 ft x 4 ft
CONSOL Energy, Inc.
26. FEM: Model Input
Ultra-close Multiple Seam Mining - Analysis and Verification
Determined through
laboratory testing
Density
Elastic
Modulus
Comp.
Strength
Poisson
Ratio
(lb/ft3) (psi) (psi) (-)
Sandstone 162 1,500,000 14,000 0.2
Fireclay 160 1,000,000 9,400 0.3
Shale 156 875,000 7,000 0.35
Massive Sandy Shale 160 1,400,000 17,700 0.25
Gob 100 3,500 max 1,440 0.49
Coal 82 210,000 900 0.3
CONSOL Energy, Inc.
27. FEM Output: Vertical Stresses
Ultra-close Multiple Seam Mining - Analysis and Verification CONSOL Energy, Inc.
28. FEM Output: DILATION - Vol. Strain
Ultra-close Multiple Seam Mining - Analysis and Verification CONSOL Energy, Inc.
No enhanced
Permeability of the
Sandstone Interburden
29. FEM Output: Strength Factor
Ultra-close Multiple Seam Mining - Analysis and Verification CONSOL Energy, Inc.
30. Ultra-close Multiple Seam Mining - Analysis and Verification
FEM Validation
0
500
1000
1500
2000
2500
3000
3500
4000
500 600 700 800 900 1000 1100 1200 1300 1400 1500
VerticalStress(psi)
Horizontal Coordinate of Pillar(ft)
Vertical stress on critical pillar (in the middle) at Lick Branch
mine provided by the FEM Phase2 model
3,083 psi
CONSOL Energy, Inc.
31. Ultra-close Multiple Seam Mining - Analysis and Verification
FEM Validation
Vertical stress on critical pillar (in the middle) at Lick Branch
mine provided by the AMSS program
0
1000
2000
3000
4000
500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600
VerticalStresses-Detail(psi)
Location of Pillar (ft)
Analysis of Multiple Seam Stability
Single Seam Stress Total Vertical Stress
2,870 psi
CONSOL Energy, Inc.
32. Ultra-close Multiple Seam Mining - Analysis and Verification
1. Mining History
2. Geological Reconnaissance
3. Purpose of the Study
4. Numerical Analyses
5. Outcome
6. Conclusions
Outline
CONSOL Energy, Inc.
33. Positive Outcome
Ultra-close Multiple Seam Mining - Analysis and Verification
New Drilling Plan Proposed CFR Sec.75.388(f)
Under Pool Areas Every X-cut Into #2 Gas mine
Elsewhere Every 200 feet Into #2 Gas mine
Under Pool Areas Every x-cut 25 feet up
Elsewhere Every 300 feet 25 feet up
CONSOL Energy, Inc.
34. Positive Outcome
Ultra-close Multiple Seam Mining - Analysis and Verification
Underground observations after 8 months of
development:
1. No water encountered during drilling
Proved FEM safe interburden results
2. Pillar rib integrity: Intact
Proved AMSS safe pillar size design
CONSOL Energy, Inc.
35. Ultra-close Multiple Seam Mining - Analysis and Verification
1. Mining History
2. Geological Reconnaissance
3. Purpose of the Study
4. Numerical Analyses
5. Outcome
6. Conclusions
Outline
CONSOL Energy, Inc.
36. Conclusions
Utilized AMSS and FEM to ensure the stability of
the pillars and interburden for Lick Branch mine.
Optimized the interburden drilling plan to monitor
the potential for inflow.
Therefore,
Guarantying a safe and productive mining
environment.
Ultra-close Multiple Seam Mining - Analysis and Verification CONSOL Energy, Inc.
……where we proved to ourselves as well as to MSHA that it was SAFE and possible to mine Lick Branch in a room and pillar mode, under rather limiting conditions.
Buck Run #1 Mine (2 Gas)
This is one of the coreholes we drilled in the property, and we can se how nice and consistent this sandstone looks. After testing for the strength and water sensitivity of all the rock units, we confirmed that this sandstone is very competent, being above-average strong and not water-sensitive.
Moreover, the most determinant factor here was the fact that we had at least 38 feet of good sandstone and we exceeded the minimum thickness required by the Bureau of Mines guidelines, which was 35 feet.
We had this scenario:
No.2 Gas seam in this region is located at least 650’ deep, and below there is Powellton seam, with a short interburden distance of at least 42’.
No.2 Gas seam has been all mined out during the late 40’s with the exception of some large barrier pillars.
So the majority of the areas are gobbed out, and SOME areas have been flooded.
On the other hand, Lick Branch mine is located along Powellton seam, and we would like to mine this in a room-and-pillar fashion.
So, the main concern for MSHA was the potential for water breakthrough, which becomes an imminent hazard to the operation of Lick Branch.
According to guidelines of the U.S. Bureau of Mines, room and pillar mining can be performed under abandoned flooded mine works when the interburden is greater than or equal to either: 5 times this entry width or 10 times the entry height. Based on the desired entry width we don’t meet these criteria, so we have a second chance to prove low breakthrough potential if we have enough competent strata in the interburden.
Fortunately, we run into very good geology in this particular site. As we can see here, all the orange dotted units are massive sandstone, over and underlaid by shale and some fireclay units, which turned out to be competent as well as the sandstone.
UNDER THESE CIRCUMSTANCES, it was determined that the potential for water breakthrough is low and that undermining IS justified.
NEVERTHELESS…we have to comply with the code of federal regulations’ section 75.388 that requires operators to drill boreholes into the coal before they extract it. In this way, the operator can determine whether mining will penetrate an area where unknown hazards may be present.
In this particular case, subsection (f) applies to us, and it states that “If mining is to be conducted within 50 feet above or below an inaccessible area of another mine, boreholes shall be drilled, as necessary, according to a plan approved by the district manager.”
An initial plan had been discussed with MSHA, such that we had to drill holes every x-cut up to the overlying mine under pool areas, and obviously, when finding water, this had to be pumped out (how long could this take??? No one knew). Additionally, under the areas that we didn’t expect to be flooded, we had to drill every 200 feet.
It was evident that this drilling plan would have been very cumbersome and not cost-efficient. Also, we felt that with this extensive drilling, the interburden might be prone to cracking and those unknown conditions (such as water or methane) in the upper seam could become imminent hazard for Lick Branch Mine.
That’s when we undertook this project to try to come up with a solution to demonstrate that this plan was not needed.
We would have to make an stability assessment of the interburden and show that it was competent enough to protect Lick Branch operation.
We have 2 approaches to study the stability of Lick Branch mine.
One is the traditional computer program AMSS provided by NIOSH to analyze the interaction between 2 coal mines. This specifically will provide the pillar size design.
The FEM approach is to try to reproduce what happens in the entire field, specifically within the interburden of the old and new mines.
the results of AMSS are limited to the calculation of the vertical stress on the underlying pillars, as well as the safety factor for these pillars,
On the other hand, we have an advance numerical model that calculates stresses, deformation and safety factors distributed throughout the whole domain of the model.
The key point here was to converge to a similar response using both of these methods.
As part of the input parameters in AMSS we have geometric details of the old works, and the specification of the TYPE OF REMNANT PILLAR.
There is another section were the geometric details of the active mine are input.
And as a result, the calculations provide the pillar stability factor.
In the background of this black outline you see the old works’ remnant pillars and the water safety zones surrounding the flooded areas in No. 2 Gas mine.
So under these water safety zones MSHA required that we demonstrated a safety factor greater than 2.0 for Lick Branch mine pillars, and greater than 1.5 in areas other than the water safety zones.
This black outline was the proposed layout of Lick Branch room and pillar panels.
1) And for each specific panel, the most critical conditions were analyzed in AMSS in order to determine the pillar size to meet MSHA’s safety factor requirements.
As you can see in this particular example panel, pillars with center-to-center dimensions of 60 by 90 produce a stability factor of 2.03.
We did the same analysis throughout all the mine plan.
So, again, although the Lick Branch mine pillars satisfied the minimum requirement for this multiple seam mining scenario, we still had to study the stability of the interburden between both mines in order to propose a different drilling plan.
That’s when we utilized the Finite Element Method approach.
***Stability factors (SF) are obtained by dividing the total load-bearing capacity of the AMZ (active mining zone) by the total load applied to it.
The Active Mining Zone (AMZ) includes all of the pillars on the extraction front (or "pillar line"), and extends outby the pillar line a distance of 5 times the square root of the depth of cover. This breadth of AMZ was selected because measurements of abutment stress distributions (Mark, 1990) show that 90% of the front abutment load falls within its boundaries.
This picture here is only a clip of the whole model that simulated far away boundary conditions.
One of the important aspects of analyzing the interaction between 2 coal seams, is the size of this overlying pillar. The smaller the pillar, the more concentrated the stresses will be on this, so higher stresses will be transferred to the underlying pillars. In this particular case, this 90-ft wide pillar is the smaller one we come across at Lick Branch, so it is in fact the most critical or hazardous case scenario.
Once gravity has been applied, we open the entries, and see how this pillar is affecting these underlying ones.
For details on the numerical model configuration you can refer to the paper.
But here is important to say that in order to have the most accurate performance of a finite element model, the input geometry and specially the material properties of all these different units have to be as realistic as possible.
In this regards, we spent a great deal of time trying to obtain accurate values for the rock, gob and even coal parameters to put into the model.
We tested in the laboratory all the rock units (present in the model) from 2 coreholes, and these are the average values of the input parameters.
We got HIGH enough values of compressive strength for the interburden rock, so we were confident that the model would provide good results.
This is a snapshot of the distribution of the vertical stress throughout the entire field.
Here is Lick Branch mine, and up here is the overlying seam.
The colors range from blue at zero vertical stress, to red being the highest magnitude.
You can see how below the Isolated Remnant Pillar we have the largest concentration of vertical stress, and it’s clearly transferred to the lower seam pillar.
And if I zoom into that critical area, we see how this particular configuration could compromise this pillar at the Lick Branch level.
It’s important to mention that there is NO support in place yet, and this is only the ground’s natural response.
This is a snapshot of the distribution of the vertical stress throughout the entire field.
Here is Lick Branch mine, and up here is the overlying seam.
The colors range from blue at zero vertical stress, to red being the highest magnitude.
You can see how below the Isolated Remnant Pillar we have the largest concentration of vertical stress, and it’s clearly transferred to the lower seam pillar.
And if I zoom into that critical area, we see how this particular configuration could compromise this pillar at the Lick Branch level.
It’s important to mention that there is NO support in place yet, and this is only the ground’s natural response.
One of the most important pieces of information from these models is the distribution of safety OR STRENGTH factors.
In this particular picture, the colors range from red being the weakest and most hazardous locations, to blue being the stronger and safer.
As far as pillar stability, MSHA’s requirement is that the average safety factor on pillars should be larger than 2.0. So we look at this distribution on the critical pillar, we have average ranging from 2 to 3. Therefore we meet the requirements.
On top of the entry roof we see how the shale unit would not stay up if it’s not supported. And furthermore, we see that beyond 19 feet high into the roof, the strength factor is greater than 1.5 within the interburden.
From these results, we should be able to determine the appropriate roof support AS WELL AS THE DRILLING EXTENT TO COMPLY WITH THE REGULATIONS.
Well, you might be thinking “how are these results reliable ??? ” Or “Has this model been validated against anything else ???”
And here is the answer: We have compared stress distributions from both AMSS and the FEM model and they compare well.
***
Generalized Hoek-Brown material model:
Strength Factor: refer to Invariants of Stress Notes sheet.
The average vertical stress on the critical pillar (under isolated remnant) is only 7% different when comparing FEM and AMSS results.
With this we feel confident of our results and we were able to use these to change our proposed drilling plan.
On a final note… we were more than pleased that with the results of our study, CONSOL Energy’s core values were successfully fulfilled.
For those of you who don’t know them, here they are:
